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Watershed , Patna District, Bihar
 
Groundwater studies
Hydrogeology
The watershed is located in the central axial part of Middle Ganga Plain. The Ganga basin is an active foreland basin formed in response to the uplift of Himalaya due to collision of Indian and Asian plates. The Middle Ganga Plain of Ganga basin lies between Munger-Saharsa ridge in the east and Faizabad ridge in the west exhibiting an asymmetrical sediment wedge, with thickness varying from less than a meter in basin margin areas with Peninsular craton towards south to more than 5 km near the Himalayan orogeny towards north.
 
Figure 1.8: Basement Depth Contour in parts of South Ganga Plain.
Source: CGWB after Bose et al, 1966.
 
Assessment of aquifer geometry based on the available data reveals the presence of a thick pile of alluvial sediments of Quaternary age comprising various grades of clay, silt and sand which constitutes the ground water reservoir. The entire alluvial thickness overlying the Precambrian basement in the area is expected to be over 700 m as inferred through deep seismic refraction survey towards the southern part of Patna district (see figure above). Towards the north, around Patna urban area, a sharp drop in bedrock depth forming a deep trough of unconsolidated alluvium is indicated.
Exploratory boreholes drilled by CGWB have revealed two major aquifers within 200 m depth separated by clay occurring in the depth range of 45 and 70 m. The shallow aquifer comprises fine to medium sand, while the deeper one is made up of medium to coarse grained sand with gravel at the base. The deeper aquifer is highly productive.
 
Water level
CGWB is monitoring 4 Hydrograph Network Stations (HNS) for ground water monitoring, which are dugwells tapping the shallow aquifer. Besides, there are 10 piezometer monitoring stations tapping the deeper aquifers.
 
Figure 1.9: Location map of ground water monitoring wells.
 
Figure 1.10: Location map of piezometers.
 
To fill the water level data gaps, 104 key dugwells and 5 piezometers have been established in the project area in addition to existing wells. These wells are distributed throughout the area.
 
Figure 1.11: Location of Key wells for monitoring unconfined aquifer.
 
The depth to water level of the shallow aquifer ranges between 2 and 10 m bgl during pre-monsoon (May 2010). The water level in most part of the area is within 4 m bgl, however towards the western part of the area and in patches, it is less than 4 m bgl.
The depth to water level for post monsoon 2011 is mapped below. The depth to water level is within 4m bgl in the major part of the area and only in patches it is below 6 m bgl.
 
Figure 1.12: Depth to water level contour, post monsoon 2011.
 
Long-term water level trends are the following:
The long term water level of the shallow aquifer at Maner (HNS well), which is located in the peri-urban part, shows a falling trend both during pre- and post-monsoon periods.
The HNS well at Patna city, located within the urban part, shows an almost stable trend for the post-monsoon season and a modest falling trend for the pre-monsoon season.
The HNS well located at Deokali adjoining the southern part of the watershed with predominantly rural land use characteristics and significant dependence on shallow aquifer system, exhibits a modest declining trend both during pre- and post-monsoon season.
 
Piezometric head analysis of deeper aquifer for 2008, 2009 and 2010 reveals (i) almost stable pre-monsoon water level during the period, and (ii) wide variation in pre- and mid-monsoon fluctuation.
The water table in the shallow aquifers and the piezometric head of the deeper aquifers follow the seasons, i.e. shallowest during August and deepest in May. The difference between their respective hydraulic heads is apparent round the year.
 
Geophysical Studies
Both vertical electrical soundings and geophysical logging of exploratory wells have been carried out by CGWB.
Vertical electrical soundings carried out help delineate the low resistivity (clay) layers and the deeper moderately resistive (sand- aquifer) layers.
 
Figure 1.13: Location of VES soundings.
 
On the basis of interpreted results, details of some geoelectrical sections are presented below.
 
Lithological Section (Naubatpur-Bhabhanpura)
The lithological section (NE-SW) based on sounding results represents the south-eastern part of the watershed. The top soil layer of varying thickness is characterized by different resistivity values, indicating variations in lithology as well as moisture content. For the clay predominating second layer, the wide variation in resistivity, from 11 to 152 ohm.m, is due to the presence of kankar. The resistivity of the underlying layer occurring at a depth of 10 to 30 m varies between 35 and 46 ohm.m, indicating the presence of medium to coarse grained sand. This layer is the shallow unconfined aquifer in the area.
 
Figure 1.14: Lithological section based on VES results (Naubatpur-Bhabhanpura).
 
Lithological Section (Pahalwanghat-Haldichapra)
The lithological section along the course of the river in the east–west direction from Pahalwanghat to Haldichapra reveals a persistent clay capping of variable thickness throughout the section. The thickness of clay admixed with fine sand increases to 60 m towards east. Patches of unsaturated sand have been also encountered. Below the clay layer, medium to coarse grained sand of considerable thickness occurs.
 
Figure 1.15: Lithological section based on VES results (Pahalwanghat-Haldichapra).
 
Exploration
CGWB has drilled 15 exploratory wells (EW) in the watershed. The locations of the wells are shown below. The maximum explored depth in the watershed is 311m bgl.
 
Figure 1.16: Location of exploratory wells.
 
The configuration of the aquifers has been established through the lithological and geophysical logs. The logs reveal the alternate beds of various grades of sand, forming prolific aquifers, with interbeds of clay and silt.
Lithological logs of CGWB wells shown below indicate persistent clay capping with thickness reaching up to 60m from the ground level at places. The boreholes drilled to a depth of 200m reveal mainly two aquifer systems, separated by 3 to 20m thick lenticular clays. The deeper aquifer is made up of medium to coarse grained sand often grading to gravelly sand at the bottom. The cumulative aquifer thickness up to the maximum explored depth of 311m is more than 250m.
 
Figure 1.17: Lithologs of Exploratory wells and Piezometers drilled by CGWB.
 
A NE-SW lithological section based on 100m deep state wells also shows clay capping from the ground level to a maximum depth of 46 m. There is almost a mono-aquifer system made up of medium to coarse grained sand below this. In SW part, an underlying clay layer is found around 75m depth, below which another aquifer is encountered.
Lithological logs of CGWB wells shown below indicate persistent clay capping with thickness reaching up to 60m from the ground level at places. The boreholes drilled to a depth of 200m reveal mainly two aquifer systems, separated by 3 to 20m thick lenticular clays. The deeper aquifer is made up of medium to coarse grained sand often grading to gravelly sand at the bottom. The cumulative aquifer thickness up to the maximum explored depth of 311m is more than 250m.
 
Figure 1.18: Lithological Section (NE-SW) along Ramnagar -Bahpura-Telhari-Darveshpur.
 
Aquifer parameters
The pumping test data of CGWB wells reveal transmissivity of deeper aquifers in the range of 5000 to 16000 m2/day, indicating their potentiality. The specific capacity of the wells ranges from 56 to 100.4 m3hr-1m-1 and the mean hydraulic conductivity is of the order of 90 m/day. Storage coefficient values obtained at two locations are 7.7x10-2 and 5.0x10-3, indicating a deeper aquifer in semi-confined condition because of the overlying aquitard zone.
In addition, data of the state tube wells constructed in the western and south-western part of the watershed area have been analyzed. The discharge of the deep tube wells, tapping aquifers in the range of 91 to 110m bgl, varies from 159 to 222 m3/hr. This corroborates the significant yield potential of the deeper aquifers.
 
Groundwater Quality
Groundwater quality in shallow aquifers
pH of the analyzed water samples varies from 6.8 to 8, with an average value of 7.42. In general, pH of the water samples is neutral to slightly alkaline in trend.
Electrical conductivity (EC) of the ground water varies from 260 to 3050 µS/cm with an average value of 825 µS/cm.
In general, concentration of Ca and Mg is found within the permissible limits.
Total Hardness (TH) values vary from 95 to 875 mg/l. The average value for TH is 262 mg/l.
HCO3 is the dominant anions with a mean value of 304 mg/l.
The chloride concentration varies between 71 and 461 mg/l.
Towards west of this watershed arsenic contamination in shallow aquifers within 50 m depth (see below).
 
Groundwater quality in deeper aquifer
pH of the ground water of deeper aquifer is marginally alkaline and varies from 7 to 8.52 with an average value of 7.7.
EC has an average value of 491µS/cm and varies from 301 to 703µS/cm. The deeper groundwater is less mineralized than the shallow ground water.
Average concentration of Ca and Mg is at 49.3 and 14.40 mg/l respectively.
TH values vary from 95 to 260. The average value for TH is 184 mg/l.
The chloride concentration varies between 4 and 34 mg/l with an average value of 12 mg/l.
The average concentrations of sulphate, nitrate and silicate are 18, 0.54 and 16.6 mg/l respectively.
 
Issues related to groundwater quality in the watershed
The major issue with groundwater quality is the high arsenic level. The arsenic contaminated area includes Maner and Danapur blocks in the northern part, aligning the old and present courses of Ganga and Sone Rivers.
 
Figure 1.19: Map of the parts where groundwater quality is affected.
 
The contamination is confined in general up to the depth of 60m. The distribution of arsenic is patchy and there is a wide spatial variability in arsenic concentrations.
In the arsenic contaminated area, the alluvial sequence can be broadly divided into two aquifer systems. The lithological log of the borewell drilled at Maner (just south of the arsenic affected area) by CGWB reveals a 15-20 m thick clay bed at depth of 80-100m dividing the Quaternary deposits into two aquifer systems within 250m bgl. The top part of the upper aquifer, within 60m below ground level, is arsenic contaminated.
Most of the drinking water supply in the rural area is dependent on shallow ground water, except for a couple of deep tubewells in the southern part of the affected area. Hand pumps sunk to a depth of 20 to 40 m are the main abstraction structure in the rural areas.
PHED, the state department for drinking water supply, has taken measures to provide safe potable water in arsenic affected areas like promoting use of dug wells which are arsenic free, increasing the depth of hand pumps so that they tap arsenic free deeper aquifers, and community based drinking water supply through deep tube wells. In Patna urban area, no arsenic contamination is reported.
CGWB has also conducted mass awareness camps at Maner and Danapur, in the contaminated part of the watershed, to create awareness in this regard.
 
Groundwater Resources
The average stage of ground water development for the blocks under this watershed as a whole is 60%, with the stage of development varying from 40.9 to 74.6%. The highest stage of development is in Patna Sadar block. All the blocks falling in the watershed area have been categorized as “Safe”. There is scope for further development of groundwater in the watershed.