SITE INFORMATION AND CORRESPONDENCE_FILE 1

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'From:-Haff To: Erin Garner ` Oats:119196 Tirne:11:42:25 Adak Page 7 of 11 Subject: Supplemental instructions to State Water Board December 3, 1995, Interim Guidance on Required Cleanup at Low Risk Fuel Sites January 5, 1996 / Page 4 LOW RISK GBOUN®WWATER SASE Definition 1) The leak has been stopped and ongoing sources, including free product, have been removed or remediated (see Low Risk Soi,s Case Definition #1). 2) The silo has been adequately characterized (see Low Risk Sails Case Definition #2). The presence or absence of horizontal and vertical conduits which could act as preferential pathways for the dissolved plume should be evaluated as a part of the site characterization process. 3) The dissolved hydrocarbon plume is not migrating. The LLNL report found that petroleum plumes in the subsurface tend to stabilize once the source is removed. Natural biodegradation of hydrocarbons is the main reason why this stability occurs. Chemical concentrations of hydrocarbons in groundwater that decrease or do not change with time are the best indicators of a stable plume. Comparison of background and hydrocarbon plume concentrations of inorganic ions such as oxygen, iron, nitrate, sulfate, and others, can provide evidence of biodegradation at a given site. These data may not be required to determine plume stability but can supplement other lines of evidence. Stable or decreasing plumes often display short term variability in groundwater concentrations. These effects are due to changes in groundwater flow, degradation rates, sampling procedures, and other factors which are inherently variable. This behavior should not necessarily be construed as evidence of an unstable plume but may be the natural variations of a stable plume in the environment. 4) No :eater wells, deeper drinking water aquifers, surface water, or other sensitive.receptors are likely to be impacted. 5) The site presents no significant risk to human health. For this analysis, the groundwater ingestion pathway need not be considered if the groundwater is not. currently used as a source of drinking water or projected to be used within the life of the plume. (See Low Risk Soils Case Definition #5) —' 6) The site presents no significant risk to the environment. RBCA has no specific guidance for evaluating environmental risk although the basic framework is appropriate if site specific exposure pathways and ecological receptors are included. If the site has a potential to significantly impact surface water, wetlands, other sensitive receptors, it should not be considered low risk. (See Low Risk Soils Case Definition #6)