A series of column studies, using site-specific soil and groundwater, were conducted to determine the feasibility of biologically active zone enhancement (BAZE) process for reductive biotransformation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in groundwater. This treatability study examined the use of four amendments (acetate, ethanol, soluble starch, and acetate plus ammonium), which served as electron donors. Triplicate columns, with groundwater residence time of about 27.5 h, were used for each amendment treatment and the amendment control. In treatment columns amendment dosing was 500 mg/L C for carbon sources and 100 mg/L N for ammonium. Each of the amendment treatments reduced RDX inlet concentrations of 100 mug/L to less than 1 mug/L. The highest first-order RDX biodegradation rate ranged between 0.140 and 0.447 h(-1) for acetate amended columns as compared to 0.037 to 0.083 h(-1) in control columns (no amendment). The addition of soluble starch resulted in increased toxicity (based on Microtox(R) analysis) that was partially removed by biological activity in the columns. Ethanol addition itself did not result in increased toxicity but biological activity in this system did induce Microtox(R) toxicity. Acetate did not have any Microtox(R) toxicity associated with it. The addition of ammonium as a nitrogen source did not significantly increase the removal rate of RDX. Based on these observations acetate was selected for the field demonstration. Published by Elsevier B.V.