Seawater recirculation drives groundwater nutrient loading from a developed estuary shoreline with on-site wastewater treatment systems: Mobile Bay, USA

Abstract

Previous studies have highlighted the influence of submarine groundwater discharge (SGD) on the environmental condition of coastal waters worldwide. Here, we focus on the quantity and quality of SGD from a developed estuarine shoreline in Mobile Bay to investigate the effects of residential development, specifically on-site wastewater treatment, on coastal water quality. A variety of techniques including a hydrologic balance, Darcy calculations, seepage meter observations, and coastal water radon-222 surveys were employed to quantify fresh and total components of SGD under varying hydrometeorological scenarios. Samples of groundwater from shallow wells, SGD from seepage meters, and surface water were analyzed for dissolved nutrients throughout the year to determine nutrient sources and loadings to the Bay. Comparisons of SGD yielded by Darcy’s calculations with seepage meter and radon-222 observations revealed that fresh, meteoric groundwater contributes only a minor fraction (~ 7%) of total SGD, thereby suggesting that seawater recirculation is the dominant mechanism driving SGD. Total dissolved nitrogen loading averaged 1960 mmol d−1 per m shoreline for the duration of the study; however, total dissolved nitrogen loading supported by the fresh, meteoric SGD component was only 11.2 mmol d−1 per m shoreline (less than 1% of the total loading), despite the presence of on-site wastewater treatment systems. These results demonstrate the importance of SGD as a source of nutrients to Mobile Bay and suggest that nutrient loading from regeneration and remineralization processes in the subterranean estuaries can exceed that of groundwater nutrient transport from developed watersheds.