The BSB Wastewater Treatment Plant (WWTP) is located within the Butte Priority Soils Operable Unit (BPSOU), which is part of the Silver Bow Creek/Butte Area National Priority List (NPL) site. To the south, the WWTP borders the Montana Pole and Treating Plant (MPTP) NPL site. During dewatering activities associated with nutrient effluent upgrades at the WWTP, Pentachlorophenol (PCP) was noted in the dewatering effluent. Because the site is within BPSOU, the dewatering effluent was being released to the Butte Treatment Lagoons (BTL), which treats metals contaminated water from area sites. The BTL; however, was not designed to treat PCP contaminated water. After confirmation that the effluent water had metals and PCP contamination, mitigation strategies were developed to facilitate the needed upgrades at the plant and protect water quality in Silver Bow Creek. Of the developed mitigation strategies, the preferred strategy was for the MPTP to increase the pumping volume in their Near Creek Recovery Trench (NCRT) several weeks before initiation of dewatering at the WWTP. Groundwater modeling of the site suggested that this strategy will result in extension of the MPTP capture zone to an area between MPTP and the WWTP that has elevated PCP concentrations; thus, reducing PCP concentrations in dewatering effluent at the WWTP.
A demonstration pilot test of the preferred mitigation alternative was completed to better define affects on water quality, while determining aquifer properties and contaminant flow characteristics. This project included coordination between multiple local, state and federal government entities, multiple meetings, review of a 3-D ground water flow model of the site, additional flow modeling of mitigation strategies and field work including a subsurface Geoprobe investigation, metering of discharge flows, groundwater and surface water sampling (using a flow through cell with YSI Environmental Water Sampling Meter and contract lab) and water level monitoring (using YSI pressure transducers in multiple site monitoring wells). The data has been analyzed and a draft report has been submitted to multiple agencies.