Abstract : This ESTCP demonstration evaluated the technical effectiveness of in situ bioremediation as a treatment technology for explosives, including RDX, HMX, and TNT, in groundwater at the Picati1my Arsenal in Dover, NJ. A recirculation cell design was employed to distribute and mix cheese whey with contaminated grotmdwater in order to promote the biodegradation of explosives by indigenous bacteria. The system was operated in a semi-passive mode, such that the whey solution was added to groundwater during active pumping cycles (3-5 days), and then the system was shut down for 6 to 12 weeks. This remedial approach proved to be highly effective for nitramine and nitroaromatic explosives. The applicable regulat01y guidance and! or action levels were achieved for RDX and TNT, and there vas no significant accumulation of degradation intennediates. Moreover, due to the semi-passive operation, operation and maintenance costs were minimal. The results from this project suggest that in situ bioremediation of explosives in groundwater using semi-passive cosubstrate addition can be a viable long-term treatment approach.