Estimation of groundwater and nutrient fluxes to the Neuse River estuary, North Carolina

Abstract

A study was conducted between April 2004 and September 2005 to estimate ground-water and nutrient discharge to the Neuse River estuary in North Carolina. Largest ground-water fluxes were observed to occur generally within 20 m of the shoreline. Ground-water flux estimates based on seepage-meter measurements ranged from 2.86 x 108 to 4.33 x 108 m3 annually and are comparable to estimates made using radon, a simple water-budget method, and estimates derived by using Darcy’s Law and previously published general aquifer characteristics of the area. The lower ground-water flux estimate (equal to about 9 m3 s–1), which assumed the narrowest ground-water discharge zone (20 m) of three zone widths selected for an area west of New Bern, North Carolina, most closely agrees with ground-water flux estimates made using radon (3–9 m3 s–1) and Darcy’s Law (about 9 m3 s–1). A ground-water flux of 9 m3 s–1 is about 40% of the surface-water flow to the Neuse River estuary between Streets Ferry and the mouth of the estuary and about 7% of the surface-water inflow from areas upstream. Estimates of annual nitrogen (333 tonnes) and phosphorus (66 tonnes) fluxes from ground water to the estuary, based on this analysis, are less than 6% of the nitrogen and phosphorus inputs derived from all sources (excluding oceanic inputs), and approximately 8% of the nitrogen and 17% of the phosphorus annual inputs from surface-water inflow to the Neuse River estuary assuming a mean annual precipitation of 1.27 m. We provide quantitative evidence, derived from three methods, that the contribution of water and nutrients from ground-water discharge to the NRE is relatively minor, particularly compared with upstream sources of water and nutrients and with bottom sediment sources of nutrients. Locally high ground-water discharges in the NRE do occur, however, and could help explain the occurrence of localized phytoplankton blooms, submerged aquatic vegetation, or fish kills.