Hydrogeochemical Investigations and Solute Transport Modeling of Polluted Coastal Aquifer

In this paper, the extent of sea water intrusion and groundwater contamination due to aqua ponds are investigated through hydrogeochemical studies along with solute transport modeling of the aquifer. The study area is an interstream region of the Pennar and Upputeru Rivers located in the Nellore district of Andhra Pradesh, India, covering an area of 529 km². The groundwater levels and samples are collected in the 196 observation wells to prepare groundwater contour maps and to analyze the groundwater samples for various chemical parameters. Occurrence of saline water in certain localized pockets due to intensified aquaculture and occurrence of saline water along the coast due to sea water intrusion are verified by various ionic ratios like Ca⁺²/Mg⁺², TA/TH, Na⁺/Na⁺+Cl^- and Cl^-/CO₃^-2+HCO₃^-.

The above investigations have revealed that all along the coast, with a strip width of 2 to 5 km, groundwater levels are below the sea level at an average depth of 3 m below ground level; consequently there is seawater intrusion with TDS concentrations ranging from 1500 to 2500 mg/L along this strip. Similarly, the TDS values were found to be as high as 5000 mg/L at a localized pocket at T.P.Gudur due to storing of sea water meant for aquaculture. The ionic ratios are confirming the presence of seawater in these localities and along the coast. In general, the seawater spreading is more during the pre-monsoon season and it is considerably diluted in the post-monsoon season. The solute transport model indicates that the lateral spread of saline water towards inland is not occurring farther beyond a 2 to 5 km strip due to higher topographic elevations and the groundwater is following the topography. In the contaminated strip, the model predictions are made up to the year 2052 with respect to its vertical and lateral spread.