Gary D. Hopkins

Field evaluation of in situ aerobic cometabolism of trichloroethylene and three dichloroethylene isomers using phenol and toluene as the primary substrates.

The Moffett field site was used for further evaluation of in situ biotransformation of chlorinated aliphatic hydrocarbons with phenol and toluene as primary substrates. Within the 4 m test zone, representing a groundwater travel time of less than 2 days, removal efficiencies for 250 μg/L TCE and 125 μg/L cis-1,2-dichloroethylene were greater than 90%, and that of 125 μg/L trans-1,2-dichloroethylene was ∼74%, when either 9 mg/L toluene or 12.5 mg/L phenol was used. Phenol and toluene were removed to below 1 μg/L. Vinyl chloride removals greater than 90% were also noted. However, only 50% of the 65 μg/L 1,1-dichloroethylene was transformed with phenol addition, and significant product toxicity was evident as concomitant TCE transformation was here reduced to ∼50%. Hydrogen peroxide addition performed as well as pure oxygen addition to serve as a required electron acceptor.

Field study of organic water quality changes during groundwater recharge in the Palo Alto Baylands

Abstract Water quality data are presented from a field study in which reclaimed water is injected directly at a rate of 61 s −1 into an aquifer in the Palo Alto Baylands on the margin of San Francisco Bay. Water quality changes are observed by analyzing samples from wells at distances of 10–40 m and in differing directions from the injection point. Data on trace organic pollutants show evidence of retardation of movement in varying degrees, presumably caused by adsorptive interactions with the aquifer. Trihalomethane compounds show evidence of biodegradation in the aquifer. The concentration of total organic substance as measured by TOC and COD is decreased significantly by biodegradation, but total organic halogen appears unaffected by aquifer passage.

Pilot scale field studies of in situ bioremediation of chlorinated solvents

This paper discusses results from pilot scale field studies that evaluated enhanced in situ bioremediation of chlorinated solvents. A stimulus-response methodology for performing controlled field experiments is exemplified. The cometabolic transformation of chlorinated aliphatic compounds by methanotrophic bacteria is of primary focus.

Enhanced anaerobic bioremediation of groundwater contaminated by fuel hydrocarbons at Seal Beach, California.

Enhanced anaerobic biodegradation of groundwater contaminated by fuel hydrocarbons has been evaluated at a field experiment conducted at the Naval Weapons Station, Seal Beach, California. This experiment included the establishment of three different remediation zones in situ: one zone was augmented with sulfate, one was augmented with sulfate and nitrate, and the third was unaugmented. This enables a comparison of hydrocarbon biodegradation under sulfate-reducing, sequential denitrifying/sulfate-reducing, and methanogenic conditions, respectively. In general, the results from the field experiment are: (1) Certain fuel hydrocarbons were removed preferentially over others, but the order of preference is dependent upon the geochemical conditions; and (2) In the zones that were augmented with sulfate and/or nitrate, the added electron acceptors were consumed quickly, indicating that enhancement via electron acceptor injection accelerates the biodegradation process. More specifically, in the sulfate-reducing zone, sulfate was utilized with an apparent first-order rate coefficient of approximately 0.1 day-1. In the combined denitrifying/sulfate-reducing zone, nitrate was utilized preferentially over sulfate, with an apparent first-order rate coefficient of 0.1–0.6 day-1. However, the data suggest that slow sulfate utilization does occur in the presence of nitrate, i.e., the two processes are not strictly sequential. With regard to the aromatic BTEX hydrocarbons, toluene was preferentially removed under intrinsic conditions; biodegradation of benzene was slow if it occurred at all; augmentation with sulfate preferentially stimulated biodegradation of o-xylene; and ethylbenzene appeared recalcitrant under sulfate-reducing conditions but readily degradable under denitrifying conditions.

Bioaugmentation with butane-utilizing microorganisms to promote in situ cometabolic treatment of 1,1,1-trichloroethane and 1,1-dichloroethene.

A field study was performed to evaluate the potential for in-situ aerobic cometabolism of 1,1,1-trichloroethane (1,1,1-TCA) through bioaugmentation with a butane enrichment culture containing predominantly two Rhodococcus sp. strains named 179BP and 183BP that could cometabolize 1,1,1-TCA and 1,1-dicholoroethene (1,1-DCE). Batch tests indicated that 1,1-DCE was more rapidly transformed than 1,1,1-TCA by both strains with 183BP being the most effective organism. This second in a series of bioaugmentation field studies was conducted in the saturated zone at the Moffett Field In Situ Test Facility in California. In the previous test, bioaugmentation with an enrichment culture containing the 183BP strain achieved short term in situ treatment of 1,1-DCE, 1,1,1-TCA, and 1,1-dichloroethane (1,1-DCA). However, transformation activity towards 1,1,1-TCA was lost over the course of the study. The goal of this second study was to determine if more effective and long-term treatment of 1,1,1-TCA could be achieved through bioaugmentation with a highly enriched culture containing 179BP and 183BP strains. Upon bioaugmentation and continuous addition of butane and dissolved oxygen and or hydrogen peroxide as sources of dissolved oxygen, about 70% removal of 1,1,1-TCA was initially achieved. 1,1-DCE that was present as a trace contaminant was also effectively removed (approximately 80%). No removal of 1,1,1-TCA resulted in a control test leg that was not bioaugmented, although butane and oxygen consumption by the indigenous populations was similar to that in the bioaugmented test leg. However, with prolonged treatment, removal of 1,1,1-TCA in the bioaugmented leg decreased to about 50 to 60%. Hydrogen pexoxide (H2O2) injection increased dissolved oxygen concentration, thus permitting more butane addition into the test zone, but more effective 1,1,1-TCA treatment did not result. The results showed bioaugmentation with the enrichment cultures was effective in enhancing the cometabolic treatment of 1,1,1-TCA and low concentrations of 1,1-DCE over the entire period of the 50-day test. Compared to the first season of testing, cometabolic treatment of 1,1,1-TCA was not lost. The better performance achieved in the second season of testing may be attributed to less 1,1-DCE transformation product toxicity, more effective addition of butane, and bioaugmentation with the highly enriched dual culture.

Field Studies: Elicitation of Fate and Transport Processes and Application to Full-scale Remediation

Although laboratory studies and studies of uncontrolled releases are essential elements in our quest to understand the fate and transport of organic contaminants in the subsurface, both types of studies have inherent limitations. Laboratory studies, though well controlled, do not recreate the complexity and variability of natural systems. Studies of uncontrolled releases, by definition, do not have the experimental controls (for instance, knowledge of the location of contaminant source in time and space) necessary to quantitatively elicit fate and transport processes. Thus, controlled field experiments are vital links, bridging the gap between laboratory and application, and helping us to understand fate and transport processes in natural systems, as well as helping us bring theory to bear on remediation of uncontrolled releases.