Helena, Montana

WET is currently supporting NorthWestern Energy with planning and execution of a remedial investigation at the former manufactured gas plant (MGP) in Helena, Montana. The site has been divided into five areas of concern (AOCs), including the MGP portion of the site and Petroleum Release 3422. Each AOC is undergoing targeted investigation to determine if historic or current practices have impacted soils or groundwater. Combining historic groundwater and soil data with additional multi-media sampling, the investigation is being completed to characterize multiple waste streams and waste forms across the site by defining the nature and extent of contamination both on and offsite.

Available hydrogeologic data indicates a groundwater mound is present, which has resulted in the migration of mobile chemicals across the site and to offsite monitoring wells. As part of this project, WET is responsible for performing semi-annual groundwater monitoring on the MGP and Petroleum Release 3422. The Montana Department of Environmental Quality (DEQ) has also required that the groundwater monitoring incorporate three newly installed wells and offsite wells that are part of the groundwater monitoring network for the closed Old Helena Landfill. Groundwater is acquired from 14 of the landfill wells as part of each event, to aid in evaluating impacts from the MGP.

The investigation has also required sampling and analysis of indoor air and sub-slab air quality and the collection of soil gas data from around both facility buildings. Both indoor air and soil gas have undergone sampling and analysis as part of each sampling event. Methane has been included in the monitoring program to support a better understanding of site conditions and if any interactions are occurring between the MGP and the landfill.