Ground Water Modeling of an Arsenic Contaminated Sandy Aquifer with Response to Transient River Level, Mekong Delta, Cambodia

Arsenic contaminated groundwater on the deltaic floodplains of Asia adversely affects 10\u27s of millions of people. While there is broad consensus that the deltaic sediments are the source of the arsenic, the mechanism(s) leading to release and transport within the aquifer remain uncertain. Based on work at a field site on the Mekong Delta in Cambodia, Polizzotto (2007) proposed a model of arsenic liberation wherein arsenic is released from near-surface sediments underlying flood-plain wetlands by seasonal water level changes and transported downward to the aquifer, ultimately discharging to the river. The objective of this thesis is to evaluate the conceptual model of groundwater flow using numerical modeling. Flow modeling was undertaken using MODFLOW and employing seasonally transient boundary conditions for wetlands and the Mekong River. Simplifying assumptions include an otherwise homogeneous and isotropic two layer (clay aquitard and sand aquifer) flow field and two-dimensional flow in a vertical plane perpendicular to the river. The model was calibrated by adjusting the hydraulic conductivity values of the clay and sand layers (within the range of field measured values) to match the observed transient head behavior along a transect of observation wells perpendicular to the river. Best fit hydraulic conductivity values for the clay and sand were 10-6 cm sec-1 and 3.5x10-2 cm sec-1 respectively. These results were further validated by comparison of model and observed head values along additional transects and observed head changes produced by daily tidal fluctuations. MODPATH calculated travel times from wetlands to the river ranged from 300 years proximate to the river to 600 years distal to river. These results indicate that the conceptual hydrologic model proposed by Polizzotto is robust, that arsenic contamination predates human influence, and that land use changes altering the hydrologic system will influence arsenic release and transport