SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

it absence of detections of volatile organic compounds (VOCs). During this period (Sept. 1996 to November 2001), the extraction wells were sampled twice per month in accordance with requirements of the Remedial Action Work Plan (Parsons, 1998). In November 2001 the two quarterly sampling events were reduced to four monitoring wells (MW -5R, MW -6, MW -11, and MW -12) with CVRWQCB approval, In January 2003 the CVRWQCB approved annual sampling of MW -7 and removal of MW -3 and MW -8 from the sampling program. The monitoring program was then formalized in the current MRP (CVRWQCB, 2003) which specified that new monitoring wells be added to the quarterly sampling program. MW -8 and MW -1 were abandoned on February 9, 2004 and December 12, 2008, respectively, due to construction of new buildings on the property. MW -15 was added to the quarterly program in August 2004 and MW -16 and MW -17 were included in January 2008 following investigation of the area offsite to the southeast of the property. Historical data plots for each well are provided in Attachment A. Figure 1 shows the locations of the wells and concentrations of 1,1-DCE and TCE in groundwater. REVISED SAMPLING FREQUENCY Based on results of more than six years of groundwater monitoring since the previous monitoring schedule revision, the data support elimination of quarterly sampling currently conducted at MW -5R, MW -6, MW -11, MW -12, MW -15, MW -16, and MW -17. Semiannual sampling would be conducted for all active monitoring wells with the exceptions discussed below. Annual sampling is proposed for two wells currently on semiannual schedule (MW -13 and MW -14) in addition to the one well (MW -7) that is already sampled annually. Measurement of water levels in all available wells will continue on a semiannual basis. The three wells proposed for annual monitoring would be sampled in August and the wells proposed for semiannual monitoring would be sampled in February and August. Rationale for Monitoring Wells The analytical history of each well was examined to develop a rationale for revising the sampling frequency: • MW 4• Northeast property corner, continuing ND results - Concentrations at this well have generally been ND for nine years. Three isolated detections of TCE occurred since 2000, but only one (TCE 2 pg/L) occurred since 2003. • MW -9 MW -11 and MW -12: Downward concentration trends — Analyses from these wells continue to show steady downward concentration trends and have reached approximately 1 pg/L of 1,1-DCE or TCE. Page 2 of 6