Laboratory investigation of explosives degradation in vadose zone soil using carbon source additions



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Explosives contamination in vadose zone soil presents difficulties in remediation. Because vadose zone contamination can extend deep into the subsurface and underneath existing buildings and utilities, excavation is often infeasible. In response, this dissertation focuses on the development and testing of a practical system to enhance the remediation of vadose zone explosives contamination.

Soil at the DOE Idaho National Engineering and Environmental Laboratory field area was characterized for explosives contamination. Of the soil tested, the particulate TNT retained on a 3 mm screen contributed approximately 2000 ppm (96.4%) of the overall soil contamination, compared to the soil that passed through the sieve, which averaged 75 ppm TNT. Contributing significantly to the contamination profile, heterogeneously dispersed, and likely point sources of contamination, the particulates thereby present difficulties in estimating the extent, risk, and treatability of explosives contamination in the soil.

For monitoring soil gases, a method was developed and validated using solid phase microextraction coupled with gas chromatography and mass selective detection (SPME-GCMS). The within-run precision (repeatability) was 3.5X tighter than the between-run precision (reproducibility) in the 4 days. The esters gave the best repeatability from 50 to 80 ppmv while the corresponding alcohols gave the best results at 10 to 20 ppmv. The method was applied to monitor gases in laboratory and field studies testing explosives remediation in vadose zone soil.

Anaerobic and microaerobic batch and column studies using soil from the DOE Pantex Facility contaminated with hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), and 1,3,5-trinitrobenzene (TNB) were performed using gaseous carbon source additions. In the anaerobic batch study, over 99 days, flasks periodically receiving headspace pulses of 330 to 570 ppmv n-propyl acetate yielded 97.5±0.3% TNB and 66.7±43.2% RDX removal. Using ethanol in place of n-propyl acetate yielded similar results. Two column studies were performed using throughputs of oxygen, nitrogen gas, and organic carbon combinations. The columns supported less robust HE degradation than the batch systems. This difference in HE degradation between batch and column work may indicate that a key factor accumulated in the headspace of batch flasks, but was continually removed in the columns.