Assessment of land use and monsoon impact on high nitrate groundwater and health risk in the hard rock aquifer, South India

Abstract

Groundwater sustainability in hard rock aquifers is compromised largely due to nitrate contamination from anthropogenic sources resulting in diminishing potable resources and attendant health issues. A purpose-driven study through an integrated approach was undertaken in the area of interest (hard rock aquifer) to assess the variations in nitrate concentration and resultant health impacts in response to variations in monsoon and land use patterns. Groundwater samples (n = 284) were collected for a period of three years (2017-2019) and analysed. From the analytical data, it is inferred that 27% and 9% of groundwater samples in the study area have high NO₃^- values of > 45 mg/l and > 100 mg/l, respectively. NO₃^- contamination zones mapping illustrates that NO₃^-contaminated area (> 45 mg/l) varied seasonally 1164 km² (2017), 1086 km² (2018) and 1640 km² (2019)) and high-risk area (NO₃^- > 100 mg/l) has reduced drastically during 2018 due to dilution by monsoon (277 km² (2017), 41 km² (2018), 634 km² (2019)). The lowest NO₃^- and Cl^-concentrations are recorded during 2018 which coincides with high rainfall (2061 mm). NO₃^- concentrations in response to land use pattern indicate that the hot spots (NO₃^- > 45 mg/l and > 100 mg/l) are observed in groundwater samples of residential areas which are vulnerable to contamination from domestic wastewater, septic tanks and other pollutants. Further, wastewater infiltration facilitated the dissolution of certain minerals in the unsaturated zone which enhanced the accumulation of NO₃^- and other ions in this aquifer. Mineral weathering, denitrification and evaporation processes also affected the groundwater chemistry. The health risk model (HQ_oral) indicates that groundwater in 1261 km² (2017), 1232 km² (2018) and 1669 km² (2019) is unsuitable for drinking (HQ > 1) and causes adverse health risks to the local inhabitants. The study has identified areas from the central and southeastern regions significantly affected by nitrate pollution underpinning the necessity of using treated groundwater for drinking purposes.

Keywords: Groundwater; Hard rock; India; Land use; Monsoon; Nitrate.