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PUBLIC NOTICE | Proposed Leaking UST Case Closure for Case File 5423.01

ADEQ is considering closure of the following leaking underground storage tank (LUST) case:

LUST Case File No.: 5423.01     

Facility ID No.: 0-003208
Maricopa County

Facility address: 
ExxonMobil Corporation #12034
4438 N. 24th Street
Phoenix, AZ 85016


The former Mobil Service Station #18613 is located at 4438 N. 24th Street in Phoenix which is the southeast corner of 24th Street and Campbell Avenue. This former service station is currently inactive but still owned by ExxonMobil. The former UST system was installed in 1984. UST system decommissioning and closure activities were conducted between July 28 and September 17, 2004. The UST closure soil samples collected at 14 feet below ground surface (bgs) were analyzed for volatile organic compounds (VOCs) using EPA Method 8260B. None of the soil samples indicated VOC contamination present over laboratory reporting limits. Additional soil borings were installed as part of a possible sale. Benzene was reported over applicable regulatory standards at 30 feet bgs (4.31 mg/Kg) and at 40 feet bgs (10.2 mg/Kg) at location KB5. This location was located in the former 6,000 gallon UST area. A release was reported to ADEQ based on this soil data. Additional drilling and soil sampling was conducted in November 2004. Soil boring B1 (at location KB5) was drilled to 97 feet bgs and completed as groundwater monitoring well MW1. Benzene contamination was present over the residential Soil Remediation Level (rSRL) in B1 between 30 and 70 feet bgs.  Benzene contamination was present over the rSRL at 30 feet bgs in B2 and at 40 feet bgs in B4. A groundwater grab sample was collected during the drilling of MW1 (before the well was completed) and (benzene, toluene, ethylbenzene, xylene) BTEX and (1, 2-dichlorethane) 1,2-DCA concentrations exceeded applicable regulatory standards. Methyl tert butyl ether (MTBE) also exceeded the Tier 1 Corrective Action Standard. Corrective actions including free product removal from MW1 between 2005 and 2009 and the installation of additional monitoring wells was completed in 2006. The Site Characterization Report was approved in 2006, and a Corrective Action Plan (CAP) was requested. In 2007, air sparge (AS) and soil vapor extraction (SVE) wells were installed and an AS/SVE pilot test was conducted in February 2007. The CAP was approved in 2008. ExxonMobil hired Groundwater & Environmental Services, Inc. (GES) to continue corrective actions. The AS/SVE system began operation in June 2010 and ran until June 2015. The system was temporarily turned on in February 2016 for a rebound test. The system has not been in operation since. In-situ chemical oxidation (ISCO) [PersulfOx®] was done at AS2 (up gradient of MW1) three times in 2016. Contamination in soil and groundwater was primarily limited to the nearby source area.

Subsurface soils consist of layers of silts and sandy silts from ground surface to a depth of approximately 60 feet bgs. Alternating layers of well and poorly graded sand of varying thickness appear to a depth of 88 feet bgs. Since 2005, the depth to groundwater has ranged from 69 to 73 feet bgs. The groundwater flow direction is approximately southwest. Post injection groundwater sampling indicated VOC concentrations were reduced to below applicable regulatory standards (in most cases to non-detect) in all wells except MW1.

Current data was provided by GES in the 2017 Corrective Action Completion Report dated March 16, 2017. A site specific risk assessment and detailed file/information search were also completed. Benzene and MTBE analytical groundwater results in MW1 remain above applicable regulatory standards.

Review Document 

Review results of remedial activities and entire public notice | View/Print >

Public Comment Period 

Dates: April 25, 2017  May 25, 2017

Comments may be submitted as follows:
     By Email | Send Email > 
     By Mail (Must be postmarked or received by May 25, 2017):

       Debi Goodwin
       Waste Programs Division
       1110 W. Washington Street
       Phoenix, AZ  85007 

Documents may also be reviewed in person at the ADEQ Record Center | Learn More >