Nearshore groundwater seepage and geochemical data measured in 2015 at Guinea Creek, Rehoboth Bay, Delaware

Assessment of biogeochemical processes and transformations at the aquifer-estuary interface and measurement of the chemical flux from submarine groundwater discharge (SGD) zones to coastal water bodies are critical for evaluating ecosystem service, geochemical budgets, and eutrophication status. The U.S. Geological Survey and the University of Delaware measured rates of SGD and concentrations of dissolved constituents, including nitrogen species, from recirculating ultrasonic and manual seepage meters, and in nearshore groundwater, on the southern shore of Guinea Creek, an estuarine tributary of Rehoboth Bay, in Millsboro, Delaware, in June, August, and October of 2015. A novel oxygen- and light-regulated seepage meter and a standard seepage meter were deployed as an adjacent pair and sampled at 0.5- to 2-hour intervals across the majority or entirety of single tidal cycles (8 to 12 hours). SGD rate was measured within an attached collection bag (0.5- to 2-hour intervals), or with an ultrasonic flow sensor (1-second intervals). Groundwater samples were collected at multiple depths (5 to 83 centimeters) in shore-perpendicular transects extending across the nearshore subtidal SGD zone. Constituents and other parameters measured in seepage meters and groundwater included: dissolved oxygen, salinity, pH, oxidation/reduction potential, temperature, nitrate, ammonium, phosphate, dissolved organic and inorganic carbon, stable isotopic ratios of carbon species, trace elements, and alkalinity. These data can be used to evaluate biogeochemical conditions and extent of chemical transformation in the upper coastal aquifer and surface sediments and to calculate fluxes of nitrogen and other constituents carried by SGD across the aquifer-estuary interface.