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Ground Water Discharge and Nitrate Flux to the Gulf of Mexico
Author(s) -
Dowling Carolyn B.,
Poreda Robert J.,
Hunt Andrew G.,
Carey Anne E.
Publication year - 2004
Publication title -
groundwater
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.84
H-Index - 94
eISSN - 1745-6584
pISSN - 0017-467X
DOI - 10.1111/j.1745-6584.2004.tb02688.x
Subject(s) - nitrate , environmental science , flux (metallurgy) , oceanography , hydrology (agriculture) , geology , chemistry , geotechnical engineering , organic chemistry
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 3 H/ 3 He and 4 He 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 × 10 8 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 4 He 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 4 He and CH 4 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 CH 4 ‐He plume and the accumulation of 4 He rate (2.2 ± 0.8 μcc/kg/1000 years), we estimate the natural submarine discharge and the replenishment rate for the A3 aquifer.