The usefulness of bioaugmentation and biostimulation in enhancing the natural attenuation of chlorinated ethanes, ethenes, and methanes at a seep site at Aberdeen Proving Ground (APG), MD was tested. The biodegradation of (1) a mixture of 1,1,2,2-tetrachloroethane (TeCA), tetrachloroethene (PCE), and carbon tetrachloride (CT), or (2) TeCA alone was compared in sediment and groundwater microcosms amended with chlorinated substrates alone, chlorinated substrates and electron donor, or chlorinated substrates, electron donor and a TeCA-degrading enrichment culture. A third experiment evaluated the usefulness of H₂ thresholds in determining the importance of co-metabolic and metabolic processes in the biodegradation of chlorinated substrates. Biostimulation alone did not significantly affect chlorinated substrate removal. Biodegradation of TeCA was significantly enhanced by the addition of electron donors coupled with bioaugmentation. However, the presence of other contaminants, especially chlorinated methanes appeared to inhibit TeCA biodegradation, even in the presence of exogenous electron donors and the enrichment culture. H₂ thresholds did not prove useful in determining the importance of metabolic and co-metabolic processes in the transformation of CT, PCE, and TeCA; however, evaluating the biodegradation of each chlorinated compound individually provided insight in regards to biodegradation pathways followed and the effects of electron donor substrates on degradation rates. Overall, the results provide evidence that when assessing a site contaminated with a mixture of chlorinated solvents, it is necessary to examine each contaminant individually and as a mixture, in order to develop a successful remediation plan.