US-Thailand Planning visits: Influence of climate on groundwater interactions with Mekong River: Implications for arsenic concentrations in alluvial aquifers

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Across South and Southeast Asia, aquifers along rivers draining the Himalayas are heavily utilized for dry-season irrigation and domestic water supply. Previous work has documented elevated arsenic (As) concentrations in the deltaic aquifers of these rivers, including the Ganges-Brahmaputra, the Red, and the Mekong. Few studies have been conducted in the alluvial aquifers upgradient of the deltaic aquifers, but those few have documented locally elevated As concentrations. The monsoonal climate of the region imposes shifts in hydraulic gradients between the river and adjacent aquifers. These gradient shifts have the potential to influence the behavior of trace elements in groundwater due to changes in flow magnitude and direction, as well as in redox conditions. This project initiates an international collaboration with investigators at Khon Kaen University, Thailand to collect preliminary hydrologic and geochemical data from an alluvial aquifer along the Mekong River in northeast Thailand. Wells and the river will be sampled during the dry season (November) and the monsoon season (May-June) and samples analyzed for field parameters, major ions, selected trace elements, dissolved organic carbon and stable isotopes. Equipment will be installed in selected wells for continuous monitoring. This data will enable tracking of flow between the alluvial aquifer and the Mekong. The group will develop a conceptual model of alluvial aquifer-river interactions, which will form the basis of a proposal to the NSF Hydrologic Sciences program for a longer-term and more spatially intensive study. Results will be shared with local residents, educators and government agencies in Thailand, and with researchers through CUAHSI?s Hydrologic Information Systems (HIS), and will provide a baseline for the aquifer prior to dam construction projects proposed on the Mekong. The conceptual model will offer a template for other alluvial aquifers in the region, such as the Irrawaddy valley of Myanmar, where dams are being built or planned. Non-technical abstract:Elevated arsenic (As) in groundwater across South and Southeast Asia is a critical public health problem. The behavior of As and other trace elements in groundwater is tied to hydrologic and geochemical processes, which are in turn influenced by climate. Another possible forcing is the construction of dams, which dramatically changes the connections between the river and the adjacent aquifers. Our goal is to collect a preliminary dataset on the geochemistry of groundwater in an aquifer adjacent to the Mekong River in northeast Thailand during two hydrologic seasons: the dry season (November) and the monsoon season (May-June). The results will be used to develop a conceptual model of how climate influences aquifer-river interactions, and if those interactions affect As concentrations in the aquifer. These data will provide a ?baseline? of water quality in the aquifer prior to construction of proposed dams on the Lower Mekong.