In-Situ, Inc.

Groundwater Flow in Arsenic-Contaminated Aquifer, Mekong Delta, Cambodia

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ABSTRACT
To advance understanding of hydrologies I influences on As concentrations within groundwaters of Southeast Asia, the flow system of an As-rich aquifer on the Mekong Delta in Cambodia where flow patterns have not been disturbed by irrigation well pumping was examined. Monitoring of water levels in a network of installed wells, extending over a 50knr area, indicates that groundwater flow is dominated by seasonally-variable gradients developed between the river and the inland wetland basins. While the gradient inverts annually, net groundwater flow is from the wetlands to the river. Hydraulic parameters of the aquifer (K^lO^ms-1) and overlying clay aquitard (Kal0_8ms_') were determined using grain size, permeameter and slug test analyses; when coupled with observed gradients, they indicate a net groundwater flow velocity of 0.04-0.4 ma-1 downward through the clay and 1-13 ma ' horizontally within the sand aquifer, producing aquifer residence times on the order 100-1000 a. The results of numerical modeling support this conceptual model of the flow system and. when integrated with observed spatial trends in dissolved As concentrations, reveal that the shallow sediments (upper 2-10 m of fine-grained material) are an important source of As to the underlying aquifer.

1. Introduction
There has been extensive investigation into the geo-chemical processes leading to elevated As in the deltaic groundwaters in Asia and there is emerging consensus that reductive dissolution of Fe oxides is an important mechanism leading to As release to the porewaters of the aquifers in the region (Nickson et al.. 2000; Harvey ct al., 2002; McArthur ct al., 2004; Islam et al, 2004). In contrast, there has been less agreement regarding the both the role of hydrology in As release and the importance of surficial processes as a source of As and/or organic C (see Harvey et al 2002 and associated discussion). Many studies pies-ent evidence that As is released from sediments at depth (e.g. McArthur et al.. 2004; Islam et al., 2004). while near-surface processes have also been evoked (Polizzotto et al., 2005; Harvey et al.. 2006). Based on work at a field site in Cambodia, the authors recently proposed that significant As is mobilized from shallow sediments with release and transport strongly influenced by surface hydrologic processes and coupled groundwater flow (Polizzotto et al., 2008). A tight linkage between As behavior and hydrology implies that ongoing and future anthropogenic activities (i.e. land use change, groundwater pumping, sediment excavation) will likely influence As behavior, potentially alleviating or exacerbating this massive health crisis.

Elucidating the role of the hydrologic flow system on release (and transport) has been hampered by the flow complexity of the heavily pumped Bangladeshi aquifers where most research has been focused. Such local flow complexity has precluded the development of a site-specific, spatially distributed flow model that can enable evaluation of changes in gcochcmical parameters, inclusive of As concentrations, along a constrained flow path- Despite this complexity, a number of recent observations indicate that hydrologic conditions influence the resulting distribution of dissolved As within the aquifer (Harvey et al., 2006; Klump et al., 2006; Postma et al., 2007; Stute et al., 2007).

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