Revised On: Feb. 18, 2024 - 9:20 p.m.

2015: COT has been dealing with numerous contracting issues pertaining to its proposed P+T system.

2014: COT evaluated options for handling the discharge water from COT’s proposed P+T system and decided that part of the treated water will be used for irrigation at a golf course lake and part of the water will be put into Tucson Water’s Sweetwater Recharge Facilities reclaimed water system.

2013: With the additional modeling of SWR site contaminants complete, COT has determined that the Kinder-Morgan gasoline release would reach COT’s proposed groundwater extraction wells within approximately eight years of pumping. Kinder-Morgan is planning on expanding their existing SVE system to continue to remove the remaining mass of gasoline contamination, and installing a groundwater treatment system to extract and treat gasoline contaminated groundwater by mid-2014. Additionally, COT has determined that injection of treated water from their groundwater extraction system will not be feasible and is undertaking an engineering study to determine how and where to discharge treated groundwater.

2012: Prior to finalizing its conceptual design for the proposed groundwater P+T system, COT is having its contractor perform additional modeling to predict when the SWR site contaminants-gasoline additives methyl tertiary-butyl ether (MTBE) and tert-butyl alcohol (TBA) will reach COT’s proposed groundwater treatment system, (Benzene is also a contaminant of concern at the SWR site). 

2011: COT hired a contractor to review the most recent field data (including data from the more recently-installed wells), revise the conceptual site model (CSM), and update the modeling study. The model was to be updated to reflect revisions in the CSM and to account for the data collected from 2006-2011. To maximize contaminant removal by COT’s proposed groundwater P+T system, the contractor also was tasked with evaluating additional extraction and injection well arrangements and pumping rates. This work was completed in the fall. The ERA system will focus on removal of the contamination from the “shallow” part of the aquifer, which contains most of the contaminant mass, and reduction (to the extent feasible) of the migration of the most contaminated groundwater towards COT’s Sweetwater Recharge Facilities (located downgradient) extraction wells.

2010: In January, COT submitted to ADEQ its revised RAP Implementation Evaluation of remedial alternatives report for review. In this report, COT proposed the installation of a groundwater P+T system to address the central part of the landfill with the highest VOC concentrations. The plan includes four extraction wells, an air stripping treatment plant with granular activated system to capture VOCs in the vapor phase, and four injection wells.  One potential complication not addressed by the plan is the commingling of the Kinder Morgan plume with the Silverbell Landfill PCE plume.

In the fall, ADEQ provided COT with conditional approval of the plan as an ERA, but asked COT to further define the extent of the plume to the northeast. COT installed two shallow- and intermediate-zone well pairs between the Silverbell Jail Annex Landfill and the Miracle Mile WQARF sites. These wells were sampled during the fall groundwater monitoring event which was coordinated with Tucson Water, ADEQ, and Kinder Morgan so that the well water gauging for the Sweetwater Recharge Facilities, the Miracle Mile site, and the Kinder Morgan site were conducted during the same period. 

2009: As of December, Kinder Morgan Energy Partners has extracted approximately 172,300 pounds (approximately 28,000 gallons) of hydrocarbons from the soil beneath the site.

2008: The COT Environmental Services, with assistance from Tucson Water, developed a groundwater fate and transport model for the site. The COT has been evaluating a P+T system for plume containment using this model to prevent further off-site migration. 

2007: In August, Kinder Morgan began operating an SVE remediation system to clean up contaminated soil beneath the site.

2006: In May, COT included the south cell in the pilot test. Based upon the pilot test results, sodium lactate was shown to be a suitable nutrient to stimulate anaerobic bioremediation and has reduced PCE concentrations within both the north and south cells. However, COT has determined that use of this remedial technology for the final remedy would be cost-prohibitive.

1999 – 2005: From October 1999 to September 2005, COT operated an SVE system to mitigate the source. This system removed a total of 2,061 pounds of VOCs. The system has remained off since achieving cleanup goals, but has operated occasionally to control methane migration.

In July 2003, a pipeline operated by Kinder Morgan broke resulting in the release of gasoline to an area near the site. Cleanup of the gasoline contamination is being conducted under ADEQ’s Voluntary Remediation Program (VRP) and the site has been named the (SWR) site. The gasoline contamination and the existing PCE and TCE plume emanating from the Silverbell Jail Annex Landfill are being monitored.

The COT has evaluated monitored natural attenuation with enhanced bioremediation for the source areas. A pilot test using sodium benzoate to stimulate indigenous microbes began in June 2003.

In 2004, Kinder Morgan identified the presence of VOCs in the soil vapors within the Silver Creek residential subdivision. ADEQ believed these soil vapors were unrelated to the gasoline pipeline rupture. ADEQ conducted additional soil vapor testing in early October 2004 to confirm and expand upon the Kinder Morgan data. With that data, the Arizona Department of Health Services conducted a Health Consultation to evaluate whether soil vapors from VOCs in the subsurface at the Silver Creek subdivision posed any health risks to residents. The report, released in June 2005, concluded that the observed concentrations of compounds in soil vapors pose no apparent public health hazard. In July, COT expanded the pilot test by constructing an automatic mixing and delivery system for the north cell area, and changed the added nutrient to sodium lactate. 

1999: The site was placed on the WQARF Registry in April with an eligibility and evaluation score of 51 out of a possible 120.

1996 – 1998: ADEQ approved COT’s request to conduct a pilot study to evaluate the effectiveness of a recirculation well to replace or enhance the approved P+T remedy.

1995: In June, COT completed an interim final remedial action proposal (RAP). The approved RAP proposed a P+T system utilizing air stripping treatment and a carbon filter to capture exhaust from the air stripper. Treated water would be reinjected into the aquifer and/or reused at Silverbell Golf Course. Although this groundwater remedy was not implemented, COT evaluated several groundwater remedial pilot projects and installed an air injection/soil vapor extraction (SVE) system.

Also in June, ADEQ closed the University of Arizona’s West Campus Agricultural Center site. Data and information collected and available as of June 1995 did not show that this site was a source of PCE and trichloroethene (TCE) contamination in the groundwater.

1983 – 1985: In fall 1983, groundwater contamination was discovered at the Tratel Mobile Home Park Production Well located east of the site. Also in 1983, COT began conducting subsurface investigations, and ADEQ began conducting groundwater investigations in 1985.  VOCs were identified in the groundwater at concentrations exceeding Aquifer Water Quality Standards (AWQS). 

1960 – 1978: The University of Arizona operated a solid waste landfill in the area east of the Santa Cruz River and south of Fort Lowell Road. The COT-managed Silverbell Jail Annex Landfill is made up of two landfill cells, approximately 40 acres in total size, which received wastes between 1966 and 1977. Neither cell accepted hazardous waste, although landfill access was not strictly controlled. The South Cell has been covered with soil, and the North Cell underlies a portion of the Silverbell Golf Course.