( . n
linn irrni fflnnrnnj|^ •imnnnt'
•iiiiiiiiinm! ■iimiiii||]Ti« .mrnrmn' iiillTTnnniii dliTTnnnn'
c mnmnni rtf'* (nn mn < •iT Tmrnrn nmiiiiiini mnnnnp .mranmr*. nmniiuTni itflmrmnil It nnrrnnni mTTTnnm lUllinTnm* iniT Inn ■II l mTTi X lilTIlliii|l nnrl hit '•uiuin* < W Tm < nnrnnr ii|f||iuuni» uiiiiini nuiiun tmiiiinr Z .^Appropriate exploitation of groundwater forms sui important practicstl aispect of any hydrogeological study in the delineated Deccan basaltic aqiiifer(s). The subsequent management of the available water forms another important aspect of such study. The shallow unconfined Deccan basaltic aquifer(s) (upto a depth of 15 metres b.g.l.) are usually tapped by means of large diameter shallow open dug wells. This water is utilised for domestic and agricviltural purposes. In recent times, partic\ilarly after the 1972 drought, bore wells have been drilled to tap the deeper confined aquifers as aui additional source suid subsequently as an independent source of water. Nowadays there is an increasing tendency to drill borewells to augment the water resources even in the agricultural sector. The inception of the 'Down The Hole Hammer' (DTH) technique has resulted in a tendency to prefer borewells to dugweUs particularly in the agricultural and industrial sectors. The following points advocate the choice of drilling of borewells over digging of open dug wells. 1) The borewells occupy negligible space on surface (0.105 metrer to 0.15 metre) because of its very small diameter as compared to open dug wells ( 8 metre to 10 metres). 2) The excavated material in case of a borewell is much less as compared to the large amount of debris derived from the excavation of dugwells. Further, the excavated debris is piled up auround the well aund occupies a lot of space. 3) The total time for drilling a borewell to an average depth of 50 metres is very less (maximum 1 day) as compared to that for an average dug well of 5 metre diameter and 12 metre depth (approximately 60 days). 4) A borewell, apart from the insertion of the casing pipe, does not require any further construction or civil work, while a dugwell usually requires lining especially in caise of areais where thick over burden is present. 5) DugweUs usually tap the shallow unconfined aquifers while borewells usually tap the deeper confined aqviifers along with the shallow unconfined aquifers. 6) The shallow unconfined Deccan basaltic aquifers, in general, have a low Trauasmissivity au:id consequently a slow well recuperation. Therefore, the large diameter dug wells are preferred because of their large storage capacity. Earlier it was thought that borewells may not be suitable for exploitation of the low transmissive Deccan basaltic aquifers especially for agriculture and industrial purposes. However, a properly exploited and planned bore well can yield a sustainable quamtity of groundwater for agriculture auid industrial purposes from the deeper Deccan basaltic aquifers. 7) A fully operational dug well (15 metres deep and 5 metre in diameter), without a pump, costs Rs.52,000 (Khutavakar,1984). On the other hand, a boreweU 150 mm in diameter and 50 metres deep, in the same area, costs only Rs.12,000. The cost of a lifting device from the well would approximately be Rs.20,000 in comparison to Rs.25,000 in case of borewells. METHODS OF DRILLING Drilling of borewells to augment the existing supply of water for agricultural, industrial or domestic supply is commonly done in the Deccan basaltic terrain. The compact nature of the terrain makes drilling conditions in this area rather tough. In earlier days, rotary rigs using rock roller or core bits were used, but presently with the advent of the DTH technique, a majority of the borewells are drilled with the DTH rigs. The DTH drilling system consists of three basic units made up of the air compressor the drill stem and the drilling hammer. The compressor, forms the heaurt of the whole system, as compressed air is the main driving force. Compressed air is supplied to the hammer through the drill stem (the hammer is attached to the lower end of the drill stem) at a working pressure of approximately 7 kg/cm2 or more.This air facilitates the rotation of the drill stem at a speed of about 10-30 rpm and the hammering action of the bit. The hammer which is made up of aUoy steel has a bit attached at the base which rapidly strikes the rock while the drill stem is slowly rotated.. This is similar to the blows delivered by a cable tool bit. The bit is cQso made from cdloy steel and consists of buttons of tungsten caurbide embedded in it (Photo 8). It is these buttons that actually provide the cutting or chipping surfaces. The rotation of the bit assures an even penetration . The cuttings are removed continuously by the air which is used to drive the hammer. Unlike a conventional cable tool bit which constantly strikes previously broken fragments, the DTH bit (or buttons on the hammer) always strike a fresh surface, thereby making the DTH technique more efficient (DriscoU, 198<). The advantages of DTH drilling method are- 1) Cutting removal is extremely rapid. 2) Aquifers are not plugged with drilling fluids as air is the 'drilling fluid'. 3) Absence of mud pumps, 4) Bit life is longer as compared to that in cable tool drilling because the bit always strikes a fresh surface thereby making it more efficient. 5) Extreme weather conditions do not hamper the drilling work. 6) Penetration rates are high even in highly resistant and compact rocks like basalt. 7) An estimate of the yield can be made during drilling by the air-lift method using the same compressed air which is the drilling fluid. The disadvantages of the DTH drilling method are- 1) It is restricted to semi-consoKdated and well consolidated material/formations. 2) Initial costs and maintenance costs of the large compressor are high, 3) The litholog prepared from the lithological samples of the DTH drilling is rather approximate as compared to the one by core drilling. The borewells drilled using the DTH technique can be broadly classified into two basic types, on the basis of their diameter. 1) The 105 mm diameter borewell and 2) The 150 mm diameter borewell. Conventionally, borewells with the above mentioned diameters are more commonly drilled for agricultural, industrial and domestic purposes even though it is possible to drill borewells with diameters otherwise. Further, there is a vast difference between the DTH rigs drilling the 105 mm diameter borewells and the 150 mm diameter borewells. Some of the relevant differences are listed below in Table-3.1. DRILL-TIME OBSERVATIONS The drilling period in the case of borewells drilled by the DTH rigs is rather short (rounging approximately from 4 hours to 24 hours). However, this drilling period offers an unique opportunity to acquire first hand information about Deccan basaltic aquifers. Certain observations can be made so as to obtain a better understanding of the subsurface Deccan basaltic aquifers tapped by the borewells and also about the approximate yields, depths and the thicknesses of the different aquifers tapped. Some details of the Drill-time observations have been described below. COLLECTION OF LITHOLOGICAL SAMPLES Use of DTH rigs for the drilling of borewells in Deccan basalts causes the pulverising of basaltic rocks into smaller pieces. These can be broadly categorised into powder, fragments or chips, and the larger flat pieces. These pieces are available for observation when they are brought to the surface by the drilling flmd, which in this case is compressed air, or if the aqiufer has been tapped, along with the mixture of air and water . The lithologic samples are collected at regxilau: intervals of approximately 4.5 metres by the rig operating crew. The usual practice is to coUect the pieces flushed out and lying around the drillstem at the time when the drilling is temporarily stopped to attach a new drillstem. The approximate interval of 4.5 metres corresponds to the length of one drillstem in csise of the 150 mm diameter borewells auid to 3 drillstems in the caise of the smaller 105 mm diameter borewells. This has been a standard methodology used by drilling agencies. Even the data supplied to Government organisations Kke the Groundwater Survey and Development Agency by its contractors is of a similsir nature. Table 3.1 : Drilling by 150 mm and 105 mm rigs- a comparison 150 MM nRILLU15_^LiaS__EQE__IQ5.....^ 1) The weight of the rig is 1) The weight of the rig considerable and is always truck excluding the Compressor is less mounted. The rig cannot be (approximately 400 kg.). These dismantled; only the mast can be are portable and can be raised and lowered. assembled and dismantled on the site itself. 2) Higher operating pressures 2) Works on a lower pressure require larger capacity of the and hence requires a smaller ccjmpressors, thereby increasing capacity compressor, thereby the diesel consumption. reducing the diesel consumption. 3) Depth is no restriction- 3) The rig normally have a borewells can eeisily be drilled limitation of a maximum depth of upto depths of 200 metres. 45 metres. 4) Drilling rate is faster. 4)Due to the smaller capacity of the compressor the rate of drilling is comparatively slower. 5) Borewells can be drilled only 5) It is possible to drill in places which are readily boreweUs in confined accessible to a truck. spaces,aswell as in existing dugweUs. 6) Drilling platfom is fixed to 6) The driUing platform is the drilling rig which is movable and is usually mounted on a truck along with connected to the compressor by the cortpressor. means of a long flexible hose pipe. 7)The connecting drill stems are 7) The connecting drill stems 4.5 metres in length. are only of 1.5 meter length. 8) Larger weight of the drilling 8) The drilling equipment is equipment requires a larger comparatively light in weight operating team. and can be managed by a small team of only three. 9) The total cost of the 9) Total cost of driUing a borewell upto 40 metres borewell upto 40 metres depth including casing of 6 metres is plus 6 metres of casing is Rs.10300 (Rs.8200 + Rs.2100). Rs.8700 (Rs.6600 +Rs.2100). 10) Any type of purrp can be 10) AU types of pumps can be installed on the drilled installed on this type of well. borewell. The lacunae in this approach lie in the fact that exact changes in Kthology with respect to depth maybe missed out and fail to be recorded. Flow or flow unit contacts can be easily missed out due to the fixed interval of collection. Hence, the exact flow contact is never marked and is always approximated. For instance, if the change of flow/flow unit occurs immediately after the collection of a sample at the start of the next rod, the exact depth of this change cainnot be recorded since the next sample is collected at the end of that paurticular rod. One csua sometimes completely miss out a flow unit if its thickness is less than 4 metres { less than one drill-stem ). Further, if this small flow unit is acting as anaquifer, and its sample is not included in the litholog, wrong interpretation would result. For instance an Amygdaloidad basailtic flow unit of small thickness say 3 metres to 4 metres forming an aquifer and sandwiched between two Compact basaltic flow unit^ if missed out, the middle portion of the Compact basaltic flow units maybe interpreted as water bearing. This is because the collected samples only represent the Compact basaltic flow units. The thicknesses of the flow units can also be either grossly enhanced or reduced. Thus, with this present methodology in vogue, a grossly approximated lithologic layering can emerge. SUGGESTED METHODOLOGY FOR THE COLLECTION OF LITHOLOGIC SAMPLES FOR THE DTH BOREWELLS The main aim of this exercise is to note the changes in the flow units with respect to depth during drilling and adso to note the variation within the flow units wherever possible. During the monitoring of the drilling operation the following practice should be followed as a standard for the collection of Litho samples to generate a reliable litholog: While drilling, any change in the nature of the material continuously coming out of the borehole should be caurefuUy observed and recorded with respect to depth. Such changes include a change in the colour of chips/powder from grey to reddish or vice versa as also the relative increase or decrease in the size of the amygdules, the change in the size of chips from coarse to fine or vice versa or the presence of abnormally large pieces. The presence or absence of dust or fine powder which is normally blown out while drilling through dry rock and presence or absence of water cind also its colour whether red, black or clear, should carefully be noted. Before collecting the samples at a particudar level where the change occurs, further drilling should be halted and the well should be subjec±ed to thorough flushing and only then should drilling be resumed to achieve a further penetration of 0.06 to 0.1 metre. The boreweU shoiild again be subjected to flushing thereafter. The samples can be collected by placing a pan near the drill stem. A sample so collected represents the change which has been noted in the earlier lithology and the contamination of the previous Hthounit is considerably reduced. If such samples are collected, a fairly acccurate Ktholog can be prepared. IDENTIFICATION OF BASALTIC LITHO UNITS FROM DTH DRILL-TIME LITHOLOGIC SAMPLES The author has used the terminology of 'Compact' and 'Amygdaloidal' to describe the basaltic flow units. Further, the alternating sequence of Compact basaltic units and Amygdaloidal basaltic units is widely observed in the Deccan basaltic terrain. Some salient features of the Uthologic samples collected during drilling (with DTH rig) have been observed by the author under a magnifying lens to differentiate between the Amygdaloidal basaltic flow units and the Compact basaltic flow units. These features have been described below. It has been observed by the author that drilling through any dry basaltic flow unit is always associated with the emergence of a lot of smoke-Kke dust. This dust is formed due to the pulverisation of the hard homogeneous basalt into a fine powdery form (Photos 9 and 10). Usually, the Compact basaltic flow units give rise to whitish smoke-like dust while the Amygdaloidal basaltic units give rise to a greenish or pinkish coloured dust. On closer observation, the dust is found to contain small chips of the respective basalt types (Photo 11 ). While drilling through dry Amygdaloidal basaltic units, the amount of dust is lesser as compared to the amount of chips whereas in case of Compact baisciltic flow units comparatively more dust is generated as compared to the amount of chips obtained. This again is due to the much harder and homogeneous nature of the Compact basciltic flow units. The chips and the fragments obtained while drilling through the Amygdaloidal basaltic flow units and the Compact basaltic flow units show different characters. These characters have been described below. =^7 Amygaloidal basaltic flow umt samples As mentioned earlier (In Chapter II ) the Amygdaloidal basaltic flow unit is very often capped by a red tuffaceous layer which forms a fairly reliable marker horizon. This red tuffaceous layer can be easily identified during drilling in the form of small red fragments/chips/powder which accumxilate around the drill stem. The Photographs 12 to 15 show a sequence of changes within the Amygdaloidal basalt flow unit associated with a tuffaceous horizon. In water bearing horizons, damp samples of the tuffaceous layer are observed (Photo 12), followed by wet mud (Photo 13), then water ( Photo 14) and finally the emergence of water in measurable quantities (Photo 15). The water issuing from the borewell in this case is reddish in colour as it contains suspended tuff particles (Photo 15 and 16). The chips of the red tuffaceous layer, when dry, appear as depicted in Photo 17, It has been observed that the red tuffaceous layer grades downward into an amygdaloidal basalt (Kulkarni,1987; Kulkarni and Deolankar ,1990). This gradational contact is characterised by fragments of the Amygdaloidal basaltic flow unit and the red coloured chips of tuffaceous material that are obtained during drilling. Many a times, this portion represents a volcanic breccia, which can also be identified from the chips (Photo 18). The brecciated and amygdule rich portion of the Amygdaloidal basaltic flow unit grades into a middle portion which is very hard and fairly homogeneous in nature. This portion contains small amounts of pyroclastic material and the number and size of amygdules also decreases in this portion. This middle portion, during drilling, is characterised by the emergence of dust and small chips if the upper portion of the amygdaloidal basaltic flow unit is dry, i.e. devoid of water (Photo 10). The lower portion of the Amygdaloidal basaltic flow unit is characterised by a higher percentage of vesicles and amygdules. The contact between the lower portion of the Amygdailoidcd basaltic flow unit and the underlying Compact bassdtic flow unit is usually characterised by a mixture of chips and fragments from the Amygdaloidal basaltic flow unit and the Compact basaltic flow unit along with a higher percentage of the secondary minerals (Photo 19). Similarly, the contact of the overlying Compact basaltic flow unit and the underlying Amygdaloidal basaltic flow unit is characterised by a mixture of tuffaceous chips and black chips of the Compact basaltic flow unit (Photo 20). The Amygdaloidal basaltic flow unit is grey coloured and usually not associated with the red tuffaceous layer. The top portion of such a grey Amygdaloidal basaltic flow unit, if weathered and water beziring, is characterised by grey coloured powder and very small chips interspersed with amygdules (Photo 21). The colour of the water flowing out of the borewell under such circumstances is bluish grey (Photo 22). The upper and lower amygdule rich portions of the Amygdaloidal basaltic flow unit are usually characterised by the development of sub-horizontal sheet joints. Such sheet jointed portions are also encountered in bore wells, e.g. in Shirgaon area the dugweU tapping the Amygdaloidal basaltic flow unit shows the development of sheet jonts (Photo 6) and the uncsised borehole close to this dugwell also shows the development of sheet joints near the surface (Photo 23). During drilling, such sheet jointed portions can be identified from the big flat pieces of the Amygdaloidal basaltic flow unit that are brought out (Photo 24). The chips of the Amygdaloidal basaltic flow units associated with very little quantity of water (slight dampness) are usually coated with a fine powder and rounded to sub-rounded in shape and quite soft in nature (Photo 25). Compact basaltic flow unit samples The Compact basaltic flow units are characterised by small chips which are angular in shape and very hard in nature. They can be recognised on the basis of their black or grey colour and are practically devoid of amygdules (Photo 26). The change from an Amygdaloidal basalt to a Compact basalt is very apparent especially in case of the pink Amygdaloidal basalt flow unit. This change is easily recognised by the harder and smaller angular dark coloured chips in the reddish background (Photo 27). The Compact basaltic flow unit is sometimes exposed on the surface and is subjected to weathering. Such a weathered portion does not necessarily produce the same kind of powder and instead, slightly bigger and angular pieces are collected around the drill stem. These pieces sometimes show a coating of iron oxide because of the circulation of groundwater (Photo 28). A water bearing Amygdaloidal basaltic flow unit changes to Compact basaltic flow unit on progressive penetration with drilling. Consequently, the colour of the flowing water also changes from reddish or bluish grey to greenish grey or Kght grey as drilling proceeds (Photo 29). The rate of drilling is dependent on the type of rock as well as the capacity of the compressor. It has been observed by the author that thereis a marked difference in the rate of drilling in the two basaltic flow units. While drilling through the Amygdaloidal basaltic flow unit the rate of penetration is about 10 -20 minutes for 4.5 metres (length of one drill stem), whereas in case of the Compact basaltic flow units it is between 40-60 minutes for 4.5 metres (length of one driH stem). This varies to a sKght degree at different locations of course. This fact has been observed at the sites at Chandannagar, Vadgaonsheri, Wagholi/Padal, Mundhwa, Talegaon etc. (the drilling rig used was equipped with a Kiloskar Pneumatic screw compressor operatingon 12.5 kg./cmz pressure and air volume of 800 CFM ). DRILL-TIME YIELD In the DTH drilling technique, the primary moving force is the compressed air which is used for achieving the up and down movement of the percussion hammer along with the rotational movement. The air which is released at the end of the hammer further acts as the drilling fluid which brings to surface the broken rock chips. The compressed air also throws up water on striking an aquifer. The water which is brought to the surface along with the compressed air and rock chips starts flowing in the form of a small streamlet. The quantity of water flowing out in the form of a channeUsed streamlet can give a fairly good estimate of the yield of the borewell. Hence, during the drilling operation,a continuous record of the quantity of water flowing out of the boreweU is of prime importance. This water which issues out of the boreweU at the time of drilling normally flows out on the surface, and can be channelised. The water flowing through such channels can be measured using different weirs. Generally, drillers using the DTH techniques have V-Notch weirs cut out of a 1 to 2 mm thick metallic/nonmetallic sheet. The angle of the V-Notch is either 60 degrees or 90 degrees. Urquhart (1959) has given the details of construction of V-Notches along with the formulae for calibration of the recorded head into a measurable unit. Hiranandani and Chitale (1960), have explained the basic prerequisites for the This variation in the measurable quantum of water is usually due to the difference in operating pressures. The high pressure, in turn, results in a faster rate of drilling due to which a larger volume of rock is pulverised to powder amd chips which, on mixing with water, forms a slurry. This slurry is not very mobile and gets plastered onto the sides of the well thereby sealing off the aquifer to some degree. More the amount of powder and chips more is the quantity of water required to make the mixture mobile. If the quantum of water flowing out of the well is less the powder and chips along with the water forms a slurry and during the flushing this slurry gets plastered along the borewell face. With subsequent drilling, due to the plastering effect, a collar is formed at and around the portion where the water inflows into the well (aquifer(s)). Finally, no more water issues out of the well while drilling. This situation results if the inflow into the well is less than 100 LPH in case of the 105 mm diameter boreweU and less than 500 LPH in case of the 150 mm diameter boreweU drilled using the low pressure compressor and less than 1400 LPH in case of the 150 mm diameter wells drilled using the high pressure compressor. On the contrary, if the quantity of water flowing into the well increases, the resultant powder and chips go into suspension and during flushing such water issues from the boreweU and can be subsequently measured on a V-Notch as continuous flow. Such borewells are usually designated as successful borewells. On the contrary, when, the yield cannot be measured on the V-Notch because of the plastering effect mentioned earlier, borewells are many a times designated as 'failure bore wells'. This is more so in case of 150 mm diameter borewells drilled using a high pressure drOling rig and showing yields less than 1000 LPH. Such 'failed borewells' with a low yield and plastering effect can be readily revitalised with a proper cleaning and flushing operation. The author himself has salvaged qmte a few such borewells by removing the plastered collar with proper cleaning and/or flushing operation. The Deccan basalts constitute a layered aquifer system, and hence, many a times, a boreweU taps more than two basaltic flows acting as aquifers and this results in the development of more than one inflow zone into the weU. During drilling, there is a substantial increase in the quantity of water flowing out of the boreweU as measured on the V-Notch and this increase in the discharge persists even as driUing progresses. This is usuaUy due to the tapping of here Q is the discharge in U.S. Gallons/ Minutes (1 U.S. Gallon = 3.785 Litres) 'H' is measured in inches and is usually measured as a difference in elevation and the apex of the V-Notch. Various workers have published charts for the 60degree V-Notch and the 90 degree V-Notch, converting the measured heads into discharges in Litres per Minute and Gallons Per Minute ( Ministry of Food and Agriculture, 1962; Raghunath,1982; Driscoll,1986). These charts are generally used by practising hydrogeologists and drillers. In pratice the drill- time yield is usually measured by the drilling crew who have no knowledge of the importance of accurate measurements on the V- Notch and therefore the prerequisites mentioned above are rarely adhered to. For instance, the special straight channel with a flat base is never constructed, the V-Notches are placed close to the drill stem where a lot of turbulence is present, the head is usually measured at the notch itself by placing a scale at the apex and there is a lot of leakage in and around the V-Notch (Photo 30). Many a times, a sump has been made and the water is allowed to pass over and above the V-Notch (Photo 15,29,31). These inadequecies cause the measured Drill- time yield to be quite inaccurate. The author has tried to adhere to the prerequisites in the measurement of Drill-time yield as much as possible. As mentioned earlier, there are two basic types of drilling rigs, the one drilling 105 mm boreweUs and the other drilling the 150 mm boreweUs. The 150 mm boreweU drilling rigs include rigs equipped with a compressor which have higher operating pressures and a higher air volume (10.5 to 17.5 kg/cm2 and air volume of 800 CFM and above). The low pressure rigs operate on 7 kg/cm2 pressure and have 600 CFM air. It has been observed that the measurable quaintity of water that flows out varies depending upon the type and capacity of drilling rigs. For instance, in case of rigs drilling 105 mm boreweUs, only yields that exceed 100 LPH can be monitored as a continuous flow over the V- Notch. In case of rigs drilling the 150 mm boreweUs and operating on lower pressures (7kg/cm2), outflows greater than 500 LPH can be measured continuously.For rigs operating on higher pressures (> 10.5 kg/cm2), outflows greater than 1400 LPH can be monitored as continuous flow over a V-notch. accurate measurement of discharge. Some of the salient prerequisites are given below, BASIC REQUIREMENTS FOR MEASUREMENT OF PLOW THROUGH V-NOTCHES 1) A stable, uniform and straight approach 2) A length of fifteen times the maudmum head over the notch is considered as minimum channel length. 3) Cross-section of the approach channel should be at least six times the depth of the flow. 4) Notch should be perfectly vertically placed in the groove. 5) Distance between the crest and the bottom of the channel and between the sides of the notch and the sides of the channel should not be less than twice the depth of flow over the crest and in no case less than 0.3 metres. 6) Flow should be free from turbulence. 7) On the downstream side, full aeration should be ensured andclinging of the nappe should be avoided. 8) A minimum distance of 0.046 metres between the base of the V-notch and the base of the channel is considered necessary for accurate measurements. 9) Notches should be free of weeds and silt on the upstream side suid should be peroidically cleared as it accumulates. 10) No leakages should be allowed from around or underneath the structure. 11) Some distance away from the notch, on the upstream side, the head 'H' is measured as difference in elevation between upstream water surface and the apex of the V-Notch. In the present study the calibration of discharge has been done by the formula given by Driscoll (1986). The equations are given below. For 90 degrees V-Notch Q= 2.4381 H(2.4S) For 60 degrees V-Notch Q= 1.4076 H (2.48) another inflow zone. The author has observed that such an increase in the DriH- time yield may occur more than once during the drilling of any well, depending upon the number of inflow zones tapped with depth. With careful monitoring of the drill-time yield it is possible to quantify the relative potentials of these different inflow zones. The author has measured such variations in yield while monitoring drilling operations, whenever possible. The data to this effect is given in Figure 3.1 along with the lithologs of borewells. A very queer phenomenon is sometimes noticed during drilling. An aquifer with a substantial yield is tapped and with further drilling, at a certain depth, the measurable quantity of water suddenly stops flowing. It is as if the outflowing water is absorbed within the subsurface. The sudden stoppage in the flow of water coincides with the drill stem suddenly dropping down by a metre or so; many a times, powder and/or cuttings also cease to come out from the well and in such cases no further drilling is possible. The author has observed that this phenomenon is not very common amd is apparently restricted to certain areas. Preparation of lithologs (Drill logs) from collected boreweU samples The lithologs are prepared using samples collected during drilling. These samples are placed in small heaps in a sequence (depth wise). Based on the characteristics of the drill samples (mentioned earlier on page-^--), major changes in the bascdtic flow units can be identified. Typical characters like the presence of red tuffaceous layers, sheet joints and a higher percentage of secondary minerals are indicated at proper places. It may not be possible to identify the exact nature of the basedtic flow unit closest to the ground surface on the basis of the samples collected during drilling because of the high degree of weathering. Under these conditions, the surface exposures of the basaltic flow units observed in the near vicinity can be used to decide the nature of the flow unit. The author hais prepared lithologs for aU the sites monitored by him. Any idecilly prepared Utholog should include the depth wise vertical variation in characteristics of basaltic flow units, the depth at which the water was struck 59 Case Numbers ■n 5 6 8 10 11 12 Or- 1600 ■1600 Damp 10 • - 1000 9^00 ■900 -2800 ■•-Damp ^900 s ? s Damp 20 2250 •-250 250 • 9^:00 •-Damp 30 6850 ■ *■ 2800 T3 c: 3 i-0 . o o> 50 o r. V;! ■k;:e Of •“8600 GOO 80 70 I 80 ] AMYGDALOIDAL BASALT FLOW UNIT. 1500 COMPACT BASALT FLOW UNIT [-»~250 INFLOW ZONE WITH YIELD IN LPH 90 RED TUFF kte n S ONLY DAMPNESS OBSERVED IN '------^ DTH SAMPLES. FIG 3-1 BOREWELL LITHOLOGS OF CASE STUDY AREAS. and the variation in the Drill-time yield with depth. Some of the representative lithologs are presented in Figure 3.1,3.2. Generally, the Hthologic samples collected at the time of driUing (in polythene bags) are discarded after the preparation of the schematic lithologs. The author feels that for better extrapolation, the samples can be stored in the form of a repository for further reference. This can be done by filling the Drill time samples in a groove, cut on a Thermocole sheet. This sample log sho\ild be preserved along with a systematic sketch. A representative repository sample, as suggested above, is presented in the jacket (Sample 1). STIMULATION OF BOREWELLS Sometimes a borewell fails to yield adequate water or is totally dry even though other bore wells in its near vicinity are yielding sufficient quantities of water. Such borewells are labelled 'failed borewells'. These failed borewells may tap the same flow units which, in the other borewells, yield a substantial quantity of water. It is conceivable that such borewells are not properly connected to the aquifer through suitable interconnecting channels like fractures and/or joints. It is qviite possible that interconnected openings like fractures and joints are not developed at all. It is possible to stimulate such failed borewells by sectional blasting. Certain selected portions are subjected to blasting with the help of explosives. The main aim of such sectional blasting is to create fracture conductivity along the well face which, in turn, connects the borewell with the surrounding aquifer, Sakare and Maldhure (1984) have described the sectional blasting technique for the stimulation of failed borewells. Driscoll (1986) has also given a detailed account of the methodology of the use of explosives for the stimulation of borewells. METHODOLOGY OF SECTIONAL BLASTING Sakare and Msddhure (1984), have explained a methodology for the sectional blasting of borewells. The author was also involved in the sectional blasting for stimulation of low yielding or dry borewells. The following methodology is adopted in sectional boreblasting . 60 Case numbers I II III Damp 10 600 20 2500 1000 ■1600 30 NOTE Z.0 Symbols and other details 50 same as in Fig 3 1. 60 FIG, 3 2 B O R E W E L L L I T H O L O G S OF CASE STUDY AREAS 1) The water level and depth of the bore well are measured, 2) Sections of the bore well are selected on the basis of the Drill-time litholog, the hydrogeological prospecting and the resistivity prospecting data interpretation. The author has given preference to the Amygdaloidal basaltic flow units, especially to the upper and lower flow unit contacts. The reasons for such a bias will be discussed in detail in chapter IV (Synthesis). 3) Usually, for the blasting, gelatin sticks of 25 mm diameter and 200 mm length of the class-3, division-1, category-ZZ sold under the brand name of NOBLE GEL 80 are used. One single stick usually weighs 200 grams. Usually at each blasting level 10 gelatin sticks are coupled together (Photo 32) i.e. two Kilograms. This amount is variable depending upon the depth and the type of basalt to be blasted, e.g. if the blasting level is close to the surface (15-20 metres b.g.l.) then a smaller charge is utilised (4-6 sticks, 800 grams to 1200 grams) and for deeper levels greater than 60 metres b.g.l. a higher charge is used. A maximum of 13 sticks (2.6 Kilograms) have been recommended by the author. 4) Each group of gelatin sticks is provided with a detonating fuse (Photo 33), These sticks are tied to a thin cotton or nylon chord at measured intervals (corresponding to the depth from surface of the section to be blasted). Finally, the fuse(s) are connected in series through a fuse wire or a detonating chord. 5) The whole assembly is lowered into the borewell with a rope which is secured to or near the casing pipe. The detonating chord or fuse wire is connected to the detonator through a cable. 6) The blasting system requires the borewell to contain water which acts as confining medium during the explosion. This is to ensure a lateral transmission of the effects of the blasting within the borehole. Therefore, if the borehole is completely dry or the blasting level is above the static water level in the borehole, then the borehole is first filled with water. As per the authors experience a minimum of 8 m water column above the explosive charge is necessary to create the required pressure due to blasting. 7) The detonator is kept approximately 50 metres away from the borehole. This is necessary because, on explosion, rock cuttings are thrown up into the air along 61 with water and this may fall as debris within the radius of 50 metres from the borehole. This detonator should be situated in the direction from which the wind is blowing, Further, the area in the direction in which the wind is blowing should be cleared. 8) The residual rock cuttings auid the shattered rock fragments and blasting are thrown up in the air along with the water in the form of a plume attaining heights of 50 to 60 metres (Photo 34) 9) Five minutes after the blasting, shattered rock fragments extruded from the borewell are collected and examined to see whether the collected samples show any characteristics of the contemplated (Amygdaloidal basaltic) flow units (Photo 35). 10) If blasting has created sufficient fracture conductivity to connect the borehole properly with the aquifer at different levels then water should start flowing into the well, raising the water level in the well. Sometimes, the sound of water falling in to the borehole from the newly stimulated inflow zone(s) can be heard. 11) The rise in water level is measured with respect to time. THE EFFICACY OF STIMULATION BY BLASTING As mentioned earlier, the author was associated with about 10 cases of stimulation by sectional blasting. Majority of the bore wells were either completely dry or very low yielding . At all these sites the author had carried out hydrogeological and the resistivity surveys. The author was mainly involved in choosing the levels at which the explosive charge was to be placed. Table 3.2 shows details of the sites at which the boreholes |/?were subjected to stimulation by sectional blasting. A few representative cases of borehole stimiilation have been discussed below. C a ^ .51...: At the village Undri, a 150 mm borehole was drilled on the farm land of Mr. Budhiwant. Figure 3.3 shows the schematic drill log noted during drilling. At a depth of 15 metres b.g.l. some water was struck which showed a measured yield of less than or equal to approximately 500 LPH on a 90 degrees V-Notch. The water level within the borehole before blasting was at about 10 metres b.g.l.. The yield was insufficient to meet the necessary requirements. Further, in Table 3.2: Stimulation of borewell data S .No . Site Borewe 11 Sectional blasting Yield in LPH Type of location Dia ./Depth levels in meters Before After Rock in meters b.g.l. (stimulation) Blasted 1 . Undr i 0 .150/76 15 , 26 , 65 600 1600 A.B.F.U 2 . Ranjegaon 0 .105/30 22 , 27 Dry 1000 A.B .F .U 3 . Sanaswadi 0 .150/50 19 , 41 Dry 600 A-B.F .U 4 . Takwe Bk. 0.150/60 21 , 39, 55 Dampness 900 A.B.F .U 5 . Vadgaon 0.150/90 20, 41 , 50 Dampness 1200 A .8.F .U Maval 6 . Lohegaon 0.150/90 18, 39 , 55 Dry 1000 A.B.F .U 7 . Bhandgaon 0.150/55 28 , 40 Dry 2000 A .8.F .U A.8.F.U - Amygdaloidal Basaltic Flow Unit 3 : a . >1 ■ ^ § Q} JH ~ g > to TS cn oi q : Q ^ 0 . o o d) U. Uj Q- 5: ■fj la Q X m j d ) o ; “o cv U c ^ 2 3 I jO g S * 0 03 u •H “ 8 IS ■P a n 0 " c H D cn •H Of L. OJ u i o 1—< U l-H I I I I o E £ o o Q O ro (U o a> E- fN ■ E c » I I < N o ■o (U cn ___ c 3 Q) 0) m 0 O c > > Within minutes of blasting, the water level was measured to be at 30 m. b.g.l. The water level subsequently rose at the rate of about 1 metre per minute. Gushing of water inflowing into the well could be heard till the water level reached a level of 15 metre b.g.l.. Later, the well was subjected to flushing with an air compressor. The pipe was lowered up to a depth of 45 metres b.g.l. and the compressor worked for 2 hours, A continuous flow of about 1500 LPH was measured over the V-Notch. On the basis of this, a suitable Jetpump of 1.5 HP was recommended. Qc^se,. 5.2,..,: At Sanaswadi, approximately 30 km from Pune, on the Pune-Nagar road, a well of 150 mm diameter was drilled on the premises of M/s Shutham Electric. Figure 3.3 shows the DriU-time litholog recorded during drilling. This borewell was found to be completly dry while drilling. There was no borewell within the vicinity of this site . However a dug well tapping the upper tuffaceous Amygdaloidcil basaltic flow unit is situated close to the site. It was reported that this dug well was characterised by a sufficient quantity of inflow through the Amygdaloidal basaltic flow unit. This was also evident by the agriculture support of this well. Sectioned blasting was recommended to enhance the fracture conductivity in the upper Amygdaloidal basaltic flow unit . As shown in Figure 3.3 the charges were lowered to the desired depths in the Amygdaloidal basaltic flow units. Water was first poured in the well until the water level reached 4 metre b.g.l. Instantly after the blasting the water level in the borewell was measured to be at 21 metre b.g.l. During the next 20 minutes, the water level rose to about 15 metre b.g.l. A hand pump was fitted on this well to meet the immediate drinking water requirements. This handpump is reported to be working on this well continously even today. Case S3 : (Case 12 in chapter II), At Ranje village near Shivapur (25 kms. form Pune on the Pune - Satara road), on the premises of M/s Pacific Labeling a 105 mm diameter borehole was drilled. Figure 3.3 shows the schematic section of the drill log observed while drilling. During drilling, this borehole only showed 63 dampness at 22 and 27 metres b.g.l. in the Amygdaloidal basaltic flow unit. There was no measureable outflow of water from the borehole. However on the day after the drilling, the water level was measured to be at 21 metres b.g.l. There were three more borewells in the vicinity which tap the same amygdadoidal basaltic flow unit under a similar situation. However, at a lower elevation them the present borewell, (where the Amygdadoidal basaltic flow unit is exposed on surface) a dug well with substantial inflow from the Amygddaloidal basaltic flow unit was observed. Hence, on the basis of the dampness recorded in the Amygaloidal basaltic flow unit during drilling and the potential of the said, dug well stimulation by sectional blasting was recommended. As shown in the Figure 3.3, the explosives were lowered to the recommended depths. Water was poured till the water level was raised upto 12 metres b.g.l. Immediately after the blasting the static water level was noticed to be at 21 metres b.g.l. In order to test its yield a deep well piston pump was installed with the piston assembly at 24 metres b.g.l. This well was tested by working the pump for one hour at a discharge of approximately 1000 LPH. Even though the piston assembly was only 3 metres below the static water level the pump worked continuosly for 1 hour, signifying that the drawdown after the pumping is less than three metres. This pump is now being used by the company to meet their daily requirement of water. ECONOMICS OF STIMULATION OF BOREWELLs Sectional blasting involves certain expenditure for which the general economics, as experienced by the author, is given below: 30 sticks of explosive @ Rs 10/stick Rs.300-00 3 Detonating fuses 0 Rs 25/fuse Rs. 75-00 For a 50 metre deep boreweU a 100 metre fuse wire @ Rs 1 /metre RslOO-00 Total Rs.475-00 Thus, the total cost of material is Rs 475/- (say Rs 500/-), including the rope for lowering the explosive charge. Professional consultants charge about Rs 1000/- to Rs 1200/- per explosive charge. The total drilling cost (including the casing pip)e of a 50 metre borewell ranges from about Rs 10,000/- to Rs 15,000/- . The actual cost of sectioned blasting (Rs 500/- material cost and Rs 500/- consultancy and labour) amounts to about Rs 1000/-, which is about 10% of the total cost of the borewell. In reality, private consultants charge upto 30% of the total cost. This above data shows that it is possible to stimulate a low yielding or dry borewell by creating a fracture conductivity at certain sections, thereby connecting the borewell to the aquifer. However, to achieve the desired effect, precise hydrogeological data is necessary. The economics also show that this is a very cheap method for salvaging expenses already incurred for drilling, which otherwise would have resulted in a totally dead investment. PUMPING EQUIPMENT For the augmentation of water from boreweUs different types of pumps are utilised. In the Deccan basaltic terrain of Maharashtra the pumps that are commonly installed on boreweUs are the hand operated or the motorised piston pumps, the jetpumps, the submersible , the volute centrifugal pumps and the air compressor pumps. These different kinds of pumps have their advantages and limitations (Campbell and Lehr,1973; Raghunath,1982;DriscoU,1986). Hand operated piston pumps are usually installed on boreweUs where the average yield is low (about 500 LPH or less) and in cases where the water requirement is very low. GeneraUy, with this pumping system the discharge of about 500 LPH ( depending upon human efficiency) can be expected. Sometimes this piston assembly can be powered by a motor (electric or fuel driven). The only disadvantage of this piston pump system is its low discharge capacity (maximum discharge is 1400 LPH). The Centrifugal Jetpump assembly operates on a wide range of suction heads and discharge capacities. It can draw water out from about 75 metres depth at a discharge varying between 900 LPH with a 5 HP pump & 2500 LPH with a 10 HP. These pumps have a high discharge capacity for low suction heads of approximately 20 metres b.g.l. and on account of this advantage auid their low initial costs, centrifugal jetpumps are widely used in the Deccan basaltic terrain. The main disadvantage of this pumping system is that the discharge capacity decreases with increase in suction head. Submersible pumps are high discharge pumps. These are usually installed in high yielding borewells having a yield of 2000 LPH or more. The submersible pumps are available over a wide range of dischaurge capacities. For a fixed horse power, the discharge head can be increased by increasing the number of stages during the manufacturing process. Submersible pumps can draw water from great depths (380 metres,17 HP/40 stages ,discharge 3300 LPH). Due to these reasons, submersible pumps are widely used in borewells in the Deccan baseiltic terrain . Normally, pumps in the range of 3 HP,5 HP or 7.5 HP are installed. The main disadvantageof this assembly is that it has to be installed within the borewell and whenever something goes wrong with the pump the whole assembly has to be taken out. The volute centrifugal pumps are often installed on borewells tapping a shallow high potential aquifer upto 10 metre b.g.l. The volute centrifugal pumps have a very high discharge ( e.g. a 1.5 HP pump at a discharge head of 10 metres discharges 1498 LPH), but these pumps have a very low suction head of a maximum of 15 m. In high potential shallow aquifers, these pumps are widely used because of their high discharge capacity and low cost. The air compressor pumps work on the principle of air Hft method. The compressor used in this pump assembly shoiald be able to develop pressures upto 5-7 kg/cm2. These pumps have a high discharge and are capable of drawing water from great depths (upto 200 metres). The main disadvantage of these pumps is their low efficiency which further decreases as the submergence of the air-Line decreases. Further the flow of water is discontinuous because of the presence of air. A proper functioning of this system requires a minimum submergence of 50% of the airline. SELECTION OF PUMPS As mentioned above, a wide variety of pumps is available for the augmentation of water from the borewells and the selection of a proper pump for a boreweU is a difficult task. Presently, pumps are selected primarily on the basis of the requirement of the user and the total depth of the borewell. DO The pump assembly is generally lowered to a maiximum possible depth in order to lift out the water from the bore well. The author has observed that in case of irrigation borewells 3 HP,5 HP or 7.5 HP submersible or volute centrifugal pumps are preferred. In this selection, no thought is given to whether the pumping unit is suitable or not for a particular borewell. Therefore, many a times such a pumping assembly either fails to give the estimated discharge or may end up dewatering the borewell very quickly (within 10 to 15 minutes), giving a false impression that the borewell is actually a fadlure. Raghunath (1982) and NABARD (1983) have given some criteria for the selection of proper pumpsets. These criteria are based on the fluctuations ii^ static water level in different seasons, the yield of the well, cropping pattern and the total requirement. The author being a freelance groundwater consultant has installed pumping units on many borewells. For the selection of the pumping assembly on a borewell the following factors need to be considered, 1) Diameter of the borewell 2) Drill-time yield 3) Depth of the main aquifer b.g.l. 4) Total depth of the borewell 5) The delivery and suction heads 6 ) Economics and discharge requirements. Taking into consideration the above points and the selection charts for various Volute and Jetpumps marketed by the M/s SUGUNA agencies (brand name), and the submersible pumps manufactured by M/s Protecto Engineering Corporation (under the brand name of ATLANTA), the author has prepared a computer program to facilitate the proper and quick selection of appropriate pumps to be installed on boreweUs for domestic, agricultural or industrisil use. This program requires five basic inputs. The diameter of the borewell, the depth of the borewell b.g.l., the depth of the main aquifer b.g.l., the drill- time yield recorded at that depth and the elevation difference between the boreweU and the level at which water is to be discharged. This data is fed into the program as input . The suction head is taken as equal to the depth of the main aquifer b.g.l. The aim behind this is to ensure that the main aquifer does not get dewatered on pumping. The program is formulated so as to get a close match of the suction heads with the depth of the main aquifer (Suction heads should always be less than or equal to main aquifer). Next, it scans the relative discharge for different capacities (i.e. Horse power of pump) for the chosen suction head (less than or equal to the depth of the main aquifer). The maximum discharge capacity of the pump, which is less than or equal to the Drill-time yield, is selected. On this basis, the data corresponding to these specifications of suction head and discharge is printed out. This includes the pump Horse power, the suction S. pressure pipe diameter in case of Jet pumps,the diameter of delivery pipe along with the discharge at that particular suction head. The flow chart for the program pertaining to selection of pumpsets is given in Figure 3.4. The program and the details of its operation are available with the author. The author has used this computer program to select pumps whenever required. About 30 pumps have been selected using this program and a few cases have been listed in Table 3.3. In a majority of the cases the pumps selected are working satisfactorily. The program has certain limitations though. In case the Drill-time yield is more than 100 LPH but less than 400 LPH a suggestion for a handpump is made directly. Further, it does not consider the possibility of intermittent pumping. In case the yield is more than 400 LPH but less than 1400 and the depth at which the water is struck is between 50 metres and 76 metres b.g.l. a deep well piston pump is suggested . A limiting factor of 76 metres, has been considered taking into account the load bearing capacity of the threaded coupled joints of the connecting rods which, under higher suction heads, are liable to give way and break. If the main aquifer is within 8 metres, checking for selection is done for the volute centrifugal pumps. For depths greater than 40 metres of the main aqiaifer checking is done for the range of submersible pumps . The above two pumps are considered even if the discharge head is more than 15 metres. For all other cases i.e. main aquifer between 8 metres and 40 metres, scanning is done for jetpumps. 68 -(STAR'^ INPUT "DDiameter of Borewell :";DIA / INPUT "2)Depth of BoreweU :";DP / INPUT ''3)Depth of Mam aquifer :”;MA / INPUT "4)Yield at Mam aquifer :";DY INPUT "5)Discharge head PRINT "Discard well' , INPUT "Specific Capacity ";SC / INPUT "Static water lever';SWL' r Safe drawdown = SWL - MA (SD) ! t Yield = SC * SD /V DY = Yield ■I MA = MA + DH PRINT PUMP * /details : PUMP HP, PIPELINE etc. T JET PUMP 1 SUBMERSIBLE VOLUTE CENTRIFUGAL RANGE PUMP RANGE PUMP RANGE Bl FIGURE 3,4 .: FLOW CHART..QF .'PROGRAM.'. FOR, P.UMP SELECTION a a a XI a a c X a Cl (/:■ T O' X X E G -1 lTj w a e 4-.' U! a a _i 4-' ■ ) n u~) D a • 0 5: >-i > .—I E lU CD c c a o < t i : 0 0 0 0 CM c o X ^ 0 0 o u a X CD r > LD Cvl iTl tT ' a r j <—< r \ j h-i u _____ 1 0 0 w U J I o o 5; a o ) - i _ j 0 0 c 0 o n m ( - ^ 0 c 0 n D 1 r i 0 _ j a r > 0>J CvJ _ i _ i t - i LU a C l > e rtJ L l E E £ c E k-j 0 a ■ 0 u ') 0 0 lii O O' U J —1 ••. O I U - • OC' NN >^0 fx kj t- Z i-i O) CnJ n Ok a M D ■ U J < r 0 J 3 Cl a ' z < i ^ 4-1 3 a EE c E O ' o N u U C r > LO ■X' o _ i •••• 0) UJ -uJ a i n LD V—j r s o x : > e lu a O' c •r-i i-iU • --I d »- JD (C E UJ 5; TESTING OF BOREWELLS Borewells are tested by pumping out water mainly to find out the yield of the well and also to find out the properties of the aquifer tapped by the well. A user is more interested in finding out "what she (borewell) will do ?" (Johnson,1972). To find out how a borewell behaves, the measurement of yield and drawdown are needed. These measurements help in selection of a proper and permanent pumping system. However, in the Deccan basaltic region, rarely is a borewell tested for its yield by pumping it and simultaneously making measurements of water levels. In a majority of the cases the Drill-time yield measured by the airlift method is used as a criterion for the performance of the borewell. Driscoll (1986) has pointed out that the airlift method is less efficient because of the variation in discharge depending on the percentage of submergence of the air line. Therefore the Drill-time yield cannot be taken as an exact criterion to indicate the capacity of a borewell. For accurate measurement of the capacity of the well and for finding out the aquifer characteristics, one needs to know the exact discharge of the well as well as obtained measurements of drawdown and recovery. However, it is rarely possible to conduct such pumping tests on borewells in the Deccan basaltic terrain. This is because of certain difficulties associated with the testing of borewells. Some of these are listed below:- 1) The smaller (105-150mm) diameter of the borewell and the conventional pump assembly leaves very Little space for the standard probes to be lowered down the borewell and many a times a probe gets entangled with the pipeline of the pump assembly. 2) The outlets of the pumps are often situated at some distance from the borwell and sometimes the pumps are directly connected to overhead tanks. This renders measurement of the discharge difficult. 3) It has been observed that the initial drawdown in a borewell is much faster as compared to that observed in a dug well, and hence it difficult to make consecutive measurements accurately. 4) Generally, high capacity pumps of 3 HP or 5 HP with high discharges are installed on borewells and the owners are reluctant to allow a long duration test. 5) There is a tendency to cover the borewells, thereby having no gap to insert the probe for the measurements. In order to overcome the above mentioned difficulties the author followed the following procedure for the testing of borewells. _ 1) A special small diameter (very thin ) probe was fabricated in order tomake measurements in the small annular places available in borewells (Photo 36). 2) The author chose some of the newly drilled borewells for testing. These borewells were pumped with a portable volute centrifugal pump or jetpump (.5 HP to 1 HP) in order to minimise the drawdown and also to find out whether steady state could be achieved (Photo 37). 3) In case of boreweUs which were already installed with pumps the instant slug withdrawal test was considered. The borewell was pumped for 5 to 10 minutes causing some drawdown (slug withdrawal) and then the recovery of water level was measured. To find out the practical efficacy of the above mentioned methods the author carried out tests on some of the available boreweUs. Table 3.4 gives the necessary details regarding these tests. In case of low capacity pumping the borewell was pumped at a rate of 12 to 56 litres per minute for 100 minutes or so. Figures 3.5 to 3. d show the details of drawdown and recovery of these tested borewells. In all these cases a steady state was aechieved with a drawdown of .03 metres to .5 metres. The discharge of the pump was measured using containers of known capacity. The Specific capacity values for borewells were calculated by dividing the measured discharge by the steady state drawdown in each weU. These Specific capacity values are given in Table 3.4 . The time drawdown data for low capacity pumping tests were analysed using Cooper-Jacob (1946) method. Table No. 3.4 : Details of Pumping test data Puipios Rite Test PT.Ho. Site of OHE W ltU . . V . i. (•etres) Coopar-Jacob Theis recovery Depth ■ b o 1 Details Storage effect 100 (1946) (1935) Type of rock schafer (1975) PI.l Mr.Kirloskar Jiwary 91’ 0.0105 0,020 10 1.3 0,26 0.25 ,IUZ 88 1.47 lOS PI.2 Mr. Advani Jaiuary 91’ 0.0105 0.036 1.4 0.46 0.45 2.1 Airo. ,iuz 108.0 190 1.09 100 PI.3 Poona Univ. Juie 92’ 0.0150 0.016 41 6.49 0.45 0.40 13.7 Atrs. ,IU2 36.0 120 6.54 95 Hr, HotMini Jawary 91’ 0.0105 0.0S6 10 2.5 2.50 2,48 4.5 Aiyg. ,iuz 23.0 30 5. 18 IOC PT.S Hr.Najraii Jawary91’ 0.0105 0.020 2.54 0.23 3.0 Ars. ,IUZ 0,22 29.0 21 4.04 90 PT.6 Mr.Thadhani Harch 92’ 0.0105 0.030 9.5 15.8 0,13 0.12 Atrs. ,ioz 13C.0 32 0.76 1440 P I.7 m.Havinknrve April 93’ 0.0150 0.022 10 5.53 7.0 5.85 5.19 Aiys. ,I0Z 18C.0 145 2.75 120 PT.S Hr. Scrte Deceiber 91’ 0.0150 0.24 14.6 48.73 50.0 4.27 4.26 41.0 Atyj. ,iuz PI .9 10 lie 2.95 H/s Globe December 91’ 0.0105 0.012 5.0 4.0 Electricals 3.25 3.22 2.t Aiyg. ,IUZ 10 15 2.95 Hiy 92’ 0.0105 0.020 27.8 4.5 PT.IO Hr.Siigh Aiyfl. ,102 6.15 INDEX TO FIGURES 3.5 TO 3.7 s : drawdown in meters } y-axis s' : residual drawdown in meters t : time in minutes since pumping started } x-axis t' : time in minutes since pumping stopped Shaded circles represent data points (s,t) for the abstraction phase Crosses represent data points (s',t') for the recovery phase PT : Fuming test Q : Discharge of the pump /k s ; Change in the value of s over one log cycle of t T : Transmissivity calculated using the Cooper-Jacob (1946) formula c : Specific capacity - calculated using the SUchter (1906) formula 10° 10’ 10- 10^, 7:1.3m 0 ' “T- ■ .j- ...... PT 1 ♦ 0=0.02 m ^/m in 0.1 As=0.06 • T = 6 8 m^/d ay c = S 0 L P M /m • • • • •••••• 0.3 1 1 0.^ U) -«— t or t' — »■ O -1 0 y.4m tn PT2 + * ^ Q=0.036m ^ / m ID 0.1 + A s=0.05 ft T =190 m^/day 0.2 - c=WS L P M / m 0.3 0.^ ______\______1------10^ 10’ 10^ 10^ Figure 3.5 : Semilogarithmic time-drawdown/recovery plots for calculating Transmissivity using the Cooper-Jacob (1946) method PT 1 : Mr. Kirloskar ; PT 2 : Mr. Advani 10° 10’ 1Q2 1Q3,y.-S.5w -I— r P T 3 Q=0.016rr?/min 0.1 - As=0.035 T = 120m^/day c=36LPM/m 0.2 - 0.3 - 0.^ - 0.5 m o—1 tn Figure 3.6 : Semi logarithmic time-drawdown/recovery plots for calculating Transmissivity using the Cooper-Jacob (1946) method PT 3 : Poona Univ. ; PT 4 : Mr. Motwani ^— t or t --- r— 1— P T 5 0.1 Q=0.012rri^/min As=0.15 0.2 T=21m^/day c=29LPM/m 0.3 0.^ • • 0.5 10*^ 10’ 10^ Q=0.03m^/min P T 6 As=0.25 0.1 T=32m^/day O —1 c=130LPM/m U) 0.2 - • • • • • • 0.3 P T 7 Q=0.022m /min 0.05 As=0m5 T=K5m^/day c=180LPM/m 0.1 • • • • • • 0.15 10° 10' 10^ 1Q3 Figure 3.7 : Semi logarithmic time-drawdown/recovery plots for calculating Transmissivity using the Cooper-Jacob (1946) method PT 5 : Mr. Nagrani ;PT 6 : Mr. Thadhani ;PT 7 : Mr. Mavinkurve INDEX TO FIGURE 3.8 s' : residual drawdown in meters t : Time in minutes since start of pumping. t' : time since pumping stopped in minutes Shaded circles represent data points (recovery phase) A s' : Change in the value of s' over one log cycle of ( t /f) T : Transmissivity calculated using the Theis (1935) formula t/t’ t I Figure 3.8 : Semi 1ogarithmic plots for calculating Transmissivity using the Thais (1935) recovery method PT 8 : Mr. Sorte ; PT 9 :M/s Globe Elec. ; PT 10 :Mr. Singh Figures 3.3 to 3.9 show the semilogarithirdc time-drawdown plots. The Transmissivity was calculated using the following formula. T = 26.4.,X...Q A s where : T = Transmissivity in mZ/day. Q = Pump discharge in m3/minute A s = change in vsdue of drawdown /unit log cycle of time in metres. Table 3.4 shows the values of Transmissivity calculated by the Cooper- Jacob method. The boreweU as against large diameter dug wells, because of its small diameter does not posses a large storage. However, Schafer (1978) is of the opinion that even though the storage in case of the borewells is small its effect is seen on the time drawdown plots. During the first initial few minutes of pumping the water is removed from the casing storage of the borewell and only later is the water derived from the aquifer. Schafer (1978) is of the opinion that if this initial portion of the time drawdown curve is used for the calculation of the Transmissivity erronuous results may be obtained. Schafer (1978) has given a formula to calculate the duration of time over which the effect of casing storage is prevalent . tc = ,(dc2...-.dp2)...X...0.,Q17 Q/s where tc = time in minutes dc = inner diameter of casing in millimetres (diameter of borewell) dp = outer diameter of the pipeline of the pumping system in millimetres Q/s = Specific capacity of the well in m3/day/m In the present study, the author has calculated the time tc with a view to find out the time after which the actual contribution of the water from the aquifer starts. Table 3.4 shows the values of time tc in minutes obtained for the tested bore wells. Recovery in all the seven cases of low capacity pumping have been plotted cdong with the time drawdown graph in Figure 3.5 to 3. 6 . This was done in order to get an idea about the relative variation in the rate of recuperation in case of various bore wells. Slug withdrawal tests were carried out on a few borewells with the idea of using Schafer's (1981) method. It was observed that 5 to 10 minutes of pumping at a rate ranging from 12 to 240 litres per minute caused a drawdown of 4 to 3 metres. Table 3.4 . However with this drawdown the Transmissivity values generated were in the range of 0.1 to 1.5 m2/day. Some of these wells show substantial Drill-time yields (Case 7, Mr. Sorte). At times the pump works for very long periods ( Case 5, Mr. Singh). In light of the above mentioned facts, the Transmissivity values (generated by Schafer's method ) appear to be inconsistent and very low. The recovery data generated from the slug withdrawal test was analysed for calculating the Transmissivity using the Theis recovery method (1935), Figure 3 « e i show*,-the semilogarithmic residual drawdownplots for the Theis recovery method. The Transmissivity was calculated using the following formula. T = ... 2$4,.,Q A s' where T = Transmissivity in m2/day Q = Pump discharge in m3/day A s' = change in the value of residual drawdown s' per unit cycle Table 3.4 shows the values of Transmissivity generated by the Theis recovery method. It can be seen from the Table 3.4 that in aU the borewells the Amygdailoial basaltic flow unit(s) were acting as the aquifer(s). Further the Specific capacity of borewells tapping this Amygdaloidal basaltic flow units as 72 aquifers ranges from 3 liter/minute/metre of drawdown to about 130 liter/minute/metre of drawdown. Similarly the Transmissivity values range from 5 m2/day to 190 m2/day. The effec± of casing storage during the pumping for these borewells was found to be prevalent from 1 minute to 6.5 minutes. Taking this into consideration the latter portion of the drawdown data was used for the calculation of Transmissivity. The author is of the opinion that the Specific capacity values generated by low capacity pumping can be utilised for finding out the magnitude of drawown if the well is pumped using pumps of higher dischcirges. It must be borne in mind that the water level should not drop below the depth of the main aquifer. In the Poona University campus, borewell no. B8 struck water in the Amygdciloidai basaltic flow unit at 13.7 and 30.4 metres b. g. 1. In this borewell, the static water level was at about 6.49 metres b.g.l. With the pumping rate of 16 litres per minute, only 0.45 metre of drawdown was caused and a steady state was achieved. Since the first aquifer tapped by the boreweU, is at 13.7 metres b.g.l. the maximum lowering of the water level without dewatering the aquifer directly could be designated as 12 metres b.g.l. This gives a maximum permissible drawdown of 5.51 metres. With the Specific capacity of 36 litres/minute/metre drawdown the borewell can be pumped at a rate of 198 litres per minute to achieve the designated 5.5 metre drawdown. These calculations show that the Poona University borewell can be pumped at a rate of less than or equal to 200 litres per minute without lowering the water level below the top aquifer. In this case according to the author, a 1 HP, high speed volute centrifugal pump would be best suited. The author is aware that the data generated by the above tests is not foolproof. For accurate Specific capacity values the step drawdown tests should be carried out. Similarly, for accurate measurement of aquifer characteristics more sophisticated pumping tests with observation weUs are necessary, MANAGEMENT OF BOREWELLS The commissioning of borewells, from the time of drilling to the installation of a pumping system involves a certain capital investment. It is very important to know whether the commissioned borewell is economically viable from the point of view of its utilisation. It is also quite possible that the borewell is overexploiting or underexploiting the available water. All these factors require a proper management of the borewells. Such management involves . 1) Effective utilisation of available water 2) Maintenance of the borewell. EFFECTIVE MANAGEMENT OF AVAILABLE WATER Users tend to drill borewells for different purposes. Borewells are drilled to suffice the water req’oirements for agric\ilture, industry and domestic supply. The water requirement for agriculture depends upon the season as well as the type of crop and the area of cultivation . In the basaltic terrain of Karnataka, a 1.2 hectare farm with 0.4 hectare sugarcane,0.4 hectare vegetable and 0.4 hectare chilly requires about 72,000 litres/day during the peak rabi/summer period NABARD, (1983). On the contrary, the industrial requirement remains fairly constant throughout the year and in general their requirement ranges from 30,000 to 100,00 0 litres per day (depending upon the actual use). Domestic water requirement is small but remains fairly constant throughout the year. A small family of four members, in a city, would require about 400 litres of water per day. The utility of a boreweU i.e. its failure or success to satisfy the purpose for which it has been drilled depends upon three factors 1) The Drill-time yield 2) The requirement of water/day 3) The type of pumping system and pumping schedules A borewell can be declared a success or a failure solely on the basis of the Drill-time yield. Sakare and Maldhure (1990) have classified borewells based on the Drill-time yield as : 0 - 100 LPH DRY ICO - 500 LPH POOR YIELDING 500 - 700 LPH MODERATELY YIELDING 700 - 3000 LPH GOOD YIELDING > 3000 LPH HIGH YIELDING The high yielding bore wells, for example, are those which are yielding more than 3000 LPH, while those yielding less than 100 LPH are considered to be dry. As mentioned earlier, the DriU-time yield is measured by the airlift method which itself is a very inefficient method of pumping and therefore the DriU-time yield recorded could be on the lower side as compared to the actual yield of the well. Further, in the user's mind there are certain misconceptions about borewells. If continuous flow of water cannot be observed during drilling then that weU is regarded as a failure. It is quite possible that such borewells have been drilled with the high pressure drilling rigs and may have recorded only dampness at certain depths and, as mentioned earlier, these wells may show a delayed yield. Another misconception is that the drilled borewells should sustain pumps of high capacity which give high discharges ( e.g. 3 HP or 5 HP). Many a times, a boreweU fails to yield the desired quantity of water to suffice the needs of the user. Under such circumstances, the bore well is declared a failure. It is quite possible that the borewell is yielding less than the water requirement of the user for his designated pumping schedules. In reality the improper pumping system as also the improper pumping schedules maybe responsible for the well to be declared a failure. All these points contribute to the success or the failure of the borewell. Hence, before designating any borewell as a failure it is necessary to find out whether the available water is being properly managed or not. A few of the salient cases observed by the author have been described below. 75 CASES WITH LOW DRILL-TIME YIELD 1) In Shivapur village of Pune district, a farmer intending to irrigate 14 hectares of land with sugarcane drilled five 150 mm diameter borewells of an average depth of 60 metres each. All the drilled wells showed a Drill-time yield of 1000 - 1200 LPH. To irrigate 1 hectare of sugarcane with 29 irrigations over a period of 350 days he required 61,000 litres/hectare/ interval of 12 days. [This requirement is needed only during the initial period (I.A.R.I, 1977)]. Failing to get the desired quantity of water of atleast 3128 LPH for each of his borewells individually, the borewells were abandoned. The author suggested that this group of wells could be successfully utilised for the cultivation of sugarcane using 1 HP jetpumps and following a longer duration pumping schedule. To meet the large requirement of water for sugarcane /interval/hectare, these wells can be pumped for 10 hours each and the water stored in a large tank and then distributed according to the requirements. Thus, he can manage to irrigate a major portion of the 14 hectares. 2) In Kondhwa, towards the south of Pune city, an industrial estate with about 300 small industrial sheds was proposed (M/s Tiny Industrial Estate). The estimated water requirement for these units was a minimum of 150,000 LPD and hence boreweUs of 150 mm diameter and an average depth of 50 metres were drilled. Of these, 3 borewells were reported to have a Drill-time yield of about 500,1000, and 1500 LPH and the rest of the borewells were declared totally dry. The boreweU having a yield of 600 LPH was also discarded later. The author suggested to the Director's of the company not to discard the low yielding well as it could be fitted with a 1 HP jetpump. This pump will give about 6000 LPD which could suffice the water requirements of a few units and/or the proposed garden. 3) In Undri village (WadachiWadi) , situated towards the south east of Pune city, a borewell of 150 mm diameter was drilled to suffice the needs of a horticulture farm/smaU poultry/dairy /and a small industrial unit. This borewell recorded a drill-time yield of 1600 LPH. The total requirement of the entire setup mentioned above was estimated to be about 20,000 LPD. Since the Drill-time yield was low and the point of discharge was at a much higher elevation form the well- about 35 metres (high delivery head)- 76 ■ the borewell was fitted with a 3 HP submersible pump. At the highest point, a storage tank was constructed. The water from the borewell (1600 LPH) was pumped into this tank with a head of about 85 metres by controlling the discharge of the pump. A twelve hour test pumping was suggested. In the morning, this stored water was distributed all over the field over a short period. 3) A dairy at Yerawada, Pune city belonging to Mr. Shirke, had a borewell (105 mm in diameter, 24 metres in depth), to suffice the need for water for the maintenance of its cattle shed. This borewell showed a Drill-time yield of 600 - 800 LPH. This borewell was discarded as it failed to deliver water with the required pressure (at 2000 LPH minimum). In this case the author installed a .25 HP volute centrifuged pump since the main aquifer was at 4 metres b. g. 1. and the static water level was measured to be at 1.2 metres b.g.l. The initial discharge from the pump was high but with the subsequent lowering of the water level it reduced to aproximately 600 LPH and a steady state pumping was achieved when the water level was at about 7.3 metres b.g.l. The water from this well was pumped into a tank of the required capacity and the same pump was attached through a 'T' to the tank, to deliver the required water (2000 LPH) to the cattle shed. 4) In Chandannagar, Pune corporation limits, Mr.Choudhary (case 5) has drilled a 150 mm diameter,50 metre depth borewell for the purpose of constructing a bungalow and later on for domestic use. This borewell showed a Drill-time yield of 250 LPH, and was discarded as it failed to sustain a 1.5 HP jetpump with its jet assembly installed at 30 metre b.g.l. The author advised him to lower the jet assembly down to a depth of 45 metres . With 45 minutes of pumping the owner gets about 500 litres of water with a drop of water level upto 30 metres. After a gap of about 4 hours, the water level almost attains its originad water level of about 3 metre b.g.l. and the owner can pump out 500 Ktres of water again. With this iterative pumping schedule the discarded borewell was put into proper use. During the summer of 1991 (drought period) this well even supplied water to some of Mr. Chaudhary's neighbours. The author actually comes across several cases of failure needing proper management and only a few have been enumerated above. SUFFICIENT DRILL-TIME YIELD Many a times, a borewell showing sufficient Drill-time yield ( > 2000 LPH) is not managed properly so as to be economically viable. The author has observed that in such cases the improper selection of the pumping system and the ignorance of the Drill-time yield are the major reasons behind the mismanagement of such borewells. Some of these cases are described below. 1) In the Greenfingers School, Akluj, Solapur district, a borewell of 150 mm diameter was drilled up to a depth of 75 metres. Water was struck at 12 metres b.g.l. in an Amygdaloidal basaltic flow unit. The Drill-time yield was about 9000 LPH and a submersible pump of 7.5 HP was installed at 60 metres b.g.l. This pump drew water at the rate of 15,000 LPH. In summer it was found that this pump does not discharge water continuously. The author found out that because of the high discharge of the pump the pumping caused a substantial drawdown, thereby reducing the discharge of the pump and causing the pump to work only in spurts. The reason for this was that the capacity of the well, as compared to the discharge of the pump, was low. The author in this case, created an artificial head by controlling the discharge of the pump to about 7000 LPH. This proved beneficial and a continuous discharge of 7000 LPH was achieved. This pump was later worked continuously for 8 hours, ascertaining the stablity of the discharge. 2) A chemical plant of M/s PCS DATA PRODUCTS, near Alandi, Pune district drilled a borewell (150 mm diameter and depth of 75 metres.) Water was struck at about 15 metres b.g.l. The Drill-time yield was about 5800 LPH. Just because the borewell is 75 metres in depth, a 5 HP jetpump (RMW make) with its jet assembly at 65 metres was installed. The author found out that pumping water with this system is not economically viable as the DriU-time yield of 5800 LPH nearly matches the discharge of the pump (discharge of 5500 LPH with suction head at 24 metres). Since the water was struck at 15 metres b.g.l., even a 3 HP jetpump with the jet assembly at 33 metres depth woiold be sufficient. As per the pump specification this pumping system will deliver water at the rate of 5800 LPH with the water level stabilising betwen 27-30 metres. However, taking into account the consumption of electricity the 3 HP system is economically more viable for drawing out a similar quantity of water from this boreweU. 3) In Aundh area of Pune city a bore well of 150 mm diameter was drilled to 60 metres depth b.g.l. in a bungalow belonging to Mr. Joshi to suffice the domestic water requirements. This well was reported to have struck water at 7.5 metres b.g.l. and the Drill-time yield was recorded to be about 3300 LPH. As the depth of the well was 60 metres, a multistage jetpump was installed with its jet assembly lowered down to 55 metres. As the requirement of water is less, this pump is operated for only 2 to 2.5 hours/day, at a rate of about 4000 LPH with a stabilised water level at 10 metres b.g.l. In this case, the author is of the opinion that a 0.5 HP volute centrifugal pump with a discharge of about 2000 LPH with water level stabilising at a much higher level would have been more economicaly viable, taking into account the initial cost of the system as weU. as the daily running costs. The above mentioned cases and many more similar examples show that a proper selection of the pumping system and efficient management of the available water is necessary to make the boreweU a real success and render it economically viable. This is essential since the common practice is to try to lower the suction assembly f, to the lowest possible depth within the borewell and then to draw out the water to reap maximum benefits from the borewell . The above mentioned cases show that a thorough knowledge of hydrogeological factors and a good understanding of the pumping system can decide the success or failure of a borewell. MAINTENANCE OF BOREWELLS To derive maximum benefits from a borewell maintenance is of prime importance. During drilling, a lot of suspended matter (lithological cuttings/chips and powder etc.) settles in the borewell. Also there is some plastering effect within the wells ( page58). In order to remove all this, a thorough flushing with water immediately after the drilling is necessary. This helps in cleaning the connections between the borewell and the aquifer(s). In order to enhance the life of the borewell this flushing should be done biannuaUy. In order to maintain the purity of well water, periodic chemical treatment (Chlorine and Potassium permanganate) is also necessary. Similarly, the perodical maintenance of the pumping system is also necessary to ensure a constant discharge. 7Q The usual complaints about the pumping system include damage to the water seal,pressure gauge, footvalve pump bearings, etc. These things need an expert attention so as to keep the borewell economically viable. SUMMARY In this chapter the advantages and disadvantages of the DTH drilling of 105 mm and the 150 mm diameter borewells has been discussed. Appropriate methodology for collection of the DTH Drill-time litho samples and the criteria for identification of different flow units from the DTH drilling samples have been enumerated. The different aspects of the measurement of the DrUl-time yield by the airlift method have been dealt with. A proper methodology for preparation and presentation of lithologs based on the Drill-time observations is given. The efficacy of the stimulation of the failed borewells by sectional blasting has been discussed. A computer program has been formulated for the proper selection of a pumping system to be installed on the borewell for permanent use. A methodology, with the help of low capacity centrifugal pumps to test the Specific capacity of the borewells at steady state and the Transmissivity of the aquifer(s) tapped by the borewells has been described. The aspect of effective management of the available water in a borewell has been delineated to judge the success or failure of the borewells. The forthcoming chapter on synthesis includes the collation of the exploration data, exploitation data and the management of groundwater.