A side-by-side comparison of bioaugmentation, biostimulation, and a recirculation-only control was implemented in a chloroethene-contaminated aquifer. The objective was to develop a contaminant mass balance based on the analysis of groundwater and aquifer solids and to quantify key dechlorinating populations during treatment to determine their relation to the rate of chloroethenes removed. The bioaugmentation strategy, using a Dehalococcoides-containing PCE-to-ethene dechlorinating inoculum enriched from the same aquifer, resulted in a nearstoichiometric dechlorination of both sorbed and dissolved chloroethenes to ethene within 6 weeks. In the biostimulation plot, continuous lactate and nutrient injection resulted in dechlorination but only following a 3-month lag period. Molecular tools targeting the 16S rRNA genes of Dehalococcoides and Desulfuromonas spp. were used to qualitatively monitor the distribution and quantitatively (Real-Time PCR) measure the abundance of the dechlorinating populations during the test. In the bioaugmentation plot, Dehalococcoides populations increased 3-4 orders of magnitude throughout the test area. Dehalococcoides populations also increased in the biostimulation plot but at a slower rate and immediately before the onset of rapid dechlorination. Terminal Restriction Fragment Length Polymorphism analysis indicated that the inoculum only impacted the bioaugmentation plot. This work extends the knowledge gained from previous field studies which reported qualitative relationships between the occurrence of Dehalococcoides populations and ethene production. Furthermore, the results demonstrate that bioreactive barriers capitalizing on reductively dechlorinating populations to control the migration of chloroethene plumes can be effectively designed once hydrologic information is incorporated.

Many hypersaline environments are often contaminated with petroleum compounds. Among these, oil and natural gas production sites all over the world and hundreds of kilometers of coastlines in the more arid regions of Gulf countries are of major concern due to the extent and magnitude of contamination. Because conventional microbiological processes do not function well at elevated salinities, bioremediation of hypersaline environments can only be accomplished using high salt-tolerant microorganisms capable of degrading petroleum compounds. In the last two decades, there have been many reports on the biodegradation of hydrocarbons in moderate to high salinity environments. Numerous microorganisms belonging to the domain Bacteria and Archaea have been isolated and their phylogeny and metabolic capacity to degrade a variety of aliphatic and aromatic hydrocarbons in varying salinities have been demonstrated. This article focuses on our growing understanding of bacteria and archaea responsible for the degradation of hydrocarbons under aerobic conditions in moderate to high salinity conditions. Even though organisms belonging to various genera have been shown to degrade hydrocarbons, members of the genera Halomonas Alcanivorax, Marinobacter, Haloferax, Haloarcula, and Halobacterium dominate the published literature. Despite rapid advances in understanding microbial taxa that degrade hydrocarbons under aerobic conditions, not much is known about organisms that carry out similar processes in anaerobic conditions. Also, information on molecular mechanisms and pathways of hydrocarbon degradation in high salinity is scarce and only recently there have been a few reports describing genes, enzymes and breakdown steps for some hydrocarbons. These limited studies have clearly revealed that degradation of oxygenated and non-oxygenated hydrocarbons by halophilic and halotolerant microorganisms occur by pathways similar to those found in non-halophiles.

A study to evaluate the dechlorination end points and the most promising electron donors to stimulate the reductive dechlorination process at the chloroethene-contaminated Bachman Road site in Oscoda, MI, was conducted. Aquifer materials were collected from inside the plume and used to establish microcosms under a variety of electron donor conditions using chlorinated ethenes as electron acceptors. All microcosms that received an electron donor showed dechlorination activity, but the end points depended on the sampling location, indicating a heterogeneous distribution of the dechlorinating populations in the aquifer. Interestingly, several microcosms that received acetate as the only electron donor completely dechlorinated PCE to ethene. All acetate-amended microcosms rapidly converted PCE to cis-DCE, whereas PCE dechlorination in H2-fed microcosms only occurred after a pronounced lag time and after acetate had accumulated by H2/CO2 acetogenic activity. The microcosm experiments were corroborated by defined co-culture experiments, which demonstrated that H2 sustained PCE to cis-DCE dechlorination by acetotrophic populations in the presence of H2/CO2 acetogens. In sediment-free nonmethanogenic enrichment cultures derived from ethene-producing microcosms, acetate alone supported complete reductive dechlorination of chloroethenes to ethene, although the addition of H2 resulted in higher cis-DCE and VC dechlorination rates. Measurements of H2 production and consumption suggested that syntrophic acetate-oxidizing population(s) were active in the enrichment cultures. These findings demonstrated that either acetate or H2 alone can be sufficient to promote complete