Processes affecting geochemistry and contaminant movement in the middle Claiborne aquifer of the Mississippi embayment aquifer system |
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| Authors: | Brian G Katz James A Kingsbury Heather L Welch Roland W Tollett |
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| Institution: | (1) U.S. Geological Survey (USGS), 2639 N. Monroe St., Ste. A200, Tallahassee, FL 32303, USA;(2) USGS, 640 Grassmere Park, Ste. 100, Nashville, TN 37211, USA;(3) USGS, 308 South Airport Rd, Jackson, MS 39208, USA;(4) USGS, 3095 West California Ave., Ruston, LA 71270, USA |
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| Abstract: | Groundwater chemistry and tracer-based age data were used to assess contaminant movement and geochemical processes in the
middle Claiborne aquifer (MCA) of the Mississippi embayment aquifer system. Water samples were collected from 30 drinking-water
wells (mostly domestic and public supply) and analyzed for nutrients, major ions, pesticides, volatile organic compounds (VOCs),
and transient age tracers (chlorofluorocarbons, tritium and helium-3, and sulfur hexafluoride). Redox conditions are highly
variable throughout the MCA. However, mostly oxic groundwater with low dissolved solids is more vulnerable to nitrate contamination
in the outcrop areas east of the Mississippi River in Mississippi and west Tennessee than in mostly anoxic groundwater in
downgradient areas in western parts of the study area. Groundwater in the outcrop area was relatively young (apparent age
of less than 40 years) with significantly (p < 0.05) higher dissolved oxygen and nitrate–N concentrations and higher detections of pesticides and VOCs compared to water
samples from wells in downgradient areas. Oxygen reduction and denitrification rates were low compared to other aquifers in
the United States (zero order rate constants for oxygen reduction and denitrification were 4.7 and 5–10 μmol/L/year, respectively).
Elevated concentrations of nitrate–N, and detections of pesticides and VOCs in some deep public supply wells (>50 m depth)
indicated contaminant movement from shallow parts of the aquifer into deeper oxic zones. Given the persistence of nitrate
in young oxic groundwater that was recharged several decades ago, and the lack of a confining unit, the downward movement
of young contaminated water may result in higher nitrate concentrations over time in deeper parts of the aquifer containing
older oxic water. |
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