Evaluation of Recharge Trench System, North Boundary Containment Treatment System, Rocky Mountain Arsenal, Commerce City, Colorado

摘要

This report summarizes hydrogeologic investigations completed in 2002 on the recharge trenches at the North Boundary Containment Treatment System (NBCTS) at Rocky Mountain Arsenal (RMA), located in Commerce City, CO. The NBCTS is a 6,740-fT (2,054-M)-long multicomponent system that precludes off- site movement of contaminated water at the north boundary of RMA. It consists of a slurry wall, dewatering wells, treatment plant, and recharge trenches. Prior to 1988, treated water was recharged back to the shallow unconfined aquifer by means of recharge wells. Over time, these wells lost their efficiency as the result of microbial fouling, and 15 recharge trenches were constructed along the length of the system to replace the recharge wells. To address concern regarding the continued hydraulic efficiency of the system, methodologies were developed to evaluate, on a year-to-year basis, trench hydraulic conductivity. lnitially, hydraulic conductivity was detennined on individual trenches and trench sets according to Darcy's Law using averaged values of recharge and hydraulic gradient, as available in annual assessment reports. In 1992, field testing was initiated, which consisted of cessation of recharge to a given trench and measurement of the decline or water levels in the trenches with time. These tests were modeled after field slug tests, and the resulting drawdown rates, which were propoflional to hydraulic conductivity, were considered to provide relative information on changes in trench condition with time. In 1996, testing was again revised, and the earlier methods were reestablished except that water levels were measured in the field and recharge (now a Imown quantity) was held constant during the water level reading. Having established steady-state conditions, hydraulic conductivity could be calculated for each trench on the basis of Darcy's Law. After 8 years of testing (including all methods), test results were remarkably uniform.