INTRODUTION TO GROUND WATER HYDROLOGY
Hydrology is defined as the science concerned with the occurrence, distribution, movement and properties of all waters of the earth and its atmosphere. Include surface water and rainfall, underground and water cycle. Geo-hydrology is a subdivision of hydrology, dealing with the water beneath the Earth’s surface. Its main concern is with water association with earth materials and with water–flow mechanics through the rocks. The term geohydrology , hydrology and groundwater hydrology are not clearly differentiate and have been used synonymously; however in hydrology there is an emphasis on the geological aspects of groundwater, while geohydrology emphasis the fluid-flow aspects.
BASIC CONCEPT OF GROUNDWATER HYDROLOGY
Water resources are mainly divided into, surface water sources and subsurface water sources. All water that occurs naturally below the Earth’s surface is called subsurface water, whether it occurs in the saturated or unsaturated zones. Groundwater referred to without further specification is commonly understood to mean water occupying all the voids within a geologic stratum. This saturated zone is to be distinguished from an unsaturated, or aeration zone where voids are partly filled with water and air.
Groundwater hydrology may be defined as the science of the occurrence, distribution, and movement of water below the surface of the earth. Geohydrology has an identical connotation, and hydrology differs only by its greater emphasis on geology.
ORIGIN OF GROUNDWATER:
Two main sources from water has been originated are,
(1) Continuous disintegration of rocks and minerals (in many minerals H20 molecule is the major part
(2) Volcanic activity.
WORLD WATER RESOURCES:
It is found that 97.2% of water on our planet occurs in sea (i.e., salt water). Remaining 2.8% is fresh water of this nearly 2.15% occurs in ice bodies and 0.65 % occurs as surface water (0.05 %) and groundwater (0.6 %). That is groundwater represents a major proportion of Earth’s usable water resources. This limited resource becomes more precious because the demand for water is growing at an alarming rate.
BASIC TERMS IN GROUNDWATER HYDROLOGY:
Hydrological cycle:
The constant interchange of water between the Oceans, atmosphere, and land areas of the Earth viz. transpiration and evaporation.
POROSITY:
Porosity or the proportion of voids within a rock is a function of all the textural parameters-grain-size, shape, sorting, packing, as well as the amount of void-filling materials, matrix and cement. The absolute Porosity of rock is given by the ratio-total pore volume/Bulk volume of the rock. The effective Porosity is given by the ratio of the interconnected pores to the total bulk volume of the rock. The effective porosity is generally 5-10% less than the absolute porosity.
PERMEABILITY:
Permeability is the property allowing passage of fluid through a rock. The unit of permeability is a dracy. Absolute permeability is defined as the ability of a rock of transmit a fluid when the saturation with that fluid is 100% effective Permeability is the ability of earth materials to transmit sufficient saturated permeable material to yield significant quantities of water to wells and springs, this implies an ability to store and to transmit water; unconsolidated sands and gravels are a typical example.
AQUA=WATER
POROSITY
PRMEABILITY
YIELD
EXAMPLE
Aquifer
(ferre= to bear)
Porous
(store)
Permeable
(transmit)
Gines a siginificant qualities of water to well and spring
Unconsolidated snad sandstone, gravels.
Aquiclude
Porous
Impermeable
Does not yield appreciable quantities of water to wells
Argelloceous rocks like clay and shale
Aquifuge
Not porous
Impermeable
Neither containing minor transmitting water
Solid granite quartzite
Aquitard (tard=slow)
Porous
Poorly permeable
These formations have a considerable number of interconnected fractures, which hold and transmit water.
Sandy clay
WATER TABLE:
The position in the earth’s crust below, which cracks, and other openings are filled with water. Above the water table ( zone of aeration ), the inserstices in earth materials are partly filled with air. In the zone of saturation, the interstices are completely filled with water. This zone extends from the water table downward. The position of the watertable below the ground surface is a function of the topography of an area and of local climatic conditions.
CONFINED AQUIFERS:
Confined aquifers, also known as artesian or pressure aquifer, occur where groundwater is confined under pressure greater than atmospheric by overlying relatively impermeable strata.
If a well is dug or drilled which is deep enough to reach the confined aquifer . the water wikll rise up in the well because the water stands in the wells defines an imaginary surfaces whose height above the aquifer depends on the pressure in the aquifer. This surface is called the Potentiometeric surface.
SPECIFIC YIELD (Sy):
Specific yield is the ratio of the volume (Vw) of water that will drain by gravity from a rock or soil that was initially saturates to the volume (vo) of the rock or soil i.e,
Sy = Vw/ Vb
SPECIFIC RETENTION (sr):
The water that is unable to drain from the pores is referred to as specifc retention (sr), whioch is the ratio of the volume (vr) of water that a rock or a soil, after being saturated, will retain against the pull of gravity, to the volume (vvvvvb) of the rock or soil.
Sr = Vr/Vm
Hydrogeology OF TAMILNADU
The rock types occurring in Tamil Nadu State and Union Territory of Pondicherry can be classified broadly under two types viz., crystalline and sedimentary formations. The former includes granite, gneiss, Charnockite etc., and are of considerable interest as they occupy nearly 73 percent of the total area. Semi-consolidated and unconsolidated formations ranging from the Mesozoic to the present times overlie the crystalline basement and their occurrence is confined to the eastcoast only.
WATER-BEARING FORMATIONS
CONSOLIDATED FORMATIONS
The hard rocks constitute 73% of the total geographical area of the State, which mostly cover the western and central parts. These rocks contain mainly fracture which act as repository for groundwater. The fracture porosity is generally not uniform either laterally or with depth. It is fairly known that the hard rock aquifers are heterogeneous, as indicated by the variations in lithology, structure and texture within short distances. This phenomenon renders it difficult to generalise any uniform hydrogeological regime in hard rocks, and as such every individual area needs to be examined in depth independently, depending upon the hydrogeological set-up of each area. The occurrence and movement of ground water mainly depends on the degree of weathering, topography and interconnection of fracture zones.
The important hard rock aquifers noticed in the State are the weathered gneiss of Coimbatore, Dharmapuri, Salem and Ramanathapuram district, weathered and jointed Charnockite and Khondalite of Madurai, Theni, Tirunelveli, Vellore, Coimbatore and Kanniyakumari district, weathered and jointed ultramafic rocks of Nilgiri district and the cavernous limestone of Tirunelveli district. The hard rock aquifers occurring in south and south-eastern part of Tiruvallur and Kancheepuram districts, western-most part of Thanjavur district, northern and north-western part of Tiruchirapalli district and western part of Pudukkottai district are not highly productive.
Ground water occurs in the hard rocks normally under water table conditions in the weathered mantle and under semi-confined to confined conditions in the fractured rocks. The folded quartzite bands and solution cavities in the limestone of the Tirunelveli district also form potential aquifers. The available hydrogeological data on hard rock aquifers suggest that the depth of weathering varies from 5 to 30m.bgl and the depth of open wells varies between 6 and 50m while depth of borewells in general varies from 30 to 100m. Potential hard rock aquifers are located in Dharmapuri, Coimbatore, Madurai, Theni and Ramanathapuram districts. The yields of open wells range from 50 to 140m3/day in Dharmapuri district and from 10 to 256m3/day in Madurai and Theni district. The yields of borewells very from 13 to 363m3/day in Madurai and Theni districts and from 10 to 256m3/day in Ramanathapuram district. The yields of open wells range from 50 to 140m3/day in Dharmapuri district and from 10 to 256m3/day in Madurai and Theni districts. The yields of borewells vary from 13 to 363m3/day in Madurai and Theni districts and from 50 to 575m3/day in Ramanathapuram district. Similar aquifer zones are also expected in the fractured and jointed rocks of the remaining districts. In the water balance studies carried out by the CGWB in collaboration with Swedish International Development Agency (SIDA) in parts of Coimbatore district, the depth of borewells ranged from 25 to 305m and their yields range from 1.4 to 42m3/hour. Transmissivity of the weathered and partly fractured aquifers varied from 3.5 to 167.8m2/day whereas in limestones it has range of 86 to 114m2/day. Transmissivity of the gneiss in Noyil varied from 0.2 to 496.9m2/day and in Ponnani basin it varied from 2.06 to 61.75m2/day.
UNCONSOLIDATED FORMATIONS
The unconsolidated formations are considered more prolific in comparison to hard crystalline formations. These includes formation ranging in age from U.Gondwana to Recent sediments, comprising Shales, Sandstones, Limestones, Laterites and Alluvium.
The Jurassic Formations, represented by the Upper Gondwanas, are encountered in Tiruvallur, Kancheepuram, Vellore, Tiruchchirappalli and Ramanathapuram districts. Due to the low transmissivity and the compact nature of these formations, they do not contribute much to the ground water resources of the State. The Tertiary formations are the most prolific aquifers and ground water in these formations is under great pressure and flowing wells have been constructed notably in Cuddalore and Villupuram districts, in the Cauvery delta of Thanjavur district, eastern part of Pudukkottai district and in the north-eastern part of Ramanathapuram district.
The Quaternary sediments in the State are represented by the laterite and older alluvium of Pleistocene age and the Recent alluvium, 'Teris' and coastal sands. Cauvery alluvium occupying a major part of the Thanjavur district includes potential alluvial aquifers. Other important alluvial aquifers occur in Tiruvallur, Kancheepuram, Vellore, Pudukkottai and Ramanathapuram districts. Ground water occurs both under water table and confined conditions in the alluvium.
Filterpoint and cavity wells in the alluvium have recorded yields in the range of 15 to 60m3/hour. Important alluvial aquifers occur between depth ranges of 7 and 68m b.g.l. The maximum discharge recorded from the tubewells constructed in alluvium is about 82m3/hour. The transmissivity of the aquifers is found up to 2000 m2/day. The thickness of these aquifers varies from place to place. The "Teri" deposits of Tirunelveli district have a thickness ranging from 5 to more than 30m. In Cuddalore, Villupuram and Ramanathapuram district, tube wells were constructed tapping all the possible granular zones of the soft rocks. The average transmissivity of the aquifer in this region is 550 m2/day with storage co-efficient of 2 x 10-4.
STRATIGRAPHIC SUCCESSION OF FORMATIONS IN TAMIL NADU
Era
Age
Stage
Lithology
Quaternary
Recent
--
Soils, Alluvium and Beach Sands
Tertiary
Pleistocene
--
Boulder conglomerate, older alluvium and laterite.
Pliocene
Karaikal Beds
Sands and Clay with fossils
Miocene
Cuddalore Sandstone
Mottled and friable sandstone, buff-coloured clays and gravel
Cretaceous Niniyur
Arenaceous limestones and sandstones
Sandstones and Clays
Ariyalur
Sandstones and Clays
Trichinopoly
Sandstone, Clays and Shell limestone
Uttatur
Basal limestones, Coral, Clay and Sandy beds
Mesozoic
Jurassic
Satyavedu
Ferruginous sandstone and Conglomerates
(Upper Gondwanas)
Sriperumpudur Clay, Shales and Felspathic sandstones
UNCONFORMITY
Archaean
Archaean
Gneissic complex, and Associated intrusives
Charnockite, Granite basic and Ultrabasic
STRATIGRAPHIC SUCCESSION OF FORMATIONS IN TAMIL NADU AND PONDICHERRY
GEOPHYSICAL SURVEYS:
Based on the geophysical properties involved various geophysical methods are used, depending in their applications and importance in ground water investigations. Among various geophysical methods the electrical method is very suitable for groundwater explorations.
RESITIVITY METHOD:
In the resistivity method, artificially generated electric current are introduced into the ground and the resulting electric current are introduced into the ground and the resulting potential difference are measured at the surface. Deviations from the pattern of potential differences expected from homogeneous of subsurface in homogenates.
FIELD PROCEDURE FOR CONDUCTING A VES USING SCHLUJMBERGER CONFIGURATION
Before conducting a ves in an area it is always useful to study the local geology, well sections, depth to the water table, quality of water, reported water yield etc.
REQUIRED EQUIPMENT:
(1) Resistivity meter.
(2) Power parks (D.C. 90 – 270v)
(3) Multimeter.
(4) Two winches with PVC covered cables of 100-m length.
(5) MN cables.
(6) Metallic electrodes (mild steel, length 45cm) and diameter 4 cm.
(7) Non-polarizing electrodes.
(8) Data sheet and log-log graphs.
(9) Calculator.
(10) Rubber mats to serve as insulators.
(11) Connecting cables.
(12) Hammers 3-4 (about 4kg).
PRINCIPLES:
The place for, conducting a sounding should be selected first. In hard rock region, the electrode expansion should be parallel to the strike direction of joints and fracture, to minimize the errors.
The non-polarizing electrodes needed for measuring the potential differences. Two metallic current electrodes are needed for passing the current in to the ground.
The current electrodes are connected to the instruments.
The ∆U and I values are then displayed on the meter after passing the currents through the current electrodes. This ratio multiplied by configuration constant K, gives the apparent resisitivity value for that separation. The configuration constant for various separations of AB/2 and MN/2 are tabled.
AB/2 = separation distance of caurrent electrode.
MN/2 = separation distance of potential electrode.
The current electrode separations(AB/2) are chosen ;in such a manner that when plotted on a log-log graph paper, the distancesbetween the neighboring points are approximately equal. This is achieved by increasing the current electrode separations by a factor of √2 or 1.5
PRE CAUTIONS TO BE TAKEN:
(1) Large voltages and currents should; not be used as they can cause gave accidents.
(2) Since the symmetric configuration is being used, can should be taken to plant electrodes on either side of the center at equal distances from the center.
(3) The place sould be away from l;industrial areas where leakage is of current hi the ground can interfere with the Resisitivity measuremens.
(4) The electrodes should be expanded parallel to the topographic contours.
INTERPRETATION OF RESISTIVITY SURVEY:
(1) Resistivity is low for Topsoil & weathered part of the rock.
(2) Resistivity is very hilgh for massive rocks compared to weathered rocks.
If any saline water intrusions are there, Resistivity will be very less. Resistivity for freshwater is high compared to saline water.
EXPLORATORY DRILLING
INTRODUCTION:
Exploratory drilling operations, aimed at delineating the potential granular zones in sedimentary formations and fracture zones in crystalline rocks have been taken up in the district by Central Ground Water Board.
SEDIMENTARY DRILLING
EXPLORATORY DRILLING IN SEDIMENTARY ROCKS
Direct rotary rigs of drilling capacity 450 mts and 900 mts equipped within 89 mm drill rods were deployed for drilling down to 400 mt and 800 m bgl respectively. The deepest drilling accomplaised was 777 m.bg
. the rigs of both capacities were provided 7½ inch × 10 inch duplex mud pumps to give desired mud flow at desired discharge pressure to meet the requirement under various conditions.
The characteristics of rocks to be penetrated which are largely unknown, govern the ease hazards of drilling and well construction. The drilling problems are directly proportional to the depth of the drilling involved. Almost all the problems encountered during the course of drilling with mud rotary technique can be combated by controlling drilling mud parameters and having suitable design of bottom hole assembly.
DRILLING AND WELL CONSCTRUCTION HAZARDS
Depending on the characteristics of formations encountered in exploratory bore holes at various sites in the state, the following drilling hazards were most commonly occurred.
Loss of circulation (mud loss)
Caving and struck drill pipe
Drilling through saline aquifer-improper cement sealing.
Slow penetration and bore hole deviation
Development of tube well
Mud loss
The CGWB invariably used bentonite mud as circulation medium in exploratory drilling. The mud density is usually maintained a specific gravity of 1.02 to 1.03 and viscosity of 35 march funnel. The drilling has been successfully completed in majority of the cases. However, at few sites, viz. Radhamangalam in Nagapattnam district loss of circulation was observed. To combat the situation the following steps were taken:
The drilling operations were suspended for a day for arranging mud-conditioning materials. As a matter of fact suspension of drilling operations temporarily allows the for nation to adjust to the new pressure conditions.
As the mud loss generally occurs due to non-formation of a thin mud cake on the bore hole wall. It is imperative that some fibrous materials are circulated so that they can provide a mat like structure on the borehole wall. Accordingly the bentonite mud was reconditioned by mixing saw-dust and the drilling was successfully completed.
CAVING AND STUCK DRILL PIPE
In mud rotary drilling practice a surface casing conductor of around 250 mm diameter to a depth of 6-7 mbgl is first installed and then drilling with 216 mm RR bit is taken up. In areas where surface soil is very unstable, the soil below the rig platform caves into the borehole as drilling progress resulting in formation of a crater below rotary table. At times it becomes big enough leading to tilting of rig. This type of problem is solved by installing conductor surface pipe down to 60-70 mbgl.
Any problem of caving involving stuck drill pipe resulting in fishing operations was not encountered. However, at some sited thickening of return mud was observed. This happens on account of water loss from mud cake formation on borehole wall which leads to a restricted passage for bottom-hole assemble (drill string). To overcome it the mud is reconditioned by mixing CMC (carboxyl methyl cellulose) or kutch into the bentonite mud. Thick mud cake also causes difficulty in development of the tube well. Hence whenever water loss from mud is observed, it must be reconditioned.
DRILLING THROUGH SALINE AQUIFERS:
Saline aquifers were encountered during in coastal tracts. The occurrence of saline aquifer was noticed by observing the condition or return mud. The bentonite mud (prepare with fresh water) become flocculated with individual a clay particles gathering into relatively inactive groups or flocks there by destroyed its thixotropic properties. Although it became necessary to use saltwater based mud for drilling through such sections, but water well drillers generally do not have the material at right time. However, the problem was tracked by mixing around 15-25 kg of lime with bentonite mud. Drilling down to desired depth has been successfully completed by adopting this technique.
DRILL STEM TESTING(DST):
A drill stem test is a temporary completion where in desired section of open hole is isolated by a packer and drill stem is used as a conduit to bring formation water to the surface by means of airlift pumping.
The critical component of a DST assembly is a packer element which isolates aquifer to be tested from rest of the bore hole. Conical packers are made from
conical rubber washers
coir rope over a conical frame made from rods with plates and
Solid cone made from steel pipes. The solid conical packers fabricated from steel pipes are found to be most reliable.
The cone type solid packer is made from a steel pipe of 250 mm or 300 mm in diameter in the form of a frustum of a cone. The length of conical section is generally 60 to 90 cm and has smooth surface to provide effective sealing when it rests. Arrangements are made to connect it to the drill pipe at the top and 100mm diameter assembly pipe at the bottom.
Procedure:
The DST is conducted in a pilot hole starting from the top most aquifer:
Step I: The borehole is reamed to the next larger size down to the depth of stiff clay formation where the packer is set. The testing zone section of the bore hole is then washed. The packer assembly is consisting of 100mm dia slotted pipe at the bottom and drill at the top is lowered and set at the pre-determined depth in such a way that the screen is against the zone meant for testing.
Step II: A ‘T’ shaped discharge pipe is connected at the top of the drill pipe at the surface and airline is lowered.
Step III: The annular space between drill pipe and reamed hole above packer is filled with mud.
Step IV: The compressed air is then let to air pipe with the help of a compressor initially the muddy water comes through the pipe. After sometime fresh formation water starts coming out of the discharge pipe. This water sample is collected for chemical analysis.
Step V: The process is repeated for testing aquifers located in deeper depth.
Precautions:
Packer should be set on hard/stiff clay formation to provide leadk proof seat.
As soon as the compresser is sarted, the mud level in the bore hole at the surface should be watched. Change in mud level indicates leaky packer under such circumstances, the packer should be reset by applying some load.
A sketch showing salient features of DST is placed in the figure 1.
WELL CONSTRUCTION IN SALINE AQUIFERS:
Construction of tubewell tapping fresh water aquifers occurring below saline aquifers atmost care and precaustion. When the well assembly has been lowered as per design and recommendations, the aquifers tapping fresh water must be thoroughly developed and annular spaces shrouded with thoroughly washed pea gravel of recommended size filling the annulus enough to the toop of aquifer. Above the top of the gravel mud balls may be put up through annulus followed by cement seal at desired depth. The cement slurry musty be pumped by a pump. It is generally done with mud pump of rig and it is immediately flushed with fresh water long enough so that mud pumps is completely free from cement particles. A sketch showing cement grouting is placed in fig 2.
PREPARATION OF CEMENT GROUT:
A cement slurry having specific gravity of 1.85 to 1.86 is prepare by mixing one sack (50kg) of cement in 24 liters of water. Their mixture yields 40 liters of cement slurry. In order to improve the fluidity of the cement slurry, bentonite powder is added. Approximately 1.5 to 2.25 kg bentonite should be mixed with 25 liter of water for preparing cement slurry from one sack of cement. It is the best to mix bentonite first with water and then gradually add cement with water slowly mixing and making up the quantity.
Sealing operations are carried out by
gravity placement and
pumping.
In gravity placement method cement slurry is poured through a pipe of 38 mm dislowererd in the annular to the depth required. After the slurry is placed in position, it is allowed to settle for 72 hours. Thereafter, depth sounding is taken to ensure that calculated thickness of seal has been provided and then annular is filled with local clay up to top.
Pumping of cement slurry through a pipe lowered up to required depth is accomplished by means of suitable pump.
Many exploratory wells and production wells and production wells have been successfully completed in Tamil Nadu adopting the above techniques.
BENEFIT ASPECTS
The exploratory drilling operations were carried out in areas where ground water condition are little known. The exploratory by way of drilling established the existence of potable potential aquifers both laterally and vertically. In fact the existence of Thiruvadunnai aquifer of potable quality which was hitherto unknown before the deep exploration was taken up by the CGWB, has become the only source for supply of drinking water in Ramanathapuram district. The deep exploratory drilling operations helped in standardization of equipments and materials and techniques and technology needed for efficient and optimal of ground water resources in the state as well as UT of Puducherry.
One of the tangible benefits is the gainful utilization of assets created in the successful exploratory wells by the state agencies. The exploratory wells after conducting pumping tests, have been handed; over the various state agencies. The exploratory wells after conducting pumping tests have been handed over the various state agencies to either execute the schemes or augment the existing irrigation/water supply. Most of the exploratory wells drilled fall in rural areas, while a few fall either within or outskirts of urban areas. From the exploratory wells handed over to water supply agencies in Tamil Nadu, about 1, 15,000 m3/day of water can be pumped out thereby benefiting a populace of about 28,68,500 to meet the drinking water requirement; and in the U.T. of Puducherry about 20,515 m3/day water can be pumped from the successful exploratory wells to benefit a populace of about 5,13,000.
DRILLING IN HARD ROCKS
DTH Rigs equipped with 85; 0f, 350 dp (Schramm) and 750 cfm 50dp (Elgi) compressors and high pressure hammer tools have been deployed for exploratory drilling operations in hard rock. The deeper drilling has been accomplished to the depth of 300 mbgl. The drilling operations have been carried out using air alone or air-foam as circulating medium. Adoption of foam drilling has increased the rate of drilling and improved cutting removal thereby enhanced the over all efficiency. Hence use of foam is always recommended.
DRILLING PROBLEMS
In DTH drilling operations the following problems are most commonly encounter.
Thick overburden
Stuck hammer and pit
Back pressure of formation water
Well completion in thick overburden
THICK OVERBURDEN DRILLING
In areas were top loose formation (overburden) mantle is thin, drilling with 216 mm button bit is straight way done by DTH method to pierce through the overburden and extending about 25 to 30 cm in to hard rocks, the tool is withdrawn and 175 mm nominal size casing is installed. Further drilling with 165 mm and or 152 mm diameter button bit is done to the targeted depth.
In case of thick overburden, if drilling is started as above, erosion of bore hole wall due to high up hole velocity versus caving in and stuck drill pipe. Under such situations some times drilling is done with injection of foam, if overburden is not very thick. Otherwise mud rotary methods are restored to. The CGWB DTH rigs are provided with enough capacity of mud pump so no problem posed by thick over burden remained unsolved. However, it is recommended to do air-foam drilling instead of only air drilling.
STUCK HAMMER AND BIT
This problem occurs due to negligence of driller and crew. It happens when slightly oversize bit is lowered into drilled hole for further drilling. Sound drilling planning and at most cares are prerequisite for successful drilling; three bits must be kept at site. All the pits though of the same nominal size, may not have exactly gauge diameter. The gauge diameter of the entire all the three bits must be measured accurately with micrometer or vernier caliper and bits be numbered 1,2, and 3 in the order of decreasing diameter. Take bit 1 that it larges bit and start drilling. When the bit is first taken out from the hole for checking and grinding, its gauge diameter must be measures sand recorded. Now lower bit number two, if it gouge diameter is equal to of slightly less than the gauge diameter of bit number 1 taken out of the hole. In no circumstances, any bit having larger gauge diameter be royal led. After re grinding the gauge diameter of bit number 1 must be accurately measured and recorded for further use at appropriate time. By this procedure any change of stuck of hammer bit is completely avoided and bit life enhanced.
BACK PRESSURE OF FORMATION WATER
The hammer tools model DHT-360 of IR and SD -6 mission or design to operate at a minimum air pressure of 180 PSI and consume around 450 cfm at the pressure. CGWB rigs are provided with these types of hammer tools. When ever high discharge in the well during drilling is encountered much of the air is used in flushing water out of the well. There for sufficient compressed air is not available for to operate hammer tool. Also when the head of water is high it counter balances the air pressure and sufficient pressure is not available to activate the hammer tool. In both the cases drilling becomes very slow. It has been practically found that whenever discharge is 9 lps or more, no further drilling is accomplished when the discharge is 7 lps. The rate of drilling becomes un economical. In such cases the drilling is stopped at the depth the well is treated competed.
This problem can be tacked by equipping the rig with reamer button bit, larger diameter hammer tool and another compressor of similar capacity and operating pressure for coupling in parallel so that the bore hole can be reamed with 216 mm diameter bit and 175 mm casing can be lowered to the bottom of the hole for casing; the high yielding fracture zone. One it is done further drilling can be accomplished. But there is not guarantee that after drilling is over, a casing will be pulled out so that to uncase the high yielding fracture zone for production. This proposition is neither economical nor desirable. Hence drilling is stopped at the depth where high discharge is encountered and further drilling becomes very slow and well treated das completed.
WELL COMPLETION IN THICK OVER BURDEN
Many of the bore holes have been found devoid of water below weathered rock. In such cases wells have been successfully completed by installing blank and slotted pipes in to the hole down to overburden depth followed with gravel shrouding and development.
CASING AND SCREEN
In all categories of well (test wells and observation wells) drilled underground water exploration MS pipe as per ISS 427- have been used. For screen this MS pipes have been got slotted with longitudinal slots of width 1.5 mm or 3 mm as per requirement. However, the CGWB have constructed some production wells in sedimentary formation with PVC casings and screen. The use of PVC casing and screen in deeper wells posses problem of installation. Hence MS pipes are recommended for deeper wells. In hard rocks PVC casing can be used for casing over burden.
BACK PRESSURE OF FORMATION WATER
The hammer tools model DHT-360 of IR and SD-6 are B-23 of mission or design to operate at a minimum air pressure of 180 PSI and consume around 450 cfm at this pressure. CGWB rigs are provided with these types of hammer tools. When ever high discharge in the well during drilling is encountered much of the air is used in flush in water out of the well. Therefore sufficient compressed air is not available for to operate hammer tool. Also when the head of water is high it counter balances the air pressure and sufficient pressure is not available to activate the hammer tool. In both the cases drilling becomes very slow. It has been practically found that whenever discharge is 9 lps or more, no further drilling is accomplished when the discharge is 7 lps. The rate of drilling becomes uneconomical. In such cases the drilling is stopped at the depth the well is treated as completed.
This problem can be tackled by equipping the rig with reamer button bit, larger diameter hammer tool and another compressor of similar capacity and operating pressure for coupling; in parallel so that the bore hole can be reamed with 216 m diameter bit and 175 mm casing can be lowered to the bottom of the hole for casing the high yielding fracture zone. Once it is done further drilling can be accomplished. But there is no guarantee that after drilling is over; a casing will be pulled out so that to uncase the high yielding fracture zone for production. This proposition is neither economical nor desirable. Hence drilling is stopped at the depth where high discharge is encountered and further drilling becomes very slow and well is treated as completed.
DRILL TIME LOG – CHIRUMANGALAM, KALLAKURUCHI.
HARD ROCK DRILLING DATA
DATE
DRILLING DEPTH
TOTAL DEPTH
DRILL TIME
TOTAL
Time in min.
FROM
TO
FROM
TO
23-05-07
00-00
03-00
3-00
10.54
10.54
04
03-00
05-60
2-60
10.54
10.57
03
05-60
06-75
1-15
11.29
11.31
02
06-75
09-75
3-00
11.35
11.46
11
09-75
12-85
3-10
11.47
11.58
11
12-85
15-85
3-00
12.03
12.13
10
15-85
18-95
3-10
12.14
12.25
11
18-95
21-95
3-00
12.30
12.40
10
21-95
25-05
3-10
12.41
12.51
10
25-05
28-05
3-00
12.56
13.07
11
28-05
31-15
3-10
13.08
13.18
10
31-15
34-15
3-00
13.23
13.33
10
34-15
37-25
3-10
13.34
13.44
10
37-25
40-25
3-00
13.50
14.00
10
40-25
43-35
3-10
14.01
14.11
10
43-35
46-35
3-00
14.15
14.24
9
46-35
49-45
3-10
14.25
14.35
10
49-45
52-45
3-00
14.40
14.50
10
52-45
55-55
3-10
14.51
15.02
11
55-55
58-55
3-00
15.07
15.19
12
58-55
61-55
3-10
15.20
15.32
12
61-65
64-55
3-00
15.37
15.52
15
64-65
67-55
3-10
15.53
16.08
15
24-05-07
67-75
70-75
3-00
14.55
15.09
14
70-75
73-85
3-10
15.10
15.24
14
73-85
76-85
3-00
15.24
15.37
13
76-85
79-95
3-10
15.38
15.52
14
25-05-07
79-95
82-95
3-00
9.40
9.53
13
82-95
86-05
3-10
9.54
10.08
14
86-05
89-05
3-00
10.13
10.28
15
89-05
92-15
3-10
10.29
10.44
15
92-15
95-15
3-00
10.49
11.03
14
95-15
98-25
3-10
11.04
11.18
14
98-25
101-25
3-00
11.25
11.45
20
101-25
104-35
3-10
11.46
12.06
20
104-35
107-35
3-00
15.57
16.16
19
26-05-07
107-35
110-45
3-10
10.40
11.00
20
110-45
113-45
3-10
11.05
11.33
18
113-45
116-55
3-10
11.35
11.46
21
116-55
119-55
3-00
11.55
12.15
15
119-55
122-65
3-10
12.26
12.46
20
122-65
125-65
3-00
12.50
13.10
20
125-65
128-75
3-10
13.20
13.47
24
128-75
131-75
3-00
14.35
14.50
15
131-75
134-85
3-10
14.51
15.06
15
134-85
137-85
3-00
15.20
15.36
16
137-85
140-95
3-10
15.37
15.55
18
140-95
143-95
3-00
16.03
16.20
17
28-05-07
143-95
147-05
3-10
11.07
11.25
18
147-05
150-05
3-00
11.35
11.55
20
150-05
153-15
3-10
11.57
12.18
21
FORMATIONS OF CHIRUMANGALAM, KALLAKURUCHI:
DEPTH RANGE (METERS)
ACESSIBLE OF LITHOLOGY
0.00-0.50
SOIL , CLAY IN BROWNISH COLOUR
0.50 – 6.00
CHARNOKITE HIGHLY WEATHERED WITH CLAY
6.00 – 126.00
HARD AND MASSIVE CHARNOKITE
126.00 – 131.00
CONTACT ZONE BETWEEN CHARNOKITE AND PINK GRANITE
131.00 – 148.00
PINK GRANITE.
DRILLING SITE IN SOFT ROCK TERRAIN
NEYVELI LIGNITE CORPARATION:
Biggest open-cast Mechanised Lignite Mines in India. Mining 24 million tonnes of lignite annually and power generating with installed capacity of 2490 MW of power.
Engaged in exploitation of Lignite Deposits for more than four decades, a resource that is fueling the Nation's development, powering progress & nourishing the green revolution.
Exploration of lignite deposits in and around Neyveli region, with due attention to quality, economy and efficiency
Neyveli Lignite Corporation has accorded the high priority to ecology development and pollution control. Continuous monitoring in respect of liquid/ gaseous effluents control is carried out at units and treated effluents meet all MINAS and the statutory requirements. To improve the environment the Corporation has a planned afforestation programmes, and reclamation of waste land development to control pollution free air in Neyveli as a regular activity of the Corporation.
Neyveli Lignite Corporation is well aware of the effects of open cast mining to the environment. It therefore gives a lot of importance to pollution control, reclaiming land and maintaining ecological balance. A pollution level in air is being continuously monitored through six air monitoring stations in Neyveli. All the guidelines of the Central Pollution Control Board have been adhered too.
NLC's success in land reclamation of mines spoils and afforestation has been overwhelming. The mined out area or the de-coaled area is refilled with overburden, since this does not contain plant nutrients. Or have the proper texture; great effort is put to successfully reclaim the land.
The dumped soil is improved in stages through modern techniques to bring back its original fertility and the agricultural operations are carried out by adding nutrients, like organic, inorganic and bio-fertilizers. Now crops and vegetables of various varieties are being continuously raised in about 250 hectares. Further it is also proposed to increase this backfilled area into cultivable land.
Water Conservation
Many water conservation measures have been taken up by NLC like
1. Optimization of ground water pumping in Mines,2. Introduction of dry ash disposal system.3. Artificial recharging of ground water etc., and4. Stoppage of ground water pumping for Township and using mine storm water after treatment for Township.
Ground water management studies
The ground water management studies included
I Field traverse toposheet reading.
II Study of the area
1) Geology, Geomorphology,drainage pattern, cropping pattern, etc
2) Importance of hydrogeology
III. Re-appraisal of the studies with respect to available base line.
IV. Well Inventories/ ground water structures, etc
1) Types of wells, extraction, water level trend, available source and source for further development, etc
V. Management studies with the available source
1) dug well/ bore well / tube well tests etc.,
2) Assessment of available ground water source.
a) Dug well test to determine the aquifer parameter and generation of field datas
1) How to measure the discharge of the well?
The methods commonly adopted is volumetric method. Measure the discharge of the well with a known capacity Bucket/Drum,
Eg. Capacity of the bucket is known – 10 litres
If the time required to fill the bucket is 10 seconds.
Discharge = 10/10 = one liter per second.
2) measurement of discharge from the storage.
That is volume of water stored in the well between pre and post pumping levels. Note down theSWL
Start pumping, monitor the water level for 1st min, 2nd min, 3rd min, 4th min, and 5th min, take the average draw down for one min.
SWL-one min draw down= draw down in (m)/min multiply the draw dawn (m)/min x area m2 that gives the discharge of the well= m3 /min
3)consumption current method.
Note down the unit consumption current , multiply with the constant factor 2.86 which will gives corresponding discharge of the well m3 /min. or m3 / hour.
(where total head is restricted 10m, (shallow wells) ref. bhujainews apr,june 1991.
b) with the data generated , the discharge draw down , time required for 90% recovery etc.
a scientific creative speaking data can be generated.
Speaking data any export specialized in the ground water can speak to the data to glorify the law by the nature.. ie. Output
In island condition the fresh water occurs as a lense, where “as is the with drawal so is the disappearance of the fresh water lense” in such cases output = input.
Eg1) manavala kuruchi – mondakedu stretch , the monzonitic factory area of kanniyakumari dist. Tamilnadu.
2) manambam willing ton island of ernakulam dist. And madakara dharmadam island of cannanure dist of kerala.
These areas are surrended by , one side sea and on the other side back water coils .
Aquifer material: river alluvium / black clay.
In all the island condition the permeability and rate of in flow in the system is nil.
Case II
Output <>
In general it holds food to all other areas.
Eg. In the Teri sand areas if a well is pumped to create a draw down , its taken a week to attain the original SWL.
Such cases the specific capacity , permeability rate ofinflow in to the system and transmissivity are determined to know the aquifer parameters.
All the parameter are analised in all the available methods and cross checked , the results , which consider with the law grounded by the nature.
Output<>
Si called creative speaking data which are presentable.
a) specific capacity:
The production capacity of the well is rated by its specific capacity , which is defined as the discharge for unit time for unit draw down.
Sp-capacity = discharge (m3/min)/ draw down (m) expressed as m3/min/ m dd
b) Permeability
the permeability of the rock is its capacity to transmit water ( or any fluid ) under differential pressure and is measure by the rate at which it will transmit water ( fluid) trough a unit cross section , under unit pressure differential , per unit distance
c) transmissity
the over all capacity of avaquiver to transmit water depends on the thickness and hydraulic condition of the components of aquifer
i.e K = T/ B
K = average hydraulic conductivity
T = transmissivity of the aquifer
B = thickness of the aquifer.
d) Rate of inflow
The inflow of the water oor ( fulid into the aquifer system expressed as : litres per second.
Scientific creative speaking data
Dug well test
Location: plirate well by shri pushparaj tothimedu . tothimedu is 22 km for cuddalore, on cuddalore viruthachalam . tothimedu road.
Tg: cuddalore dist, cuddalore.
Toposheet/ co-ordinate 58M/11
N- 79° 43’10”
E-11°27’50”
Depth : 7 mbgl , area : 3.9*4.7 mts. MP : GL
SWL: 2.56 mbgl .
Aquifer material : laterite / weathered zone / s.st.
Type and make of motor : 7.5 Hp centrifugal pump.
Main crops : sugarcane and cashew plantation.
Area irrigated :1/4
Discharge pipe diameter 3”
Discharge of the well : 18 lps , 1.08 m3/min.
Duration of pumping : 120 min.
Draw down after 120 min – 3.74 mts.
Residual draw down after 480 min-1.91 mts
After 1150 min 90% recovery - .60 m.
I.a. specific capacity by karanth’s method.
Depth : 7 mbgl ,Area : 3.9*4.7 mts = draw down after 120 min – 3.74 mts
Pumping rate 18 lts , 1.08 m3/min , 64.8m3/hour.
Total pumping in 120 min – 120*1.08 = 129.6m3.
Volume of water developed in the well
Length * breath * height
3.9*4.7*3.7 = 67.82m3.
Pumping from the aquifer for 120 min 67.82 -64.8 = 3.02m3.
Pumping from the aquifer for one min = 3.02/120 = 0.025m3.
Pumping for 60min – 0.03* 60 = 1.2m3 .
Specific capacity = 1.2/(dd)3.7 = 0.324m3/hour/mdd
(dd)3.7 7.78m3/day/mdd
0.00540m3/min/mdd
B, unit area specific capacity = 7.78m3/ day/mdd = 0.42m3/day/m2
A=(18.33m2 )
II.a . specific capacity sititcher’s method
From the graph , fig 3
S1/S2 = 2, t = 400 min, aA = 18.33m2
Specific capacity = A/ t*2.3log S1/S2
= 18.33 * 2.3 log2
1100
Specific capacity = 0.0317 m3/min/mdd
b. unit area specific capacity = 45.6 m3/min/mdd
A= (18.33) m2
= 2.4910 m3/day/ m2 .
III. a. selitcher’s modified method
Specific capacity = A/t*2.3log S1/S2
= 18.33 * 2.3 * log 3.74
120 0.60 A =area
S1 = draw down
S2 = R.ss
T = pumping time
Interpretated R for 90% recovery - 0.60m
Specific capacity = 0.278 m3/min/mdd
402.04 m3/day/ mdd
b. unit area 8p- capacity = 402.04 m2 /day/mdd
A= (18.33) m2
= 21.93 m3/day / m2.
Cross checking with all the 3 methods:
910 a. karanth’s method
Specific capacity = Discharge let the discharge = x
Draw down
0.0054 = x___
3.7
X = 0.0199m3/min.
Which is much lower than the observed discharge 1.08m3/min data is not reliable.
3) .a. silitcher’s method:
Specific capacity = X ____ let the discharge = x
Draw down
0.0253 = ____X____
3.7
X = 0.09/3 m3/min.
This is much lower than the observed discharge.
Disharge 1.08 m3/min.
Data is not reliable.
(3).a . solitcher’s modified method:
Specific capacity = X____ let the discharge = x
3.47
0.278 = ___X____
3.7
X = 1.028m3/min.
Which is equal to the observed discharge 1.08m3/min
Result: from all the 3 methods the silitcher’s modified methid is presentable
Speaking data.
Permeability by kumaraswamy method:
Depth 7.0 mbgl , mp- GL , discharge 18 Lps , SWL : 2.56mbgl
Duration of pumping 120 min
Draw down (d.d) after 1150 min interpretated data for 90%
Recovery- 60m
Depth to the water coloum (depth –SWL) = 4.44m
d1 = 0.74(D-d.d)
d2 = 3.84(Dd.d)
tr = 120 min pumping time
A = area -18.33 m2 .
Rate of inflow by karanjack’s method:
QR = Qp tp___
Tp+tr
Qp = 18 * 120 ___
120+1150 Qp = 18 lps
Tp = 120
Qp = 2160 tr = 1150
1270
Qp = 1.700lps
1) @ 18 lps for 120 min of pumping – total pumping of water 18*60*120 = 1,29,600 litres.
2) @ 1.7lps the in flow in to the aquifer system for 90% recovery = 1.7*60*1150 = 1.17300 litres.
Mani and adyalkar method:
T = specific capacity * 527.7 *logR/r R = radius of influence of well-40m.
= 0.278 * 527.7 * log 40
297
= 0.278 * 527.7 * 16.597
= 0.27 * 527.7 * 1.22
= 178.97 m2 / day.
The radius of the influence in the hard larartic area weathered zone is 40 m , hence the spacing of the well should be 80 to 100m .
Location: Shri pushparaj, Totimedu.
SWL : 2.56mgbl
Date : 05-June 2007
s.no
Time in
Hour
Time in
Min.
PWL(mgbl)
D>D
(m)
1
0800
1
2.62
0.06
2
2
2.67
0.11
3
3
4
4
2.74
0.18
5
5
2.80
0.24
6
6
2.85
0.29
7
7
8
8
3.03
0.47
9
10
3.31
0.75
10
12
3.32
0.76
11
14
3.44
0.88
12
16
3.54
0.98
13
18
3.63
1.09
14
20
3.71
1.15
15
25
4.0
1.44
16
0830
30
4.02
1.46
17
35
4.43
1.87
18
40
4.54
1.98
19
50
4.84
2.28
20
0900
60
5.07
2.51
21
70
5.27
5.71
22
80
5.50
2.94
23
0930
90
5.65
3.09
24
100
5.85
3.29
25
1000
120
6.30
3.74
Draw down after 120 min. = 3.47m.
Recoupration data
SWL = 2.56mbgl
S1 = 3.74
Date : 05 – june 2007 to 06- june 2007
S.no
Time in hours
Time in min.
PWL
(mbgl)
R.dd
S1/S2
1
1010
10
6.10
3.54
1.05
2
20
6.02
3.46
0.08
3
30
5.90
3.34
1.11
4
40
5.97
3.41
1.096
5
50
5.74
3.18
1.176
6
1100
60
5.66
3.1
1.20
7
70
5.54
2.98
1.255
8
80
9
90
2.90
1.28
10
1200
120
2.7
1.38
11
1300
180
2.5
1.496
12
1400
240
2.42
1.54
13
1500
300
2.20
1.7
14
1600
360
2.15
1.73
15
1700
420
2.00
1.87
16
1800
480
1.91
1.95
17
90% Recovery
1150
0.06
18
06/06/07
0800
1310
2.92
0.36
1033
State Profile
Tamil Nadu
Area (Sq.km)
1,30,058
Rainfall (mm)
995
Total Districts / Blocks
30 distt. 384 Blocks
GROUND WATER MEDIUM (Hydrogeological Conditions)
Nearly 73% of the total area of the State is occupied by a variety of hard & fissured crystalline rocks like charnockite, gneisses and granites. The depth of open wells varies from 6 to 30mbgl. While the depth of borewells generally varies from 30-100m. The sedimentary formations consist of sand stones, limestones and shales whereas Quaternary sediments in the State represented by Older alluvium and Recent alluvium and coastal sands. In the Cauvery delta of Thanjavur district, the artesian pressure head ranges between 4.5 m to 17 magl with free flow up to 270 m3/hr. The yield of wells in the alluvium varies form 27 to 212 m3/hr. The yield of wells in the fissured formations varies from 7 to 35 m3/hr.
GROUND WATER EXPLORATION/SOURCES FINDINGS
Dynamic Resources
Annual Replenishable Ground water Resource
23.07 BCM/Yr
Net Annual Ground Water Availability
20.76 BCM/Yr
Annual Ground Water Draft
17.65 BCM/Yr
Stage of Ground Water Development
85 %
Developmental Monitoring
Over Exploited
142 Blocks
Critical
33 Blocks
Semi- critical
57 Blocks
Exploratory Tube wells Constructed (as on 31.03.2006)
965
Exploratory Tube wells handed over (as on 31.03.2006)
598
No. of ground water observation wells
906
Ground Water User Maps
29 districts
Artificial Recharge to Ground Water (AR)
§ Area identified for AR: 17292 sq km
§ Quantity of Surface Water to be Recharged: 3597 MCM
§ Feasible AR structures: 8612 percolation tanks, 18170 check dams, 5 lakh rain water harvesting structure
AR schemes completed during VIII Plan: 3AR schemes completed during IX Plan: 10
Ground Water Quality Problems
Contaminants
Districts affected in parts
Salinity
Karaikal, Pondicherry, Nagapattanam, Quide-Millet, Pudukottai, Ramananthapuram, North Arcot-ambedkar, Dharampuri, Salem, Trichy, Coimbatore.
Fluoride
Dharampuri, Salem, North Arcot-Ambedkar, villipurampadayatchi, Muthuramalingam, Tiruchirapalli, Pudukottai.
Water is the main source for the development for all human beings, cattles, and industries. From the History of the Global system we learn that, in ancient days, perennial rivers became the major source of water, and the people, who were far away from the river sources, practiced the utilization of ground water and stored rain water for their sustenance. In India ground water utilization has been practiced for many centuries, in the form of open wells owned by private individuals or farmers or communities, for both domestic and irrigation purposes. Well irrigation has been in practice for several centuries. To meet shortage of tank/canal waters, cultivators made conjunctive use of ground water through wells. But there was no specific planning behind this, because each source was developed without due consideration of the other.
Efficient planning and management of water sources, for irrigation/industrial and other uses, is an important aspect for the development of any system. Recognition of the fact that ground water and surface water are not separate entities but are two forms of the same total water source, leads to the recognition of the importance of conjunctive water use in the management of ground water - surface water system. But due to the land intended for recharging, the surface water is wasted as run-off into the sea whereas ground water table depletes to an alarming level. This in turn results in the reduction in well yield, drying up of shallow wells, deterioration of water quality, sea water intrusion into the coastal aquifers, increased energy required to lift water from greater depth and its consequent high cost, which becomes uneconomical to poor farmers to continue agriculture. Further many of the agricultural fertile lands have become barren in coastal area like Minjur. (Thiruvallur District), Kuttam, Athisayapuram in Thuthukudi District and Nagapattinam and Thiruvarur in Thiruvarur District of Tamil Nadu. Further about 89 blocks have been identified as overexploited and dark blocks where the balance available ground water potential shows a negative sign. The status of the categorising of blocks are based on the ground water extraction over the various periods is shown in the chart. In order to check this disturbing trend, the rain water is to be conserved.
In ancient days itself, people, especially Indians, know the methods of conservation of rainwater. There are evidences that, even during Harappan period, there was very good system of water management as could be seen in the latest excavation at Dholavira in Kachch. During independence period, the people use to manage water resources considering it as part of the nature which is essential for their survival. This could be seen from the rain water harvesting structures in the low rainfall areas of Rajasthan, harvesting springs in hilly areas and mountainous region and percolation ponds and tanks in southern India.
In Tamil Nadu, the ancient people stored rainwater in public placed separately one for drinking purposes and another for bathing and other domestic purposses and called them as Ooranies. They also formed percolation tanks or ponds, for the purpose of recharging irrigation or domestic wells. They periodically clean the water ways so as to get clean water throughout the year. These are instances in the history that people constructed crude rubble bunds across river courses either for diversion of water or for augmenting the ground water. The various methods of rainwater harvesting are classified below under two category, Traditional and Modern methods.
Traditional methods
Traditional rainwater harvesting, which is still prevalent in rural areas, was done in surface storage bodies like lakes, ponds, irrigation tanks, temple tanks etc. In urban areas, due to shrinking of open spaces, rainwater will have to necessarily be harvested as ground water, Hence harvesting in such places will depend very much on the nature of the soil viz., clayey, sandy etc. The below listed are the various kinds of traditional rainwater harvesting methods
Modern methods
The Modern methods of rainwater harvesting are categorised under two, they are Artifical Recharging and Rain Water Harvesting. The former is classified into Absorption Pit Method, Absorption Well Method, Well cum Bore Method and Recharge trench cum injection well. The later is categorised into Individual Houses and Grouped Houses which are further classified into Percolation Pit Method, Bore Well with Settlement Tank, Open Well Method with filter bed Sump and Percolation Pit with Bore Method.
Roof top rain water can be diverted to the existing open / bore well. Along with this, rain water available in the open spaces around the building may be recharged into the ground through the following simple effective methods.
Percolation pits (Small houses)Recharge Trench (Big houses / Apartments)Recharge wells (Large buildings / industries)
Based on the size /area of the building and the underlying lithological nature of the formation the said methods may be used either individually or in combinations.
To enhance nature recharge of rain water avoid pavements since unpaved surfaces have more percolation rate.Various Methods of Recharging In Individual Houses
Components of Rooftop Rainwater Harvesting System
In domestic Rooftop Rainwater Harvesting Systems rainwater from the house roof is collected in a storage vessel or tank for use during the periods of scarcity. Usually these systems are designed to support the drinking and cooking needs of the family at the doorstep. Such a system usually comprises a roof, a storage tank and guttering to transport the water from the roof to the storage tank. In addition, a first flush system to divert the dirty water which contains roof debris collected on the roof during non-rainy periods and a filter unit to remove debris and contaminants before water enters the storage tank are also provided. Therefore, a typical rooftop Rainwater Harvesting System comprise of following components:
Roof catchment
· Gutters· Down pipe and first flush pipe· Filter unit· Storage tank· Collection pit
Among the above components, storage tank is the most expensive and critical component. The capacity of the storage tank determines the cost of the system and reliability of the system for assured water supply. Following is the brief description of each component with a study of different materials used for each component.
catchment
The roof of the house is used as the catchment for collecting the rainwater. The style, construction and material of the roof effect its suitability as a catchment. Roofs made of corrugated iron sheet, asbestos sheet, tiles or concrete can be utilized as such for harvesting the rainwater. But thatched roofs are not suitable as it gives some colour to water and also the water carries pieces of roof material (such as palm leaves).
Gutters:Gutters are channels fixed to the edges of roof all around to collect and transport the rainwater from the roof to the storage tank. Gutters can be prepared in semi-circular and rectangular shapes as shown in figures. Locally available material such as plain galvanized iron sheet can be easily folded to required shapes to prepare semi-circular and rectangular gutters. Semi-circular gutters of PVC material can be readily prepared by cutting the PVC pipes into two equal semi-circular channels. Bamboo poles can also be used for making gutters if they are locally available in sufficient quantity. Use of such locally available materials reduce the over all cost of the system.
Downpipe and First Flush Pipe:Downpipe:Down pipe is the pipe, which carries the rainwater from the gutters to the storage tank. Down pipe is joined with the gutters at one end, and the other end is connected to the filter unit of the storage tank as shown in figure below. PVC or GI pipes of diameter 50 mm to 75 mm (2 inch to 3 inch) are commonly used for down-pipe. Bamboo can also be used wherever available in suitable size.
First Flush Pipe:Debris, dirt and dust collect on the roofs during non-rainy periods. When the first rains arrive, this unwanted material will be washed into the storage tank. This caused contamination of water collected in the storage tank thereby rendering it unfit for drinking and cooking purposes. Therefore, a first flush system is incorporated in the Rooftop Rainwater Harvesting Systems to dispose off the 'first flush' water so that it does not enter the tank. There are two such simple systems. One is based on a simple manually operated arrangement, where by, the down pipe is moved away from the tank inlet and replaced again once the first flush water has been disposed.In another simple and semi-automatic system, a separate vertical pipe is fixed to the down pipe with a valve provided below the "T" junction as shown in figure.After the first rain is washed out through first flush pipe, the valve is closed to allow the water to enter the down pipe and reach the storage tank.
Filter Unit:The filter unit is a container or chamber filled with filter media such as coarse sand, charcoal, coconut fiber, pebbles and gravels to remove the debris and dirt from water that enters the tank. The container is provided with a perforated bottom to allow the passage of water. The filter unit is placed over the storage tank. Commonly used filters are of two types. One is a ferrocement filter unit, which is comparatively heavy and the other is made of either aluminium or plastic bucket. The latter is readily available in market and has the advantage of ease in removing, cleaning and replacing.Another simple way of filtering the debris and dust particles that came from the roof along with rainwater is to use a fine cloth as filter media. The cloth, in 2 or 3 layers, can be tied to the top of a bucket or vessel with perforations at the bottom.
Storage Tank :Storage tank is used to store the water that is collected form the Rooftops. Common vessels used for small scale water storage are plastic bowls, buckets, jerry cans, clay or ceramic jars, cement jars, old oil drums etc. For storing larger quantities of water the system will usually require a bigger tank with sufficient strength and durability.There are unlimited number of options for the construction of these tanks with respect to the shape (cylindrical, rectangular and square), the size (Capacity from 1,000 lt. to 15,000 lt. or even higher) and the material of construction (brickwork, stonework, cement bricks, ferrocement, plain cement concrete and reinforced cement concrete). For domestic water needs, taking the economy and durability of tanks into consideration, ferrocement tanks of cylindrical shape in capacities ranging between 4,000 lt. and 15,000 lt. are most suitable. Plain cement concrete and reinforced cement concrete are used for tank capacities usually more than 50,000 lt. Brick, stone, cement brick may be used for capacities ranging between 15,000 lt. to 50,000 lt.The ferrocement tanks are usually constructed above ground level because of the advantages, such as, a) ease in finding structural problems/leaks, b) easy to maintain and clean and c) easy to draw water. It is difficult to detect the leaks and take corrective measures in case of under ground tanks. Water from under ground tanks cannot be drawn by gravity. Some kind of manual or power lifting devices need to be used for drawing the water. Further, in coastal areas, under ground tanks are prone to water contamination due to fluctuation in groundwater table and leakage of stored water.The storage tank is provided with a cover on the top to avoid the contamination of water from external sources. The cover will be in dome shape having a raise of about 20-30 cm. in the middle. The dome is provided with two circular openings, one for manhole and another for accommodating the filter. A lid covers the manhole avoiding exposure of stored water to the outside environment. The storage tank is provided with pipe fixtures at appropriate places to draw the water, to clean the tank and to dispose of the excess water. They are named tap or outlet, drainpipe and over flow pipe respectively. PVC or GI pipes of diameter 20 mm to 25 mm (¾ inch to 1 inch) are generally used for this purpose.
Collection Pit:A small pit is dug in the ground, beneath the tap of the storage tank and constructed in brick masonry to make a chamber, so that a vessel could be conveniently placed beneath the tap for collecting water from the storage tank. A small hole is left at the bottom of the chamber, to allow the excess water to drain-out without stagnation. Size of collection pit shall be 60 cm x 60 cm x 60 cm.
PERCOLATION / ABSORPTION PIT
HYDROCHEMISTRY
Parameter affecting the quality of water
S.No.
PARAMETERS
BIS PRECRIBED LIMIT
PROBABLE HEALTH EFFECT
DESIRABLE LIMIT
PERMISSSIBLE
LIMIT
1
COLOUR (HAZEN UNIT)
5
25
MAKES THE WATER AESTHETICALLY UNDESIRABLE
2
ODOUR
ESSENTIALLY FREE FROM OBJECTIONABLE ODOUR
MAKES THE WATER AESTHETICALLY UNDESIRABLE
3
TURBIDITY
(NTU)
5
10
HIGH TURBITITY INDICATES CONTAMINATION / POLLUTION .
4
pH
6.5
8.5
BEYOND THIS RANGE MUCUS MEMBRANE AFFECTED
5
TOTAL DISSOLVED SOLIDS 9TDS)MG/L
500
2000
UNDESIRABLE TASTE GASTROINTESTINAL IRRITATION , CORROSION,
6
HARDNESS AS CACO3 mg/l
300
600
affectects water supply system (scaling) , excessive soap consumption , diseases of kidney or bladder stone , stomach disorder , boiled meat/food becomes poor in quality.
7
Calcium (ca) mg/l
75
200
SCALE formation , insuffieciency causes sever kkidney or bladder stone and irritation in the urinary passage
8
Magnesium (mg) mg/l
30
100
Poor lathering and deterioration of cloths , laxavative with sulphate, and encrustation in watersystem. It essential as an activator of many enzymes system
9
Chloride (cl) mg/l
250
1000
Taste affected , indigestion , may be injurious to some people suffering from diseases of heart or kidney.
10
Fluoride (f)mg/l
1.0
1.5
Reduce dental carries , very high concentration may cause
Crippling skeletal flurosis ,less than 1.0 mg/l is essential.
11
Nitrate (No3) mg/l
45
100
causes infant methmagalobineamia ( blue baby diseases ) at very high concentration.
12
Sulphate(SO4) mg/l
200
400
Tatse affected Laxative effect, gastro intestinal irritation.
13
Iron (Fe) mgl
0.3
1.00
Causes staining of laundry and porcelin. In traces it is essential for nutrition
14
Copper (Cu) mg/l
0.005
1.5
Deficiency results in nutritional anemia in infants,large amount may result in liver damage
15
Lead (Pb) mg/l
0.05
No relaxation
Sever inflammation of gastro intestinal tract with vomiting and diarrhea, visual disturbance.
16.
Cadmium (Cd) mg/l
0.001
Highly toxic, causes itai-itai disease (painful rheumatic condition), cardio vascular system affected.
17
Chromium (Cr) mg/l
0.005
Hexavalent state of chromium produce lungs tumor
18
Arsenic (As) mg/l
0.05
Causes skin damage, risk of skin cancer (arsenocosis)
19
Antimony (Sb) mg/l
0.006
Increase blood chlostral
20
Mercury (Hg)mg/l
0.001
Neurological disorder
21
Selenium (Se) mg/l
0.01
Leads to hair and finger loss, numness in fingers
22
Barium (Ba) mg/l
2
Increase blood pressure. Affect central nerve system
23
Cyanide (CN) mg/l
0.05
Causes nerve damage, Thyroid problem
24
Pathogen/100ml
A-Total colilform
B-Faecal coliform
1
Absent
10
Absent
Causes water borne diseases like Jaundice, Typhoid, etc.
GROUND WATER EXPLORATION-CONCEPTS AND GUIDE LINES
1.OBJECTIVES:
The objectives of Ground Water Exploration is to locate aquifers capable of yielding water of suitable quality and in economic quantities for drinking, irrigation, agricultural and industrial purposes by employing as required hydrogelogical, geophysical, remote sensing, drilling and other techniques. Quality and quantity of water are relative terms that depend on the purpose for which water is intended to be supplied and the exploration therefore is accordingly aimed at. Other information sought during the course of exploration are the depth location of aquifer, its areal extent, type of rock formation to be encountered, quality of water in the aquifer, its sustained yield, feasibility for artificial recharge etc.
Type of exploration needed and sequence of exploration technique to be employed are declared by economic status and availability of data, degree of the ground water development taking place in the area and the hydrogeological conditions.
The above objectives can be achieved combinedly or separately through proper planning of ground water exploration program. At present the objective of exploratory drilling being undertaken by the Central Ground Water Board is to explore to the depth of 300-500 m in hard rock formation and upto 900 m depth in alluvial formations. Under Groundwater Exploratory drilling program four types of wells namely Exploratory Construction of Slim Holes is basically to delineate the aquifer at different depth horizons and to fill data gaps in areas for delineating the geometry of aquifers in alluvial/sedimentary formations. The Exploratory Wells or Test Wells are taken up at key locations for conducting comprehensive studies such as pumping tests, computation of aquifer parameters, well characteristics etc.
It is therefore quite essential to plan out the disposition and locations of different types of wells meticulously. However, construction of Slim Holes take much less time as lowering of well assembly is not required and there is no problem of handing over of wells to the user agency. In hard rock formation and bouldery formations where down the hole hammer rigs and Percussion rigs are respectively utilized, the concept of Slim Holes is not applicable.
2. PREPARATION OF SCHEME
Following steps should be observed for formation of the scheme proposals for an are.
i. Selection of an are for exploratory drilling based on the systematic/Reappraisal hydrogeological survey undertaken or need for scientific exploration to fill up data gaps.
ii. Consultation with state Government/user Organizations for ascertaining their requirements/ priority and to assess the exploration requirement in the area of study vis-á-vis needs of the states.
iii. Preparation of comprehensive scheme for ground water exploration bringing out objectives, justification, type of and number of rigs required, depth to be drilled, number of exploratory, Observation Wells, Slim Holes to be drilled, size and type of well design to be adopted with material specifications, tentative geological formations likely to be encountered types of aquifers, expected yield from the well., type of tests, zone test, aquifer performance test to be undertaken, quality regime to be monitored and any other related items. For every Exploratory Well site at least 2 to 3 alternative sites must be available so as to accommodate the request of user agency in view of the constraints on availability of Government land.
iv. Ground water prospects map is to be considered to draw a scheme for ground water exploration. These maps are prepared for the Rural Development Ministry under the guidance of the National Remote Sensing Agency. Most of the State Remote Sensing agencies are involved in the preparation of these maps. Central Ground Water Board is one of the agencies providing data for these maps. These maps are submitted to the Central Ground Water Board for quality checks and for approval for publication.
The scheme report should be made available to all the parties concerned at least one season in advance to plan material procurement and diversion of rigs and equipments.
3. NORMS FOR DENSITY OF EXPLORATORY WELLS
Perusal of the area so far explored by the Central ground Water Board indicates that most of the districts in various states still remain unexplored. This is indicative of the fact that most of the areas where exploratory drilling has already been carried out are again and again covered under various schemes such as accelerated drilling programme, drought mitigation programme and other special studies. This has resulted in to a situation where proportionate increase in coverage of the area under ground water exploration is not being achieved inspite of drilling considerable number of boreholes year by year.
It is therefore opined that while deciding strategy for second phase of exploration, priority may be given to such areas hitherto unexplored, so that the entire country is covered from the view point of ground water exploration for delineating ground water potential areas and to estimate ground water resources.
The proposed density of borehole for ground water exploration will be one well/200 sq.km in consolidated formations, one well/300 sq.kms in bouldery formations and one well/500 sq.km in unconsolidated formations. The above norms are not sacrosanct and as per the prevailing hydrogeological situationandobjectives of ground water exploration, the density of Exploratory Wells can be decided.
WORK ESTIMATES
Based on the scheme report estimates for the work to be carried out during the AAP indicating type of machine and equipments proposed to be deployed, materials proposed to be used during the operations, period of operation etc. should be submitted in April of the current AAP for obtaining sanction of the Competent Authority.
4. SITE SELECTION METHODOLOGY FOR GROUNDWATE EXPLORATION
Selection of successful site is a difficult task, especially so in hard rocks to study the status of information available for the area needs
· Reconnaitory surveys in the field for studying geology, geomorphology and hydrogeology of the area.
· Identification of rock type and the environment in which it was formed
· Stratigraphy, geological history and the depositional environment marine, lacustrine, estuarine in respect of sedimentary rocks.
· Nature of sediments and geomprphology in respect of alluvial formations
· Geomorphology, geological structure and the secondary porosity in respect of hard rock.
· Aeromagnetic maps indicating fracture system, remote sensing and aerial photos.
The concept of the ground water basin is an important factor for locating the sites. In ground water recharge areas (uplands), the depth to water level is deep and water level fluctuations are high. Therefore, even if it is high, a well in the recharge area is subjected to excessive drawdowns on pumping. The valley bottoms form the ground water discharge areas where depth to water level are near surface and show very less water level fluctuations. Nearly flat to low gradients of water table prevail in discharge area unlike in ground water recharge areas. Though the Transmissivity is moderate to low, well drawdown will be low and equilibrium conditions will be reached early during pumping. Therefore, a well sited in valley bottom is an advantage.
Similarly, the slope factor is high and the success rate of drilling vary highly from upland to valley bottoms, when compared to soft sedimentary rocks or the alluvial formations.
Well Siting Methodologies in different rocks:
Unconsolidated formations (Alluvial areas):
There are fourteen major river basins (catchments area more than 20.000 sq.km) and forty four medium river basins (catchments are between 2000 to 20,000 sq.kms). Apart from them, there are minor river basins with catchment of less than 2000 sq.km mostly in coastal areas. Most of these rivers contain recent alluvium, older alluvium and coastal alluvium which for important unconsolidated sediments that include potential aquifer.
The important alluvial basins having considerable dimensions Sindu-Ganga basin, Upper Yamuna and Ghaggar basin. The river basins of Godavari, Krishna, Narmada, Tapti, Cauvery and their tributaryu rivers also contain river alluvium in pockets. They comprise clay, silt, sand, gravel, pebbles, cobbles, boulders, etc., of which sand and gravel for potential aquifers. Clays and silty clays which are aquicludes form confining layers. To tap this aquifer, one has to identify their geometry. If they form shallow aquifers, the remote sensing techniques will help in mapping these aquifers identified by the fluvial features like palaeo channels, river meanders, ox bow lakes etc. However, in broad river valley extensive deposits/forming lenses of sand of considerable sizes occurring at several depths. Generally, they are topped and bottomed by clay or sandy clay beds. Depending on the geologic history, sedimentation and cyclic repetitions of the gravel sand-clay sequence. The sand lenses are well connected and gets recharged to form good aquifers. Their deep lying aquifers however are to be explored systematically in the entire basin and their geometry has to be worked out from the pre existing exploratory/production borehole data. Several cross sections and fence diagrams are to be constructed from this subsurface data so that their geometry of the aquifers can be deciphered. The transmissivity and specific yield/storage coefficients have to be estimated from aquifer tests conducted in tube wells constructed.
Coastal Aquifers:
Ground water exploration in coastal aquifers will pose certain unique problems. The problem faced in general is the brackish/saline nature of water near the sea. Ground water occurs in such areas in interbedded sands, silts and clays deposited under lagoon, estuarine and marine environment. The fresh water aquifers generally occur in the estuarine mouths along the present stream courses or their palaeo-channels. In all, such cases the fresh water floats at shallow depths on the saline water. Fresh water aquifers do exist in a similar way along the palaeo or present beach ridges or strand lines. Aerial photographs/satellite imageries help greatly in locating the present Paleocene channels or beach ridges. The presence of seasonal strand lines occurring as parallel ridges can be seen several kilometers inland in the coastal districts of Andhra Pradesh and Tamil Nadu. They are conspicuously seen especially in Ramanathapuram district, Tamil Nadu.
Semi-consolidated formations(soft sedimentary rocks):
The soft sedimentary basins in India comprise the Gondwana sediments of late carboniferous to early cretaceous age and Cenozoic sediments (Eocent to Pliocene). To understand the nature of the lateral and vertical extent of the sedimentary aquifer, the lithology, stratigraphy and structure of the rock has to be worked out. By rigorous field work to map the surface out crops or the aerial photographs or high resolution satellite imagery can help in mapping them. However, the subsurface geology and hydrogeology can be worked out from the test well drilling only through geological sections and fence diagrams.
Consolidated formations:
The consolidated formations or hard rock formations can best be classified as igneous, metamorphic or consolidated sedimentary rocks. The igneous rocks among them are crystalline rocks including non-volcanic/volcanic igneous rocks and metamorphic rocks. The sedimentary rocks based on the nature and degree of consolidation, loose the primary porosity and so are treated as hard rocks.
Selection of exploratory or evern production well sites in hard rock area is entirely different from that of alluvial or sedimentary formations. Since the primary porosity is absent in the hard rock aquifer, we may have to look for the secondary porosity. For this purpose, we should first understand how the secondary porosity is imparted to a particular type of rock.
In hard rock areas, ground water occurs under water table (Phreatic) conditions in the weathered and joined portions near surface, which generally support the dug wells (large diameter wells) and shallow bores or dug-cum-boreholes within 30m below ground level. The deep seated fractures when encountered at deeper depths sometimes constitute potential aquifers and supper large yields even up to 20 lps. To study the geometry of these fracture system, the regional and local tectonics are to be understood properly. Study of aerial photographs and satellite imagery are very useful in mapping the fracture in various litho units. The Dolerite dykes, major and minor fractures, outcrop boundaries can be mapped using these techniques. The results of the exploratory drilling programme demonstrated that the boreholes located in tensile fractures have yielded significant quantities of water in relation to the other type of fractures.
Crystalline rocks:
The permeability of these types of rocks depends on the nature of the weathered material and on the size of the open spaces of the fractures. In general, the granitic terrain contains good aquifers as compared to gneiss, schist, jphyllite, slate, pegmatities, dolerites, volcanic (igneous) rocks.
They include a variety of rock types like basalts, rhyolites, agglomerate etc., when the lava flows cools down, vesicles are developed towards the surface and bottom as the entrapped gases escape out. Sufficient time interval between the lava flows give rise to the top weathered portions of the flows. Basaltic flows cover an area of about 5,10,000 sq.km in parts of the States of Gujarat, Maharashtra, Madhya Pradesh, Karnataka and Andhra Pradesh. In basalts the water table configuration is similar to the subsurface topography.
Carbonate rocks:
The carbonate rocks like the other hard rocks lack the primary porosity but develop secondary porosity when fractured or weathered. They differ from other rocks in that they are highly soluble in water rich in carbon dioxide. Dolomites are some what less soluble. Limestone and karstificiation are the other rock types.
Geophysical prospecting:
A number of geophysical techniques and methods have been developed for use in exploration for ground water in alluvial, semi-consolidated and hard rock areas. The most common method adopted for site selection for ground water is Vertical Electrical Sounding(VES).
Geophysical methods use the principle of physics to know the subsurface geological conditions. The aim is to find the anomalies in physical properties and interpret them in terms of subsurface geological and/or hydro geological conditions.
Besides commonly used method i.e. electrical resistivity, other like electromagnetic and seismic methods are also used.
The objectives of Geophysical Surveys are to estimate overburden thickness, characterization of litho logy, delineation of fracture, joints, etc., and to know the quality of ground water.
Vertical Electrical Sounding (VES) is carried out in the area selected for electrical resistivity surveys. During VES, the site hydro geologist must be associated with the geophysical party. Using standard curve matching and computer interpretation techniques the obtained data is interpreted. The layer parameters are adjusted till theoretically generated curves match with the field curves and the resistivity variations with depth are obtained. Based on this, the sites for exploratory boreholes are recommended giving the
Depth of overburden, the depth to basement, ground water potential zones and depth to be drilled etc.
The site characteristics may be described on the format and a drawing of site location (not to scale) with respect to the known respect points should be made on the back side of the format.
SELECTION OF TYPE AND CAPACITY OF RIG
The Central Ground Water Board has got five types of drilling rigs i.e. Direct Mud Rotary rig, Down the hole hammer rig, Direct Rotary-cum-DTH rig, Percussion rig, Direct Rotary-cum-Percussion rig. Some of the DTH rigs are provided with additional attachments like Simultaneous Casing Driver, ODEX system to tackle top loose formation in hard rock areas and in bouldery formations. The Schramm rig is provided with angular drilling attachment also.
The general practice is to carryout exploratory drilling up to the present depth capacity of rig. However, due to ageing the depth capacity of some rigs has been lowered to far less than fifty percent. Such rigs are either deployed for work other than groundwater exploration or in some cases to shallower depths in areas already explored to much deeper depths. This has resulted into a situation where explored area by the organization the past years is not increasing proportionate to the number of exploratory wells drilled. Many unexplored areas, which are inaccessible to the heavy rigs, are yet to be taken up by the board. In the present situation CGWB needs to equip itself with the following state of the art rigs to retain its status of apex organization.
1. All terrain rigs for far flung tribal areas.
2. Environmental rigs for ground water pollution studies.
3. Separate fleet of medium capacity DTH drills for natural calamity mitigation programme.
Selection of type and capacity of rig is therefore to be made jointly by the
Engineer and Hydro geologist from the available fleet of rigs in the operational area.
DRILLING OPERATIONS
Flow charts of all the activities required to be performed for drilling an Exploratory Well by Percussion drilling. Direct Mud Rotary Drilling, Open Hole Percussion Drilling and Down the hole hammer drilling techniques are enclosed.
The bore hole depth vis-à-vis bit size program to be followed for carrying out drilling operations by above mentioned techniques are given below:
Percussion drilling
Borehole depth (mts)
Borehole size (inches)
0 to overburden
36
Overburden to 60
24
60 to 90
20/16
90 to targeted depth (120 or 150)
14/12
Direct Mud Rotary drilling
Borehole depth (mts)
Borehole size (inches)
0 to Overburden
14
Overburden to 300
9-7/8 and 8½
Overburden to 600
12-14, 9-7/8 and 8-½
DATA COLLECTION
The site hydro geologist and the Driller In charge should invariably camp at the drill site and record the following on the prescribed formats.
Detailed description of the lithology of borehole cuttings through out at every meter of drilling in case of soft formations and every 3 meter in case of hard rock drilling.
Details ofl the type of Rig deployed, size and type of bit used, nature of drilling fluid, size and depth of casing pipe, etc. loss of drilling fluid (air or mud) while drilling if any due to leakage into the formations must be observed and recorded. The pressure drop in the drilling fluid if noticed during the operations should also be recorded.
The time in minutes should be recorded for every meter of drilling in soft formations and 3 meters of drilling in case of hard formations or whenever change in formation takes place Formats of drill time log enclosed.
The bit record must be maintained in the formats enclosed at
The depth at which first wet sample is encountered and the depth where water is stuck first (in case of air rotary drilling).
Wherever essential coring must be carried out to decipher information on the nature and type of formation.
In case of direct mud rotary drilling, mud parameter such as density, viscosity and sand contents must be measured and recorded at site.
Discharge measurement in case of hard rock drilling whenever increase or decrease in discharge is noted.
The temperature and specific conductance measurements may be carried out after each discharge measurement is taken.
Water sample should be collected from each aquifer encountered or whenever change in specific conductance is noted for chemical analysis. The quality of water sample should not be less than half la liter.
In case of drilling by Percussion/Down the hole hammer technique static water level measurement should be recorded before start of drilling every day till the borehole is completed.
A record of water level fluctuations in neighboring wells due to drilling of the exploratory bore hole should be maintained with details of location, type of well and so on.
In case of hard rock drilling following information may be recorded and tips noted for better drilling.
Trip-in and trip-out size of bits should be recorded.
RPM of the drill string must be maintained between 12 and 30.
Load on the bit may be given @ 4.5 kg to 9 kg/min diameter of the bit.
Reaming of borehole in hard rock drilling should be avoided.
Heavy intrusion of water into the borehole affects performance of the hammer since the standing water column causes increased back pressure at the exhaust. The rate of penetration therefore drops considerably. Whenever the rate of penetration due to hydrostatic back pressure falls to 1/3rd of the normal rate of penetration further drilling becomes uneconomical and hence the operations may be called off. However, use of drilling foam, (5 to 10 percent) facilitates better flushing of the cuttings and formation water and improves penetration rates marginally.
WELL ASSEMBLY RECOMMENTATIONS
Based on the preliminary investigations carried out in the area, data generated from the lithology, sieve analysis of the formation material and geophysical logging well assembly consisting of housing, blank casing pipe, slotted pipe and ball plug is decided by the site hydro geologist for lowering into the borehole. The slot size and gravel size are to be determined from the sieve analysis of the litho logical samples collected during pilot hole drilling.
The depth of well should be adequate to penetrate saturated aquifer thickness completely. The housing size of the well should be adequate to accommodate pump and the column pipes during development and testing. The well assemblies may be lowered after joining the pipes either by welding or by screw and socketted system.
WELL CONSTRUCTION AND COMPLETION
Two types of Exploratory Well; completions in unconsolidated and semi-consolidated formations are generally adopted.
Type I: Single length well assembly consisting of Housing, reducer, blank and screen pipes (well intake) and a bail plug. The length of housing must be sufficient enough to enable lowering of pump up to the maximum pumping water level with an additional allowance of 5 meters. The diameter of housing must be sufficient to accommodate the pump with a clearance of 5cm to 10cm. Lengths of bail plug 1-3 m depending on the requirement.
Type II: Two length well assemblies is lowered under special circumstances when the conductor pipe lowered during pilot hole drilling operations is retained as
adequate to accommodate the pump. The well intake portion with bail plug is lowered separately with a reverse socket at the top. After placing the gravel pack, seal is provided and pipe length above reverse socket is unscrewed and withdrawn.
Gravel pack material used in well construction should be of appropriate size and well rounded quartz material chemically inert smooth, hard and free from dust/clayey material plate-4.
Following types of well construction are feasible in consolidated formations depending on the site specific conditions.
Type I: Top overburden is cased with appropriate size of blank pipe grouting around annulus and rest of the borehole left uncased.
Type II: Aquifer in the top weathered zone tapped with desired lengths of screen blank pipe and rest of the hole left uncased.
Type III: An assembly of appropriate size lowered in the borehole with or without pack material when the borehole walls are not stable.
Type IV: Intermediate unwanted zones (zones with poor quality of water /casing zones) cased with part assembly borehole telescoped, drilled and rest of the borehole left uncased. However top overburden is cased.
Type V: Sealing of bottom undesired zones by cement grouting and adopting any of the above type design as per site conditions.
Type VI: Sealing of top undesired zones by extending casing depths and resorting to any type of designs.
HYDROLOGICAL TESTS
For determination of aquifer properties like transmissivity, storativity specific capacity well losses etc., hydro geological tests such as preliminary yield test (PYT), Slug Test, Step Drawdown Test (SDT) and Aquifer Performance Test (APT) are conducted on exploratory wells.
Preliminary Yield Test
This test is conducted on wells in hard rock areas when the bore well yielding less than 3 liters per second. Preliminary yield tests are carried out with the air compressor attached with rig for 100 minutes duration. During the test, drawdown and recuperation readings are recorded to compute aquifer parameters and the specific capacity of the well.
In hard rock areas PYT is to be conducted for the first zone and also the cumulative of each successive zone encountered during drilling.
In soft formations PYT has to be carried out after the well is developed by the air-compressor.
Slug Test
Slug tests are useful to estimate the transmissivity of aquifers when no facility exists for carrying out pump test. The test provides preliminary information on the aquifer characteristics, but the transmissivity characterizes only a very small are around the Test Well. Generally a slug of 10 to 20 liters of water is introduced instantaneously and observe the dissipation of water for a period of 30 minutes to 60 minutes. In most cases the complete dissipation will take place within 15 to 20 minutes. The residual head can be measured with the help of DWLR’s for accuracy and more number of observations. Similarly slug of say 5000 to 10,000 liters of water will be introduced to fine out the intake capacity of the bore well for studies in connection with artificial recharge processes. Slug test may not feasible when SWL is very deep and also in formations having high transmissivity.
Step drawdown test
A step drawdown test is a single well test in which the well is pumped at a low constant discharge rate until the drawdown within the well stabilizes the pumping ;rate is increased to a higher constant discharge rate and the well is pumped until the drawdown stabilizes once more. This process is repeated through at least three steps, which should all be of constant duration say from 30 minutes to 2 hours each. Step draw down test need not be conducted on tube wells with discharge less than 2 lps or the specific capacity of the well is less that 6 lps/m drawdown and also on boreholes drilled in hard rock areas.
Aquifer performance test
An aquifer test is designed to impose a hydraulic stress on the aquifers in such a way that measurements of the response to the stress well fit in a theoretical model of aquifer is subjected to stress and one or more observation well in which the response is measure. By conducting such tests the following are determined.
i. Hydraulic characters of aquifer and the confining beds.
ii. The possible influence and nature of aquifer boundaries
The arrangements for an aquifer test must permit the following controls and measurements:
i. Constant pumping rate, even though the pumping level may vary during the pumping period.
ii. Accurate measurement of drawdown in the pumped well and in or more observation well (if constructed), some distance away.
iii. Accurate record of time each measurement is taken as pumping proceeds.
iv. Accurate record of water level recovery in each well with rate of pumping of nearly well if such wells cannot be idle during the period of pumping and during the period of recovery after pumping is stopped.
In hard rock areas on boreholes with less than 3 lps discharge APT need not be
conducted.
Schematic diagram showing equipment used during slug test, PUT, SDT AND APT is given at plate-8.
All the hydrological test must be completed within a weeks time from completion of the wells. Testing of wells drilled in hard rock areas must be carried out by the rig unit itself. Wells drilled in soft formation will be tested by pumping unit attached with the rig.
SOME PHOTOGRAPH OF DRILLING EQUIPMENT
AND LITHOLOGICAL SEQUENCE
About Me
Tuesday, March 25, 2008
LATEST GROUNDWATER MONITERING OF TAMIL NADU
Subscribe to:
Post Comments (Atom)
No comments:
Post a Comment