GEOLOGIC CONTROLS ON GROUNDWATER ARSENIC DISTRIBUTION IN THE UPPER MEKONG FLOODPLAIN, KIEN SVAY REGION OF CAMBODIA

Geologic and geochemical investigations of shallow fluvial aquifers in the 180 km˛ study area of Kien Svay district, south of Phnom Penh, Cambodia indicate that composition and age of aquifer sediments control arsenic concentration of groundwater. The sedimentary facies association along the Mekong differs from the larger, sandier fluvio-deltaic environments of Bangladesh in that distinct channel-levee–flood basin sequences are closely associated across the floodplains of the subparallel Mekong and Bassac Rivers. Organic-rich marshes and shallow lakes within the flood basins lie adjacent to levees where most of the villages and arsenic-contaminated water wells are located. Of 27 water wells sampled (20 to 65 m depth), 16 exceed the WHO As drinking water standard (10 µg/L) and 5 produce water containing more than 500 µg/L. Strong correlations of dissolved arsenic with alkalinity and ammonia are consistent with degradation of organic matter as part of the arsenic release mechanism. Excavations as deep as 5m exposed sediment with as much as 11 wt.% Corg interbedded with red paleosols. Juxtaposition of contrasting redox conditions results in arsenic accumulation in peaty layers and gray muds (up to 450 ppm As) and in Fe-stained sands (230 ppm As). High dissolved arsenic likely results from interaction of organic carbon-rich sulfate-poor water from marshes with ferric oxide coated sand. Drillers logs indicate aquifers are more tabular and continuous from 40 to 60m than overlying lensoidal levee-channel complexes of present rivers. Yet the co-occurrence of high arsenic wells with modern facies distribution suggests the modern environment influences arsenic concentration. Possible explanations may be: a) juxtaposition of organic-rich facies against channelized or tabular sand bodies containing significant Fe-hydroxide secondary coatings or b) a hydrologic connection of facies produced by the common practice of lining well casings with coarse sand to the ground surface. An area of consistently low arsenic water draws from erosional remnants of Pleistocene sediment based on surface exposures. The likely irregularity of the buried Holocene-Pleistocene contact further complicates interpretation of aquifer distribution. Although wells in the Pleistocene sediment are low in arsenic, high manganese may limit these aquifers as drinking water supplies.