Tidal and Groundwater Fluxes to a Shallow, Microtidal Estuary: Constraining Inputs Through Field Observations and Hydrodynamic Modeling
Increased nutrient loading to estuaries has led to eutrophication, degraded water quality, and ecological transformations. Quantifying nutrient loads in systems with significant groundwater input can be difficult due to the challenge of measuring groundwater fluxes. We quantified tidal and freshwater fluxes over an 8-week period at the entrance of West Falmouth Harbor, Massachusetts, a eutrophic, groundwater-fed estuary. Fluxes were estimated from velocity and salinity measurements and a total exchange flow (TEF) methodology. Intermittent cross-sectional measurements of velocity and salinity were used to convert point measurements to cross-sectionally averaged values over the entire deployment (index relationships). The estimated mean freshwater flux (0.19 m3/s) for the 8-week period was mainly due to groundwater input (0.21 m3/s) with contributions from precipitation to the estuary surface (0.026 m3/s) and removal by evaporation (0.048 m3/s). Spring–neap variations in freshwater export that appeared in shorter-term averages were mostly artifacts of the index relationships. Hydrodynamic modeling with steady groundwater input demonstrated that while the TEF methodology resolves the freshwater flux signal, calibration of the index–salinity relationships during spring tide conditions only was responsible for most of the spring–neap signal. The mean freshwater flux over the entire period estimated from the combination of the index-velocity, index–salinity, and TEF calculations were consistent with the model, suggesting that this methodology is a reliable way of estimating freshwater fluxes in the estuary over timescales greater than the spring–neap cycle. Combining this type of field campaign with hydrodynamic modeling provides guidance for estimating both magnitude of groundwater input and estuarine storage of freshwater and sets the stage for robust estimation of the nutrient load in groundwater.
KeywordsEstuarine hydrodynamics Coastal groundwater discharge Total exchange flow Estuarine modeling Index-velocity method
Funding was provided by the USGS Coastal and Marine Geology Program and by National Science Foundation Award #0420575 from the Biocomplexity/Coupled Biogeochemical Cycles Program. Access to private property was granted by Alan Rottenberg, Michael Jackson, and Jonathan Harley. Jonathan Borden, Jennifer Thomas, Lane Boyer, and Alex Nunez performed tidal-cycle surveys with support from Marinna Martini and Christine Sabens. Rocky Geyer, Kevin Kroeger, David Ralston, Christopher Sherwood, Richard Signell, John Warner, and two anonymous reviewers provided feedback on this study and/or manuscript. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
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