Linking surface flooding behavior to groundwater arsenic heterogeneity across scales in Southeast Asia

Abstract:

Geogenic arsenic (As) contamination in groundwater poses a serious health risk for several hundred million people worldwide, especially those living on floodplains in Southeast Asia. An inherent feature of alluvial and deltaic floodplain aquifers in Southeast Asia is that dissolved As concentrations vary markedly over very small lateral (10-100m) and vertical (0-10m) spatial scales. This extreme spatial heterogeneity makes it challenging to identify key variables that effectively capture the principal biogeochemical and hydrologic processes linked to the distribution of As in groundwater. Improved understanding of these key variables is needed to predict site-specific groundwater As levels with enough accuracy to manage this staggering public health crisis. In this study, we reveal that the occurrence and frequency of surface flooding serve as master variables, integrating the coupled biogeochemical, hydrologic, and geomorphic features that regulate As levels in shallow (<75m) alluvial and deltaic floodplain aquifers in Southeast Asia. The magnitude of surface water occurrence and inter-annual recurrence effectively captures floodplain hydrologic conditions linked to the availability of organic matter and other important geochemical reactants, which affect aquifer redox status and drives the liberation of As from sediments. Remotely-sensed measures of flooding occurrence and frequency were used with a Random Forest model to accurately predict groundwater As concentrations spanning three orders of magnitude (~ 1 to 1000 ug/L) as well as the probability that As levels exceed 10, 50, and 100 ug/L in Cambodia, Vietnam, and Bangladesh. We demonstrate that flooding is directly linked to the necessary environmental conditions that regulate As levels within alluvial and deltaic floodplain aquifers of Southeast Asia, and thus serve as principal features of statistical models to predict As concentrations in groundwater at the household level. We anticipate our approach can be reliably applied to similar aquifers systems worldwide to evaluate potential levels of groundwater As contamination and the associated health risks of excessive As exposure with improved accuracy.