Information will be presented on Phase I and Phase II of a multitask effort to achieve biological closure of an abandoned hazardous waste site on the Mississippi River. Waste materials, consisting primarily of aliphatic and polycyclic aromatic hydrocarbons in the form of buried sludges and lagoon wastes, were examined. Optimal loading levels were evaluated on the basis of biodegradative potential tests and acute toxicity of leachate. Microbial adenosine 5^a triphosphate (ATP) and microbial diversity were used in conjunction with Microtox™ tests to establish an acceptable land treatment experimental design. Methodology is presented on ascertaining optimal waste loading rates, based on percent oil and grease, for both laboratory mesocosm and field plots. Microbial ATP data indicated that both waste types stressed the indigenous microbial populations through Day 10. However, the microbial population showed recovery by Day 17. Lagoon wastes, applied at 4 and 8%, showed minimal stress and high microbial ATP levels by Day 17. Buried wastes showed comparable results for 2 to 4% waste loadings at Day 17. Microbial diversity indices confirmed ATP estimates, which suggested a maximum loading rate of 8% for lagoon wastes and 2 to 4% for buried wastes. EC₅₀ levels of leachates predicted 3.0 to 9.0% waste weight (wet basis) for lagoon waste and 3.0 to 6% waste weight (wet basis) for the buried waste. A loading rate of 4% for lagoon waste and 2.5% for buried waste was selected as the acceptable mean loading rate. Gas chromatography/mass spectroscopy (GC/MS) data in subsequent Phase II studies (mesocosms) documented significant biotransformation and biodegradation of the wastes at these optimized loading rates.