THE ROLE OF AQUIFER HETEROGENEITY ON METAL REDUCTION IN AN ATLANTIC COASTAL PLAIN AQUIFER AS DETERMINED USING PUSH-PULL TESTS

Push-pull experiments were conducted to determine the factors controlling Fe(III) and Mn(IV) reduction in a well-characterized, shallow, coastal plain aquifer near Oyster, VA. Sixty push-pull experiments were simultaneously conducted in 5 wells equipped with 12 port multi-level samplers (MLSs). The MLSs sampled a heterogeneous portion of the aquifer from 4.4 to 8 m bgs. Each well was injected with groundwater that contained nitrate and bromide along a distinct combination of electron donor and humics. These were 1) just groundwater (control treatment) 2) humics alone, 3) lactate (conducted twice) and 4) lactate and humics. Microbially mediated Fe(III) reduction caused the aqueous Fe_Tot and Mn_Tot concentrations to increase at every depth in the lactate treatment with significant increases within 1 day even while nitrate was present. Peak Fe_Tot concentrations reached 0.252±0.113 mM in the lactate treatment as compared to only 0.031±0.026 mM in the control treatment with peak values varying by over a factor of 4 with depth. Humics may have acted as an electron shuttle to increase Fe(III) reduction as the peak concentration of Fe_Tot was greater in the lactate and humics treatment than the lacate treatment at the 95% confidence level. The amount of Mn(IV) reduction was significantly lower than Fe(III) reduction. Geochemical modeling indicated that gas formation, sorption on reactive surfaces, and mineral precipitation were important processes. Conditions were favorable for the precipitation of Fe-carbonates, Fe-sulfides, and Fe-silicates and Fe(III) and sulfate reduction were co-occurring during the experiment. In the lactate treatment protist concentrations increased by over an order of magnitude before decreasing and planktonic cell concentrations increased almost two orders magnitude whereas no changes were observed 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.