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Quantifying Geologic and Temporal Controls on Water and Chemical Exchange between Groundwater and Surface Water in Coastal Estuarine Systems

Conceptual models for submarine groundwater discharge
Project Status: 
Project Contact(s): 
A. Scott Andres

H. Michael (UD Dept. of Geological Sciences), J. Bratton (U.S. Geological Survey), D. Krantz (Univ. of Toledo), L. Konikow (U.S. Geological Survey), K. Kroger (U.S. Geological Survey), and A.S. Andres
Student investigators: C. Russoniello, C. Fernandez, K. Myers, A. Musetto
Funded by National Science Foundation

Eutrophication is one of the most common and most severe problems facing coastal bays in
populated and agricultural areas. Unnaturally high quantities of nutrients enter fresh groundwater and surface water as a result of human activities. These nutrients contribute to the overpopulation of phytoplankton and macroalgae in coastal surface waters, which results in deterioration of water quality and animal habitat. This is a particular problem in the Delmarva region, where poultry farms, agricultural activity, and growing human populations have contributed to rapidly declining populations of blue crabs, striped bass, and many other species which live and breed in estuarine waters. The economic value of these species has, in part, prompted political action and efforts to manage nutrient inputs to groundwater and surface water, the primary pathways for nutrient loading to coastal waters. Despite significant reductions, coastal water quality has largely remained poor. A better understanding of the processes that moderate nutrient loading to coastal waters, particularly via groundwater, which is much more difficult to monitor than surface water inputs, is essential for improved management methods that will result in healthy coastal ecosystems. This project will improve understanding of where nutrients are coming from and how loading may be reduced, and may aid in identification of activities that exacerbate negative impacts.

To test our hypotheses, we will study the geology, hydrology, and geochemistry of the coastal groundwater flow system at Indian River Bay, Delaware. The overarching aim of the project is to characterize the controls of geologic heterogeneity and temporally variable hydraulic forcing on groundwater flow and fluid and chemical fluxes between aquifers and coastal surface water bodies. We seek to connect these controls to physical flow processes, to connect physical processes (flow rates, patterns, heterogeneity, and mixing) to geochemical transformations, and to combine these in order to better estimate fluxes of fluid and individual chemical species. The information obtained, though focused on a specific site, will improve the general understanding of freshwater-saltwater interaction and geochemistry of the subsurface, with implications for coastal systems worldwide.

Project staff have now drilled, logged, installed, and sampled 4 standard and 8 7-channel wells to depths of nearly 20 m below bottom in water depths up to 1.3 m and distances up to 225 m from the shore of Indian River Bay. CTD loggers have been installed in 2 standard wells.

Downhole geophysical logs and well samples have documented the presence of a vertical salinity profile that crosses two salinity interfaces: saline (30-45 mS) to fresh (1-3 mS) at depths of 2-6 m below bottom and fresh to saline at depths of 9 to 13 m below bottom (see log at right).

Physical and chemical monitoring in an additional 14 wells installed onshore are being used to characterize the flow field and geochemistry of the freshwater system. This work is integrated with broader project efforts to characterize SGD that include experiments using marine and land-based geophysics, seepage meters, pore water samplers, geochemical analysis, and numerical simulation.

Lithologic, geophysical, and water quality data were interpreted to develop a conceptual model of the fresh water-salt water interface. Fresh water flow is confined beneath low-permeability sediments offshore, creating a plume of fresh water below the bay bottom.

Two publications from the project have been released:

Russoniello, C.J., C. Fernandez, J.F. Bratton, J.F. Banaszak, D.E. Krantz, A.S. Andres, L.F. Konikow, and H.A. Michael, 2013, Geologic effects on groundwater salinity and discharge into an estuary: Journal of Hydrology, v. 498, p. 1-12.

Russoniello, C.J., L.F. Konikow, K.D. Kroeger, C.F. Fernandez, A.S. Andres, H.A. Michael, 2016, Hydrogeologic controls on groundwater discharge and nitrogen loads in a coastal watershed: Journal of Hydrology,

A DGS publication is awaiting the Director's approval for release (August 2015).

Andres, A. S., Michael, H. A., Russoniello, C.J., Fernandez, C., He, C., and Madsen, J. A., in review, Investigation of submarine groundwater discharge at Holts Landing State Park, Delaware: Hydrogeologic framework, groundwater level and salinity observations: Delaware Geological Survey Report of Investigations.