Stream Pesticide and Nutrient Loads from Baseflow, Surface Runoff and Sediment Contributions on Tutuila Island, American Samoa
National Institute for Water Resources, Water Resources Research Institute Program
The goal of the project was to determine the contribution of baseflow, surface runoff, and sediments to pollutant fluxes in Faga’alu watershed in Tutuila, American Samoa. The work utilized a watershed model (SWAT) and a groundwater model (MODFLOW) in combination with pesticide, nutrient, and sediment measurements. SWAT simulated suspended sediment fluxes and the model captured observed sediment values across the watershed. The results suggested that a lot of the sediment flux is related to human disturbance (or urbanization) at the lower part of the watershed. In order to optimize restoration efforts and to eliminate sediment sources, hypothetical land-use scenarios were introduced into the model to predict sediment fluxes under various restoration scenarios.
In order to assess the groundwater’s role as a pollution pathway, in situ samples were collected in the watershed. These show comparable levels of glyphosate, DDT, imidacloprid and nitrate as found in other watersheds on the island with anthropogenic influences. We found that stream and stream bank groundwater had comparable levels of pesticides in both gaining and losing reaches of the stream. In addition, pesticides were detected in all coastal springs. Groundwater is therefore a possible pathway for their migration. Groundwater flowpaths of all samples were simulated using a hydrological model. These were then linked to land-use to identify the sources of pesticides. This research provides important information for land and water resource managers (ASPA, AS-EPA) on the distribution and transport of pollutants so that protective measures can be developed for pollution sources and migration in the watershed.