Wednesday, October 21st, 2015
CSL 422 – 1:00pm
At Pine Valley Trailer Park in the town of Guatay, the Hilltop Mobil Station, as it was known most recently, provided gasoline, radiator water, and other services to travelers on Old Highway 80. After removal of underground gasoline storage tanks in 1992 and limited excavation of impacted soil, it was determined that groundwater, occurring at a depth of 25 to 30 feet, was impacted with petroleum hydrocarbons, including free product. The San Diego County Department of Environmental Health required assessment and remediation of the leaking UST case.
The site is underlain by fractured rock of the Peninsular Ranges batholith. According to Todd (2004), these rocks include Cuyamaca Gabbro and Chiquito Peak Monzogranite. Fracture trace analyses suggested fractures associated with plutonic emplacement, stress-release, and regional tectonic stresses are present at or in the vicinity of the site. Assessment included 16 shallow groundwater monitoring wells to assess the extent of groundwater impacts in a shallow water-bearing zone at a depth of 25 to 30 feet, and one deep well to assess a regional deep water-bearing zone at a depth of 150 to 160 feet. Coring the deep boring, along with analytical results, illustrated the shallow and deep water-bearing zones have minimal connectivity.
Remediation of free product and reduction of dissolved-phase hydrocarbons were accomplished using high-vacuum, dual phase extraction (HVDPE) in multiple remediation wells installed at the site. However, once remediation was largely completed, several impediments to closure still remained. A nearby supply well had low concentrations of aromatic hydrocarbons, suggesting possible connectivity between that well and the site. Aquifer testing confirmed the separation between the shallow and deeper water-bearing zones, and little to no connectivity between the Site and this well. Remaining high TPH concentrations, another impediment to closure, were addressed by additional HVDPE to evaluate whether additional mass was available for extraction. Upon determining that the hydrocarbons had been removed to the extent practicable, and with confirmation that sensitive receptors in the area were not affected, the case was closed.