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Austin Road Landfill Section 3 Contaminant Plume Characterization Report Field Investigation Results The highest concentrations of PCE were measured in the monitoring wells north of the landfill which are screened within the sand layer which extends over the 80-104 ft bgs interval,as described in Section 3-1 and shown on Figure 3-5. The wells screened in this interval are MW-10,MW-11, MW-13,and MW-14. Most of the wells on the perimeter of the landfill do not detect contamination. These wells are generally screened at intervals which are relatively shallow or relatively deep (i.e. above or below the shallow sand layer present between 80-100 ft bgs). MW-2,MW-3,and MW-4 are screened between 58 and 80 feet bgs while MW-6 and MW-7 are screened in the deep sand layer between 142.4 and 169.5 feet bgs. The only well on the perimeter of the landfill which detected - VOCs during the fourth quarter 1997 sampling event is MW-1,located at the northwest corner (Figure 3-5). Historically,MW-5,which is also screened in the shallow sand unit but is located on the upgradient edge of the landfill,has also detected PCE and TCE. While no VOCs were detected in MW-4 and MW-5 during the fourth quarter 1997 monitoring event,previous and later sampling events have detected VOCs in these wells. Detections have occurred as recently as the second quarter of 1998. The MW-6 and MW-7 screen intervals are below the thick layer of clay observed in well MW-6 from 117 to 115 ft bgs(Figure 3-1). This clay layer likely serves as an aquiclude, preventing contamination from moving vertically into the deeper sand layers in the area beneath the landfill. The absence of this clay unit in areas north of the landfill may allow downward migration of constituents to occur. North of the MW-14 location,the plume appears to be moving downward within the aquifer system,as contamination has been measured in deeper samples but not in shallower ones.VOCs were not detected in any of the samples collected by CDM between 65 and 90 feet bgs,suggesting the absence of contamination at these shallower depths. However,VOCs were detected in some of the samples collected at depths from 140 to 180 feet bgs (ARL-1A and 1C;see Figure 3-1). The only samples taken at these depths which were not contaminated were from ARL-1B and ARL-2A which yielded no samples impacted with VOCs. For this reason,these boring locations are considered to be beyond the edge of the VOC plume. Based on the existing sampling results,the plume appears to extend as far north as boring location ARL-3A where PCE was detected at 140 feet bgs at a concentration of 1.3 ug/L(Figure 3-5). This concentration is considered relatively low when compared to the MCL and likely represents the - downgradient edge of the plume. The two samples which were taken at intermediate depths(ARL- 2A-115 and ARL-3A-105;Figure 3-5)did not contain detectable concentrations of VOCs. Thus the plume is believed to be at greater depths at its northern end. The apparent downward migration of constituents emanating from the landfill may be controlled by the position of sand layers in addition to vertical flow conditions. This is consistent with the idea of contaminant migration occurring in the deeper sand units as depicted on cross section AW(Figure 3-1). These deeper sand and silt layers located to the north of the landfill are probably in hydraulic connection with the shallow sand layer beneath the landfill. The field parameters and inorganic quality data collected are presented in Table 3-2. Based on these results and a review of similar data in the CH2MHILL Quarterly Monitoring Reports,in general, the parameters measured in water samples do not exceed the concentration limits established for the site by CH2MHILL. Table 3-3 presents the total organic carbon(TOC)results for the soil samples which were collected. • CDM Cam Dresser&McKee 3-5 Camp CS 010775 W:\.REpOFITs\sTOCKTON\ARLPLUME.98\SEC3.WPD City of Stockton 290855-0006