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Diisopropylmethylphosphonate (DIMP) is an organic compound produced as a by-product of the manufacture of a chemical munition at the Rocky Mountain Arsenal, Colo. Groundwater contamination occurred when industrial effluent containing DIMP seeped into the water table below unlined industrial waste-disposal ponds during 1952 to 1956. In 1975, groundwater containing DIMP was found in a 73 km2 (28-mile2) area to the northwest of the arsenal and was detected within 1.6 km (1 mile) of municipal water wells supplying the city of Brighton, Colo. The DIMP concentrations in the groundwater ranged from 0.5 µg/litre, the analytical detection limit, to as much as 44 000 µg/litre near the abandoned waste-disposal ponds. A mathematical model was used to simulate the movement and concentration of DIMP in groundwater near the arsenal to determine the historical distribution and concentration of the contaminant and to predict the possible future distribution of the contaminated groundwater.
Model simulations of historical DIMP movement in the alluvial aquifer indicate that the contaminated groundwater flowed rapidly to the northwest and began discharging into the South Platte River about 4 years after the contaminant first entered the aquifer. It is estimated that from 1958 to 1960 the mean annual DIMP concentration was 30 µg/litre in the river. Modeling indicates that mean annual DIMP concentrations in the river have decreased since the period 1958 to 1960 to less than 1 µg/litre in 1976.
Model simulations of future DIMP movement in the aquifer indicate that groundwater containing DIMP in excess of 0.5 µg/litre will not reach the Brighton municipal wells by 1995, even if no remedial measures are taken to prevent the discharge of DIMP-contaminated groundwater into an irrigation canal which recharges the aquifer near Brighton. A barrier impermeable to groundwater movement, now under construction at the northern arsenal boundary, is shown by the model to be an effective means of stopping off-arsenal movement of the contaminated groundwater without producing adverse effects on the water levels in the aquifer.
solute-transport modeling, water pollution, industrial wastes, model studies, groundwater pollution, diisopropylmethylphosphonate (DIMP), permeability, groundwater
Hydrologist, U.S. Geological Survey, Lakewood, Colo.