Groundwater and Associated Solute Contribution to a Pristine Semi-Arid Estuary Using Resistivity Imaging, Naturally Occurring Radioactive Tracers, and Geochemical Methods.

Abstract:

Geophysical, geochemical, and Naturally Occurring Radioactive Tracers (NORTs) were used in a semi-arid National Estuarine Research Reserve to estimate Submarine Groundwater Discharge (SGD) rates and nutrient fluxes. SGD has been identified as an important source of limiting nutrients to coastal ecosystems, and is conceivably even more critical in a dry climate. Groundwater inflows are rarely considered in estuary mass budgets, therefore sampling stations were chosen over the entire bay system to identify spatial differences in SGD and to estimate direct groundwater contributions. This interdisciplinary approach proved necessary to differentiate between the bay recirculation and terrestrial components of SGD. Resistivity measurements using a towed electrode array were used to identify sampling stations at areas favorable to SGD. While NORTs may provide an estimate of basin-wide groundwater input, these sampling stations ascertain the favorable geologic environment for hydraulic interconnectivity between surface water and groundwater in the system. A sampling suite (including metals and nutrients) of surface, bottom, and pore waters during winter and summer months display spatio-temporal changes in nutrient fluxes. This study reinforces the understanding of the necessity to use multiple methods to accurately quantify SGD and associated solute fluxes and advances the knowledge of coastal groundwater interaction in semi-arid environments.