Design, Construction, and Operation of a Field Demonstration for In Situ Biodegradation of Vadose Zone Soils Contaminated with High Explosives

Abstract

Abstract : This report describes the first field demonstration of a technology for in situ remediation of vadose zone soils contaminated with high explosives (HE). The HE contaminants of primary concern at the demonstration site were hexahydro-1,3,5-trinitro-1,3,5- triazine (RDX) and 1,3,5-trinitrobenzene (TNB), with concentrations typically ranging from 5 to 35 ppm, well above the risk reduction cleanup criteria of 2.6 and 0.51 ppm for RDX and TNB, respectively. The shallow (<30 ft (9.15 m) depth) soils at the site could not be excavated due to the presence of buried utilities and operating facilities that could not be interrupted. Based on previous laboratory studies by the research team and others, it was found that the impacted soils had indigenous facultative and anaerobic microbial populations that could be encouraged to reduce the RDX and TNB in the soil. In this field application, wells were arranged in a five-spot well pattern with the injection well at the central position and sample extraction wells at the four outer positions (each 15 ft (4.57 m) from the injection well). A continuous flow of nitrogen (injection flow rate of 4.8 Limin) was injected into the target vadose zone soils with the intent of creating an anaerobic environment to stimulate the HE degraders. The system was monitored periodically for gas composition (oxygen, methane, and carbon dioxide), HE concentrations, and microbial activity in retrievable soil samples. After 295 days of treatment, the target HE concentrations dropped from the initial site averages of 18 i 2.8 ppm RDX and 17.1 i 3.3 ppm TNB to the final site averages of 10.8 i 1.9 ppm RDX and 10.3 i 2.1 ppm TNB. These values represented 40-percent reductions in concentration. Microbial activity was monitored using an indirect RABIT method, which indicated the presence of an active microbial community during the entire treatment period. Operation of this remediation technology continues at the site.