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ARCO STATION BACKGROUND INFORMATION • February 1988: Prior to the removal of multiple gasoline USTs, exploratory borings were advanced that de- termined significant total petroleum hydrocarbons as gasoline (TPHg) and benzene, toluene, ethylbenzene, and xylene (BTEX) impacts to the subsurface. • June 1988: Multiple USTs were removed from the southeast portion of the site. Confirmation soil samples were collected (E-1 through E-5) that revealed TPHg and benzene concentrations up to 43,000 and 496 parts per million (ppm), respectively. • March 1993: A groundwater monitoring program was established under the Santa Regional Water Quality Control Board (RWQCB) oversight. Groundwater sampling is still presently being conducted, with up to 17 groundwater monitoring wells having been installed around the site and vicinity. • June 1994 to December 1996: A soil vapor extraction (SVE) and carbon treatment system was operated, which removed approximately 1,952 pounds of TPHg. • September 1999: Oxygen -release compounds (ORC) in the form of magnesium peroxide was injected into the subsurface from 14 direct -push borings (approximately 30 pounds per boring) in September, to increase the level of oxygen aiding in the natural biodegradation of TPHg. TPHg and volatile organic compounds (VOCs) continued to persist after the injection of the ORC. • September- October 2000: A high -vacuum dual-phase extraction system (HVDPE) was operated, which extracted approximately 74,000 gallons of groundwater and 376 pounds of TPHg. • October 2003: A HVDPE system was operated, which extracted approximately 43,000 gallons of groundwa- ter, 294,000 cubic feet of vapor, and 11.5 pounds of TPHg. Based on the low amount of TPHg recovered, HVDPE was considered cost ineffective and discontinued as a viable remedial option. • July 2009: Groundwater monitoring was changed from quarterly to semi-annually based on a letter from the Santa Ana RWQCB. • May 2012: Confirmation soil borings were advanced in the vicinity of former USTs and dispenser islands. High TPHg and benzene concentrations were reported at 10 feet bgs in two of the borings, but in general concentrations demonstrated a reduction in concentrations of petroleum hydrocarbons. • October 2013: A soil vapor survey and health risk assessment was conducted, which found low vapor con- centrations and insignificant health risks to workers at the ARCO station and nearby residences. • October 2014: The Santa Ana RWQCB accepted groundwater monitoring and sampling to be conducted on an annual basis. The Santa Ana RWQCB also indicated that the site was eligible for regulatory closure. • April 2017: Groundwater sampled during the second quarter 2017 event indicated residual concentrations of TPHg and benzene at maximum concentrations of 3,200 and 180 micrograms per liter (pg/1), respectively. TPHg and VOC contamination is generally confined to the site, and therefore natural attenuation and moni- toring is the ongoing remedial action at the site. Appendix B-BAckSround tufo