Assessment of the impact of flood on groundwater hydrochemistry and its suitability for drinking and irrigation in the River Periyar Lower Basin, India

Owing to the consequences of Anthropocene climate changes, extremes of weather and climate disasters such as heavy precipitation, tropical cyclones, and floods, have strengthened in every region across the globe and are badly affecting the earth system processes. The climate change has a tremendous impact in the Western Ghats, the UNESCO heritage site in India, and has to be studied in detail. Floods are the most common (and among the most deadly) natural disasters bringing heavy destruction in the Western Ghat river basins of India, and the Periyar basin is one among them. The low-lying areas of Periyar river basin (RPLB) is one of the worst affected river basins in the 2018 Kerala floods. Since the basin was continuously monitoring, the groundwater quality has been assessed in Periyar basin during three periods (pre-flood, flood, and post-flood-April, August, and October 2018 respectively) and is described in this paper. Since the water table is shallow in RPLB, floodwaters quickly reach the groundwater table, thus influencing the groundwater quality. So, the groundwater samples were collected from 26 sites in RPLB, and analyzed for quality determining parameters. Parameters like pH, TDS, EC, TH, NH₃^-, NO₂^-, HCO₃^-, SO₄^2-, SiO₄^4-, Mg²⁺, and K⁺ decreased during flood compared to pre-flood times. On the other hand, Ca²⁺, Na, and Cl^-, turbidity values were increased during flood times. The values of pH, TDS, EC, turbidity, NH₃^-, HCO₃^-, SO₄^2-, SiO₄^4-, Mg²⁺, Na⁺, and K⁺ were slightly increased in post-flood period compared to flood period. When compared to the pre-flood values, turbidity, NH₃^-, SO₄^2-, Na⁺, and K⁺ were slightly increased in post-flood period. It was also observed that pH, TDS, EC, TH, NO₂^-, Cl^-, HCO₃^-, SiO₄^4-, PO₄^3-, Ca²⁺, Mg²⁺, and E. coli values were higher in pre-flood periods compared to the post-flood scenario. Except pH, turbidity, and E. coli, the averages of all other parameters are falling under the recommended drinking quality values of WHO. The pH belongs to acidic nature throughout the study period. The Piper plot indicates the dominance of Ca-Mg-HCO₃ type in pre-flood, and mixing of Ca-Mg-HCO₃ type in both flood and post-flood periods. The Gibbs plot reveals that the concentration of elements is mainly depending on rock weathering and sub-surface water flow. The USSL plot depicted that the 92-96% of samples are noted as lower salinity and low sodium hazard at the source of C2S1 and C1S1; however, the 4-11% of samples are noted as high salinity but low sodium hazard at a source of C4S1 and C3S1. The Wilcox diagram indicates that the 88-96% of groundwater was found as higher suitability for irrigation during the study periods. Based on GIS based WQI model, out of the 26 samples studied, 69% of wells showed improvement in water quality after flood; meanwhile, 19% well water samples in flood were observed for lower quality compared to pre-flood times and 12% of samples remain unchanged during flood. Though floods are having positive and negative impacts, from this study, it is clear that quality of the groundwater in the RPLB is not severely affected, but they became diluted to permissible limits during flood and post-flood periods except some locations. Since the flood impact studies on groundwater systems are meager, this data from Periyar basin can be used as baseline groundwater reference data for all future flood-related river basin studies and will be very beneficial for the policy and planning needs in the context of climate change. It is high time to establish the baseline data of all river basins of Western Ghats since the normal earth system processes are worse affected by the recurrence of floods which are reporting every year.