Kinetics of biodegradation of chlorophenols were studied in six sandy aquifer columns (0.06 m I.D.; 1.00 m L). Remediation of chlorophenols was enhanced by using a "closed-loop" configuration system, where local groundwater was recirculated through the polluted site in a controlled manner. Consecutive accidental spills of phenol, 2-monochlorophenol (2-MCP) and 2,4,6-trichlorophenol (2,4,6-TCP) as single pollutants were removed following first order kinetics. The removal of chlorophenols increased by one order of magnitude following consecutive accidental spills demonstrating adaptation of the resident micro flora. The biodegradation rate constants in this study were in the same range and agreed with those reported in the literature for biodegradation in aerobic aquifers. Following the fate of the resident micro flora (enhanced by adding NH4Cl and KH2PO4 at a ratio C/N/P equal to 120:10:1), biomass growth was observed in the sandy aquifer columns and particle size analyses of the aqueous phase recirculated through the polluted site experimentally proved aggregation of cells. Aggregation of cells has been hypothesized as one of the causes for low biodegradation rates found in the field compared to those calculated using biodegradation rate constants determined in batch culture.