HR: 11:20h
AN: B31F-05    [PDF]
TI: Assessing the effect of natural attenuation on oxygen consumption processes in a sewage-contaminated aquifer by use of a natural-gradient tracer test
AU: * Mathisen, P P
EM: mathisen@wpi.edu
AF: Worcester Polytechnic Inst., Civil and Environmental Engineering Dept., Worcester, MA 01609
AU: Kent, D B
EM: dkbent@usgs.gov
AF: USGS, MS 465, 345 Middlefield Rd., Menlo Park, CA 94025
AU: Smith, R L
EM: rlsmith@usgs.gov
AF: USGS, 3215 Marine Street, Boulder, CO 80303
AU: Barber, L B
EM: lbbarber@usgs.gov
AF: USGS, 3215 Marine Street, Boulder, CO 80303
AU: Harvey, R W
EM: rwharvey@usgs.gov
AF: USGS, 3215 Marine Street, Boulder, CO 80303
AU: Metge, D W
EM: dwmetge@usgs.gov
AF: USGS, 3215 Marine Street, Boulder, CO 80303
AU: Hess, K M
EM: kmhess@usgs.gov
AF: USGS, 10 Bearfoot Road, Northborough, MA 01532
AU: LeBlanc, D R
EM: dleblanc@usgs.gov
AF: USGS, 10 Bearfoot Road, Northborough, MA 01532
AU: Koch, J C
EM: jckoch@usgs.gov
AF: Univ. of Arizona, Dept. of Geosciences, Tucson, AZ 85721
AB: Processes associated with aquifer restoration subsequent to cessation of treated-sewage loading in a sand and gravel aquifer are being investigated at the USGS Toxic Substances Hydrology Site on Cape Cod, MA. Restoration has been slow because of significant oxygen depletion resulting from biogeochemical processes associated with residual sorbed pools of organic carbon, ammonium, and reduced metals in the aquifer. The in situ interaction of the physical, chemical, and biological processes governing oxygen consumption was examined by using a natural-gradient tracer test in fall 2001, 6 years after sewage disposal had been discontinued. Ground water with a high dissolved oxygen (DO) concentration was withdrawn from an uncontaminated zone of the aquifer and re-injected with a conservative tracer, bromide, into an anoxic zone directly below a former sewage-effluent disposal bed where Fe and sulfide concentrations were below detection and the DO was less than 5 uM. An injection with negligible ammonium, a nitrate concentration of 22 uM, and DO of approximately 260 uM was maintained at approximately 15 L/hr for a period of 75 days. An array of multi-level samplers (MLS), placed at distances ranging from 1 to 7 m down-gradient from the injection well, was sampled prior to and throughout the 75-day injection, and during a 25-day period after the injection. Water samples from the MLS were analyzed for DO and a variety of aqueous constituents. The DO decreased from approximately 260 uM to 210 uM over 7 m of transport, indicating the presence of rate-limited oxygen consumption. An increase in nitrate from 22 to approximately 36 uM indicated the presence of rate-limited ammonium oxidation. However, this ammonium oxidation was not sufficient to account for all of the DO consumption. Further characterization of these processes was accomplished by use of PHREEQC, a one-dimensional, geochemical reactive transport model. The 1D model is based on an ion association model for aqueous speciation. Surface complexation reactions were used to define the speciation for surface-complexed species, including ammonium and major cations. Kinetic reactions were defined for oxidation of ammonium and aerobic respiration of sorbed organic carbon. The initial chemical condition was specified as an equilibrium between a background solution, determined by using field data, and the surface sites, while flow was specified with constant flux boundary conditions on the basis of a steady-state injection. The model was calibrated by adjusting equilibrium constants and matching simulations to breakthrough curves observed in the tracer test at several distances downstream of the injection along the plume centerline. Modeling of breakthrough curves at different MLS ports suggested that aquifer heterogeneity and variability in the injection affected the geochemical reactions to some extent. Model simulations indicated that consideration of both aerobic respiration and ammonium oxidation was necessary to account for the observed oxygen consumption. This result, which was consistent with bacteria and dissolved carbon data, demonstrates that aerobic respiration and ammonium oxidation are important processes to consider when modeling the aerobic restoration of anaerobic, sewage-contaminated aquifers.
DE: 0400 Biogeosciences
DE: 1829 Groundwater hydrology
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
SC: Biogeosciences [B]
MN: 2003 Fall Meeting