HR: 17:45h
AN: B42E-08    [PDF]
TI: The Role of Aquifer Heterogeneity on Metal Reduction in an Atlantic Coastal Plain Aquifer as Determined by Push-Pull Tests
AU: * Mailloux, B J
EM: bjm2103@columbia.edu
AF: Columbia Earth Institute, Columbia University, New York, NY 10025
AU: Devlin, S
EM: sd248@cornell.edu
AF: Department of Earth & Atmospheric Sciences, Snee Hall Cornell University, Ithaca, NY 14853
AU: Fuller, M E
EM: Mark.Fuller@shawgrp.com
AF: Shaw Environmental & Infrastructure, Princeton Research Center 4100 Quakerbridge Rd, Lawrenceville, NJ 08648
AU: Onstott, T C
EM: tullis@princeton.edu
AF: Department of Geosciences, Princeton University, Princeton, NJ 08544
AU: DeFlaun, M F
EM: mdeflaun@geosyntec.com
AF: GeoSyntec Consultants, Research Park, Princeton, NJ 08540
AU: Choi, K
EM: khchoi@sfsu.edu
AF: Romberg Tiburon Center, San Francisco State University, Tiburon, CA 94920
AU: Green-Blum, M
EM: mxgreen@odu.edu
AF: Ocean, Earth, and Atmospheric Sciences, Old Dominion University, 4600 Elkhorn Ave., Norfolk, VA 23529
AU: Swift, D J
EM: dswift@odu.edu
AF: Ocean, Earth, and Atmospheric Sciences, Old Dominion University, 4600 Elkhorn Ave., Norfolk, VA 23529
AU: McCarthy, J
EM: jmccart1@utk.edu
AF: Department of Geological Sciences, The University of Tennessee, Knoxville, TN 37996
AB: Field-scale push-pull experiments were conducted to determine the factors controlling Fe(III) and Mn(IV) reduction in the well-characterized, shallow, coastal plain aquifer near Oyster, VA. Sixty push-pull experiments were simultaneously conducted in 5 wells equipped with multi-level samplers (MLSs), each with 12 ports equally spaced from 4.4 to 8 m below ground surface; a zone that sampled a heterogeneous portion of the aquifer. Four distinct treatments, one treatment across the twelve ports of each MLS, were utilized to determine the effect of an electron donor and humics on metal reduction. The treatments were 1) control, 2) humics, 3) lactate (conducted twice), and 4) lactate and humics. Microbially mediated Fe(III) reduction caused the aqueous Fe and Mn concentrations to increase at every depth in the lactate treatment with significant increases within 1 day even while nitrate was present in the aquifer. Peak Fe concentrations reached 0.252$\pm$0.113 mM in the lactate treatment but only 0.031$\pm$0.026 mM in the control treatment with peak values varying by over a factor of 4 with depth in the lactate treatment. Humics may have acted as an electron shuttle to increase Fe(III) reduction in the presence of lactate. The amount of Mn(IV) reduction was significantly lower than Fe(III) reduction with peak Mn values only reaching 17.24$\pm$37.67 $\mu$M in the lactate treatment as compared to 0.75$\pm$0.42 $\mu$M in the control treatment. Mass balance and thermodynamic calculations indicated that sorbed sulfate was probably reduced and that significant amounts of Fe(II) sorbed to mineral surfaces and precipitated. Geochemical modeling suggested that conditions were favorable for the precipitation of Fe-carbonates, Fe-sulfides, and Fe-silicates. In the lactate treatment protist concentrations increased by over an order of magnitude to 2914$\pm$1482 protists/ml before decreasing and planktonic cell concentrations increased almost two orders magnitude to 1.4$\pm$1.3x10$^{7}$ cells/ml whereas no change in protist and cell concentrations occurred in the control treatment. Increased rates of Fe(III) reduction may have occurred in the low permeability non-peat rich zones but the correlations with the physical and chemical descriptions of aquifer heterogeneity were weak, probably as a result of Fe precipitation, Fe sorption, and the abundance of Fe(III) bearing minerals and bacteria at all depths. This work has important implications for developing methods to remediate subsurface sites contaminated with metals and radionuclides.
DE: 0400 Biogeosciences
DE: 1030 Geochemical cycles (0330)
DE: 1045 Low-temperature geochemistry
SC: Biogeosciences [B]
MN: 2003 Fall Meeting