An estimated 100 million people living in floodplains in Asia are exposed to arsenic in groundwater that is derived from Himalayan sediments. Arsenic is a toxin linked to cancer and a variety of other serious ailments through direct ingestion of contaminated groundwater. Growing exploitation of these contaminated aquifers increases the number of people facing these health risks, and exposes still greater populations to the hazard of consuming agricultural products irrigated with arsenic-contaminated groundwater. Despite widespread awareness of the arsenic hazard, understanding of the relationship between groundwater exploitation and arsenic contamination remains limited, particularly in deep aquifers, which are increasingly providing a larger portion of total pumped groundwater.
In this work, we focus on the Mekong Delta, where we have obtained a comprehensive, unique, unanalyzed dataset consisting of >42,000 dissolved arsenic measurements from southern Vietnam showing widespread contamination (>1000 sq km) in deep aquifers (>200m) that are used extensively for water supply. One hypothesis is that deep pumping has induced shallow dissolved arsenic or arsenic-mobilizing solutes to move deeper. However, preliminary analysis does not support this mechanism in the Mekong Delta given the observed widespread deep arsenic contamination in the presence of thick clay deposits that serve as relative vertical flow barriers. We hypothesize a previously unrecognized deep arsenic source mechanism in which water containing arsenic is expelled from storage in clays that compact when overlying and underlying deep aquifers are exploited. This work combines spatial statistical modeling of groundwater arsenic observations, 3D aquifer flow and compaction simulation, and remote measurement of land subsidence using satellite radar imagery (InSAR). Our goal is to explore the notion that deep groundwater arsenic may be due to the release of pore-waters containing arsenic trapped in clay beds deposited millions of years ago.
This research has important implications to science and society. First, our formerly unrecognized contamination mechanism may be fundamental to understanding arsenic occurrence in aquifer systems and the associated health risks of deep groundwater exploitation. Our investigation will have implications for water resources development and human health in the arsenic-affected basins of Southeast Asia where some regions of planned deep aquifer exploitation may unknowingly expose people to deep-source arsenic. This work has consequences for analogous arsenic-affected aquifer systems in sedimentary basins around the world containing interbedded compressible clays that may harbor arsenic and other contaminants. Second, in terms of methods, the link between subsidence and arsenic release has significant potential as a reconnaissance tool, particularly in underdeveloped regions where the impacts of excessive pumping on land subsidence have not been recognized. The use of satellite radar to detect land deformation can serve as a means to identify areas where clay compaction and consequent arsenic release may be occurring.
