Article pubs.acs.org/est
Arsenic in the Multi-aquifer System of the Mekong Delta, Vietnam: Analysis of Large-Scale Spatial Trends and Controlling Factors Laura E. Erban, Steven M. Gorelick,* and Scott Fendorf Department of Environmental Earth System Science, Stanford University, Stanford, California 94305, United States S Supporting Information *
ABSTRACT: Groundwater exploitation is rising in the Mekong Delta, Vietnam, potentially exacerbating arsenic contamination from natural sources. We investigate trends and controls on contamination patterns throughout the Delta’s multi-aquifer system as observed in a spatially exhaustive data set of arsenic measured in >40,000 wells, 10.5% of which exceed the WHO drinking water standard for arsenic (10 μg/L). We relate strong trends in the distribution of contamination among well samples to explanatory variables derived from 3D ancillary physicochemical data sets using logistic regression models. Parsimonious models describe much of the observed variability in arsenic occurrence, which differs considerably between subsets of wells tapping shallow versus deeper aquifer groups. In the shallowest Holocene-Pleistocene aquifers, arsenic occurrence is best described by distance to the Mekong river channels and delta front, depth, and location within fault-bounded zones of the region. The same model, however, fails to explain observations in the deeper group of Pliocene-Miocene aquifers. Among these deeper units, arsenic occurrence is rare except among older wells in near-river, heavily pumped areas. Our analysis is the first to examine both natural and anthropogenically mediated contributions to the distribution of arsenic throughout the Mekong Delta’s multi-aquifer system, with implications for management of similarly affected basins throughout Southeast Asia.
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INTRODUCTION In the Mekong Delta, naturally occurring groundwater arsenic poses serious health risks to a large and growing population. The Delta, shared by Cambodia and Vietnam, is home to over 20 million people, many of whom extract groundwater for a variety of domestic, agricultural, and industrial purposes. In the densely settled floodplain, arsenic concentrations in wells have exceeded 100 times the World Health Organization’s (WHO) drinking water standard (10 μg/L).1−3 The dissolved arsenic burden tends to be most significant, in terms of both concentration and proportion of affected wells, among wells that are proximate to the main river network and screened in the most shallow, Holocene-Pleistocene age aquifers (typically 10 μg/L). Considering all wells tapping aquifers that predate the Pleistocene, 20.2% are contaminated, which is nearly twice the proportion in the Delta overall. Two other contamination trends, less obvious than those related to depth and river distance, are nonetheless apparent. First, an inverse relationship between arsenic concentrations and TDS (max 20 g/L) is observed in the data set as a whole (see SI Figure S8). Second, in a large cluster of nearly 1,400 deep wells, more than half of which are contaminated, well age appears to be a distinguishing feature. In fact, this deep cluster of contaminated wells (see SI Figure S6) is primarily responsible for the rise in mean concentration and occurrence probability in the group of Pliocene-Miocene aquifers (Figure 3). In addition to the advanced median age of wells in the deep cluster, which is ∼10 years older compared to nearby (
