NIBW Site History
NIBW Site History
NIBW Site History
In October 1981, industrial solvents known as volatile organic compounds (VOCs), chiefly trichloroethene (TCE), were detected at concentrations above the drinking water standard in two water supply wells operated in South Scottsdale by the City of Phoenix. As a result, the Environmental Protection Agency (EPA) listed the Indian Bend Wash Superfund Site on the National Priorities List in September 1983. Originally, the Indian Bend Wash Site was defined as a 13 square-mile area of south Scottsdale and north Tempe. The EPA subsequently designated the portion north of the Salt River as the North Indian Bend Wash (NIBW) Site. The EPA currently defines the site as any zones of soil and groundwater affected by VOCs, which require remediation.
Various manufacturing and other industrial facilities, including the former Motorola plant located on the southeast corner of Hayden and McDowell Roads, operated at the NIBW site beginning in the late 1950s. Some of the facilities used organic solvents, such as TCE, that subsequently entered the soil and groundwater in portions of the site. The use of TCE was discontinued at the Motorola facility in the early 1970s.
Remedial actions at the NIBW Site consist of three principal activities:
• Site Characterization
• Groundwater remediation of the middle and lower aquifer units
• Soil remediation of known source areas
The EPA, Motorola, and other responsible parties have conducted extensive studies to identify sources of VOCs affecting soils and understand the occurrence and movement of VOCs in groundwater.
Motorola installed 26 groundwater monitor wells at the NIBW site between 1983 and 1985. TCE was found in groundwater in three distinct aquifer units present at the site: the upper, middle, and lower alluvium units (the different layers of sediment at the site). More than 150 groundwater monitor wells have now been installed at the site to help understand the extent of groundwater impacts.
Based on results of shallow soil gas and groundwater monitoring investigations conducted by the EPA and Motorola, 15 potential source areas of VOC impacted soils were identified in the NIBW Remedial Investigation/Feasibility Study, prepared by the EPA's technical consultant in 1991. Subsequent investigations and analyses conducted by the EPA and the involved parties focused investigation efforts on 10 potential source areas at the site.
Groundwater remediaton of the middle and lower aquifer
In September 1988, the EPA issued a Record of Decision for Operable Unit I (OUI) to address groundwater contamination in the middle and lower alluvium units. This aimed to contain the middle and lower alluvium unit groundwater in areas where water is extracted to supply the City of Scottsdale and where VOCs occurred at concentrations in excess of federal drinking water standards (note, this is after COS had acquired the City of Phoenix wells in South Scottsdale). Remediation was carried out by extracting groundwater at four existing City of Scottsdale production wells for treatment at a central facility with air stripping and vapor-phase carbon adsorption.
The Central Groundwater Treatment Facility (CGTF) began operating to implement the OUI remedy in October 1993. Motorola and the other participating companies (Siemens, and GlaxoSmithKline) voluntarily made the following improvements to the OUI remedy to improve capture and containment of middle and lower alluvium unit groundwater:
• Replacing a chiefly middle alluvium unit extraction well, COS75, with a new lower alluvium unit only extraction well, COS75A, in 1995
• Installing a downgradient lower alluvium unit extraction well PCX-1 in 1997
• Constructing the Miller Road Treatment Facility for treatment of groundwater extracted from PCX-1 and two Arizona American Water Company supply wells
• Implementing a middle alluvium unit source control program at the Motorola facility in early 1999, comprised of two extraction wells capable of a combined pumping rate of 1,800 gallons per minute (gpm)
• Implementing a middle alluvium unit source control program at Siemens' Area 7 in late 1999, comprised of three extraction wells capable of a combined pumping rate of 500 gpm.
These voluntary enhancements became part of the NIBW remedy when they were included in the Amended Consent Decree (CV-91-1835-PHX-FJM) signed in June 2003. The Amended Consent Decree supersedes the OUI and the OUII Consent Decrees for the NIBW site.
Soil remediation of VOC source areas
In September 1991, the EPA issued a Record of Decision for Operable Unit II (OUII) to address VOC impacts in the vadose zone (unsaturated sediments above the water table) and upper alluvium unit groundwater system. This aimed to stop further migration of VOCs to upper alluvium unit groundwater and to assess whether the VOCs in the groundwater were migrating to other areas. It required vadose zone investigations and, if warranted, remediation at several potential source areas and monitoring of mass migration of groundwater to the middle and lower alluvium units for extraction and treatment by OUI.
Vadose zone and upper alluvium unit groundwater investigations and groundwater impact analyses associated with OUII were conducted between 1992 and 1995. Based on the results from these programs, vadose zone remediation was implemented at the Motorola facility and three Siemens source areas. The soil cleanup effort has removed more than 9,000 pounds of TCE and assured that the source areas are no longer a threat to groundwater.
Optimization of Long Term Upper Alluvial Unit (UAU) Groundwater Monitoring
The Groundwater Monitoring and Evaluation Plan (GMEP) requires an annual assessment of the scope and frequency of monitoring activities to optimize program effectiveness over time. In the first Five-Year Review of the NIBW Superfund Site (2011), EPA comprehensively reviewed groundwater monitoring data obtained pursuant to the GMEP and concluded significant progress had been achieved toward restoration of the UAU. Based on this finding, EPA and the NIBW PCs agreed to reassess and revise the UAU groundwater monitoring program as part of an optimized approach to be adopted in an updated GMEP.
Since May 2012, the NIBW Technical Committee had discussed an approach for future UAU groundwater monitoring that included streamlined groundwater data acquisition. This approach was documented in the NIBW’s PC’s revised long-term UIAU groundwater monitoring program proposal entitled, Final Technical Memorandum - Recommendations for Upper Alluvial Unit Aquifer Long-Term Groundwater Monitoring Program, North Indian Bend Wash Superfund Site, Scottsdale, Arizona, dated March 11, 2013. This proposal, which was approved by EPA, recommended formal abandonment of 30 UAU monitoring wells that were no longer needed to define either water level or water quality conditions in the UAU. The UAU wells were successfully abandoned in accordance with all Arizona Department of Water Resources (ADWR) requirements during field work conducted from August 2, 2013 to January 9, 2014.
The stated goals of the revised UAU monitoring program, consistent with the GMEP, are to make the NIBW groundwater monitoring program less repetitive, and to make resulting groundwater data reports more valuable to all interested parties. The UAU well abandonment program helped accomplish these goals by:
• Abandoning wells in areas where the groundwater has consistently been significantly below cleanup levels defined in the NIBW Site ROD.
• Reducing the number of UAU wells that are monitored to 23.
• Reducing the monitoring frequency to an annual water level and sampling event for UAU wells in October.
• Focusing ongoing UAU groundwater monitoring activities on aquifer restoration in the localized areas associated with Area 7 and Area 12 where low levels of TCE still occur.