SURFACE WATER AND GROUNDWATER INTERACTION STUDIES USING ISOTOPE TECHNIQUES

Abstract

Fresh water available from surface water bodies, such as, lakes and rivers, is a precious resource for mankind. As fresh water is becoming increasingly scarce in recent times, it needs proper attention. Improper management of fresh water has resulted in extensive environmental degradation of many lakes and rivers in India. The environmental damages caused to surface water bodies, may propagated further into hydraulically connected groundwater system and would drastically reduce the availability of this alternative source of potable water. Surface water and groundwater interaction studies provide useful information about environmental impact and for better management of available fresh water resources. In this present study, an attempt has been madeto study surface water and groundwater interaction by using the stable isotopic technique in two different hydrogeological settings, namely, the Nainital lake(mountain terrain) andthe upper reaches of RiverGanga (alluvial riverine system) in western Uttar Pradesh. These water bodies have been selected for the present investigation, as they form the main fresh water resource for large area of irrigation besides domestic use. Nainital Lake - Groundwater Interaction Nainital lake (29° 23' 09" N and 79° 27' 35" E) is a high altitude (1937 m above m.s.l.) natural lake located in Nainital district of Uttar Pradesh. It is a crescent - shaped lake with maximum length and width of 1.4 km and 0.45 km respectively and maximum and mean depths of 27.3 m and 18.5 m respectively. The surface area of the lake is 0.46 Km2 with a maximum capacity of 8.57 Mm3. The average annualrainfall in the basin is 2488mm and is geologically characterized by Krol and Tal formations, which are folded and faulted. Direct precipitation over the lake has been estimated by Theissen Polygon method using rainfall data collected from four raingauges installed in the lake catchment. The surface inflow to the lake, due to rainfall has been estimated by two different methods, viz. Soil Conservation Service - Curve Number method (SCS-CN) and Lake Level Trend Analysis method (LLTA). The latter method was developed in the present study. SPOT satellite imagery, ERDAS™ software, Survey of India toposheet (#53 0/7) and Nainital guide map were used for obtaining land use information, which in turn was used intheselection of appropriate runoff curve number for SCSCN method. The weekly rates of reduction of water level in the lake level were considered for the LLTA method. Surfaceinflows into the lake through the drains were measured at discrete time intervals, with anassumption that the flow in the drains does not vary significantly during intermittent period. The change instorage ofthe lake was estimated from daily lake level records. Surface outflow from the lake was computed by empirical method applicable for submerged rectangular sluice openings. Evaporation, from the lake was computed from the meteorological data collected near the lake by modified Penman method. Water samples collected during 1994 - 1996 were used for chemical and isotopic characterization of the springs and the lake. A total of 63 samples were collected from the springs, 273 from Nainital lake at different locations and depths, and 39 from the drains for chemical characterisation. Majorions were analysed by standard procedures. The 5180 and SD isotopic characterisations were done for local rainfall (16 samples), drains (7 samples), Nainital lake (184 samples) and springs (35 samples). Isotopic composition of the lake evaporates was calculated by Craig and Gordon model. 6180 was used as tracer to calculate theproportion of the lake water drawn bythe wells. Sub-surface components ofthe lake were estimated byisotope mass balance method, by using isotopic data in conjunction with conventional data. Estimates of uncertainties in the measured / estimated values of a few water balance components were made based on literature. The propagated errors in the estimated sub-surface components were evaluated by standard methods. It was noted that the lake temperatures during December to February vary from 8° to 10°C bothin epilimnion andhypolimnion zones, indicating that the lake is thermally well-mixed and homogeneous and the thermocline, which had developed in March disappears totally in November. During the period from March to November, the temperatures in epilimnion zone vary from 15° to 25°C, whereas in the hypolimnion zone, they vary from 8° to 10°C, identical to the range that was observed in winters. In winters, 8180 in epilimnion andhypolimnion zones, ranges from -8.2%o to -9.7%o and -8.1%o to -9.8%o respectively, whereas 6D remains -49%o and ranges from -52%0 to -55%o respectively, indicating that the lake is isotopically homogeneous and well-mixed. During ii summers, 6180 in epilimnion and hypolimnion zones varies from -5.5%o to -8.1%o and -6.6%o to -7.7%o respectively, and the 5D varies from -38%o to -53%o and -46%o to -50%o respectively, suggesting that during summer, the effect of evaporation is pronounced in the epilimnion zone and during monsoon stratification renders the warmer surface inflow, float over the cooler hypolimnion zone. Isotopic investigations carried out in Nainital lake catchment reveal that the weighted annual mean 6180 of rainfall is about -11.3%o. The estimated altitude effect on the rainfall in Nainital area in 5180 is about -0.34%o and in 8D about -2.4%o. Amultiple linear regression model has been developed for generating 5180 data of rainfall in Nainital area, by employing the meteorological parameters such as, monthly mean relative humidity, monthly rainfall andmonthly mean air temperature. 8E was estimated by Craig and Gordon Linear Resistance model. The results ofthe model indicate that 8E is controlled mainly by the relative humidity. The estimated 6E is heavier during months ofhigher relative humidity, as compared to the months oflower relative humidity. A lower limit for isotopic enrichment ofa fresh water lake has been identified by analysing the Craig and Gordon Linear Resistance model. The lower limit is determined by the difference between isotopic composition ofthe lake (5L) and that ofthe atmospheric vapour above it (6a). The evaporative enrichment ofthe water body will continue if (8L - 8a) is greater than the equilibrium enrichment factor. The total cations ofthe springs, normalised to those ofthe lake, suggest that the springs located in Balia ravine (downstream side ofthe lake), are hydraulically connected to the lake. This is confirmed by isotopic data obtained for Balia ravine, where during summers and monsoons 6180 and 6D values remain -5.9%o to -8.8%o and -55%o to -64%o respectively and during winters, they remain -7.9%o to -9.5%o and -44%o to -56%o respectively. Data pertaining to summers and monsoons, indicate that the epilimnion zone of the lake is the main contributing source for Balia ravine springs. It implies that reduction in the lake level below the present level, might result in drying up ofthe springs in Balia ravine. iii Conventional water balance studies, carried out for estimating inflow components show that the groundwater contributes 50%, surface runoff from the catchment 30%, drains 10% and direct rainfall over the lake surface contributes 10% ofthe total annual inflow. Similarly, studies carried out for estimating the outflow components indicate that sub-surface outflow accounts for about 55%, evaporation 10% and surface outflow for about 35% of the total annual outflow. Annual pumping data indicate that outflow through seepage towards northern bank ofthe lake is about 40% of the total annual outflow from the lake. This is substantiated by the long-term surface outflow and annual rainfall data analyses, which indicate reduction inthe surface outflow for agiven amount of annual rainfall in the past three decades. It is, therefore, inferred that any change in the quantity ofpumping will affect the availability ofwater in the lake. Hydrogeological investigations indicate that the shale formation, which occupies about 50% of the lake catchment area, has an hydraulic conductivity of about 5.4 * 10"8 m/s and a specific yield ofabout 0.015%, suggesting it is not asuitable aquifer. However, the catchment area has well developed lineaments and faults. The hydrologic investigations conducted along these lineaments indicate higher infiltration capacity (about 58 cm/h). Therefore, it is inferred that most of the groundwater inflow to the lake might be occurring along these zones. Further, due to higher infiltration capacity of Sukhatal lake, its seepage appears to be amajor recharge source for Nainital lake and therefore, any activity in Sukhatal lake catchment may affect the water quality and availability in Nainital lake. The slope ofthe 6180 - 6D water line ofthe lake is 7.1, which is very close to the Local Meteoric Water Line of7.5, indicating that the lake is rainfall dependent and any change in annual rainfall might be reflected on the isotopic characteristics of the lake. Water retention time was computed for the Nainital lake by different methods. The isotopic mass balance method gives 1.93 years, chloride mass balance technique 1.77 years and the conventional water balance method yields 1.92 years. Sensitivity analysis carried out for the isotope mass balance method, indicates that the method is highly sensitive to the difference between the 6180 values ofgroundwater inflow and that oflake seepage. It was also found that the relative error decreases with increase inthe difference between these two isotope indices. iv River Ganga - Groundwater Interaction The River Ganga flows in one of the largest alluvial basins of the world and the enters the plains at Hardwar after flowing nearly 260 km in hilly terrain. For the present study, the river reach between Hardwar and Narora of western Uttar Pradesh has been chosen. The study area falls between 28° to 30° N latitudes 77°45' to 78°15' E longitudes. This reach of the river, caters water to extensive land developed for agricultural use through four major canals, namely, Upper Ganga Canal, Madhya Ganga Canal, Lower Ganga Canal and the Eastern Ganga Canal. Groundwater levels were observed using the piezometers installed at Balawali, Rawalighat, Brijghat, Anupshahr and Rajghat (near Narora). Groundwater hydraulic gradients were estimated from this data. Single well tracer dilution experiments were conducted to estimate the hydraulic conductivity. Specificdischarge of groundwater to the river at different sites were estimated by Dupuit's method. Two- and three- component mixingmodelswereused to estimate groundwater discharge to the river. The rainfall isotopic index for the study area has been computed by using long-term (1961-1995) monsoon isotopic data ofNewDelhi station. For isotopic characterisation, 63 water samples from riverGanga and 122 groundwater samples from handpumps existing closerto the river werecollected during November, 1992 to May, 1995. Another 35 groundwater samples were also collected from either side of the river at a distance of 1, 5 and 10 km perpendicular to the river course, during January 1994 and analysed for their isotopic characteristics.. It has beenfound from the groundwater levelmeasurements thatbothwestern andeastern aquifers contribute to the river flow at Balawali, Brijghat and Anupshahr, in thenon-monsoon seasons. Whereas, 8180 data of groundwater in the eastern aquifernear Anupshahr, showsthat the influence of the river decreases with increase in distance. At Rajghat (Narora), it appears from thegroundwater level data that the river receives groundwater from the western aquifer, and seeps towards theeastern aquifer during thesame period. These indicate that the nature of river and groundwater interaction is different in different reaches of the river, in the study area. The 8180 variation in the river at Brijghat, Anupshahr and Rajghat, during pre-monsoon seasons is comparable to the variation observed in western aquifer, and therefore, the western aquifer seems to be a major source ofwater to the river during pre-monsoon period. Analysis ofthe rainfall isotopic data from New Delhi station shows that the long-term monsoon weighted average for 8180 is -6.496o. Therefore, it is likely that the groundwater 8180 in the study area might be around -6.4%o. The frequency distribution analysis of6'80 were carried out by using samples pertaining to groundwater collected closer to the river, during pre-monsoon season. Theresults show the western and eastern aquifers have different peak values viz. -8.5%o and -9.5%o respectively. It indicates that different processes are involved in influencing groundwater isotopic characteristics ofthe two aquifers. Spatial variation in 6I80 values ofthe aquifers is considerable and it is due to various hydrological processes. The western aquifer is recharged mainly by precipitation, seepage from Upper Ganga Canal, and also by the irrigation return flow. On the other hand, the eastern aquifer is recharged mainly by precipitation, seepage from Eastern Ganga Canal (restricted only to the northern part ofthe study area), Ram Ganga Feeder Canal and incertain zones by the river Ganga. The frequency distribution analysis of6180 were carried out by using samples pertaining to groundwater collected closer to the river, during monsoon and post-seasons. The results show the western and eastern aquifers have different peak 6I80 values of -6.5%o and -5.5%0 respectively. The enrichment of 6180 during these seasons reflects the influence of rainfall recharge to the groundwater. The enrichment is also possible due to evaporation ofgroundwater, as groundwater table closer to river at many sites which are found to be less than one to two metre below the ground level. Aplot of 8180 and 6D pertaining to river Ganga and those pertaining to New Delhi rainfall shows that there isno discernible enrichment effect on isotopic characteristic ofthe river. This signifies that evaporation may not bea major factor for isotopic variations in the river reach between Hardwar and Narora.

The mean 6180 values indicate a progressive enrichment in 6180 during pre-monsoon season from Hardwar to Brijghat river Ganga, whereas it becomes more negative between Brijghat and Rajghat. It is observed from the isotopic data of both river and aquifer systems that the enrichment of 6180 in the river could be due to the contribution of groundwater from western aquifer rather than from eastern aquifer. The specific discharge of groundwater into Gangariver betweenHardwar and Balawali has been computed using two component mixing model and it ranges from 13.4 m2/day in June 1993 to 26.8 m2/day in March 1994. These are much higher than those calculated by Dupuit's method. Thisdiscrepancy maybe dueto linear alignment of the piezometers normal to the flow direction of the river and consequent estimation of the apparent hydraulic gradient of groundwater than true gradient. Groundwater contribution to river Ganga between Rawalighat and Brijghat has been estimated by using a three-component mixing model. The results compare well with that of channel water balance method. This substantiates the conjecture that the 6I80 value of groundwater occurring close to the river, particularly when the groundwater level is higher than the river level, is the appropriate isotopic index of groundwater. The contribution of groundwater to theflow in river Ganga at Balawali, Anupshahr and Rajghat during non-monsoon season ranges from 21% to 67% at different sites, while during monsoon season it ranges from 17% to 60%. The quantity ofgroundwater in the river atBrijghat varies from 5% to 76% during non-monsoon season, and from 18% to 35%during monsoon season. During monsoon season, seepage from river Ganga is retained as bank storage, which subsequently flows back into the river.