Laserfiche WebLink
19 October 2012 <br /> AGE-NC Project No. 97-0312 <br /> Page 12 of 22 <br /> 5.0. CONCEPTUAL SITE MODEL <br /> Based on the data collected to date from CPT and soil borings and monitoring well <br /> installations,AGE has developed a conceptual site model(CSM)for the release, migration, <br /> and distribution of the contaminants in the subsurface: <br /> Generally, the stratigraphy from surface grade to approximately 60 feet bsg is composed <br /> of fine-grained material consisting primarily of silt and clayey silt, with laterally <br /> discontinuous intervals of clay and sandy silt. A layer of coarser-grained material (sandy <br /> silt, silty sand, and poorly graded sand) is encountered throughout most of the site at <br /> depths between 20 feet and 30 feet bsg. At depths ranging from 50 feet to 60 feet bsg, a <br /> layer of clay was identified in most soil borings. Below 60 feet bsg,coarser-grained material <br /> (sandy silt, silty sand, and sand)with lesser intervals of silt and clayey silt is encountered <br /> throughout most of the site to the total depths of the borings(up to 109 feet bsg); this deep, <br /> coarser-grained zone is not encountered in soil borings MW-8 and B17, located on the <br /> eastern edge of the site, below approximately 65 feet bsg. Based on the sediment <br /> two stratigraphy, the preliminary subsurface hydrogeologic model consists of three informal <br /> zones, a vadose (unsaturated) zone and two hydrogeologic units; the vadose zone and <br /> Hydrogeologic Unit 1 are composed dominantly of fine-grained, generally clays and silts, <br /> U.0 whereas Hydrogeologic Unit 2 consisted of sand and gravel along with stiff, fine-grained <br /> soil. <br /> Soil boring locations are depicted in Figure 2 and geologic cross sections and areas used at1 <br /> for the volume estimations are illustrated in Appendix C. ,r ,, `` <br /> ss <br /> y v,14 6 . <br /> Since initiation of groundwater monitoring by AGE in March 2001, depth to groundwater <br /> measurements have fluctuated between approximately 54 feet and 73 feet below the tops <br /> of the well casings. The inferred groundwater flow direction beneath the site has generally <br /> been toward the northeast at gradients ranging between 0.0001 ft/ft and 0.007 ft/ft. <br /> 5.1. MIGRATION OF PETROLEUM RELEASE <br /> Release(s) of petroleum hydrocarbons occurred from the former on-site USTs, including <br /> USTs Nos. 1 through 5, located along the northern edge of the site, UST No. 6, located at <br /> the eastern edge of the site, and two waste oil USTs located in the southwest corner of the <br /> site (Figure 2). The majority of the UST release occurred from dispenser line leaks, based <br /> on sampling from the UST removal. During UST and product piping removal activities <br /> conducted in September 1997, it was noted that USTs Nos. 1 through 5 appeared to be <br /> y in good condition; however, a hole was observed on the bottom of UST No. 6, beneath the <br /> fill port. Obvious field indications of hydrocarbon-impacted soil were observed immediately <br /> surrounding the USTs and product piping. TPH-g and TPH-d were detected at <br /> L <br /> Advanced CeoEnvironmental,Inc. <br /> V <br />