COI Funded Project: Groundwater Discharge into North Carolina Estuaries: A Time-Series Comparison of Radiocarbon-Based and Radium-Based Geochemical Estimates

Daniel McCorkle , Geology and Geophysics
Matthew A. Charette, Marine Chemistry and Geochemistry

Proposed Research

We propose a year-long comparison of two independent geochemical methods for studying groundwater discharge into estuaries and the coastal ocean. Our goals are: (1) To develop and test our radiocarbon-based method for quantifying the groundwater fraction of total freshwater inputs into coastal waters, with a particular focus on understanding seasonal/interannual variability in groundwater discharge, and (2) To carry out the first direct comparison of the radiocarbon and radium methods for estimating groundwater discharge into coastal waters. This project will be a key component of Joint Program student Carolyn Gramling's dissertation research.

The time-series aspect of this project (goal 1) is motivated by our ¹⁴C results from two RCRC-funded sampling trips to Pages Creek, a small estuary in the Onslow Bay area of coastal North Carolina. These ¹⁴C data suggest that in November 1999 artesian groundwater discharge accounted for essentially all of the freshwater input to Pages Creek, while in July 2000 surface waters (streams, and perhaps surficial (water table) groundwater) provided all of the freshwater input. We hypothesize that this dramatic shift reflects the annual cycles in precipitation, evapotranspiration, and groundwater withdrawal - November is in the low-rainfall season and yet hydraulic heads are high, while July is in the high-rainfall season and hydraulic heads are low. The proposed work will test this hypothesis, linking tracer-estimated discharge variations to hydrologic forcing, and enabling us to assess the annually-integrated importance of groundwater discharge.

The second goal (the ¹⁴C-radium method comparison) is a high priority for both PIs, and for the growing community of scientists studying groundwater - seawater - surface water interactions in coastal environments. Geochemical methods are in many respects the most promising way to estimate the cumulative impact of groundwater discharge from countless small seeps and springs along a given length of coast. However the carbon and radium isotopic methods are each influenced by different aspects of estuarine geochemistry. We expect that the proposed comparison will provide valuable insight into the strengths, weaknesses, and applicability of each method, and at the same time will yield a more-complete, more compelling overall picture of coastal groundwater discharge and groundwater — seawater interactions.

Project Funded: 2001