A carbon isotope method to quantify groundwater discharge at the land-sea interface

We present a new method to characterize and quantify groundwater discharge to estuaries and the coastal ocean. Using data from the Pages Creek estuary in the Cape Fear region of southeastern North Carolina, we show that the concentration and carbon isotopic composition (D14C and d13C values) of dissolved inorganic carbon (DIC) can provide a tracer of a single, well-defined component of the surface water-groundwater system in coastal regions— the integrated freshwater discharge to an estuary from confined aquifers. Groundwater from the two shallowest confined aquifers in the Cape Fear region (the Castle Hayne and the Peedee) has DIC D14C values ranging from 2282‰ to 2829‰, significantly lower than the radiocarbon content of surficial (water table) groundwater, rivers and streams, and seawater in the area (D14C 5 238‰ to 197‰). DIC additions from salt marsh decomposition and DIC removal via photosynthesis and gas evasion can influence estuarine DIC concentrations and DIC d13C values. However, none of these processes results in strongly depleted DIC D14C values. Because artesian springs are the only significant low-D14C DIC input to the Pages Creek estuary, flood-ebb 14C budgets provide a direct measure of the fraction of the total freshwater inputs to the Pages Creek estuary that is derived from artesian discharge. With this method, we have observed a striking range in the relative contribution of artesian flow to the Pages Creek estuary freshwater budget. During November 1999 and April 2001 (both periods of low precipitation in southeastern North Carolina), artesian groundwater discharge could account for essentially all of the Pages Creek freshwater inputs. In contrast, during July 2000 (a period of high precipitation in this region), artesian groundwater made a negligible contribution to the creek’s freshwater budget. Fresh groundwater can discharge into the coastal ocean wherever there is a land-sea hydraulic connection with a seaward head gradient (Johannes 1980), and it is widely recognized that groundwater-borne nutrients and pollutants can 1 Corresponding author ([email protected]). 2 Current address: Marine Policy Center, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543.