Groundwater Influence on Pollution Transport to Nu'uuli Pala Lagoon, American Samoa

Nu'uuli Pala Lagoon, located on the island of Tutuila, American Samoa, is the largest remaining and most threatened wetland in American Samoa. Various non-point pollution sources, such as solid waste, sewage, and animal feeding operations, have been documented in the vicinity of the Pala. In this study, the natural tracers radium (Ra) and radon (Rn) were used to characterize spatial and temporal variability in submarine groundwater discharge (SGD) into the Pala, estimate the residence time of water in the Pala, and quantify SGD. Kayak-based Rn surveys were conducted to map areas of high and low SGD, and samples were also collected from streams and springs flowing into the Pala. Fieldwork was conducted in August 2022 (dry season) and March 2023 (wet season). River discharge (m3/d) was greater in the wet season (18,854 – 34,157 m3/d) than in the dry season (1,780 m3/d). SGD into the Pala was also higher in the wet season (46,754 m3/d) than in the dry season (26,879 m3/d). This study also calculated radon fluxes (Bq/d) and SGD fluxes (m3/d), where the dry season had lower levels (22,925,592 Bq/d and 33,177 m3/d, respectively) than the wet season (33,006,545 Bq/d and 49,589 m3/d, respectively). Concentrations of nutrients (nitrate, ammonia, and phosphate) were measured, and nutrient fluxes into the Pala from rivers and SGD were estimated. Sampling of benthic macroalgae for δ15N in March 2023 found that all primary producer samples had a δ15N value (4.77 - 7.05‰, mean of 5.84‰) higher than fertilizer or atmospheric levels. Therefore, sewage or manure must be contributing some of the nitrogen supporting the Pala’s primary productivity. This study will be useful to local residents and environmental managers as it will allow for better understanding and better mitigation of pollution that threatens the ecological health of the Pala.