INVESTIGATING NUTRIENT RELEASE VIA SUBMARINE GROUNDWATER DISCHARGE ALONG A DEVELOPED ESTUARY SHORELINE

Submarine groundwater discharge (SGD) has recently been recognized as a diffuse and nearly ubiquitous source of water and dissolved nutrients to the coastal environment along continental margins. Previous studies have highlighted the influence of SGD on the environmental condition of coastal waters throughout the world. Here we focus on the quantity and quality of SGD from a developed estuarine shoreline (i.e. Mobile Bay, AL) to investigate effects of mixed residential development on coastal water quality. Various techniques including a hydrologic balance, Darcy’s law calculations, seepage meter measurements, surface water salinity mapping, radon-222 inventory estimations, and piezo-manometer measurements were used to quantify the total and freshwater components of SGD under differing hydrometeorological scenarios. Samples of groundwater from shallow onshore wells, SGD from seepage meters, and Bay water were analyzed for nutrients (i.e. ammonia, nitrate, total nitrogen, and reactive phosphorus) throughout the year in order to determine nutrient sources and fluxes to the Bay. Our results indicate that SGD in Mobile Bay and associated nutrient fluxes vary over both temporal (e.g. tidal and seasonal) and spatial scales. Although daily SGD per meter of shoreline (0.1 – 6.1 m³ per day per m shoreline) was relatively insignificant throughout the year relative to the volume of Mobile Bay, the shoreline distance considered for our study is approximately 13 km, thus yielding daily SGD approaching the reported median discharge of the nearest monitored river system (i.e. Fowl River). Concentrations of nutrients in SGD were consistently enriched an order of magnitude over surface waters, further substantiating the role of SGD in affecting the nutrient balance of coastal waters. Despite the presence of onsite wastewater treatment systems throughout the study area, concentrations of nutrients in shallow groundwater samples collected from wells were consistently lower than SGD samples collected from seepage meters thereby indicating additional nutrient sources within the subterranean estuary. These results demonstrate the importance of SGD as a source of nutrients to Mobile Bay and highlight the application of both hydrogeologic and oceanographic techniques to study the effects of SGD on coastal water quality.