GROUNDWATER CONTRIBUTION TO GLYPHOSATE CONCENTRATIONS IN THE FAGA`ALU STREAM, AMERICAN SAMOA

Pesticide use is a growing concern in today’s society. Pesticides in natural waters may present problems not only to terrestrial and marine flora and fauna, but also to humans. We investigated groundwater flow as a vector of pesticide dispersion in the watershed. Although contaminant transport through aquifers may take several decades, problems may persist for extended periods and eventually reach drinking water supplies, streams, and the marine environment. Our study focused on surface water-groundwater interactions in streams and the coastal ocean, focusing on pesticide fluxes. Groundwater and base flow samples were taken across the island of Tutuila in American Samoa to analyze for the herbicide glyphosate (“Roundup”). In addition, a more detailed stream-study, involving samples along the mountain to ocean gradient was conducted in the developed Faga`alu watershed on the outer Pago Harbor region of the island. Samples were collected at base flow and high stream flow conditions, at coastal springs, and from groundwater wells run by the American Samoa Power Authority (ASPA). Glyphosate concentrations were determined using an ELISA method at the University of Hawai`i. Concentrations were low in general, with the highest being 0.301 ppb. The trace amounts recorded do not indicate a threat to aquatic habitats, as the EPA’s set MCL value for glyphosate is 700.0 ppb. Although the observed concentrations indicate minimal toxicity, the results show presence of pesticide in the groundwater that likely contributes to the stream’s base flow as well as to submarine groundwater discharge (SGD) in the coastal region of the studied watershed.