TECHNOLOGY FOCUS - ENVIRONMENT - Preferred Method: The contaminant in the groundwater is biodegraded in situ by forming biological permeable barrier by excavating a trench to intersect the in-ground flow of the groundwater and filling the trench with the immobilized microorganism, or by removing the groundwater from the ground and passing the contaminated groundwater through the biological barrier. Before immobilization, the selected microorganism is conditioned to increase the tolerance to the contaminant or to the contaminated water. The microorganisms are immobilized on a carrier medium comprising polyvinyl alcohol having a molecular weight greater than 20000, preferably greater than 50000 and less than 110000, or greater than 75000 and less than 100000. The microorganism is immobilized on a carrier medium comprising polyvinyl alcohol treated with an aqueous acid or comprising granular activated carbon. The method also comprises providing nutritional supplements to the immobilized microorganism, and selecting a second microorganism capable of biodegrading a second contaminant contained in the contaminated groundwater. The second microorganism is also immobilized on a carrier medium to provide a second immobilized microorganism. EXAMPLE - The removal mechanism of trichlorophenol (TCP) from groundwater by polyvinyl alcohol (PVA) immobilized cells and 3 % granular activated carbon (GAC) immobilized cells/sand mixture was examined. The average influent TCP concentration was 10.50l0.83 mg/L. The C:N:P ratio was kept at 100:18:188 by adding nutrients to the influent feed solution. The influent solution was aerated with laboratory compressed air 24 hours/day to maintain the dissolved oxygen (DO) above 8.4 mg/L. Rate flow for all 4 columns was 1.0 mL/minute. Applied loading for columns 1 and 3, and 2 and 4 were 0.074 and 0.037 g L-1 d-1, respectively. PVA columns 1 and 2 reduced the influent TCP concentration to 0 on day 17 and 13, respectively. Polyvinylalcohol (PVA) column 2 with a 20.0-cm bed height provided longer contact time between cells and TCP than PVA column 1 with a 10.0-cm bed height. Both PVA columns maintained 100 % TCP removal for remaining time. No TCP was ever detected in the effluents of GAC columns over entire course. TCP was removed initially by adsorption on GAC that is no leakage was seen, and later by biodegradation. For aerobic mineralization of each mg of TCP, 0.89 mg of oxygen was expected to be consumed by bacteria. All 4 columns consumed oxygen at the same extent (2.5 mg/L), except PVA 1, which had a slightly higher consumption (3.3 mg/L). The reduction in dissolved oxygen in the effluent indicates the biodegradation process going on in the columns.