Hydrogeological characterization of southeast coastal plain aquifers and groundwater discharge to Onslow Bay, North Carolina (USA)

The natural geochemical tracer 222Rn was used to quantify submarine groundwater discharge (SGD) onto the continental shelf of Onslow Bay, North Carolina. Water column samples were collected aboard the R/V Cape Hatteras on July 21-26, 2002, and an additional nearshore water column transect and groundwater samples were collected in 2005/2006. Assessment of SGD was accomplished using a mass balance approach that quantified sources and sinks of radon, including benthic flux, exchange across the pycnocline or air-sea interfaces, horizontal transport into and out of the study area, and a water column inventory. Four independent geochemical box models were developed to quantify SGD regionally and with distance from shore. Overall, water column inventories and diffusion rates decreased with distance from shore. Average water column inventories were 8520 ± 1310, 7230 ± 1190, and 760 ± 510 dpm m^-2 for three shore-parallel boxes from nearshore to offshore, and resulted in a regional average of 5800 ± 1050 dpm m^-2 for the Regional box model. Diffusion rates of radon through the sediment-water interface were 0.9 ± 0.2, 0.6 ± 0.1, and 0.4 ± 0.1 dpm m^-2 min^-1 for the near to offshore models, and averaged 0.6 ± 0.1 dpm m^-2 min^-1 for the Regional box model. SGD estimates were calculated using two end-member 222Rn activities for the advecting fluids which allowed a distinction between terrestrially-driven SGD and total SGD. Terrestrially-driven and total SGD estimates averaged 0.2 and 0.7 cm d^-1, respectively. The calculated terrestrially-driven SGD is as important in the delivery of fresh water as riverine sources to Onslow Bay and a significant contributor to the South Atlantic Bight.