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Geosynte& consultants 2. BACKGROUND 2.1 Site History The Site, located at 26700 South Banta Road in Tracy, California(Figure 1), is a triangular parcel bounded by South Banta Road to the west, agricultural property to the south, and the California Northern Railroad line to the east (Figure 2). The Site is currently developed with a former production building, tank farm, and maintenance and storage areas. A stormwater pond is in the southeast portion of the Site.The surrounding area is primarily agricultural lands with some mixed- use light industrial and low-density residential areas along South Banta Road. The Site was agricultural land prior to 1947. An alfalfa dehydrating plant was constructed in 1947 and operated seasonally between 1948 and 1957. From 1957 to 1970, Anderson Tire Recapping Company (Anderson) occupied the Site for sorting and selling used tires (SECOR International Inc. [SECOR] 2001). The Site was vacant from 1970 until 1975, when it was purchased from Anderson by All-Pure Chemical Company(All Pure)(The Traverse Group Inc. 1989).The original All Pure owner remains the current property owner. Between 1975 and 2022, the Site operated as a bleach manufacturing and distribution facility. During this period, All Pure occupied the Site from 1975 to 1989 and redeveloped it into the current 15-acre manufacturing facility. In 1989, Pioneer Americas Inc. (Pioneer) acquired the All Pure business and began leasing the Site from the property owner. Site operations during Pioneer's period of operations included production of sodium hypochlorite (liquid bleach) from chlorine and caustic soda, storage and distribution of calcium hypochlorite, repackaging of chlorine gas and sulfuric acid, and truck maintenance in an on-site shop. Olin acquired Pioneer and the property lease in 2007 and continued liquid bleach production and chlorine gas repackaging. Chlorine gas repackaging operations ceased in 2009. Olin ceased liquid bleach production during the second quarter of 2022. 2.2 Local and Regional Geology and Hydrogeology The Site is in the San Joaquin Valley and overlies recent Quaternary deposits and the Tulare Formation.The upper 30 feet of the subsurface is comprised of recent Quaternary alluvial deposits, which are difficult to distinguish from the underlying deposits of the Tulare Formation. In this area, the Tulare Formation is divided into Upper, Middle, and Lower Members. The Upper Tulare Member consists of interbedded gravels,sands, silts,and clays. The Upper Tulare Member reaches thicknesses of 200 feet near the Site in the northwest part of the San Joaquin Valley. The Middle Tulare Member is the Corcoran Clay aquitard, estimated to be 220 to 250 feet thick near the Site. The Lower Tulare Member, like the Upper, consists of permeable material (SECOR 2001). The Upper Tulare Member is subdivided into four geologic units, designated "horizons," as follows: Above Upper, Upper, Middle, and Lower (in this Work Plan, the Above Upper Horizon is included in the Upper Horizon).The depth,thickness,and lithology of these horizons vary across the Site and surrounding areas. The upper 100 feet of lithology at the Site and surrounding area has been described as primarily silty clays containing layers of fine-to-medium sand and silt (SECOR 2001; Amec Foster Wheeler Environment& Infrastructure, Inc. [Amec Foster Wheeler] 2017). Well Destruction Work Plan 2 May 2026