Mobility of arsenic in soil from the Rocky Mountain Arsenal area

From 1942 to the early 1980s, Rocky Mountain Arsenal (RMA), a superfund sit e northeast of Denver in central Colorado, served as a facility for the dev elopment, manufacture, and disposal of toxic organic and inorganic chemical s including US Army surety agents (including nerve gas and blistering agent s), munitions, propellants, and pesticides. Arsenic (As) in the form of Lew isite (blistering agent), arsenic trioxide (herbicide), trisodium arsenate, and arsenic trichloride (process intermediate) was present in extremely la rge quantities at RMA's South Plants Processing Area. Even though current c leanup efforts are likely to remove the vast majority of As presently pollu ting the soil and groundwater, there is still a future potential threat for the movement of residual levels of As into groundwater supplies. The distr ibution and movement of As were monitored over a 2.5-year period to evaluat e the threat to groundwater by low levels of As. Because of access restrict ions to RMA, an off-site meso-scale (0.6 m diameter by 1.83 m in height) we ighing lysimeter study was conducted using excavated soil (i.e., Ascalon sa ndy Flay loam) associated with As contamination at RMA's South Plants Proce ssing Area. The long-term study revealed the persistence of As under aerobi c 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 phys ical, chemical, and biological factors. Results suggest that even though th e movement of As is significantly retarded due to adsorptive processes, pre ferential flow and chemical factors (i.e., pH and redox potential) can mobi lize As at point locations above permissible levels, if precautionary measu res are not taken. (C) 1999 Elsevier Science B.V. All rights reserved.