Candidate: Franklyn D. Legall

Degree of: Doctor of Philosophy

Department: Geosciences

Title: Geochemical and Isotopic Characteristics Associated with High Conductivities in a Shallow Hydrocarbon-contaminated Aquifer

Committee:
Dr. Estella Atekwana, Chair
Dr. R.V. Krishnamurthy
Dr. William Sauck
Dr. Eliot Atekwana

Date: November 1, 2002 Friday 4:00 p.m.-6:00 p.m. 1122 Rood

Abstract:
High conductivities observed at sites contaminated with hydrocarbons have been attributed to enhanced mineral weathering resulting from the generation of inorganic and organic acids produced during microbial hydrocarbon mineralization. The use of vertical resistivity probes has been effective in identifying zones of contamination by their anomalous conductivity. Vertical resistivity profiling at a hydrocarbon-contaminated site in Carson City, Michigan, showed depth variations postulated to relate to biodegradation. The objectives of the study were to: 1) provide geochemical evidence of biodegradation in order to test the hypoTheses that the high soil conductivities were due to biomineralization of hydrocarbons; 2) investigate the redox processes occurring within the anomalously conductive zones; and 3) evaluate the role of mineral weathering in the aquifer as the source of ions responsible for the anomalous conductivity. Groundwater samples collected from closely spaced well screens (~30 cm) within the high conductivity zones were analyzed for major ions, terminal electron acceptors (TEAs) such as NO3 and SO4 and concentrations of educts such as NH4, Fe(II), Mn(II),
(over)
dissolved organic carbon (DIC), and its isotopic ratio.
The results show a pattern of decreasing resistivity (increasing conductivity) within the impacted zone. Broad conductive zones are aligned with the intervals that exhibit high total petroleum hydrocarbons (TPH), depleted NO3 and SO4 and increased NH4 Fe, Mn, alkalinity, and DIC. The close correspondence between these zones suggest that the bulk conductivity is probably linked to redox processes. The d13C values range from +1.8 to-1.9‰ in the contaminated zone compared to background (-14‰). The results also show higher concentrations of Na, Ca, and Mg associated with high conductivities in the contaminated portion of the aquifer compared to background that is consistent with the weathering of carbonate and Na and Ca feldspars, the dominant minerals in the aquifer. This ion enrichment translated to higher TDS at the contaminated locations relative to background. The higher TDS at the contaminated locations was also coincident with higher DIC, and significant CO2 production. The isotopic data indicate that biodegradatin was primarily due to methanogenesis. This suggest that the observed changes in conductivity were due to the microbial mineralization of hydrocarbons. A simple weathering model using native aquifer minerals (including minor calcite, dolomite, Na and Ca feldspars) suggests that weathering reactions in the aquifer resulting from CO2 production were responsible for dissolution of the aquifer matrix and the enrichment of ions in the groundwater at the depth of impact, thus increasing the ionic content of the groundwater as reflected in the conductivity measurements. The implication of the study is that geophysical measurements via vertical resistivity probes could provide an important window on the nature and extent of microbial biodegradation in the subsurface. The study also suggests that geophysics can be used as a proxy for monitoring biogeochimical processes occurring in contaminated, dominantly unconsolidated sandy aquifers.

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