Regional-Scale Controls on Arsenic Contamination in the Multi-Aquifer System of the Mekong Delta, Vietnam

Abstract:

Geogenic groundwater arsenic poses a considerable health threat to a large, largely agrarian population in the Mekong Delta, Vietnam. Demand for groundwater continues to rise despite limited information about where arsenic contamination occurs and how it may be affected by excessive pumping. We investigate arsenic measurements from over >40,000 wells found throughout the Delta’s seven major exploited aquifers, relating trends in contamination to ancillary physico-chemical datasets. Logistic regression modeling shows that among wells in the most shallow aquifers, arsenic occurrence above the WHO standard (10 µg/L) is best described by a well’s distance to 1) the Mekong River network and 2) delta front, its depth and location within fault-bounded areas of the delta. The shallow model is inadequate to capture contaminant occurrence in deeper aquifers (generally 200-500m) where wells are rarely contaminated except in near-river areas undergoing heavy pumping. Our results are the first to quantify how the probability of arsenic contamination varies in 3D throughout the complex aquifer system in relation to both natural and anthropogenically-mediated factors, and suggest that excessive extraction may be degrading the quality of deep aquifers. Findings may serve as a baseline for managing groundwater use for optimal human welfare and assessing any future changes in arsenic occurrence.