A carbon isotope method to quantify groundwater discharge at the land-sea interface
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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 (Delta(14)C and delta(13)C 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 Delta(14)C values ranging from -282parts per thousand to -829parts per thousand, significantly lower than the radiocarbon content of surficial (water table) groundwater, rivers and streams, and seawater in the area (Delta(14)C = -38parts per thousand to +97parts per thousand). DIC additions from salt marsh decomposition and DIC removal via photosynthesis and gas evasion can influence estuarine DIC concentrations and DIC delta(13)C values. However, none of these processes results in strongly depleted DIC Delta(14)C values. Because artesian springs are the only significant low-Delta(14)C DIC input to the Pages Creek estuary, flood-ebb C-14 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.