51

Chapter – III

Present Status of Constructed Percolation Tanks

The State of Maharashtra is divided into five river basins, namely

Godavari, Krishna, Tapi Narmada, and westerly flowing rivers in the

Konkan Coastal Strip (Fig. 1.8). The total area covered under these basins

is 30.88 million hectares (mha), of which 22.54 mha can be cultivated.

The water availability in these river basins annually on an average is

131.56 Billion Cubic Metre (BCM). To harness the above surface water

potential, various major dams, medium projects and minor tanks have

been constructed. Water from the dams is supplied for the different needs

of agriculture, domestic and industrial. At the strategic level, Water

Resource Development (Erstwhile, Irrigation Department, Government of

Maharashtra) through five Irrigation Development Corporations (IDCs) is

responsible for managing the surface water resources and it allocates

water for irrigation, industrial, drinking and sanitation, purpose. A major

portion of the water is consumed by the irrigation sector in the state of

Maharashtra. It has increased gradually from 13.98 BCM during 2002-03

(FY) to 16.49 BCM during 2006-07 (FY). Minor Irrigation Projects, local

sector of the State completed 32 major dams, 178 medium projects and

approximately 2274 minor tanks till the end of June 2005. Maharashtra

ranks fifth from the top in descending order of ultimate irrigation

potential (Ref. NARAA). It is important to note that Punjab, Haryana,

Tamil Nadu and Karnataka have only about half of the ultimate irrigation

potential of Maharashtra and yet Maharashtra has a lower share of area

under irrigation. This is in some way a clear reflection of the failure of

efforts to tap the available potential as well as the difficult topography of

the state.

52

Three significant points emerge from this discussion which are,

1. The potential of minor irrigation covers a larger portion of the

ultimate irrigation potential, hence focus should be directed towards

this source.

2. Despite the numerous committees and commissions and volumes of

work on irrigation, the state could utilise only 39.3 percent of its

ultimate irrigation potential compared with the share of potential

utilised by Tamil Nadu (64.6 per cent), Rajasthan (82.7 per cent) and

Gujarat (51.5 per cent).

In this context and notwithstanding the controversy on potential vis-a-vis

utilisation, a question arises about the intensity of efforts in tapping the

potential. In a comparison of the States across the Nation, Maharashtra

has about 15.8 per cent of the total capacity of live storage of water

created in the country (including proposed), which is the second highest

in the country. Thus Maharashtra does not seem to have fallen short in

creating the water storage or at least the State is comfortably placed at the

top rank as far as the creation of the storage capacity is concerned. But in

terms of achievements, the proportion of cultivated area under irrigation

is less than 15 per cent of gross cropped area. Thus the situation is that,

the State has the highest number of dams, high storage capacity created,

but still can claim only one of the bottom ranks in proportion of

cultivated area under irrigation. In addition to surface water, groundwater

constitutes to be a large part of the water supply, especially in rural areas

of Maharashtra, where over 50% of the total water use comes from

groundwater (NRAA, March 2011).

The groundwater resources are regulated and monitored by the

Groundwater Survey & Development Agency under the Water Supply

53

and Sanitation Department, Government of Maharashtra. Groundwater

development and management in hard rock aquifer areas, in India and

many other countries have traditionally played a secondary role compared

to that in the areas having high-yielding unconsolidated or semi-

consolidated sediments and carbonate rocks. This has been due to the

relatively poor groundwater resources in hard rocks, low specific capacity

of wells, erratic variations and discontinuities in the aquifer properties

and the difficulties in exploration and quantitative assessment of the

resource.

The Groundwater development in Maharashtra has been traditionally

neglected with the assumption of the relatively poor groundwater

resources in hard rocks, low to moderate yield of wells, erratic behaviour

and variations in the hydrogeological characters of basaltic aquifers and

the limitations in exploration and quantitative assessment. Groundwater

occurring under the basaltic terrain in Maharashtra resides in the soft

mantle of weathered rock under phreatic condition. Under this soft

mantle, groundwater is mostly in semi-confined state in the fissures,

fractures, cracks, and joints. (Deolankar 1980) In basaltic terrain the lava

flow junctions and weathered red tachylitic basalt pockets sandwiched

between two layers of lava flows, also provide additional porosity. The

ratio of the volume of water stored under semi-confined condition within

the body of the hard rock, to the volume of water in the overlying

phreatic aquifer depends on local geohydrological conditions.

A possible management solution, encouraged by Government agencies

over the recent years, is to intervene on the supply side of the

groundwater balance, i.e. by artificial recharge. Artificial recharge

structures encompasses percolation tanks, check dams and, injection

54

wells to recharge around defunct dug wells. In many places, a high

density of tanks exists as they were the traditional water source for many

centuries. Artificial recharge is widely promoted through construction of

percolation tanks. The residence time of water in the basins is thus

increased from a few months to a few years and the percolated water is

available in the wells even during the summer season of a drought year

Irrigation based on groundwater, that is extracted through shallow wells

is one of the age old established practice in most of the semi-arid tropical

regions of India. Natural depressions served as an important source of

groundwater recharge. Ponds refer to a reservoir impounding run-off

water behind earthen/cement bunds and embankments constructed across

the slope to harvest and store water in the rainy season, and to use it for

irrigation through water extraction structures like open and bore well, and

other purposes. Village ponds are historical innovation to address

monsoon irregularities and reduce the risk of uncertainties in water

availability in the dry zones. As per the 4th Minor Irrigation Census that

refers to the year 2006-2007, about 6 lakh tanks and storages are created

in the country with 58.9 lakh ha of irrigation potential out of which 39.31

lakh ha has been utilized.

3.1 Percolation Tank

A percolation tank can be defined as an artificially created surface water

body submerging a land area so that the surface runoff is made to

percolate and recharge the groundwater storage (Fig. 3.1). They are not

provided with sluices or outlets for discharging water from the tank for

irrigation or other purposes, but as a safety measure they are provided

with arrangements for spilling away the surplus water that enters the tank

in case of heavy rain spells, so as to avoid over-topping of the tank bund.

55

The purpose of percolation tank is to conserve the surface runoff and

recharge groundwater storage. The objective for the construction of these

tanks is to accumulate water and overcome the difficulty of erratic

monsoon in the changing climate scenario. The water collected in these

tanks shall be useful in recharging the groundwater and if needed it can

be used for protective irrigation in the dry spells. Thus the water

accumulated in the tank after the monsoon is made to percolate at the

earliest, without much loss due to evaporation (Fig. 1.11).

Fig. 3.1 What is a percolation tank?

Percolation tanks are to be normally constructed on second or third order

streams, as the catchment area of such streams would be of optimum size.

The field conditions which are necessarily studied during the

identification of the site for construction of percolation tanks are

described below.

The positioning of the tank and its submergence area should be in

non-arable land and in natural depressions requiring lesser land

acquisition.

56

There should be cultivable land in the downstream area of the tank in

its command with a number of wells to ensure maximum benefit by

such efforts.

Steps should be taken to prevent severe soil erosion through

appropriate soil conservation measures in the catchment. This will

keep the tank free from siltation which otherwise reduces the storage

capacity resulting in a decrease in percolation efficiency and life of

the structure.

The initial efficiency of a percolation tank is reduced due to silting of its

bottom by receiving muddy runoff from the watershed. If the watershed

is well-forested and has a cover of grass, bushes and crops, the silting

is minimal. But in an average of 5 to 6 Monsoon seasons the tank bed

accumulates about 0.20 to 1.00 meters of silt. Silt reduces the storage

capacity of the tank and also impedes the rate of vertical flow of

recharge because of its low infiltration rate. The efficiency gets reduced

due to silting hence de-silting of tank bed becomes necessary when it

dries in summer. (Limaye S D. 2010).

3.2 Design of Percolation Tank

A percolation tank, like an irrigation tank, has a structure to impound

rainwater flowing through a watershed, and a waste weir to dispose of the

surplus flow in excess of the storage capacity of the lake created. A

masonry waste weir is also necessary to pass surplus water. A percolation

tank is designed to ensure maximum capacity utilization, long life span,

cost effectiveness and optimum recharge to groundwater. Storage

capacity, waste weir, drainage arrangement and cut off trench (COT) are

the important features of percolation tank which need proper design. The

overall design of the percolation tank is similar to an earthen dam

constructed for minor irrigation without the canal.

57

Fig. 3.2 Cross section of a bund for percolation tank

These are the most prevalent structures in India as a measure to recharge

the groundwater reservoir in hard rock formations. The efficacy and

feasibility of these structures are more in hard rock formation where the

rocks are highly fractured and weathered. The catchment yield and basin

configuration drawn from topographic surveys at site, determines the

height of the percolation tank. The top of the dam wall is normally kept

2-3 m wide. Upstream and downstream sides of the dam wall are

normally taken as 2.5:1 and 2:1 respectively.

The hydrogeological condition of the site for percolation tank is of utmost

importance. The subsurface rock strata in the submergence area should

have high permeability. The degree and extent of weathering of rocks

should be uniform and not just localized.

The objective of this study is to compare the contribution of artificial

recharge based on the hydrological characters of the various formations to

the overall recharge and discuss alternative options.

To undertake a comprehensive survey of the existing percolation tank and

study and map their inefficiency if any, a study was carried out and the

information was collected through a simple survey form which is given

below.

58

Annexure 3.1

Geo hydrological Studies of poorly permeable

Percolation Tanks in Deccan Basaltic Terrain

Questionnaire to short List Percolation Tanks

Village ………… Tahsil …………… District ……………

Gut No. …………… Name of the Farmer ……………….

Well No. ……………

Location of the well ………… Farm,

Bank of Nala, In the Nala

Location Name ……………

User ………… Personal Community

Year of the Digging ………..

Construction year If yes type …… parapet Ht. ………

Shape ………… Circular Square Depth …………

Water level from ground …………… m.

In rainy season …………… winter ……………

Percolation : Bottom / Lateral

Direction in case of lateral

If the bore is taken in horizontal direction, Length..… m.

and or vertical Depth of vertical bore …………….. m

Location at the bottom …….

Use - Drinking Irrigation ……….. Acre.

Rainy seasons ………………. Acre

Winter seasons ………………. Acre

Summer seasons ………………. Acre

59

Type of withdrawals Electric motor Diesel Pump

HP ………………. dia. of outlet ……………….. cm.

Quantity of withdrawals Daily …….. Hr. Seasonal

………………... day

Time require for a full recharge

Rainy season ……….. Hrs. Summer………….. Hrs.

winter …………… Hrs.

Name of the surveyor Signature

A detail analysis was made of the data collected from this well survey.

(Fig. 3.3) Consequently the percolation tanks were shortlisted for the low

performance. After analysis of the primary data collected the reasons for

the lower efficiency of the percolation tanks are discussed as follows.

3.3 Reasons for Lower Efficiency

The reasons that contribute to low recharge efficiency can be identified as

follows.

3.3.1 Inadequate catchment : During this study it was observed that some

of the percolation tanks are constructed not at proper site in spite of the

sufficient availability of catchment from where the surface runoff could

be directed towards the tank. For example the Percolation tank of village

Pathri, block Phulambri of Aurangabad District, constructed towards the

western side of the Aurangabad Sillod road demonstrate this condition. It

hardly receives any water even during good monsoon due to the

insufficient catchment area (Fig. 3.4). It was also observed, that at places,

the catchment is disconnected because of the construction of the road or

urbanisation in the catchment areas. These kinds of tanks become defunct

and hardly collect any amount of water.

60

Fig. 3.3 Well survey in progress to

shortlist percolation tanks

Fig. 3.4 Percolation tank at village Pathri, Tq. Phulambri,

hardly receives any water due to insufficient

catchment area

61

3.3.2 Inadequate attention to the geohydrology : It was observed that

most of the percolation tanks are constructed as per the suitability of the

site from the impounding point of view. A percolation tank which is

primarily constructed for the recharging the groundwater is completely

ignored towards the geohydrological study, surface geology as well as

subsurface geohydrology, while finalising the location. The percolation

tank constructed without paying due attention to the geohydrology, many

times become a the storage tank (Fig. 3.5).

For example percolation tank of Pokhri located in Jafrabad Taluka of

Jalna district is constructed on a gently sloping topography with the good

catchment area. The south and southeastern part of the village bears few

hillocks which generate run off during the monsoon. The top soil layer

varying from the 0 to 4 m is followed by vesicular basalt up to 9 m. This

is underlain by massive basalt which is partially fractured and devoid of

joints. Therefore the percolation through this tank is restricted up to a

depth of about 10-12 m. This is quite evident from the impounded water

in the summer months also. This tank is getting filled up to the capacity

in the rainy season, but percolation from this tank is restricted due to

adverse geohydrological condition and it almost serves as a storage tank.

It is expected that due to percolation of water the tank should become

empty in the month of February. Out of the surveyed 45 tanks, 30 ( i.e.

65%) tanks have never become empty in the month of February or

summer since the time they were constructed. From this it can be

concluded that the selection of sites of such tanks were not proper from

the geohydrological point of view.

The importance of the geohydrology is discussed in detail and is

presented in this study.

62

3.3.3 Incomplete/Faulty construction : The construction of many of the

percolation tanks is incomplete. Even though the gorge is not filled up the

embankments are complete, due to which all the water accumulated in the

percolation tanks flows down through the gorge area. Some of the tanks

are not provided with the proper spillway due to which the water

accumulated in the percolation tank spreads on the surrounding low lying

area and finally finds its route towards the downstream. This also causes

lots of erosion of the soil in the surrounding areas. The percolation tanks

for want of proper attention paid to the strengthening of the Hearting and

Casing, are washed away in the heavy intensity rain spells (Fig. 3.6).

Such tanks without the proper consideration of the geohydrological,

topographic and engineering aspects can be categorised as faulty and

unsuccessful.

Out of the two percolation tanks located in Kerul, Tq. Ashti, District

Beed one was found to be empty by the end of November month due to

heavy leakages from the main wall of the percolation tank. The reason

behind this state of affairs is improper or faulty construction of the dam

wall right at the centre.

3.3.4 Silting of percolation tanks : It was also observed that, the rate of

silting, was more than the expectation which could be attributed to

untreated upper catchments of the tanks. The initial storage capacity and

efficiency of a percolation tank is reduced due to silting of its bottom

through muddy runoff from the watershed. On an average of 5 to 6

Monsoon seasons the tank bed accumulates about 0.20 to 1.00 meters of

silt. Silt reduces the storage capacity of the tank and also impedes the rate

of vertical flow of recharge because of its low permeability causing a loss

to groundwater potential.

63

Fig. 3.5 Poorly permeable percolation tank turned into a

storage tank where water stands till the advent of

summer

Fig. 3.6 Casing of percolation tank had been washed away

following spells of heavy rains due to faulty

construction

64

The percolation tank located near Pachan wadgaon and Sarvadi located in

Taluka (Tq. ) and Dist. Jalna have become defunct due siltation (Fig. 3.7).

The untreated upper catchments bring in the huge amount of silt in the

water during the rainy season and it gets deposited in the percolation

tank. The silting reduces the infiltration of the water into the subsurface

though geohydrology of the area is favourable.

3.3.5 Poor / lack of repairs and maintenance : Lack of repair and

maintenance of the percolation tank also reduces its efficiency. At some

places it was found that the spillway is either broken or completely

diminished due to which the water cannot be stored in the tank. In a few

of the places it was noticed that out of 45 tanks studied 7 to 8 need to be

repaired. The leakages in the dam wall were also observed in some of the

tanks (Fig. 3.8).

The percolation tank located in village Asai Tq. Jafrabad Dist Jalna was

observed to be leaking from the centre of the dam wall. The leakage was

found to be from the casing of the percolation tank and was in a sorry

state of affair for the want of repair.

3.3.6 Inappropriate site location : The percolation tank is constructed to

serve as a groundwater recharge structure. The site for this structure is

selected so as the open wells or bore wells in the near vicinity, used for

the irrigation should get maximum benefit. It was observed at a few

places that the percolation tanks are constructed at lower elevations where

as the agricultural lands and the open wells located in it are at higher

elevation than that of the percolation tank (Fig. 3.9).

The percolation tank in Chincholi Budruk Tq Phulambri Dist.

Aurangabad is constructed at the lower elevation than that of the village.

In other words the village and its agricultural land; fall in the catchment

65

Fig. 3.7 Percolation tank at Panchan Wadaon has become

defunct due to siltation

Fig. 3.8 Leakages in the dam wall owing to lack of

maintenance

66

of the percolation tank. Hence the wells located in these agricultural land

do not get any benefit from this percolation tank though the subsurface

strata is favourable for percolation of stored water in the tank.

After ascertaining the failure/success of the percolation tanks the

percolation tanks were shortlisted on the basis of the performance

attributing to the geohydrological condition. The details of the total,

Percolation tanks surveyed and shortlisted for the detail survey are given

in the table below.

The physical dimensions of the percolation tank were also measured to

know the impounding area of the tank.

Table 3.1 Details of the PT surveyed and studied in detail

Sr.

No.

Name of the

district

No. of

PT.

No. of PT. surveyed

for short listing

Detailed

PT. Studied

1. Aurangabad 1393 12 3

2. Jalna 487 10 4

3. Beed 1006 08 2

4. Osmanabad 788 08 3

5. Latur 606 07 2

Total 4280 45 14

To assess the detail geohydrological condition of these tanks, a detail well

inventory of the surrounding wells a detail well inventory was carried out

in the surrounding areas by demarcating representative deep wells. It was

ensured that all the wells in the downstream were below the level of the

bottom of percolation tank and the geological lithologs were drawn was

carried out as per the form given below in Annexure 3.2. The wells which

were studied in detail through well inventory were plotted accurately on

the cadastral map of the respective village.

67

Fig. 3.9 Percolation tank constructed at a lower level than

the adjacent agricultural lands and the open wells

Fig. 3.10 Geohydrological study of open well

68

Annexure-3.2

Geo hydrological Studies of poorly permeable

Percolation Tanks in Deccan Basaltic Terrain

Well Inventory Form

Date :- ……./……./………

1. Name of the Village………………… .

Taluka…………… District. ………………….

2. Toposheet No. ……………. . Coordinates: …………….

Census Code ……………….

3. Population …………………. Altitude range ……………

Area of village ………………

4. Accessibility to Village and percolation tank.

a) Village from main town ………. km. along road …………….

b) Interior road from ………. direction N/S/E/W……………. km.

c) Percolation tanks from the N/S/E/W Distance : ……………….

d) Near gut No. …… right / left / exactly downstream or ……….

5. Percolation tank study

a) Area of village under influence of percolation tank …………….

b) Total submerged area of percolation tank in September ………

c) Total submerged area of percolation tank in March / April …. 25%

d) Catchment area, degraded / steep / gentle / uneven or …………

e) Construction related leakage from ………. Location ………….

Rate of flow: …………………. Quantity …………………. if any.

……….………………….……………………..….………………

f) Field characters of rock type exposed in submergence and at right

and left banks of Percolation Tanks ………………….……….

g) Siltation Thickness …………… m. type …………..

69

6. Geomorphology of the area:

a) Hills / hillocks slope / steep / gentle ………………….

b) Main stream flow N/S/E/W ……………………….….

c) Drainage pattern ……………………………………...

d) Location of Percolation Tanks ……………………….

e) Any other ………………………………………….….

7. Soil formation of the area.

a) Black cotton soil. Area / location ………… depth ……….. m.

b) Sandy / Yellow / coarse soil / canker formation / mimic soil

(gravels) waterlogged soil any other ………………………..

Area / location …………………… depth …………… m

8.Geo-hydrological survey, observations.

a) Surface geology….………………….

i. Traverse along ………………….

ii. Exposures / Flow demarcation ………………….

iii. Type of basalt ………………….

iv. If jointed, closely spaced / broadly spaced : ………………….

v. If weathered, spheroidal / sheet / or any ………………….

Geo-hydrological characters (from well section)

i) Compact basalt

1) Thickness (from G. L.) ……….. m. from ……… to …….m.

2) Jointing pattern– closely spaced/broadly spaced open/closed

Columnar / vertical 3 sets / Interconnected or Not. …………

3) Fresh /weathered. If weathered type – highly / moderately /

Spheroidal / any other. ………………..

70

ii) Amygdaloidal basalt

1) Thickness (from G.L.) ………. m. From ……… to …… m

2) Fresh / Weathered / highly / moderately sheeted or any

other…………………

iii) Hydrothermally Altered Basalt.

1) Thickness (from G.L. …….. m., From ……… to ……..

2) Fresh / weathered, highly / moderately or any:

iv) Tachylytic Basalt

1) Thickness from (G.L.) …….. m., From ……… to ……… m.

2) Red / Green / black

Fresh or weathered / highly / moderately / sheeted or any

other…………………….

v) Volcanic Breccia

1) Thickness (from G.L.) ……….. m. from …….. to ……….. m.

2) Fresh / weathered, highly / moderately. Any …………………

3) Matrix of zeolite / lava / tachylitic or any other.

9. Well inventory

a) Total number of wells in the village: ……………….

b) Wells before percolation tank construction: …………………

c) Wells after percolation tank construction : …………………..

d) Total No. of wells getting benefit from percolation tank :……

71

10. Classification of wells according to:

a) High yield: Well No.…………. total………….

b) Depth (+ 10 m) :- Well No. ………….total ………….

c) Different section :- Well No. ………….total ………….

d) Perennial well :- Well No. ………….total ………….

e) Seasonal well :- Well No. ………….total ………….

f) Pump 2-3 / hrs (summer) : Well No. ………….total ………….

11. Measurement Details of Percolation Tank

a) Height of Percolation Tanks …………….. .

b) Length of wall ……………...

c) Width of wall at the bottom ……………..

d) Width of wall at the top ……………..

e) Height of spillway – ……………...

f) Length of the spillway ……………..

Various open wells observed and studied during this detail

geohydrological study were selected from the immediate downstream of

the percolation tank (Fig. 3.10). The deeper wells which were exhibiting

distinct geological section were considered. The immediate adjacent

wells and with similar geological structures were not considered. A

comprehensive format was designed for the geohydrological study, which

also includes various details like geographic area of the village,

accessibility to the percolation tank, soil formation and measurement

details of the percolation tank apart from the geological and

geohydrological details.

72

During the field study and filling up the information in the ‘well

inventory form’, the village in which the percolation tank is located was

identified and marked on the toposheet accurately and Mean Sea Level

(MSL) of the village was ascertained. A revenue map of the village was

obtained from the revenue department or Gram Panchayat and the

percolation tank under study was marked on that map with respective gat

numbers. The surrounding gat numbers were also collected during the

well inventory so as to confirm the location of the percolation tank on the

village revenue map.

The population of the village and the area of the village was noted which

could lead to the requirement of water for the irrigation.

Accessibility to the village and the shortlisted percolation tank was

assessed and distance and direction for the approach of the percolation

tank was also noted. This was carefully done because in some of the

villages there were more than two percolation tanks located in different

directions and locations. This detail geographical information was also

recorded during the geohydrological study about the road, direction and

distance of the percolation tank from the village. This helped to avoid the

confusion where more than one percolation tank in a village was existing.

Area of the village and the area under the influence of the percolation

tank was noted either from the secondary information available with the

revenue officer of the village or during the group discussion. On the field

visit to the tank the total area under submergence after the monsoon (in

the month of September) and during the post monsoon (in the month of

March) was noted. This was very useful to assess the permeability and

the geohydrological condition of the percolation tanks.

73

The nature of catchment plays an important role in bringing the run off to

the percolation tank. Hence, the area of the catchment was also assessed

along with the type of the catchment like degraded and steep or gentle

catchment. During the focused group discussion if the villagers reported

that the percolation tank was not holding the water till the month of

February and also the surrounding area was not getting benefitted, then

the possibility of leakages or faulty construction was assessed. The

leakages if any, were located on the map and approximate quantity of

water along with the rate of flow was also noted. At few places, though

the geohydrological condition and the catchment were favourable but no

impact was seen. In such situation the percolation tank was found to be

filled up with the silt. The thickness of siltation at various locations was

also noted in the form (Fig. 3.11).

Geomorphology of the surrounding area was assessed through the

toposheet as well as on the field. The stream flows drainage pattern with

regard to the position of the percolation tank was critically examined.

Soil layer has an important role while percolating the water to the

subsurface. The impounded water in the percolation tank has to pass

through the soil layer to contribute to the groundwater. The penetration of

the water depends on the soil character. Different soils like black cotton

soil, sandy soil, coarse soil, Kankar formation have different ability to

percolate water through them. The type of soil type was noted along with

the thickness so as to understand their role in groundwater recharge.

In the Geohydrological observations, Surface geology of the area was

ascertained with the surface exposures seen along the Nala sections or

along the hills where ever distinctly visible. On the basis of the

demarcation criteria like irregular top surface, pipe amygdales at the

bottom of the flow etc. demarcation was done and their superimposition

was carefully noted. This could give a brief idea about the geology and

types of flows occurring in the region.

74

Fig. 3.11 Percolation tank with excessive silt deposition

Fig. 3.12 Topographic survey of percolation tank

75

After identification of the flow and their geohydrological characters were

also noted. The joint spacing in the case of compact basalt flows, whereas

sheet or spheroidal jointing in case of amygdaloidal basalt flows was also

noted. This geohydrological character like jointing pattern, weathering

were very useful to understand the occurrence and availability of the

groundwater in that particular rock formation. The degree and extent of

weathering were useful for understanding the influence of geology in

recharging the shallow aquifer.

After completion of the detailed geohydrological study of the short listed

percolation tanks, a topographic survey was carried out in the command

area of the percolation tanks (Fig. 3.12). The fly levels were taken in the

surrounding area specially marking the representative wells in which the

contact of the two flows was visible. The levels were converted to the

MSL with the help of the toposheet and the Geo-Hydrological sections of

selected percolation tanks were drawn. The detail percolation tank wise

discussion and geohydrology is discussed in the next chapter.

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