Ground water discharge and nitrate flux to the Gulf of Mexico

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Authors:Dowling, Carolyn B.; Poreda, Robert J.; Hunt, Andrew G.; Carey, Anne E.
Author Affiliations:Primary:
Ohio State University, Byrd Research Center, Rochester, NY, United States
Other:
University of Rochester, United States
U. S. Geological Survey, United States
Volume Title:Ground Water
Source:Ground Water, 42(3), p.401-417. Publisher: National Ground Water Association, Westerville, OH, United States. ISSN: 0017-467X
Publication Date:2004
Note:In English. 86 refs.; illus., incl. 3 tables, sketch map
Summary:Ground water samples (37 to 186 m depth) from Baldwin County, Alabama, are used to define the hydrogeology of Gulf coastal aquifers and calculate the subsurface discharge of nutrients to the Gulf of Mexico. The ground water flow and nitrate flux have been determined by linking ground water concentrations to 3H/3He and 4He age dates. The middle aquifer (A2) is an active flow system characterized by postnuclear tritium levels, moderate vertical velocities, and high nitrate concentrations. Ground water discharge could be an unaccounted source for nutrients in the coastal oceans. The aquifers annually discharge 1.1±0.01×108 moles of nitrate to the Gulf of Mexico, or 50% and 0.8% of the annual contributions from the Mobile-Alabama River System and the Mississippi River System, respectively. In southern Baldwin County, south of Loxley, increasing reliance on ground water in the deeper A3 aquifer requires accurate estimates of safe ground water withdrawal. This aquifer, partially confined by Pliocene clay above and Pensacola Clay below, is tritium dead and contains elevated 4He concentrations with no nitrate and estimated ground water ages from 100 to 7000 years. The isotopic composition and concentration of natural gas diffusing from the Pensacola Clay into the A3 aquifer aids in defining the deep ground water discharge. The highest 4He and CH4 concentrations are found only in the deepest sample (Gulf State Park), indicating that ground water flow into the Gulf of Mexico suppresses the natural gas plume. Using the shape of the CH4-He plume and the accumulation of 4He rate (2.2±0.8 µcc/kg/1000 years), we estimate the natural submarine discharge and the replenishment rate for the A3 aquifer.
Subjects:Absolute age; Aliphatic hydrocarbons; Alkanes; Aquifers; Cenozoic; Chemical composition; Coastal aquifers; Coastal environment; Contaminant plumes; Discharge; Environmental analysis; Geochemistry; Global Positioning System; Ground water; He-3; He/He; Helium; Holocene; Hydrocarbons; Hydrochemistry; Hydrology; ICP mass spectra; Isotopes; Mass spectra; Methane; Movement; Neogene; Nitrates; Noble gases; Organic compounds; Pliocene; Pollutants; Pollution; Quaternary; Sampling; Sediments; Spectra; Stable isotopes; Tertiary; Alabama; Alabama River; Atlantic Ocean; Baldwin County Alabama; Gulf of Mexico; Mississippi River; North Atlantic; United States; Meyer gas sand; Pensacola Clay
Coordinates:N301300 N311800 W0872200 W0880200
Record ID:2005022935
Copyright Information:GeoRef, Copyright 2018 American Geosciences Institute.
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