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V Excavation Remediation Report Advar:ced Environmental Concepts,Inc. V 3.0 SITE CHARACTERISTICS San Joaquin County is part of four separate physiographic regions in California. The majority of the county is located within the lower San Joaquin Valley,which extends from north to south through the eastern portion of the county. Most of the remainder of the county, in the western area, is located in the Sacramento-San Joaquin Delta. Foothills of the Sierra Nevada occupy the easternmost edge, and portions of the Coastal hod Ranges occupy the southwestern portion. Landforms of the lower San Joaquin Valley include flood plains, alluvial fans, fan terraces, basins, dunes, and low and high terraces. Surface topography is generally flat, although relief in some areas ranges from undulating to hilly due to dissection and erosion. Nearly level a.. flood plains are located along the Stanislaus, Calaveras, San Joaquin, and Mokelumne Rivers. All of these flood plains were subject to regular flooding prior to development of levees and reservoirs. Natural drainage ways in San Joaquin County generally flow from east to west; the San Joaquin River, largest in the county, flows from south to north. Flood plains are also located along Hospital, Lonetree, and Corral Hollow Creeks. �.• Flood waters in these areas dissipate on leveled alluvial plains before reaching the San Joaquin River. The Stanislaus and Mokelumne Rivers deposit alluvial material originating the Sierra Nevada Mountains, forming extensive alluvial fans and fan terraces. Wind-deposited sand dunes were once common along the Stanislaus River. Many of these dune areas have been artificially leveled and reclaimed for agricultural use. Many low and high terraces along the eastern edge of the San Joaquin Valley have developed complex surface drainage patterns and microrelief. One of these drainage patterns occurs as a meandering or disarranged pattern of intermittent channels which double back on each other. In some areas, short +.� segments terminate at small closed basins that commonly fill with water during the winter and spring. These features are identified as vernal pools and provide habitat for sensitive ecosystems. The property is located on gently sloping southwest grade with an average elevation of 50-feet above mean sea level. Surface sediments consist of unconsolidated, poorly bedded clays and silts with minor sand components. The majority of groundwater pumped from the area occurs within unconsolidated continental deposits. Regional groundwater flow direction (gradient) beneath the site area is generally toward the north r✓ and west. Groundwater was encountered at depths ranging from 12 to 15 below grade level (BGL) within exploratory borings advanced at the site during the course of this project. The local gradient beneath the subject property is estimated to be toward the northeast. 4.0 EXCAVATION AND SOIL TREATMENT '-" Soil in the vicinity of the former underground tank emplacement was excavated from surface grade to total depths ranging from 13 to 20 feet below grade in two phases beginning in October 1997 and ending in July 1998. This remedial excavation project was performed in two phases due to delays caused by a legal dispute between the onsite residential tenants of the Paulson Life Estate and Prudential combined with adverse site conditions caused by the unusually heavy seasonal precipitation which occurred during 1997 - 1998. Total vertical excavation depths varied based on daily field observations, interpretation of the analytical results of samples collected during both exploratory baring phases, continuous PID screening of 'r excavated material, and laboratory results of confirmation samples collected during the excavation phases. The horizontal extent of the excavations were based on the same criteria and continued until all indications of contaminated soil had been removed. W AEC performed the excavation procedures using a track-mounted excavator and a front loader. A total estimated volume of 3,490 yds' of soil was excavated during the course of this project. Of this volume, approximately 935 yds'of gasoline-impacted soil and 27 yds' of diesel-impacted soil were excavated and removed for treatment on an adjoining parcel; approximately 2,528 total yds' of "clean" overburden was Project: AEC 96C-1707 6 i+ •ENVIRONMENTAL CONCEPTS WITH DESIGN IN MIND 4400 ASHE ROAD.#206 • BAKERSFIELD.CA 93313 8051831-1645 FAX 8051831-1771 r✓