The Solid State Circuits Inc. (SSC) site is an area of trichloroethylene (TCE) contamination originating from a former printed circuit board manufacturing facility. SSC operated the facility in the northern portion of a late 19th-century grain mill building at the site. The site is located on less than one acre at the southeast corner of Elm and Main streets, in downtown Republic, Missouri. SSC manufactured printed circuit boards in the building's northern portion from 1968 until 1973. The manufacturing process involved the use of TCE as a cleaning solvent, which was stored and used in the building's basement. A photographic processing firm operated at the site from 1973 until 1979, when the northern portion of the building burned. The burned portion was demolished and debris was pushed into the basement. The basement was filled in; the area is now a vacant grassy lot surrounded by fencing. The land bordering the site is now used for a mix of commercial, industrial and residential purposes.
Missouri Department of Natural Resource's Role
The Missouri Department of Natural Resources collected water samples from Republic's public water system in June 1982. TCE contamination was detected in Republic's municipal well CW-1, located 600 feet south of the SSC facility. The city took immediate action to remove the well from public use. The discovery triggered further investigations. Between April 1983, and March 1984, a subsequent investigation conducted by SSC contractors identified the facility as the source of TCE contamination in the well. No TCE has been detected in any of Republic's other public or private wells. In 1985, the U.S. Environmental Protection Agency removed more than 2,000 cubic yards of soil from the basement, the soil underneath the basement and assorted debris. EPA filled in the basement with gravel and soil cover to bring it up to grade. In addition, the abandoned basement well was plugged and two wells were installed to extract contaminated groundwater.
EPA placed the SSC site on the Superfund National Priorities List on June 10, 1986. The department and EPA entered into a consent decree with the Missouri Remedial Action Corporation Inc. (MRAC), an entity formed following a series of corporate successions and acquisitions by SSE, and the responsible party for cleanup. The department has overseen site investigation and cleanup actions that MRAC has conducted since that time.
Site Location Map
The SSC is organized into four areas of investigation:
- Area 1 is the parcel where SSC operated
- Area 2 includes the Main Street corridor between Area 1 and the former location of City Well #1
- Area 3 includes the area around former City Well #1
- Area 4 is the L-shaped area that extends farther south along the Main Street corridor before extending east along Brooks Street
Summary of Investigations and Cleanup
- Between 1985 and 2018, MRAC contractors collected and analyzed more than 400 soil samples from multiple boring locations and depths in all four areas of investigation. Results showed the highest TCE levels were in the soil directly beneath the former manufacturing building and in several locations near underground sewer lines to the south, along Main Street.
- In 1985, EPA contractors excavated more than 2,000 cubic yards of contaminated soil and building debris from Area 1 and transported it off-site for disposal at an approved facility. In 2012, MRAC contractors treated approximately 2,500 cubic yards of contaminated soil in Area 1 by physically mixing the soil in place with a substance designed to reduce TCE. In 2015, MRAC contractors treated approximately 6,000 cubic yards of contaminated soil in Areas 2 and 3 by injecting substances into the soil to reduce TCE and its breakdown chemicals. In 2020, MRAC treated an additional 2,900 cubic yards of soil in Area 1 by injecting a combination of treatment substances into the subsurface to reduce additional TCE and its breakdown chemicals.
- To date, MRAC contractors have installed more than 50 groundwater monitoring wells at the site. Regular well sampling has identified TCE and its breakdown chemicals in groundwater that extends south of the former facility, primarily along the Main Street corridor to U.S. Highway 60.
- In 1993, MRAC contractors installed a groundwater recovery and treatment system. The system consisted of six groundwater extraction wells, four of which were within area 1; two were between Area 1 and U.S. Highway 60. Groundwater extracted from the wells was treated to strip TCE from the water before discharging it to the sewer. However, the system's effectiveness dropped significantly after the initial 10-15 years of extraction. After the Dec. 8, 2011 fire destroyed the treatment system, groundwater extraction has been conducted only at the southernmost extraction well, located near U.S. Highway 60. TCE levels in the recovered groundwater from that location are low enough to allow direct discharge of recovered groundwater to the sewer, without treatment.
- In 2015, MRAC contractors also treated contaminated groundwater in Areas 2 and 3 by injecting substances into the subsurface to degrade TCE and its breakdown chemicals. In 2020, MRAC contractors treated groundwater in Area 1 by injecting a combination of treatment substances to reduce TCE and its breakdown chemicals.
Vapor Intrusion Investigation:
- From 2007 to present, MRAC contractors have conducted several sampling efforts to evaluate the potential for TCE vapors to travel through the subsurface and enter overlying structures- a process known as vapor intrusion (VI). TCE vapors can enter structures at the site in two ways: by traveling within soil gas beneath building foundations or crawlspaces, then entering through utility accesses, sumps, floor drains, foundation cracks and seams; or by traveling through sewers, then entering through poorly sealed plumbing connections within buildings.
- TCE vapors have been detected in sewer mains and shallow soil adjacent to subsurface utilities along the Main Street corridor and east along Brooks Street. Based on proximity to these source areas; in 2017, MRAC contractors requested access to conduct VI sampling at eight properties, property owners granted access to three of them. TCE was detected above the health-based screening level in one residential building with a basement foundation. TCE was not detected at the other two residential buildings with crawlspace foundations.
- In December 2020, MRAC contractors conducted additional sewer vapor sampling at manhole locations along Main Street, Mill Street and Brooks Street to further define the extent of sewer gas impacts. In 2021, MRAC will repeat sampling at those same locations during warm weather conditions. Based on those findings, additional properties may be identified for VI sampling.
Vapor Intrusion Mitigation:
- To potentially lessen health impacts on residents, between 2018 and present, MRAC contractors conducted several vapor-reduction activities at the only residence where TCE was detected. These actions included installing equipment to purify air inside the building and to remove vapors from beneath the building. In November 2020, MRAC contractors conducted a smoke test at the residence, which showed sewer vapors entering the basement through a faulty plumbing connection. In December 2020, MRAC contractors repaired the plumbing at the residence. Follow-up smoke testing and indoor air testing showed that the connection was successfully sealed, preventing further sewer vapor infiltration into the residence.
Additional Site Information:
- Federal, state and MRAC actions are addressing the site. The department is the lead regulatory agency; EPA plays a supporting regulatory role. MRAC installed the site's remedy, which included extraction of contaminated groundwater using extraction wells; on-site treatment of extracted groundwater using two air strippers; discharge of treated water; and implementation of a city ordinance to prevent construction of drinking water wells in or near the contaminated groundwater plumes. Later, EPA updated the remedy to include use of a horizontal reinjection well. Following the 2011 fire that destroyed the groundwater treatment system, on June 15, 2012, MRAC and the department entered into a force majeure/ excusable delay agreement. Over time, the parties have modified and extended the agreement. Under the agreement, MRAC is performing site investigation, vapor intrusion sampling, cleanup and groundwater monitoring activities.
- The agencies have conducted several five-year reviews of the site's remedies; these reviews evaluate whether the remedies put in place protect public health and the environment, and function as intended by site decision documents. The September 2017 review indicated that the protectiveness of the remedy could not be determined until further data and information is obtained- due primarily to the destruction of the groundwater treatment system. Further data and information will be obtained by taking the following actions: completing a comprehensive vapor intrusion study of all potentially impacted structures; addressing all soil source areas; delineating the contaminant plumes in all three water-bearing zones; and fully containing the contaminant plumes in groundwater.
Future Site Cleanup Activities
- In 2021 and 2022, MRAC contractors will conduct soil and shallow groundwater sampling in Areas 1, 2 and 3 to evaluate the effectiveness of the 2015 and 2020 subsurface injections.
- MRAC contractors plan to install additional groundwater monitoring wells in 2021 to define the boundary of the impacted groundwater.
- MRAC contractors will sample groundwater in Areas 1, 2 and 3 to continue to assess the effectiveness of the 2015 and 2020 treatments in those areas. Following installation and testing of additional monitoring wells in 2021, MRAC contractors will assess conditions and evaluate potential remedies for addressing remaining TCE contamination in groundwater.
Vapor Intrusion Investigation:
- MRAC contractors conducted additional sewer vapor sampling at manhole locations along Main Street, Mill Street and Brooks Street in December 2020, to define the extent of sewer gas impacts. In 2021, MRAC will repeat sampling at those same locations during warm weather conditions.
- In 2021, the department will increase communication and education efforts to expand public awareness of potential health concerns associated with TCE contamination. The department will also assist MRAC in gaining access to conduct vapor intrusion sampling at potentially affected properties, mainly residential properties, to more completely define the extent of TCE contamination from the site and to identify potential exposures to TCE above health-based levels.
Vapor Intrusion Cleanup:
- In 2021, MRAC contractors will conduct additional indoor air sampling at the only residence where TCE was detected, to assess the effectiveness of cleanup measures.
Solid State Circuits Site Cleanup Timeline
1984 - Proposed to National Priorities List
1985 - Initial Assessment Completed, Remedial Investigation Started
1986 - Site Finalized on National Priorities List
1989 - Final Remedy Selected
1991 - Final Remedial Action Started
1993 - Construction Completed
2008 - Site Ready for Reuse and Redevelopment
2012 - Force Majeure/Excusable Delay Agreement
2017 - Most Recent Five-Year Review
2021 - Continuing Focused Feasibility Study
2022 - Sixth Five-Year Review
MoDNR Site Sheet (Coming soon)
MoDNR Environmental Remediation Program - a Citizen's Guide to Vapor Intrusion Investigation at the Solid State Circuits Inc. Site (Coming soon)
Missouri Department of Health and Senior Services Facts About Vapor Intrusion
Agency for Toxic Substances and Disease Registry TCE Fact Sheet