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small brandlManufactured Gas Plant Remediation


Addressing our manufactured gas plant sites

From the early 1800s until the 1950s, prior to the availability of natural gas, communities relied on manufactured gas – or town gas – to produce fuel for street lamps, cooking, heat and lights. During this time, at least 1,500 manufactured gas plants (MGPs) operated within the U.S., including several throughout our service territory. In the 1930s, utilities began installing natural gas pipelines gradually eliminating the need for MGPs. A by-product of the gas manufacturing process was coal tar, a black substance that has a viscosity and appearance similar to driveway sealer. Typically, operators burned the coal tar in a generator, used at the plant sites or sold it for use in manufacturing preservatives, coatings, resins and other products. Unfortunately, MGP sites released coal tar into the environment at many sites. Today, we are working diligently to address the environmental impacts associated with our predecessor companies’ operation of former MGPs at three sites in Nebraska, three in Montana and one in South Dakota.

South Dakota MGPs

The Aberdeen Gas Company, one of our predecessor companies, operated a MGP from 1888 to 1926 near the intersection of 1st Avenue NE and Jackson Street in Aberdeen, South Dakota. We have owned this location since 1926 and razed the structures associated with the MGP operations in 1946. Coal, delivered by rail, was the main raw material for the Aberdeen MGP near what is now Browne Park. The rail bed remains and now serves as a bike path for the community.


Historically, some coal tar entered municipal storm and sanitary sewer lines with residual coal tar entering a city storm sewer that runs through the NorthWestern Energy property over to Moccasin Creek. In 2006, NorthWestern Energy constructed a treatment system to handle this water, eliminating ongoing releases of coal tar to this storm sewer.


In 2007, we removed the creek sediments affected by this old discharge. We lined a sanitary sewer pipeline running north of the Aberdeen MGP site in 2008 to prevent coal tar from infiltrating into the pipe through leaking joints and flowing to the municipal wastewater treatment facility. In 2011, we began collecting coal tar after we installed engineered collection trenches.


We continue to work with the South Dakota Department of Environment and Natural Resources on the Aberdeen MGP site cleanup as we make improvements to the water treatment system and design additional coal tar removal projects.

Nebraska MGP sites

Nebraska MGPs

Our predecessor companies operated MGPs in North Platte, Kearney and Grand Island. We work at all three sites using state of the art technology to characterize the nature and extent of impacts associated with former MGP operations. Two technologies -- Tar-specific Green Optical Screening Tool (TarGOST®) and ground penetrating radar (GPR) – in particular have been very useful for our investigations.

TarGOST, works by taking advantage of the fluorescent properties of coal tar. A tool with a sapphire window is pushed through the soil column and emits pulses of laser light, which shine on the soil touching the window. If fluorescent molecules are present, a fiber optic cable captures the fluorescence that is processed in an analyzer at the surface of the ground. Using this tool, we efficiently map the vertical and lateral extent of coal tar impacts. After we use TarGOST at various locations across a site, we load the TarGOST results into visualization software. Using the visualization software helps us understand the impacts in three dimensions and aids in determining where to place soil borings and monitoring wells, and ultimately an optimal approach to remediate the site.  

Montana MGP sites

The former MGP sites in Butte and Helena are on the Montana Comprehensive Environmental Cleanup and Responsibility Act (CECRA) list. For these sites, the Montana Department of Environmental Quality (DEQ) has accepted our voluntary Cleanup Plans per the Voluntary Cleanup and Redevelopment Act (VCRA). Although, the Montana DEQ did not add the Missoula, Montana, MGP site to the CECRA list, we perform site assessment and environmental monitoring activities consistent with the other two MGPs to assess the impacts of the former MGP operation.

NorthWestern Energy performs ongoing groundwater monitoring at the Helena, Butte and Missoula MGP sites. For the past several years, we have performed consistent semi-annual groundwater monitoring at all of these sites. The objective for groundwater monitoring has been to determine the concentration of constituents of concern in groundwater and continuously monitor to ensure that contaminants do not pose a risk to human health.

Due to its successful implementation at the Nebraska and South Dakota MGP sites, the TarGOST® technology is being explored as a potential tool for providing a more extensive and detailed characterization of the Helena MGP site. To help guide placement of TarGOST application locations, we used another technology known as ground penetrating radar (GPR) to model the subsurface features beneath the historical footprint of the facility. GPR scan will be used primarily to identify the presence or absence of known or conjectured underground features that may remain from MGP operations. GPR is especially useful at this site because the location of two gasholders was previously identified, and as such will function as comparative reference features for the rest of the site.

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