Date of Graduation

1998

Document Type

Thesis

Degree Type

MS

College

Eberly College of Arts and Sciences

Department

Geology and Geography

Committee Chair

Joseph J. Donovan

Abstract

An experiment involving in situ neutralization of acidic ground water was performed on a mine-spoil aquifer in central West Virginia. The 45-acre aquifer was subjected to infiltration of lime slurry through a sludge pond located near the center of the mine. Ground-water and spring-water samples collected before lime infiltration indicate that a localized zone of neutralized ground water (pH 7) existed close to the sludge pond prior to the experiment. Concentrations of elevated sodium, pH and alkalinity correlate with the extent of neutralized ground water. This alkalinity is attributed to infiltration of sodium hydroxide-treated sludge, pumped into the sludge pond from treatment ponds prior to the experiment. Despite infiltration of the lime slurry during the experiment, the spatial extent of aquifer neutralization did not increase within the timeframe of this study. Ground-water chemistry (pH, alkalinity, and calcium) indicates that lime slurry is actively converted to calcite following recharge. A steady-state ground-water flow model using MODFLOW was calibrated based on field measurements of aquifer head and discharge. Advective transport of groundwater flow from recharge to discharge areas was simulated by particle-tracking methods. Results show that alkalinity introduced as recharge through the sludge pond should eventually contribute to the neutralization of three springs approximately 1300 feet from the basin. Simulations also show that two springs closer to the sludge pond (500 feet) are predominantly fed by acidic ground water upgradient of the sludge pond. Binary mixing models developed with the geochemical program PHREEQC simulated reactions for mixing of alkaline sludge with acid water. Results show

decreases in aluminum concentration above pH 5.5 caused by Al(OH)3(a) precipitation. iii Concentrations of dissolved iron decrease above pH 5.4, consistent with precipitation of iron hydroxide. Ground-water samples were characterized by elevated partial pressure of carbon dioxide (pCO2(g) = 10-1.0 atm), suggesting the aquifer may be modeled as a closed system with respect to atmospheric pCO2(g) (10-3.5 atm). A coupled flow and geochemical reaction model was developed to simulate both acid-producing and acid-neutralizing reactions along ground-water flow. Based on results from the steady-state MODFLOW model, mass balances for solutes were calculated through a series of 12 successive cells within a “flow channel” extending from the sludge pond to a spring. Reactions controlling solute concentrations along flow include pyrite oxidation, iron and aluminum hydroxide precipitation, clay mineral buffering, and dolomite dissolution.

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