HR: 1330h
AN: H43B-08    [Abstracts]
TI: Recharge-Driven Sulfur Cycling and its Role in Natural Attenuation in an Alluvial Aquifer Riparian Zone Contaminated With Landfill Leachate
AU: * Scholl, M A
EM: mascholl@usgs.gov
AF: U.S. Geological Survey, 431 National Center, 12201 Sunrise Valley Dr., Reston, VA 20192 United States
AU: Cozzarelli, I M
AF: U.S. Geological Survey, 431 National Center, 12201 Sunrise Valley Dr., Reston, VA 20192 United States
AU: Christenson, S C
AF: U.S. Geological Survey, 202 NW 66th St., Bldg. 7, Oklahoma City, OK 73116 United States
AU: Healy, R W
AF: U.S. Geological Survey, Denver Federal Center, P.O Box 25046, MS 413, Lakewood, CO 80225 United States
AB: In an alluvial aquifer riparian zone contaminated with leachate from an unlined municipal landfill, recharge represented a possible mechanism for natural attenuation. This study, at the USGS Toxic Substances Hydrology Program Norman Landfill research site in Oklahoma, examined whether recharge is supplying electron acceptors to stimulate microbiological activity in the transition zone between leachate and recharge water, facilitating biodegradation of organic compounds in the leachate. Water samples were collected monthly from May 1998 to May 2000 at 15-cm depth intervals in the top 2 m of the aquifer at contaminated and uncontaminated sites. At both sites, the temporal concentration-depth profiles showed a seasonal cycle of dissolved sulfate production and loss within the aquifer. The average concentration of dissolved sulfate present within the top 1 to 2 m of aquifer was 2.2 mM, with maximum concentrations of 15 mM. Sulfides were oxidized near the water table by dissolved oxygen in the recharge water, and were also oxidized at depths up to 1 meter below the water table, possibly by colloidal iron transported vertically in the recharge water. Sulfate concentrations then declined, due to root uptake by phreatophytes and microbial reduction in the mixing zone between the recharge water and the methanogenic leachate plume. Estimated net sulfate reduction rates (reduction exceeding oxidation) in the groundwater ranged from 0.007 - 0.61 mmol L^-1 d^-1 (bulk rates, calculated between sampling periods, over depth intervals ranging from 0.45 - 1.75 m). Higher rates of sulfate reduction followed periods of little or no recharge at both sites. Numerical modeling with VS2DTI, a variably saturated aquifer solute transport model, helped to quantify the relative importance of sulfate reduction and transpiration root uptake on sulfate concentrations in the shallow aquifer. These results suggest that sulfate reduction associated with infiltration of recharge can be a significant contribution to natural attenuation processes in similar alluvial plain riparian environments.
DE: 1818 Evapotranspiration
DE: 1829 Groundwater hydrology
DE: 1831 Groundwater quality
SC: Hydrology [H]
MN: 2005 Joint Assembly