Importance of Submarine Groundwater Discharge as a Source of Nutrients for the Neuse River Estuary, North Carolina

Seepage rate and chemical composition of groundwater discharge entering the Neuse River Estuary (NRE) were quantified over an annual cycle from July 2005 through June 2006. Lee type seepage meters were deployed at eight locations within the NRE to quantify the amount of submerged groundwater discharge (SGD) entering the system. Sediment porewater nitrate (NO3 −), ammonium (NH4 +), and phosphate (PO4 −3) were also quantified at each of these locations to determine groundwater chemical composition. Seepage rates for the system ranged from 0.004 to 0.035 m3 m−2 d−1. Both the average and median value for the system-wide SGD were 0.01 m3 m−2d−1. There were no significant differences between upstream and downstream seepage rates or between those at the north and south side of the estuary. Seepage rates varied greatly in time and space. Discharging groundwater was NO3 − deplete but highly enriched in NH4 +. Porewater PO4 −3 levels varied but were usually present below Redfield values due to NH4 + enrichment. SGD nutrient loading represented a small part of watershed nitrogen and phosphorus loading, 0.8% and 1.0%, respectively.