Location

The West Plume B project area is a narrow northwest trending groundwater plume located between Valencia and Drexel Roads, just north of the 162nd Fighter Wing Arizona Air National Guard (AANG) project area, and east of the main TIAA plume in Tucson, Arizona.

Contaminants of Concern

The current contaminants of concern in groundwater include volatile organic compounds (VOCs), mainly trichloroethylene (TCE) ranging from non-detect to about 8 parts per billion (ppb). The federal drinking water standard for TCE is 5 ppb.  Contaminants of concern at the site may change as new data become available.
Health Concerns:  The City of Tucson is the municipal water provider at this site. All municipal wells in the area that were contaminated with TCE have been shut down. Most of the domestic wells have either been shut down or converted to irrigation wells. If you are drinking water from a private well within the boundaries of the site, please contact the ADEQ Project Manager.

Site Hydrogeology

In Area B of the TIAA site, which includes the West Plume B project area, the regional aquifer is comprised of two aquifer zones separated by a middle aquitard. The middle aquitard limits the vertical extent of contamination to the upper zone of the regional aquifer. The upper zone of the regional aquifer extends from the water table located at a depth of about 75 to 90 feet below ground surface (bgs), to the top of the middle aquitard at a depth of about 175 feet bgs.

The upper zone of the regional aquifer can be further subdivided into upper and lower subunits. The lateral continuity of the upper and lower subunits is difficult to estimate due to heterogeneities resulting from meandering streams that deposited these sediments. In some places, the streams deposited relatively course-grained sands and gravels, but in other areas fine-grained overbank and floodplain deposits were left behind. The coarse-grained upper subunit at West Plume B appears to correlate with the upper subunit at the AANG project area, and is only five to ten feet thick. The upper subunit is defined as the first course-grained water bearing unit encountered at depth, and groundwater contamination is thought to be confined to the upper subunit in the West Plume B project area.

The groundwater flow direction in the upper zone of the regional aquifer is generally toward the northwest, but flow within the coarse-grained upper and lower subunits is more variable. More detailed descriptions of the hydrogeology of the West Plume B project area can be found in reports and studies available at the TIAA Information Repository.

Action Taken

In 2002, ADEQ completed a Remedial Investigation report for West Plume B. The report described the extent and magnitude of groundwater contamination, and it identified the AANG as the probable source of this contamination. Also in 2002, EPA completed a feasibility study (FS) report. In 2004, EPA issued a Record of Decision (ROD) for the site that specified pump and treat, but also stated that if TCE concentrations in groundwater continued to decline that they would reconsider a monitored natural attenuation (MNA) remedy.

In 2012, EPA issued a ROD Amendment that specified MNA as the appropriate remedy for West Plume B. The ROD Amendment also selected In-Situ Chemical Oxidation (ISCO) to replace the pump and treat remedies at the AANG, West-Cap and Texas Instruments Project Areas.

Status

The AANG monitors groundwater at the West Plume B project area along with the monitoring they do at the AANG project area.  Currently, one monitoring well has 8 ppb TCE.  This is now the only well still above the 5 ppb federal drinking water standard for TCE.