O-18 studies on recharge of phreatic aquifers and groundwater flow-paths of mixing in the Delhi area

Widespread decline in the groundwater level and growth of salinization are the critical factors controlling future groundwater quality and availability in the Delhi area. delta(18)O and salinity data have been integrated for studying recharge conditions of phreatic aquifers and tracing the flowpaths of groundwater mixing. Wide range (-2.8 parts per thousand to -8.6 parts per thousand) and spatial inhomogeneity of delta(18)O 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 delta(18)O-Cl 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.