O-18 Studies on Recharge of Groundwater and Flow-paths of Mixing in the Urbanised Area

P.S.Datta et al, Journal of Hydrology 176 (1996) 25-36.

Groundwater is the primary water resource for agricultural activities, domestic use and industrial purposes within the Delhi area. More than one third of the domestic supply is met found groundwater. Widespread decline in the groundwater level and growth of salinization are the critical factors controlling future groundwater quality and availability in the Delhi area. Generally, studies on regional groundwater flow systems adopt a hydraulic approach, based on gravity induced groundwater flow from high to low hydraulic head.

However, sometimes in the absence of adequate hydraulic data it is difficult to estimate key hydraulic parameters by numerical simulations. In the present study, water chemistry and stable isotopic data have been integrated for tracing groundwater movement and flow paths of mixing. O-18 and salinity data have been integrated for studying recharge conditions of phreatic aquifers and tracing the flowpaths of groundwater mixing. Therefore, spatial distribution of chemical species gives some idea about the direction of groundwater movement. The stable isotope oxygen-18 is a conservative tracer and remains constant in the direction of groundwater flow unless affected by physical processes such as mixing with water of different isotopic composition.

The chloride ion is also considered to be conservative in groundwaters of the Delhi area. Therefore, physical processes such as mixing with another groundwater body of different Cl concentration may result in changes in Cl content of groundwater. In addition, changes in evaporation rate during recharge, either spatially or temporally, can also result in changes in Cl concentration. Hence, for planning and management of groundwater resources, a knowledge of the regional groundwater flow regime is important. 

Wide range (-2.8%0 to -8.6%0) and spatial inhomogeneity of O-18 in groundwater indicate that the aquifer is characterized by a slow process of natural mixing of groundwater in its lateral extent. There is infiltration of river water to the groundwater, in addition to the recharge from rainfall. Groundwater recharge in the area also occurs through stagnant water pools in low elevation areas where water surface run-off collects. Chloride concentration in groundwater has a wide range from 15 ppm to 5070 ppm with a log-normal frequency distribution indicating that the salt deposits in the area are not homogeneously distributed.

The O-18 vs C1 relationship clearly suggests mixing of multiple sources of high salinity groundwater with low salinity groundwater or river water along specific pathways. The data also indicates leaching of salts present in the soil during vertical recharge from rainfall and irrigation. Large lateral variation in chloride concentration indicates changes in evapotranspiration rate during recharge. Detailed investigations on identifying the potential recharging zones, flow-paths of mixing and recharge conditions will be useful to protect the groundwater resources from depletion and salinization.