Groundwater exploitation and arsenic occurrence in the Mekong Delta aquifer system [electronic resource]
Keywords: Environmental Earth System Science.

The Mekong Delta is home to over 20 million people living in Cambodia and Vietnam, many of whom rely on groundwater contaminated with arsenic. Arsenic occurs naturally there, as in many other river basins across South and Southeast Asia, but the extent and spread of contamination is also mediated by human activities. This dissertation is concerned with the consequences of intensive groundwater exploitation for arsenic contamination in the Vietnamese portion of the Mekong Delta, with implications for other similarly-affected regions. The current distribution of arsenic throughout the Delta's multi-aquifer system is strongly related to large-scale variability in physico-chemical conditions. This work relied upon measurements from more than 40,000 wells extracted from a nationwide arsenic survey and ancillary datasets describing natural properties of the subsurface. Parsimonious sets of explanatory variables describe the probability of well contamination above the WHO drinking water standard (10µg/L). Among shallow aquifers, the exceedance probability is largely predicted by well depth, distance to channels of the Mekong river, distance to the Delta front, and location within different depositional zones within the broader Delta region. These findings are consistent with known, natural contamination generating and remediating processes. Among deeper aquifers, however, widespread arsenic contamination in a cluster of nearly 900 wells in an area > 1000 km2 observed in the Mekong Delta is unprecedented based on surveys in similar Asian systems, and unexplained by presently-accepted mechanisms of arsenic contamination. Further investigation of the deep well cluster suggests a human influence on this deep contamination. It cannot, however, be explained by pumping-induced vertical transport of arsenic or arsenic-mobilizing solutes, the leading explanation until now. A shallow source is contraindicated by the facts that 1) the group of shallow wells overlying the area of deep contamination has a lower mean arsenic concentration and 2) downward groundwater velocities through the complex stratigraphy, which is marked by thick sequences of confining clays, are extremely low. Yet, pumping is implicated since deep contaminated wells are considerably older than surrounding clean wells in the same aquifers and are found in a region of the aquifer system where pumping rates are elevated. Hydromechanical modeling suggests that these pumping rates induce compaction of interbedded clay layers on the order of 1-3 cm/yr. This estimate is confirmed by direct measurements of land subsidence, the consequence of subsurface compaction, made using satellite-based Interferometric Synthetic Aperture Radar (InSAR). The principal finding of this component of the work is that there is a previously unrecognized mechanism of arsenic contamination: water containing arsenic or arsenic-mobilizing solutes expelled from compacting clays to adjacent aquifers is responsible for contaminating deep wells. Pumping occurs throughout the Delta, but is uneven over space and among different aquifers. The distribution of wells and their influence on hydraulic heads is understood, owing to Delta-wide monitoring of all major exploited aquifers. However, the effects of over-exploitation on land subsidence, and potentially arsenic contamination related to it, remain unknown. A final piece of this work extends InSAR analysis to the entire Delta in Vietnam and parts of Cambodia, estimating subsidence rates throughout the region. Subsidence is then related to spatially-explicit effects of groundwater overdraft and variability in compaction of the aquifer-confining bed system. A clear correlation exists between InSAR-based subsidence rates and those determined using a hydrogeologically-based approach. In addition to providing the first estimates of land subsidence that can be used independently for planning and management purposes, these results could be further used as a reconnaissance tool for detecting regions that may potentially be at risk of compaction-induced arsenic contamination. In summary, this work comprehensively describes the current burden of groundwater arsenic in the Mekong Delta aquifer system, and serves as a baseline for assessing future changes. In the process, it identified a regional-scale human-induced impact on arsenic mobilization and consequent contamination of groundwater. The new contamination mechanism proposed here, based on observations from the Mekong Delta, may also be relevant in similarly-arsenic affected systems throughout South and Southeast Asia. The potential for exacerbation of the arsenic crisis via pumping-induced compaction and other human-mediated mechanisms must be considered as these groundwater systems continue to be developed.