Understanding surface water–groundwater interaction, submarine groundwater discharge, and associated nutrient loading in a small tropical island watershed

Abstract

Submarine groundwater discharge (SGD) is recognized as an important nutrient delivery mechanism in coastal ecosystems. However, water quality management in these settings is typically focused on surface waters, often ignoring SGD and nearshore groundwater-surface water interaction. In this study, we integrate a comprehensive radionuclide tracer based field investigation with watershed modeling to examine groundwater – surface water partitioning and to quantify nutrient loading from fresh SGD and streamflow in a small embayment located in American Samoa. Measurements included streamflow, SGD rate, and environmental tracers, including 222Rn concentrations, nutrient levels, and nitrogen isotope values in groundwater and surface water samples. We then used the Soil and Water Assessment Tool (SWAT) to validate measured baseflow and SGD rates, and also to estimate storm flows, which were not measured in the field. Field results showed SGD was a significant delivery mechanism for coastal nutrient loads, whereas baseflow-nutrient loading from the upper-watershed was minimal during the study period. Seepage run measurements informed a conceptual hydrogeologic model of groundwater, surface water, and coastal water interaction, which we applied in developing the watershed model. The SWAT model simulated flow observations satisfactorily, and indicated baseflow accounts for only 39% of the total annual stream flow with surface runoff and lateral flow (i.e. interflow) making up the rest. By examining water and nutrient exchange between groundwater, surface water, and SGD, this study provides a more complete understanding of the fate and transport of water and nutrients in small-island watersheds where anthropogenic activities potentially threaten the health of coastal ecosystems.