Congressional District # 07
LONG PRAIRIE GROUND WATER CONTAMINATIONEPA ID# MND980904072
Last Updated: January, 2012
The Long Prairie Groundwater Contamination Site is a 7,000 square-foot source area underlain by a one-half mile long groundwater plume, located in Long Prairie, Todd County, Minnesota. The contaminant plume originates in the commercial area of Long Prairie and extends through an older residential area of the city. The Site is also located in the downtown commercial district of Long Prairie.
The city of Long Prairie is a community of less than 3,000 persons, only a few of whom still have private wells. About 350 homes supplied by private wells were affected by the groundwater contamination until municipal water main extensions and service connections were provided.
The state's investigation identified the source of contamination to be a dry-cleaning facility at 243 Central Avenue, which operated from 1978 until mid-1984. Improper waste disposal of the dry-cleaning solvent tetrachloroethylene (PCE) and related sludges from the facility resulted in the eventual migration of volatile organic chemicals (VOCs) to the soil and groundwater.
The site was discovered in 1983 during the Minnesota Department of Health's (MDH) statewide VOC analysis of public water supplies. The municipal water supply of Long Prairie was found to be contaminated with PCE and its degradation products, trichloroethylene (TCE), cis-1,2-dichloroethylene (DCE), and vinyl chloride (VC). The United States Environmental Protection Agency (U.S. EPA) placed the site on the National Priorities List (NPL) on June 10, 1986.
Site ResponsibilityThis site is being addressed through federal and state actions.
Threats and Contaminants
The groundwater and soils are contaminated with VOCs, specifically PCE, TCE, DCE, and VC. Most of the residents are connected to the City of Long Prairie water supply and have abandoned their private wells. The cleanup is operating to protect the city wells that are currently being used and to restore the groundwater quality of the aquifer. Institutional Controls (ICs) have been implemented at the site to prevent exposure to contaminated groundwater. These include the Health Advisory issued in 1983 by the MDH for residential wells in a 15-block area of the city. The affected residents were provided bottled water until they were connected to the municipal supply in 1984. An Extended Health Advisory was set in 1994 after additional testing showed five private wells contaminated with PCE. These residences and 15 others in this extended area were connected to the municipal water supply in January 1994. Together, these advisory areas fully cover the area where the groundwater exceeds cleanup goals.
In January 2007, the MDH designated a Special Well Construction Area (SWCA) to prevent the installation of new water supply or monitoring wells without the approval of the MDH in consultation with the Minnesota Pollution Control Agency (MPCA). The SWCA geographically encompasses both the full contaminant plume area and the two Health Advisory Areas. The U.S. EPA and MPCA are currently determining whether additional mechanisms to restrict groundwater use will be needed at the site.
From 1983 through 1985, municipal and private wells were closed. The State of Minnesota provided emergency bottled water and funded a study of interim cleanup alternatives. The City of Long Prairie installed a new municipal well and additional water mains. The residents who connected to the mains were reimbursed by a state compensation program for 75 percent of the costs.
In 1984, U.S. EPA funded a remedial investigation and feasibility study (RI/FS) through a multi-site cooperative agreement with the MPCA. The RI studied the nature and extent of site contamination; the FS was a study of site cleanup alternatives. The potentially responsible parties (PRPs) are those parties U.S. EPA determined may be legally responsible for the site's contamination, were financially unable to pay for this work. The MPCA completed the work in 1988.
The Record of Decision (ROD), a document that explains the selected site cleanup plan, was signed in June 1988. It called for treating the soil with a soil vapor extraction (SVE) system, extracting the contaminated groundwater and treating it with an air stripper to remove the VOCs ("pump-and-treat" system).
From 1988 through 1991, the remedial design and remedial action (RD/RA) was performed by MPCA under the existing multisite cooperative agreement. Due to a concern that the contamination was simply being transferred from the groundwater to the air, a 1991 Explanation of Significant Differences (ESD) amended the ROD by expanding the treatment to include a granular-activated carbon (GAC) system. The GAC system was to remove the VOCs from the air stripper vapor stream before it was discharged to air. The MPCA completed the design of the GAC system in 1993, but no construction bids were received until January 1995. In the mean time, the plume had expanded, necessitating more bottled water provisions and a second ESD in 1994. This ESD specified an alternate water supply for the affected residences and monitoring of the residential wells.
The groundwater project construction (operable unit 1, or OU 1) was completed in 1996. The construction of additional water mains and the connection of additional homes to the municipal supply (OU 3) were completed in 1996. Construction of the SVE system (OU 2) was completed in spring 1997.
The groundwater pump-and-treat system operates at a rate of 200 to 250 gallons-per-minute (gpm) and is expected to continue for about 15 more years. The groundwater data show that the plume is shrinking in size. The SVE operated for four years and was decommissioned on March 22, 2000, after the soil cleanup goals were achieved. Two additional extraction wells were installed in 1999 to fill breaches in the plume capture area.
The preliminary close-out report, which documents the RA construction completion of all three OUs, was completed in September 1997. The first five-year review report was completed in September 2002 and found that the remedy was protective, and that long-term protection will be achieved upon attainment of the groundwater cleanup goals.
In June 2003, the MPCA completed a well and groundwater receptor survey to identify any new wells in use within the groundwater advisory area. Residential well sampling was also conducted in 2003, 2004 and 2006. In October 2004, two additional monitoring wells were installed to better detect any contaminants moving toward one of the city wells. The city wells are tested twice each year and the site monitoring wells are tested one to two times each year. Private wells are sampled periodically and their usage is checked to ensure that human health and the environment are being protected. Routine groundwater monitoring will continue to ensure that the system is effectively containing the plume and that contaminants are being recovered from the groundwater.
The MPCA conducted groundwater modeling in 2006 and 2007 to evaluate plume capture and discharge to the Long Prairie River and to verify that the city wells were still being protected by the onsite groundwater pump-and-treat remedy. The evaluation concluded that under existing conditions, the groundwater pump-and-treat system is protective of human health and the environment and is operating as intended, but that cleanup goals would not be attained for another 15-20 years. Therefore, in March 2007, the MPCA injected a mineral oil mixture into the groundwater to increase the biodegradation rate of the groundwater contaminants. This was done because the reduction of PCE levels had slowed down the natural attenuation rate. Monitoring since March 2007 has shown a decrease in PCE levels in the area. Since this pilot test was successful, MPCA injected identified hot spots within three additional areas in September 2008. Follow-up monitoring is ongoing to determine whether this is a successful remediation approach.
The second five-year review was completed in September 2007. Groundwater data showed that the contaminant levels continue to decline and the upgraded groundwater remediation system is effectively containing the groundwater that is still contaminated above site cleanup goals. One of the contaminants (DCE) has migrated into the wetlands adjacent to the Long Prairie River on the north end of the site; however, the detected levels are too low to impact the river ecosystem. At the time, the review also determined that there was little potential for subsurface vapors to migrate into buildings overlying the plume area. The five-year review found the site to be protective of human health and the environment, but that long-term protectiveness will be achieved when all necessary ICs are in place.
As a result of a Spring 2009 follow-up investigation of the pilot injections in the source area, MPCA discovered high soil vapor readings where the original SVE system had operated. Sub-slab vapor samples and indoor air measurements indicated that industrial indoor air standards were being exceeded. The SVE system had been removed in 2000 after it sucessfully treated VOCs in the source area soil to below the ROD cleanup criteria. In 2010, MPCA installed a new SVE system in the source area. Soil vapor investigations and sampling are expected to continue.
ContactsRemedial Project Manager, U.S. EPA
Karen Mason-Smith (firstname.lastname@example.org)
Community Involvement Coordinator, U.S. EPA
AliasesLONG PRAIRIE GROUNDWATER CON