Refining aquifer heterogeneity and understanding groundwater recharge sources in an intensively exploited agrarian dominated region of the Ganga Plain

Abstract

Densely populated region of Ganga Plain is facing aquifer vulnerability through waterborne pollutants and groundwater stress due to indiscriminate abstraction, causing environmental and socio-economic instabilities. To address long-term groundwater resilience, it is crucial to understand aquifer heterogeneity and connectivity, groundwater recharge sources, effects of groundwater abstraction etc. In this context, present study aims to understand factors responsible for vertical and spatial variability of groundwater chemistry and to identify groundwater recharge sources in an intensively exploited agrarian region of the Ganga Plain.

Interpretation of chemometric, statistical, and isotopic analysis categorises the alluvial aquifer into zone 1 (G1; ground surface to 100 m) and zone 2 (G2; >100 m-210 m). The group G1 samples are characterized by a wide variation in hydrochemical species, noted with pockets of F^– and NO₃^– rich groundwater, and fresh to more evolved water types, while group G2 groundwater is characterized by a sharp increase in freshwater types and limited variation in their isotopic and hydrochemical species. The G1 groundwater chemistry is governed by soil mineralogy, local anthropogenic inputs (SO₄^2-, Cl ^-, and NO₃^–), and manifested by multiple recharge sources (local precipitation, river, canal water, pond). The G2 group is dominated by geogenic processes and mainly recharged by the local precipitation. Geospatial signatures confirm more evolved water type for group G1 in northwestern region, while freshwater type covers the rest of the study area. Fluoride rich groundwater is attributed to sodic water under alkaline conditions and enriched δ¹⁸O values emphasizing role of evaporation in F^- mobilization from micas and amphiboles abundant in the soil. The findings provide insight into potential groundwater vulnerability towards inorganic contaminants, and groundwater recharge sources. The outcome of this study will help to develop aquifer resilience towards indiscriminate groundwater extraction for agricultural practices and aim towards sustainable management strategies in a similar hydrogeological setting.