In 1965, Zenith purchased the parcels making up the Site and constructed a radio manufacturing facility. There were no industrial facilities on the Site before this time. Zenith constructed six underground storage tanks to store acetone, isopropanol, white gas, lacquer thinner, 1,1, 1-trichloroethane (1, 1, 1-TCA), and gasoline.
In 1980, General Motors (GM) purchased the Site and began testing throttle-body injection fuel systems at the Site. As part of its operations, GM used an above-ground Stoddard solvent tank farm, but did not use Zenith’s underground storage tanks. In 1984, GM removed the underground storage tanks. GM stopped production at the Site in 1993, and removed the tank farm in 1994. The chemicals known to be used on Site by Zenith and GM do not coincide with the chemicals making up the groundwater contamination, except for the 1,1,1-TCA.
In 1993, the Site underwent Phase I and II assessment by GM in preparation for its sale. These assessments identified the existence of CVOCs on Site, which GM then reported to the Iowa Department of Natural Resources (IDNR). After completing a preliminary assessment, EPA deferred the site to the IDNR for cleanup oversight in 1996. Under IDNR, a remedial investigation and feasibility study were completed. These investigations revealed levels of CVOCs in the groundwater above the EPA-established maximum contaminant levels (MCLs) for drinking water. They also revealed an area of contaminated soil that could be the source for the groundwater contamination. The soil contamination was not found at levels that pose a risk to human health from direct exposure.
In 2001 a state record of decision (ROD) was signed. In accordance with the state ROD, GM operated Municipal Well 3 (MW3), and constructed a hydraulic capture system (HCS) and a butane biostimulation
system. These systems were operational by the end of 2006. The HCS is a series of pumps designed to
keep contamination from migrating. MW3 is pumped to waste, and intercepts contamination off site before
it reaches the rest of the Riverfront well field. It was formerly used as a municipal drinking water well, but
was disconnected from the water supply when it was found to be contaminated. Its usage to intercept
contamination was concluded in 2016 due to reduced well productivity. The butane biostimulation system
was meant to bioremediate contaminated soil and groundwater on site. A butane biostimulation pilot study
for groundwater in the source area was initiated and showed some concentration reductions.
However, the pilot study was not conducted for the source area soils and ended when GM declared
bankruptcy. The butane injection system’s infrastructure installed on the site was decommissioned in 2014.
In 2009, GM declared bankruptcy and sold its assets to General Motors, LLC, a separate and independent entity. At this point, GM became Motors Liquidation Company (MLC), which is responsible for settling the company’s liability. MLC reached a settlement with the Department of Justice for the liability associated with the GM Sioux City Site for $6.5 million, to be disbursed as soon as the bankruptcy order is filed. The state was unable to ensure use of the funds for the cleanup, so the money will be administered by the EPA. GM has not maintained the HCS since it declared bankruptcy.
An Action Memorandum was signed in April 2011. Activities conducted since that time include:
• Extensive maintenance of the HCS and subsequent operation.
• Evaluation of the contaminated soil in 2012.
• Work with EPA’s Office of Research and Development (ORD) on modeling.
• Several Regional Decision Team (RDT) meetings.
Based upon the findings of the amount of soil contamination, it was initially thought that soil removal would expedite remediation of the site. Modeling by ORD however, indicated that soil removal would have limited impact on down gradient ground water concentrations. It was subsequently decided at the latest RDT meeting to not excavate soil.
In 2014 the Site's current owners have completed the construction of a warehouse annex that has covered approximately 2 acres of the Site. The area is now covered with concrete under the pad of the warehouse. The improvement is located
immediately west of the existing warehouse building.
Since 2015, Semiannual samples have been collected from the HCS recovery wells, select wells from the
Sioux City riverfront well field, and from the monitoring well network. Maintenance for the HCS was conducted on an as needed basis. To continue these activities, an Action Memo Amendment was signed May 4, 2022, to increase the funding ceiling.
In 2023, a fourth Action Memorandum was signed to conduct more significant maintenance of the HCS
which is reaching the end of its working lifespan. Once the HCS has been refurbished, management of the
HCS and site were planned to fully transition to the EPA Remedial program. In 2025, a memo to file documented the intended
program management transition, and a Post Removal Site Control plan was developed by the EPA removal
program to document continuing requirements to maintain the protectiveness of the current remedy. At the
conclusion of this removal action full site management will transition to the EPA Remedial program.
