The ongoing production and bioaccumulation of drilling fluids and muds has resulted in an increased concern for reducing the environmental impact of contaminated sites. The use of bioremediation as a treatment option is an important focus of research. Objectives of this study were: (i) to assess initial toxicity of drilling mud from an agricultural setting in Galliano, Sicily (Italy) and (ii) to develop a low intervention bioremediation approach to ensure that US and international soil/ground water quality standards have been met. Total petroleum hydrocarbon content of the drilling mud was reduced to 617.0 +/- 176.0, 446.0 +/- 195.0, and 533.0 +/- 138.0 mg kg(-1), respectively, from 5000.0 +/- 530.0 mg kg(-1) after treatment via conventional ex-situ soil slurry reactor mixing (84.2-95% reduction). Using an in-situ approach with bioreactors placed vertically into the zone of contamination, hydrocarbon concentrations went from 217.12 +/- 43.38 and 149.68 +/- 45.51 mg kg(-1) to 15.16 +/- 3.35 and 34.27 +/-15.86 mg kg(-1) for control test beds (85.1 +/- 11.2% reduction), and from 89.20 +/- 67.42, 141.71 +/- 64.80, and 197.87 +/- 77.38 mg kg(-1) to 5.24 +/- 6.15, 15.02 +/- 10.20, and 9.65 +/- 9.37 mg kg(-1) for experimental test beds (92.9 +/- 3.0% reduction), respectively. Cadmium (Cd) and Selenium (Se) had exceeded Italian and state soil leachability standards (0.005 mg dm(-3) Cd) and (0.010, 0.050 mg dm(-3) Se), at the conclusion of the study. In-situ treatment of drilling mud provided a framework for monitoring the fate of organic and residual metals toxicity in soils. (C) 2003 Society of Chemical Industry.