CORRESPONDENCE_2016-2017 - Laserfiche WebLink

Home Browse Search

naturally occurring in groundwater, surface water, or the vadose zone; their presence is a tentative indication of a release. The sporadic occurrence of VOCs at low concentrations is not uncommon. The occurrence of VOCs at waste disposal sites indicates that low-level, sporadic detections of VOCs are commonly found in approximately 5 percent of trip blanks, field blanks, and clean upgradient wells. Possible explanations for these detections include laboratory false positives, impacts from ambient air, or contaminants induced during sampling or laboratory procedures. Consequently, sporadic low-level detections are not viewed as indications of water quality impact, unless confirmed in subsequent sampling events. The proposed concentration limits for synthetic constituents will be their respective MDLs. Since, by definition, synthetic organic compounds are not naturally occurring, their presence and subsequent verification is normally an indication of a release, and statistical analysis is unnecessary. Thus, for all synthetic organic compounds no statistical evaluation procedure is proposed. The nonstatistical procedure described below is proposed for all synthetic organic compounds (VOCs, semivolatile organic compounds, pesticides, and herbicides). A release will be tentatively indicated if either of the following conditions is met: • Two or more qualifying organic compounds are detected at concentrations equal or exceeding their respective MDLs. • One qualifying organic compounds is detected at a concentration equal to or exceeding the practical quantification limit (PQL) set by the laboratory. PQLs represent the concentration of the lowest standard used to calibrate the analytical procedure, and they reflect the quantitation capabilities of the procedure and equipment used by the laboratory. Common laboratory solvents (i.e., methylene chloride, acetone, diethylhexyl phthalate, and di-n-octylphthalate) are not considered qualifying organic compounds, and their detection will not be considered a tentative indication of a release. If the nonstatistical method for remaining organic compounds constituents indicates a tentative indication of a release, a discrete retest will be performed within 30 days. Retest procedures are described below. 4.2.2 Inorganic Constituents Groundwater - The concentration limits for the groundwater wells will be calculated based on historical water quality data. Tolerance limits, one of the statistical methods described in Title 27 will be used to set the limits. This statistical method is described in Appendix B. The inorganic data from the wells will be evaluated using the interwell tolerance limit method based on data collected from background well MW-4. The groundwater concentration limits for inorganic constituents are shown on Table 1. Foothill WQPS Rev Ldoc 4-3