Co-occurrence, hydrogeochemical association, and risk assessment of arsenic and iron in deep and shallow aquifers of Punjab, Pakistan

Abstract

The safety of drinking water and irrigation supplies greatly depends on groundwater quality, but is vulnerable to geogenic contaminants that are challenging to predict. The concurrent release and accumulation of geogenic arsenic (As) and iron (Fe) in freshwater systems pose significant global environmental challenges, yet their co-occurrence dynamics and source linkages remain poorly understood. This study investigates the spatial correlation and source apportionment of As and Fe in shallow (<19.8 m) and deep (>19.8 m) aquifers of Shorkot, Punjab, Pakistan, through hydrogeochemical analysis of 74 differently-scattered groundwater samples. The sampled groundwater serves as a source of drinking/irrigation water for the local community. Results revealed As concentrations ranging from 0.05 to 22.3 µg/L, with 19% of samples exceeding the WHO guideline (10 µg/L), though none surpassed the Pak-EPA threshold (50 µg/L). Iron levels varied between 0–218 µg/L, with higher mean concentrations in deep aquifers (As: 6.1 µg/L; Fe: 74 µg/L) compared to shallow ones (As: 3.7 µg/L; Fe: 66 µg/L). Health risk assessments demonstrated carcinogenic potential, with cancer risk (CR) values exceeding the acceptable threshold (CR > 0.0001) for both adults (0.005) and children (0.003). Hazard quotient (HQ) and average daily dose (ADD) values further indicated non-carcinogenic risks, particularly for children (HQ: 0.5; ADD: 0.0002 mg/kg/day). A strong positive correlation (R² = 0.80) between As and Fe highlighted the critical role of Fe-(hydr)oxide mineral dissolution in mobilizing As into groundwater. These findings underscore the geogenic origin of contamination, driven by redox-driven weathering processes in aquifer sediments. The study emphasizes the urgent need for targeted mitigation strategies to address chronic exposure risks and advocates for comprehensive source characterization to inform sustainable groundwater management. This work advances the understanding of As-Fe synergies in alluvial aquifers and provides a framework for assessing similar geogenic contamination in vulnerable regions globally.