Source Partitioning of Anthropogenic Groundwater Nitrogen in a Mixed-Use Landscape, Tutuila, America Samoa

Abstract

On Tutuila, the main island in the territory of American Samoa, nearly all of the island's residents rely on groundwater resources. However, the municipal water system has been subjected to a boil water notice since 2009 due the influence of surface based contamination of groundwater in the island's most developed region, the Tafuna-Leone Plain. The American Samoa EPA has identified three predominant anthropogenic non-point pollution sources of concern: On-Site Disposal Systems (OSDS), agricultural chemicals, and livestock (pig) manure. Development has placed all of these sources within close proximity to many drinking water wells, and water quality analyses indicate that elevated levels of total dissolved groundwater nitrogen (TN) are linked to population density. This suggests the potential to use TN as a tracer of groundwater contamination from the aforementioned activities, which may act as sources of more harmful pollutants. In this study, land-use and hydrological data are integrated with water quality analysis in an N-loading and transport modeling framework for the purpose of quantifying and partitioning the water quality impacts from human land use in the Tafuna-Leone Plain. The integrated framework consists of a numerical groundwater flow model coupled with a GIS based N-loading model and a multi-species contaminant transport model. Nitrogen from each source is modeled as a separate species in order to trace the impact from individual areas. The results are validated with nutrient and isotopic data, and also by assessing historical water quality records and land-use changes over time. Results indicate that OSDS contribute significantly more TN to the aquifer than other sources, and thus should be prioritized in future water quality management efforts.