ESTABLISHING GEOLOGIC CONTROLS ON ARSENIC CONTAMINATION OF GROUNDWATER: KANDAL PROVINCE, CAMBODIA

The spatial variability of groundwater arsenic in deltaic environments makes it difficult to predict the occurrence of dangerous concentrations based on limited well sampling alone. The ability to make such predictions is of high value because it would aid in the placement of new wells, and help to identify populations vulnerable to exposure. Recent research has shown that arsenic concentrations in wells are potentially controlled by surface characteristics such as sediment type and composition as well as the geomorphology of the area. We undertook a field study in the Kandal Province of Cambodia to examine these relationships. Our study focuses on two field areas, each of which contain both As-contaminated and As-free aquifers. Along transects in these regions, we took shallow cores (2-5 m), well samples and conducted Ground Penetrating Radar surveys. We also drilled two wells to 40m depth. By combining an analysis of the surface soil and sediment and deeper sediments captured during well drilling with an examination of remotely sensed data, we relate the fluvial geomorphology and sedimentology of the field sites to the geochemistry of the aquifers, and thus the groundwater arsenic levels. We find that groundwater arsenic is primarily associated with aquifers of large paleochannels where the combination of groundwater recharge and active reduction is stimulated. Using this relationship and radar data to define the boundaries of paleochannels, we can more accurately identify regions with As contamination in the underlying groundwater.