Hydrogeochemical evidences for targeting sources of safe groundwater supply in arsenic-affected multi-level aquifer systems.

Delineation of safe aquifers becomes highly imperative and challenging to ensure sustainable drinking water supply in rural areas of multi-level aquifer systems with complex water circulation under the impact of both geogenic and anthropogenic contamination. This work characterized hydrogeological and hydrogeochemical features of a multi-level Quaternary aquifer system of the central Yangtze River Basin to search for evidences of locating aquifers least contaminated. The results indicate modern hydrologic cycling is active in shallow phreatic aquifer (SPA) and in upper part of middle confined aquifer (MCA-1). The lower part of middle confined aquifer (MCA-2) and deep confined aquifer (DCA) have ages ranging from 200 to 2000 years and 4000 to >20,000 years, respectively. Vertical variations of hydrochemical compositions and Cl/Br ratios suggest the gradually decreasing and increasing contribution with depth from anthropogenic activities and natural water-mineral interactions, respectively. The SPA is characterized by high levels of anthropogenic components (Cl, SO, NO, organic pesticides and antibiotics). The MCA-1 is the main aquifer where most private tube wells were installed, typically contains geogenic As, Fe, Mn and NH at concentration levels several hundred times higher than the national drinking water standard values. Organic-abundant geochemical and stagnant hydrogeological conditions favor their enrichment. Both MCA-2 and DCA are slightly influenced by geogenic contamination and groundwater pumping from public wells screened in MCA-2 does not or slightly perturbs the groundwater flow condition in MCA-2 and DCA. The cost-effective water supply strategy is either to maintain safe yield of groundwater from public wells screened in MCA-2, or to use the groundwater after simple filtration and aeration treatment in case that groundwater pumping in MCA-2 were over a "safe yield" or induced leakage of groundwater containing As, Fe, Mn and NH from the overlying MCA-1. And DCA could serve as a source for water supply under emergency circumstances.