Groundwater salinity source identification in the Godavari delta, A.P. (PDS No: SP-28/2017-18/PDS-13)

Abstract

The groundwater quality in the Godavari delta, Andhra Pradesh has been evaluated in terms of salinity. It was found that the average salinity (EC) in shallow and piezometer wells has increased from 1664 to 2428 and 2515 µS/cm to 3606 µS/cm from the years 2005 to 2017 respectively. The surface water bodies mapping has been carried out using the Normalized Difference Water Index (NDWI) for the years 2005, 2009, 2014, and 2019 in the Godavari delta. The percentage of water bodies in the delta has increased from 13.6 to 21.17 from the year 2005 to 2019. These increased water bodies are compared with agriculture and aquaculture data and found that these changes are mainly due to aquaculture activities. A monitoring network of shallow wells (47) and piezometer wells (51) has been used to identify salinity zones in the Godavari Delta using water types and Cl/HCO3 ratio (molar). The identified salinity zones are validated with the improved network of shallow wells (100) and piezometer wells (46) of the year 2020. Five salinity zones (Zone I to Zone V) have been identified in the Godavari delta. The stable isotope characteristics have helped to identify the salinity sources of each zone with the confirmation of hydrogeochemical evaluation. Zone I is classified as fresh water and recharge sources to the groundwater are canal seepage/precipitation. Zone II and Zone III are classified as slightly brackish and brackish respectively. The groundwater salinity in shallow wells is more when compared to piezometer wells in these two zones, this is mainly due to the impact of anthropogenic activities on shallow wells. Zone IV is identified as a saline zone, and the salinity source is the evaporation process for shallow and piezometer wells. Zone V is classified as high saline. Evaporation and marine clays are the dominant sources for the high salinity in the piezometer wells, however, there is not much evaporation in the shallow wells since rainfall is recharging shallow groundwater. The tritium data of shallow wells and piezometer wells further enhance the salinity sources in the Godavari delta.