Stochastic management modeling of a pump and treat system at the Rocky Mountain Arsenal near Denver, Colorado

Past activities at the Rocky Mountain Arsenal in Colorado, now the Rocky Mountain Arsenal Urban Wildlife Refuge, have resulted in groundwater contamination of the shallow alluvial aquifer. In response the US Army and Shell Chemical Company, the responsible parties, have implemented a series of pump and treat systems to intercept the contaminated groundwater. This paper discusses a stochastic groundwater model for the management of a pump and treat system located in the Offpost operable unit at the Rocky Mountain Arsenal. The local hydrogeology at the site is complex, which resulted in an intricate array of pumping wells, recharge wells and trenches. The goal of the modeling effort was to develop operational guidelines for the pump and treat system, which took into account uncertainties in hydraulic conductivity while minimizing the likelihood that contaminated groundwater would bypass the system. The modeling approach treated hydraulic conductivity as a stochastic variable and incorporated its uncertainty in system performance by using geostatistical conditional simulations. The Turning Bands and Kriging methods were used to generate a series of equally probable hydraulic conductivity distributions, which preserved the measured hydraulic conductivity values at sampled locations, as well as maintained the same statistics and spatial correlation structure of the known data. Generated hydraulic conductivity distributions were implemented as input to a finite element flow and transport model in order to simulate the performance, within a Monte Carlo framework of management scenarios, of the operation of the pump and treat system. Particle tracking simulations were utilized to determine the effectiveness of the system to intercept the plume under varying uncertainties in the hydraulic conductivity distribution in order to identify an optimal pumping and recharge strategy to prevent contaminated groundwater bypassing the system. The analyses were carried out statistically based upon the probability to capture the plume and to reduce the concentration levels to regulatory standards for a given operational time period. Results allowed improvement in the operation of the system and provided an excellent tool in the decision-making process for the management of contaminated groundwater.