SITE INFORMATION AND CORRESPONDENCE

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0 Well Monitoring Program -3- 11 September 1992 DDRW, Tracy where elevated concentrations of these contaminants have been confirmed in ground water samples. (These areas are discussed in more detail below.) Comprehensive monitoring of pesticides and metals should be deferred until the completion of the site-wide RI. The Draft Report suggests that the elevated metals tend to occur in turbid samples which are generally associated with the older monitor wells. Turbidity in these wells has been attributed to a larger well screen slot size and inadequate development. The issue of whether a metals ground water contaminant plume exists at the site should be deferred until the site-wide RI and until additional data are collected by the Regional Board. We plan to collect samples for turbidity and total and dissolved metals analyses from adjacent new and old monitor wells that are screened in the same horizon, and will determine whether the elevated concentrations of metals are associated with turbid samples. This sampling should occur in September 1992. My general concerns with the monitoring recommendations are discussed below. 1 . The recommendation to continue annual monitoring of all monitor wells for pesticides and metals appears to be excessive. The analytical results from the four quarters of sampling did not reveal consistently high concentrations of these contaminants. Elevated concentrations of both metals and pesticides were detected only in samples from LM-3, LM-15, LM-16 and LM-27A. The elevated contaminant concentrations may be associated with specific WMUs. LM-3 and LM-15 are down gradient of the industrial waste water pond and LM-16 is immediately down gradient of the old industrial waste water pond. On-going monitoring for pesticides and metals in these wells may be appropriate in the interim until the completion of the site-wide RI. At that time, other existing and newly installed monitor wells should be included in an evaluation of the monitoring program. With respect to hydrocarbons, comprehensive on-going monitoring of all of the wells does not appear to be needed at this time. Only three monitor wells (LM- 27A, LM-39 and LM-48) had detectable concentrations of diesel , which were not confirmed in subsequent quarters. On-going monitoring for hydrocarbons should be deferred until the completion of the site-wide RI when potential UST source areas have been identified. 2. The Groundwater Sampling and Analysis Program (GSAP) was used to develop recommendations for an on-going monitoring program. This program used a Concentration Criteria (CC) (Table 7-1), or baseline concentration, to determine the sampling frequency for each monitor well . Frequently, the CC was established at the Primary Maximum Contaminant Level (MCL) for the organic constituents. However, the Primary MCL may be a higher concentration than what would be accepted by the Regional Board to protect the beneficial uses of the waters of the State. For example, the CC for toluene was established at the MCL (1000 µg/1 ) . However, the Regional Board uses 40 µg/l as the cleanup level for ground waters polluted with petroleum based contaminants. Ideally, the CC should be established at background or water quality standard concentrations, which ever is less. The CC for all of the constituents should be revised accordingly to reflect the more restrictive numbers.