Mobility of arsenic in soil from the Rocky Mountain Arsenal area

Abstract

From 1942 to the early 1980s, Rocky Mountain Arsenal (RMA), a superfund site northeast of Denver in central Colorado, served as a facility for the development, manufacture, and disposal of toxic organic and inorganic chemicals including US Army surety agents (including nerve gas and blistering agents), munitions, propellants, and pesticides. Arsenic (As) in the form of Lewisite (blistering agent), arsenic trioxide (herbicide), trisodium arsenate, and arsenic trichloride (process intermediate) was present in extremely large quantities at RMA's South Plants Processing Area. Even though current cleanup efforts are likely to remove the vast majority of As presently polluting the soil and groundwater, there is still a future potential threat for the movement of residual levels of As into groundwater supplies. The distribution and movement of As were monitored over a 2.5-year period to evaluate the threat to groundwater by low levels of As. Because of access restrictions to RMA, an off-site meso-scale (0.6 m diameter by 1.83 m in height) weighing lysimeter study was conducted using excavated soil (i.e., Ascalon sandy clay loam) associated with As contamination at RMA's South Plants Processing Area. The long-term study revealed the persistence of As under aerobic soil conditions, and a limited, but perceptible, mobility of As (0.87% of the total applied As drained beyond 1.5 m) resulting from interacting physical, chemical, and biological factors. Results suggest that even though the movement of As is significantly retarded due to adsorptive processes, preferential flow and chemical factors (i.e., pH and redox potential) can mobilize As at point locations above permissible levels, if precautionary measures are not taken.