GROUNDWATER EVALUATION OF A TYPICAL WATERSHED IN PIEDMONT ZONE OF HIMALAYA, INDIA

Abstract

Groundwater forms one of the most important sources of potable water. It is believed to be safe, free from pathogenic bacteria and from suspended matter. The rate of withdrawal of groundwater is increasing continuously due to faster pace of population growth accompanied by agricultural and industrial development. This has increased the concern on groundwater resource evaluation and its management for sustainable development. The most accessible groundwater reservoirs have already been tapped and are being heavily utilized. Therefore, future withdrawals must come from deeper aquifers where water quality may also be a problem. Aviable methodology for exploration of groundwater to assess aquifer system in the water scarcity areas need to be developed. In the present study, it is aimed to delineate the aquifer system in the piedmont zone situated in Himalaya foothill region of India. The Himalaya is skirted by an upper piedmont zone along its southern margin and is referred differently as "Kandi" in the northwestern India and "Bhabhar" in the northern India. The lower piedmont zone located further southward is referred as "Tarai". The piedmont zone is located to the south of Siwalik foothills of Himalayas and presents several difficulties in groundwater exploration and development due to occurrence of thick deposits of poorly sorted unindurated sediments, deep water table, and the associated problems in drilling. Due to these problems, the groundwater availability in the area becomes cost prohibitive and is generally unavailable to the rural users for the societal needs. Further, much of the groundwater is recharged by percolation to deep aquifers situated further downstream, because of the southward gradient of the land surface in the Bhabhar zone. The groundwater thus recharged into the deep aquifers in these areas is not fully accounted for by the currently available methods of groundwater assessment. Thus, there is a need to develop available methodology for groundwater evaluation in the piedmont zone in Himalaya foothill region. The objective of present study is to delineate the aquifer system in the Bhabhar and Tarai tracts of Ratmau- Pathri Rao watershed in District Haridwar, Uttaranchal, India, using an integrated geohydrological and geoelectrical techniques. The study will also result in assessment of groundwater recharge into the aquifers, age of groundwater and area of recharge using isotope technique, thus leading to creation of a groundwater resource data base for the study area. An integrated approach of geomorphological, geohydrological, geoelectrical and isotope techniques has beenused to evaluate groundwater in the piedmont zone of Himalayan foothill region of District Haridwar, Uttaranchal, India. The study area is located between Latitude 29° 50' 00" to 30° 11' 21" North and Longitude 77° 54' 19" to 78° 06' 21" East falling in Ratmau-Pathri Rao watershed covering an area of approximately 430 km . Hydrogeomorphologically, the Piedmont deposits of the study area are divided into Bhabhar and Tarai zones. The Bhabhar is a steeply sloping relatively dry tract made up of assorted sediments brought down by rivers emerging from Siwalik hills. Geologically, the Bhabhar wasformed in the Recent era by integration of piedmont of alluvial fan depo sits consisting of unconsolidated sand, boulders and clays. The Tarai zone occurs immediately below Bhabhar and is composed of alternation of sand and clays, at places impregnated by calcareous nodules commonly referred as Kankar. Systematic studies have been conducted in the present work to develop the methodology for the groundwater evaluation in the study area. Hydrogeomorphological investigations have been carried out using remote sensing and GIS techniques to have a preliminary assessment of the groundwater resources in the study area. Various thematic maps have been prepared for, hydrogeomorphology, slope, drainage density and landuse. Based on the integration of various thematic maps, compiled from processed digital satellite data of IRS-1D-LISS-III, identifying potential areas for delineate gioundwater exploration ii has been done. Based on this study, the most promising area for groundwater development is the Tarai area in the southern part whereas the high area occupied by Siwalik rocks indicates poor groundwater potential. Twenty one observations well have been used to monitor the water table depth to groundwater in the entire study area. For consecutive two years (i.e. from 2002 to 2003). Thematic maps of water table depth for premonsoon and postmonsoon period generated using Arc View, a GIS software (3.1 version) exhibit variation in the depth of groundwater in different parts of the area. The water levels are found to occur at shallow depth (1.5m to 5m below ground level) in the vicinity of spring line between Upper piedmont (Bhabhar zone) and Lower piedmont (Tarai zone). Also water levels are found to occur at deeper levels (16m to 30m (b. g. 1)) in the Bhabhar zone. The seasonal groundwater fluctuation (from premonsoon to postmonsoon) is large towards north and northeast in the Tarai zone which gradually decreases southwards and towards the northwest Bhabhar zone. A Fence diagram has been prepared for the study area based on lithologs available from the tube well division Roorkee. It has been observed that the top layers are generally comprised of silty sand in the Tarai zone and boulder with sand in the Bhabhar zone. Shallow and deeper aquifers are generally present in the area separated by clay lenses. In the Tarai zone the shallow aquifer is comprised of fine grained sand whereas in the Bhabhar zone the shallow aquifer consists of admixtures of sand and gravel of varying sizes. The shallow aquifer is less than 6 m thick in the Bhabhar whereas in the Tarai, it has greater thickness of 12-16m. The depth of the upper aquifer in the Bhabhar is over 18 m, however, in the Tarai it is at a shallow depth of 1-8 m occurrence of the spring line. The aquifers get isolated in places, where clay lenses are present between them. Tn the Bhabhar zone, the clay layer is often impregnated by kankar but it decreases towards the Tarai zone. Further, hard clay is in present in the Bhabhar zone at the 40m depth but its thickness decreases towards the Tarai zone. To delineate the detailed aquifer geometry, seventy Vertical Electrical Sounding (VES) have been conducted in the study area using Schlumberger configuration with maximum electrode spacing of about 900 m and at the station interval of about 2 km and less. An automatic computerized interpretation method has been used to obtain true resistivity and depth from the measured apparent resistivity data at each site. For the geological interpretation, the resistivity values are correlated with the known lithology from available boreholes data in the study area. This correlations help in defining the resistivity ranges for various formations in the area which shows the resistivity of unsaturated zone generally varies in range from 89 - 506 Ohm-m. The large variations in resistivity of unsaturated layer suggest to the varying nature of subsurface materials (boulder in the Bhabhar zone, fining to silty sand in the southern Tarai zone). The thickness of unsaturated layer reaches up to 30 m in the area. The clay is characterized by resistivity range of 10 - 20 Ohm-m which may increase up to 37 Ohm-m depending upon the proportion of kankar present in clays. The resistivity range of20 - 40 Ohm-m, generally indicates saturated silty sand. The resistivity ranging from 22 -165 Ohm-m indicates water bearing sand with gravel and forms the shallow aquifer in the Bhabhar and Tarai zones. The thickness of this aquifer varies between 2 to 8m in the Bhabhar and 5 to15 min the Tarai. The depth to the shallow aquifer zone varies from 17 to 32 min the foothills (Bhabhar zone) and 1mto 7min the Tarai. Due to occurrence of the finer materials in the Tarai belt, the groundwater occurs in confined, semi- confined and unconfined condition. The resistivity of the deeper aquifer ranging from 20 to 90 Ohm-m in the study area indicates presence of lower proportions of gravel in this aquifer. The thickness of this aquifer IV varies between 15 to 30m in the Bhabhar and 5 to 50 min the Tarai. Depth to this aquifer varies from 15 mnear the spring line to more than 90 min the northern part of the study area. Results of calibrated vertical electrical resistivity sounding data have been integrated with other geohydrological (depth to water table) and hydrogeomorpholgical data (hydrogeomorphology and slope) in GIS environment to workout groundwater potential in the area. The final thematic map showing the various groundwater potential zones has been generated. It can be seen that the groundwater potential in the northern part of the study area (covering about 134 km2 of the area) is very poor, due to very steep slopes, resulting in low infiltration and high runoff. In the middle part, because of groundwater table is moderate shallow to shallow, depth of clay unite, and gentle slope, the groundwater potential is good to moderate (covering 110 km2 of the area). In the southern part, about 87 km2 of area has very good groundwater potential due high aquifer thickness, low depth of clay unit, low water depth and gentle slope in the Tarai geomorphic unit. Of the remaining study area, about 99 km2 has poor groundwater potential. The generated groundwater potential map is validated from the yield data obtained from 10 tubewells located in the different groundwater potential zones shows that a zone of very good potential is having water yield ( 121-257 m3/h) and moderate to good having yield ( 63-225 m3/h). Vertical recharge in the study area has been estimated using tritium tagging technique. For this purpose, the study area has been divided into different zones in which eight representative sites have been selected. Recharge estimation at the sites has been made by monitoring the vertical movement of tritium at each site. The study indicates that the maximum vertical recharge (8- 13 percent) in the Bhabhar zone and (3-4%) in the Tarai zone. This is due to presence of different size range of grains in top surface layer consisting of permeable sand and gravel which gradually decreases to afine grained material in the Tarai zone. A linear relationship is obtained between recharge percent and water table fluctuation in the study area. A groundwater recharge model has been developed by integrating the result of vertical electrical sounding and tritium tagging study. The model can be used for estimation of recharge from electrical resistivity measurement in similar geological formations. Conceptually, the model is based on the analogy between the current flow in the unsaturated soil and flow of water in porous media, which follow Ohm's law and Darcy's law respectively. The method points to a new application of electrical resistivity in estimating recharge due to infiltration. A groundwater recharge map for the entire study area has been generated using surface electrical resistivity data. Environmental Tritium has been used to identify the recharge sources, flow rates and age of groundwater. Tritium concentration in the shallow aquifer varies from 4.2 to 11.24 Tritium Units (TU) and deeper aquifer tritium varies from 0.9 to 12 Tritium Units (TU). An attempt has been made to correlate the amount of recharge obtained from the tritium tagging technique and the tritium concentration found in the shallow aquifer of the study area. It shows good correlation between the percent recharge estimated by tritium tagging technique and the tritium concentration estimated in the water samples. This indicates that the precipitation is the major source of recharge to shallow aquifer in the study area. The estimated uncorrected age of groundwater in the area is a minimum in the Bhabhar zone which increases towards the Tarai zone. The estimated age plotted with the tritium concentration indicates that the age of groundwater increases with depth and with the direction of lateral movement of groundwater. The tritium unit (TU) values are comparatively lowered in case of deeper aquifers in Bhabhar zone that the shallow aquifer. Thus if these values are convert -in the uncorrected tritium age, a difference of ~ 4 years is obtained. This indicates poor interconnections of VI shallow and deeper aquifers. Similarly, the EC (Electrical Conductivity) value are higher in case of shallow aquifers than deeper aquifers which also indicates that deeper aquifers have recharge source other than shallow aquifers in this region. Also if the geological aspect is taken into consideration the clay lenses appears between the shallow and deeper aquifers. Therefore, it appears that the deeper aquifers have the recharge zones located in Siwalik region. The mean flow rates ofgroundwater for the deep aquifers in the study area have been evaluated to 1.2 m/d. The groundwater flow pattern concluded on the bases of TU values have been conformed to groundwater flow pattern deduced from the groundwater table elevation data. The study has led to meaningful and relevant findings about evaluation of groundwater potential of the piedmont aquifers in the Himalayan foothill region of India, in GIS environment. Further, use of isotopes has provided significant clues about groundwater recharge, age and area of recharge.