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Rema-dial Action Plan June 14, 2017
<br /> ' Project#Z054000052 Page 3
<br /> In May 1996, AquaGeo Environmental Consulting (AquaGeo) installed six wells (VW-2, VW-4, VW-5, VW-6,
<br /> MW-10NW-1 and MW11/VW-3) to depths ranging from 16.5 to 58.5 ft bgs. Monitoring wells MW-10NW-1 and
<br /> MW-11NW-3 were installed as nested wells. Well VW-2 was completed as a dual-completion vadose zone
<br /> monitoring well. Wells VW-4, VW-5, and VW-6 were completed as triple-completion (A, B, C) vadose zone
<br /> monitoring wells. Groundwater was encountered during drilling activities at depths of approximately 44 to 50
<br /> feet bgs. TPH, benzene, and methyl tertiary butyl ether (MTBE) were detected at maximum concentrations of
<br /> 6,600 mg/kg (VW-6-25), 69 mg/kg (MW-11-40), and 670 mg/kg (VW-6-40), respectively. Groundwater
<br /> samples collected from wells MW-10 and MW-11 during the third and fourth quarters of 1996 indicated that
<br /> TPH, benzene, and MTBE were detected at maximum concentrations of 370,000 pg/L (MW-11), 33,000 pg/L
<br /> (MW-11), and 880 pg/L (MW-11), respectively (AquaGeo, 1997).
<br /> In March 1999, The Park Corporation (Park) installed one groundwater monitoring well (MW-12) to a depth of
<br /> 73 feet bgs. Groundwater was encountered at a depth of approximately 33 ft bgs. Laboratory analysis of soil
<br /> samples detected TPH concentrations from 1.4 mg/kg to 6,400 mg/kg, benzene concentrations from 0.26
<br /> mg/kg to 100 mg/kg, and MTBE concentrations up to 35 mg/kg (EPA Method 8020). The presence of MTBE in
<br /> the soil samples was confirmed at concentrations ranging from ND to 1.1 mg/kg (EPA Method 8260). The
<br /> highest concentrations of aromatic volatiles in soil samples were detected in the 50 foot bgs sample.
<br /> Laboratory analysis of groundwater samples detected 21,000 pg/L TPH, 3,200 pg/L benzene, and 150 pg/L
<br /> MTBE (EPA Method 8020). The presence of MTBE was confirmed at a concentration of 120 pg/L (EPA
<br /> Method 8260). In the surrounding groundwater monitoring wells, TPH was detected at concentrations up to
<br /> 760 pg/L, benzene was detected at concentrations up to 2.1 pg/L, and MTBE was detected at concentrations
<br /> up to 540 pg/L (EPA Method 8020). The presence of MTBE was confirmed at concentrations up to 490 pg/L
<br /> (EPA Method 8260). Monitoring well MW-7 was redeveloped during assessment activities to enhance the
<br /> well's productivity (Park, 1999).
<br /> In April 2004, Terra Vac observed the advancement of six soil borings (131 through B3 and B5 through 137) in
<br /> the vicinity of the USTs and dispenser islands. Terra Vac performed this work during due diligence site
<br /> assessment. Borings were advanced to depths of 10 to 25 ft bgs, and groundwater was not encountered. Soil
<br /> samples were collected from borings B2, B3, and B5 for laboratory analyses. Petroleum hydrocarbon
<br /> concentrations were not detected in the soil samples (Terra Vac, 2004).
<br /> On June 8, 2006, Terra Vac performed a baseline soil assessment prior to temporary closure of the USTs and
<br /> lease termination. Three soil borings (131-06, 132-06, and 133-06) were advanced to 45 ft bgs. Results of the
<br /> soil sampling indicated concentrations of up to 452 mg/kg TPH, 1.97 mg/kg benzene, and 0.128 mg/kg MTBE.
<br /> Based on the decreasing trend of petroleum hydrocarbon constituents observed in soil, Terra Vac concluded
<br /> that a new release of gasoline had not occurred (Terra Vac, 2006).
<br /> In December 2007, GeoHydrologic Consultants, Inc. (GHC) performed oversight of UST removal, well and
<br /> piping removal and soil sampling activities conducted at the site. Three 12,000-gallon USTs, associated fuel
<br /> lines, and three dispenser islands were removed from the site. Soil sample results for the samples collected
<br /> from beneath the gasoline USTs, fuel lines, and dispensers did not indicate detectable concentrations of
<br /> petroleum hydrocarbons. The related conveyance piping from MW-3, VW-4, and MW10/VW-1 in the proximity
<br /> of the excavation areas was disconnected, severed, and removed. The existing lines inside of the proposed
<br /> excavation areas were cut back and capped to prevent damage and plugging of the conveyance lines.
<br /> Groundwater monitoring well MW-12 and dual nested wells MW-11/VW-3 and VW-2 were properly destroyed
<br /> by over-drilling the depth of the wells. Following the tank removal and well destruction activities, GHC
<br /> supervised the installation of replacement wells for MW-11/VW-3 and MW-12 (MW-11R and MW-12R) in
<br /> January 2008. At the direction of the SJCEHD, VW-3 was reportedly installed in a separate borehole
<br /> approximately three feet southwest of well MW-11 R. Replacement wells MW-11 R, MW-12R, and VW-3 were
<br /> installed in the UST area. After UST removal activities, soil samples were collected from beneath the USTs
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