GROUNDWATER-STREAM WATER INTERACTION, SUBMARINE GROUNDWATER DISCHARGE, AND QUANTIFICATION OF ASSOCIATED NUTRIENT LOADING IN FAGAALU WATERSHED, AMERICAN SAMOA

Coastal ecosystems in the Territory of American Samoa contain some of the most pristine and diverse reef communities in the U.S.A. Fagaalu Bay, a small arm of Pago Pago Harbor on the island of Tutuila, has been designated as a federal priority watershed due to degraded reef health and reduction of stream water quality from anthropogenic pollution sources. Previous studies have implicated Fagaalu Stream as a delivery mechanism for sediment and nutrient pollution. However, the degree of nutrient delivery to the coast via groundwater, as well as our understanding of groundwater-stream interactions remains poorly constrained. To address these knowledge gaps, this study integrates field measurements from a two-week long sampling campaign with the widely-used watershed model SWAT to estimate and validate coastal nutrient loading from both stream and groundwater sources. Field measurements of ²²²Rn inventories, water quality, and streamflow were used to quantify Submarine Groundwater Discharge (SGD) and to assess exchange between the basal aquifer and the stream. Geochemical results indicated that over 30% of the stream’s baseflow coastal-nutrient load was derived from basal-groundwater discharge, while seepage run data suggested that losing reaches in upper sections of the stream contribute water to the basal-groundwater lens. Results also clearly indicate that the role of SGD as a coastal nutrient-delivery mechanism cannot be ignored. At the time of measurement, coastal nutrient loading from groundwater sources was estimated to be over two times that of baseflow-stage stream derived nutrient loading. Additionally, the SWAT model was used to simulate water and nutrient fluxes throughout the watershed, and three of the region’s most prevalent anthropogenic nutrient sources - on-site wastewater systems, small scale livestock operations termed piggeries, and agricultural areas were incorporated. The model provides an estimate of the impact that each type of land-use has on Fagaalu's reef, and the study as a whole offers information that may be useful for future coastal resource management and land-use planning.