Spatial Distribution of Dissolved Radon in the Choptank River and Its Tributaries: Implications for Groundwater Discharge and Nitrate Inputs
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The Choptank River, Chesapeake Bay’s largest eastern-shore tributary, is experiencing increasing nutrient loading and eutrophication. Productivity in the Choptank is predominantly nitrogen-limited, and most nitrogen inputs occur via discharge of high-nitrate groundwater into the river system’s surface waters. However, spatial patterns in the magnitude and quality of groundwater discharge are not well understood. In this study, we surveyed the activity of 222Rn, a natural groundwater tracer, in the Choptank’s main tidal channel, the large tidal tributary Tuckahoe Creek, smaller tidal and non-tidal tributaries around the basin, and groundwater discharging into those tributaries, measuring nitrate and salinity concurrently. 222Rn activities were <100 Bq m−3 in the main tidal channel and 100–700 Bq m−3 in the upper Choptank River and Tuckahoe Creek, while the median Rn activities of fresh tributaries and discharging groundwater were 1,000 and 7,000 Bq m−3, respectively. Nitrate-N concentrations were <0.01 mg L−1 throughout most of the tidal channel, 1.5–3 mg L−1 in the upper reaches, up to 13 mg L−1 in tributary samples, and up to 19.6 mg L−1 in groundwater. Nitrate concentrations in tributary surface water were correlated with Rn activity in three of five sub-watersheds, indicating a groundwater nitrate source. 222Rn and salinity mass balances indicated that Rn-enriched groundwater discharges directly into the Choptank’s tidal waters and suggested that it consists of a mixture of fresh groundwater and brackish re-circulated estuarine water. Further sampling is necessary to constrain the Rn activity and nitrate concentration of discharging groundwater and quantify direct discharge and associated nitrogen inputs.
KeywordsGroundwater discharge Radon Choptank River Chesapeake Bay Nitrate
We thank Adina Paytan and Rick Peterson for the use of their RAD7 radon detectors and for advice about carrying out the study. Micah Ryder helped obtain and analyze geographic data, and David Culver generously provided access to GIS software. Dana Brenner, Nancy Goff, Christina Hill, Ryan Ihnacik, Alanna Lecher, Ginny Leviton, Joe Miklas, and Amelia Snyder assisted with sample collection and analysis. Rick Peterson, Thomas Fisher, and two anonymous reviewers provided comments that improved the quality of this manuscript. Financial support for this research came from a Smithsonian Institution Postdoctoral Fellowship (to K. Knee) and National Science Foundation grants (DEB-0919181 and DEB-0919141 to T. Jordan and T. Fisher).
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