AN ISOTOPIC AND MICROBIOLOGICAL MULTI-TRACER APPROACH TO ASSESSING RECHARGE MECHANISMS IN SURFACE WATER AFFECTED WELLS ON TUTUILA, AMERICAN SAMOA

On Tutuila Island, in the Territory of American Samoa, the municipal water supply system is currently unable to provide potable drinking water to the island’s growing population. In 2009, a boil-water advisory was issued throughout much of Tutuila’s water service area due to elevated turbidity and Escherichia coli detections in supply wells from the island’s most productive aquifers located below the Tafuna-Leone Plain. The affected wells are designated by the U.S. Environmental Protection Agency as groundwater under the direct influence (GUDI) of surface water, and federal drinking water quality standards require a cost-prohibitive level of treatment for their water to be potable. Although it is clear that surface water is infiltrating certain wells on Tutuila, the mechanism of this rapid recharge remains unknown. This study’s objective is to develop a better understanding of recharge mechanisms and timescales by collecting and analyzing data from geochemical and isotopic tracers, physical aquifer parameters, and bacteriological indicators. Methods include assessing temporal variation in δ²H and δ¹⁸O values in precipitation and groundwater, using microbial indicators and cosmogenic isotopes to assess aquifer material properties, and assessing physical aquifer response to rain events. Monthly sampling of water isotopes in precipitation has shown distinct trends in seasonality, whereas groundwater samples lack this trend, indicating that significant flushing of these aquifers does not occur on a seasonal basis. Bacteriological sampling shows a significant difference between non-GUDI wells and GUDI wells that consistently test positive for Total Coliform and E. coli, indicating that filtration ability of the GUDI aquifers is low. Results of this study are intended to inform development of Tutuila’s future water resources, which depends on a clear understanding of the hydrogeological dynamics of the Tafuna and Leone Aquifers.