Nutrient and Radium Fluxes from Submarine Groundwater Discharge to Port Royal Sound, South Carolina

Abstract

Water exchange between the coastal ocean and underlying aquifers provides a newly-recognized source of materials to the ocean. The flux of materials into the ocean from this process is termed submarine groundwater discharge (SGD). Both surficial and semi-confined aquifers contribute to SGD. Here we use 226Ra and 228Ra to quantify fluxes of SGD to Port Royal Sound, South Carolina, and to separate fluxes from the Upper Floridan (UFA) and surficial aquifers. Higher activity ratios of 228/226Ra in the surficial aquifer make this separation possible. We estimate total SGD fluxes of about 100 m3 s-1 with about 80% being derived from the surficial aquifer. The SGD flux provides about1.8 × 106 mol d-1 of NH4 with almost 90% from the surficial aquifer. Because of strong differences in the concentration of PO4 within the UFA, PO4 fluxes areless certain. Using the UFA wells with low PO4 concentrations yields a flux of 1.2 × 105 mol d-1; using wells with high concentrations yields a flux of 2.0 × 105 mol d-1. In the first case virtually all of the PO4 flux is from the surficial aquifer; in the second case, 40% is from the UFA.