You are here:
Plume Response to Source Remediation: Case Study of Active Bioremediation
Citation:
WILSON, J. T. Plume Response to Source Remediation: Case Study of Active Bioremediation. Presented at the Work Shop on MNA of Poly Aromatic Hydrocarbons at Manufactured Gas Plant Sites, Dallas, TX, May 27 - 29, 2009.
Impact/Purpose:
To explore the role of hydraulic conductivity in a case study at the Public Services .
Description:
The three-dimensional distribution of hydraulic conductivity has a profound influence on the prospects for cleaning up contaminated ground water. When there are wide variations in texture (and associated hydraulic conductivity) organic contaminants can find their way into the low conductivity materials over time. This makes the contamination difficult to reach with technology that circulates fluids. However, high conductivity flow paths can bring in soluble electron acceptors such as oxygen or sulfate that can be used to support natural biodegradation of the contaminants at the NAPL source area. The role of hydraulic conductivity was explored in a case study at the Public Services Site, a RCRA site in Denver, Colorado. The site was a garage used to service trucks. Gasoline, motor oil, and transmission fluid was disposed to a dry well under the floor of the garage. Ground water was cleaned up using aerobic in situ bioremediation. After remediation was finished, push tools were used to recover core samples for extraction and analysis of TPH and benzene in the sediment, on a vertical spacing of 0.3 feet. After active remediation, the mass fraction of benzene in the most contaminated material was near 0.4% of TPH. Assuming mass fraction is near mole fraction, the expected benzene concentration in ground water was 6,000 μg/L. The measured concentration in the monitoring well at the same location was 0.8 μ/L. The concentrations of benzene and BTEX in ground water are much lower than would be expected from their concentrations in the residual hydrocarbon. To determine why benzene was not present at high concentrations in ground water, push slotted rods were used to profile concentrations of contaminants and electron acceptors in ground water on a spacing of 1.5 feet. The slotted rods were also used to conduct a miniature specific capacity test to estimate hydraulic conductivity. The residual NAPL contaminant and the capacity to move ground water in the aquifer were separated in space. The intervals in the aquifer with the capacity to move ground water had low concentrations of contaminants and high concentrations of electron acceptors. Contaminants must have entered the aquifer by vertical diffusion through the hydraulically non-conductive layer. Active treatment during in situ bioremediation and natural weathering had reduced the diffusion of hydrocarbons to achieve two important conditions; 1) The total loading of electron acceptor demand to the transmissive unit of the aquifer did not exceed the supply of soluble electron acceptor and 2) The downward diffusive flux of benzene and other materials with an electron acceptor demand was balanced by an upward diffusive flux of sulfate. The natural biodegradation of benzene diffusing out of the LNAPL occurred in the non transmissive material, not in the transmissive portion of the aquifer. Benzene never moved away for the residual source. If active remediation can weaken the diffusive flux of hydrocarbons, such that the supply of soluble electron acceptors is not depleted in the time required from ground water to move through or underneath the residual NAPL, then natural biodegradation can prevent the formation of a plume in the ground water moving away from the source area