Adsorption of RDX to Soil with Low Organic Carbon: Laboratory Results, Field Observations, Remedial Implications

The purpose of this article is to review both laboratory and field observations of RDX adsorption to soils and to use those results to estimate the effects of a planned remedial action. Adsorption isotherms for RDX are generally observed to be linear and reversible. Statistical tests were performed to determine the relationship between Kd and various soil characteristics. A linear relationship between Kd and soil organic carbon was observed, as expected, but regression of Kd to organic carbon content indicated a non-zero intercept, suggesting that other sorbents may also be significant at low OC (e.g., > 0.5 %). No other soil properties were significantly related to Kd so the mechanism of adsorption at low organic carbon was not determined. These results were used to interpret observations of RDX in the vadose zone at Milan Army Ammunition Plant (MAAP), TN. MAAP exhibits widespread soil contamination by RDX. Depth to groundwater ranges from 40 to 80 ft. Unsaturated soils are fine grained near the surface, and sandy near the water table. RDX is concentrated in the upper 2 ft, where concentrations in some places exceed 1 %. Subsurface concentrations are generally less than 50 mg/kg. The distribution of RDX in soil, soil moisture and groundwater, and soil physical testing data were interpreted using simple models. The distribution of RDX is consistent with the following conceptual model: • Water containing RDX was dis charged to the land surface (prior to 1983); • Crystalline RDX remains in surface soil (remedial activities are ongo ing); • Infiltrating rainwater leaches RDX from surface soils; • This leachate carries RDX through the deeper vadose zone, resulting in significant soil contamination through out the full thickness of the vadose zone; these soils can generate leachate and adversely affect ground- water quality for many years to come. Field results were consistent with the adsorption studies. Simple models consistent with the field and laboratory observations indicate that deeper soils that are not planned to be remediated may continue to leach unacceptable concentrations to groundwater for approximately 180 years. The Army intends to evaluate whether it will be most cost-effective to address this continuing source by treating soils or groundwater.