HR: 0830h
AN: H11D-0888    [PDF]
TI: Coupled Vapour Phase and Dissolved Gas Analysis to Elucidate Physical and Geochemical Processes in Reactive Zones
AU: * Amos, R T
EM: ramos@eos.ubc.ca
AF: Dept. Earth and Ocean Sciences, University of British Columbia, 6339 Stores Rd, Vancouver, BC V6T 1Z4 Canada
AU: Mayer, K U
EM: umayer@eos.ubc.ca
AF: Dept. Earth and Ocean Sciences, University of British Columbia, 6339 Stores Rd, Vancouver, BC V6T 1Z4 Canada
AU: Bekins, B A
EM: babekins@usgs.gov
AF: U.S Geological Survey, 345 Middlefiled Road, Menlo Park, CA 94025 United States
AU: Delin, G N
EM: delin@usgs.gov
AF: U.S Geological Survey, 2280 Woodale Drive, Mounds View, MN 55112-0049 United States
AB: Dissolved and vapour phase gas data, including Ar, N$_{2}$, O$_{2}$, CH$_{4}$ and CO$_{2}$, were collected at a crude oil spill site near Bemidji, MN. The dataset includes sampling points in the saturated and unsaturated zones from upgradient of the source zone, within the source zone, and down-gradient extending beyond the anoxic vapour and dissolved phase plumes. In the source zone dissolved Ar and N$_{2}$ concentration are depleted to concentrations below 50% of water equilibrated with the atmosphere. This is indicative of gas bubble formation caused by increased levels of CH$_{4}$ from methanogenic degradation of the oil, and partitioning of Ar and N$_{2}$ into the bubbles. In the unsaturated zone, above the most methanogenic areas, vapour phase Ar and N$_{2}$ concentrations also show a depletion, to concentrations less than 90% of atmospheric levels, indicating significant methane production in this zone and/or gas influx from the saturated zone. Directly surrounding these areas of depletion vapour phase Ar and N$_{2}$ concentrations show an enrichment whereas CH$_{4}$ and O$_{2}$ concentrations are low indicating advective gas flux from the atmosphere toward this zone. Furthermore, directly downgradient of the methanogenic zones dissolved Ar and N$_{2}$ concentrations return to levels that approach water equilibrated with the atmosphere where advective flow calculations would predict depleted concentrations to be transported much further. This observation suggests either limited flux through the source zone or increased mixing of water from the dissolved plume with stagnant water in low permeability zones, or possibly with trapped gas bubbles. The results of this study demonstrate the use of naturally occurring non-reactive gases as effective tracers of both physical and geochemical processes in contaminated systems. This includes quantifying rates of methanogenesis, gas bubble formation and ebullition, methane oxidation in the unsaturated zone, and mixing within the groundwater plume. Additionally, this study highlights the importance of considering the saturated and unsaturated zone as a coupled system. In the methanogenic source zone, geochemical parameters such as CH$_{4}$ and CO$_{2}$ and pH will be affected by the gas flux from the saturated zone to the unsaturated zone. An accurate accounting of these parameters can be particularly important for models where rate estimates of geochemical processes depend on accurate mass balances. Future work will involve laboratory experiments and the use of reactive transport modelling to demonstrate these principles in a quantitative manner.
DE: 1055 Organic geochemistry
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
DE: 1875 Unsaturated zone
SC: Hydrology [H]
MN: 2003 Fall Meeting