SITE INFORMATION AND CORRESPONDENCE_FILE 1

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ovr:lrznen 1b:02 925-463-2559 RTC PLEASANTON .I PAGE 04 j STAFF DRAFT mtbe public STAFF DRAFT 121x3/99 c) If necessary,require development and implementation of remedial action plan within three years. I II i` Priority Class (A, B, or C) I I Distance to drinking Maximum MTBE concentration in groundwater(ppb): water weR(ft) 5-500 500-5,000 5,000-50,000 over 50,000 0-1000 B A A A !'+ 1000-2500 C B A A 2500-5000 C C B A over 5000 C C C B 6 f Step 3. Interim Remedial Action Sites with high concentrations and a large release mass should have those concentrations and mass reduced before the plume can spread regardless of their response category. For example, sites with free product orstern concentrations over 10 000 ppb MTBE in the groundwater are candidates for source area remeppn as an interim remedial acroLong-tem impacts to water quality and financial resources are likely to be reduced if interim remediation is performed in these situations. Conversely,if the investigation data indicate a low potential threat,either because the mass of MTBE released is small,migration to drinking water wells is highly / unlikely,or other relevant factors exist,then this interim remedial will not be necessary. J/ Step 4. Site Characterizationanvestigation If the conceptual model has data gaps that may affect decisions made at the site,fin Ither investigation may be warranted. Geologic data that has already been collected from nearby sites can provide an overview of what conditions may be encountered beneath the site. ;This regional information combined with the data gathered in the scoping phase allows the regulator to choose a level of effort for the investigation of the site.This is a very important step because all sites do l not require the same detail.High-risk plumes may need a more detailed level of assessment that inc u es pth-discrete sampling to determine the plumes three-dimensional profile and to target remediation so that it is most effective. An uninterrupted vertical profile of the stratigraphy by continuous core or cone penetrometcLcw. help verify if the regional,conceptual model of the geology applies to this site. if the conceptual model using regional data implies that persistent downward verticat&roundwater gradients may exist,these gradients and the vertical errtent of MTBE impacts should be investigated using cluster yells or other methods. ee co, nvis also true;sites located in areas with'known IP upward gradient may not require site specific assessment of vertical migration. It is extremely important for the agency providing cleanup oversight and the tank permitting agency to work together to identify the source of the MTBE in the subsurface(tank,pipe joint, STAFF DRAFT S'T'AFF DRAYr Page 8 of T8 I k i e