Enhancement of Microbial Sulfate Reduction for the Remediation of Hydrocarbon Contaminated Aquifers - A Laboratory and Field Scale Demonstration

Description:

Objective(s) of the Research Project:

The primary goal of this project is to design, implement, and monitor a simple but effective enhanced anaerobic bioremediation technology for treating hydrocarbon contamination in aquifers. The specific goals of this project are to:

1. Evaluate existing data for long-term trends in benzene, toluene, ethylbenzene and xylenes (BTEX) concentrations.
2. Use laboratory experiments coupled with geochemical, hydrological, and contaminant characterizations to design the sulfate injection process and to evaluate the feasibility of this approach.
3. Establish baseline contaminant, geochemical, and microbiological conditions prior to nutrient injection.
4. Monitor the performance of the enhanced anaerobic biodegradation process.
5. Identify the applicability and limitations of enhancing sulfate reduction for stimulating BTEX biodegradation

Progress Summary/Accomplishments:

A survey of groundwater and sediments for geochemical, hydrocarbon biodegradation metabolites, and contaminant concentrations, indicate that conditions are suitable for stimulating benzene biodegradation by the addition of sulfate to the aquifer. Laboratory biodegradation experiments confirmed that the addition of sulfate stimulated benzene biodegradation in samples obtained from most of the sampling locations. Groundwater samples continue to be collected from approximately 35 wells on a routine basis for a variety of geochemical and hydrocarbon analyses. These analyses have established the baseline conditions to which post-sulfate injection data will be compared. Recent accomplishments in the laboratory include the evaluation of the impact of sulfate reduction on hydraulic conductivity. Preliminary results indicate that sulfate reduction significantly reduces the hydraulic conductivity in sediment column experiments only when dissolved iron is available for reaction with produced sulfide (Fig. 1).

Figure 1. Impact of sulfate reduction on hydraulic conductivity in sediment cores.

Results show the pressure curves over 200 minutes of pumping. The cores were incubated for two weeks prior to conducting the pressure tests. The cores were incubated in the presence of: 1) lactate+sulfate+iron (pink line), 2) sulfide only (yellow line), or 3) sulfate + lactate (blue line).

Toxicity experiments indicate that benzene concentratios over 100 mg/L do not impact sulfate reduction rates indicating that benzene toxicity may not be an important variable governing benzene biodegradation. Other results suggest that sulfate addition stimulated the biodegradation of alkanes that were present in sediments collected from a region where benzene biodegradation was not detected. Additional work will be required to determine the impact that alkanes and other hydrocarbons may have on benzene biodegradation.

Additional experiments are being conducted to identify whether factors in addition to sulfate availability control sulfate reduction activity and benzene biodegradation. We are continuing experiments to evaluate the impact of anaerobic sulfide oxidation on sulfate reduction activity and hydraulic conductivity.

Supplemental Keywords: biodegradation, bioremediation, groundwater, BTEX, benzene, anaerobic