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/^ A-` LISTO ENGINEERING GROUP r July 15, 1996 v_....:.__.�r_=------7 Mr. Scott Hooton BP Oil Company f;;= Environmental Resources Management 295 S.W. 41st Street Building 13, Suite N Renton, Washington 98055 Subject: Contouring Groundwater Chemical Data Dear Mr. Hooton: In response to your request during our recent telephone conversation, we have evaluated the technical basis and applicability of calculating the mass of petroleum hydrocarbons in the groundwater using isoconcentration maps. We understand that your request was based on a letter from the San Joaquin County Public Health Services requiring that an isoconcentration map of chemical data be prepared for use in evaluating the stability and behavior of a petroleum hydrocarbon plume. As we discussed, contouring chemical data for groundwater is a subjective interpretation dependent on several variables with no set:technical basis or standard criteria to provide consistent and comparable results. The interpretation of chemical contours is not only dependent on the inherent variability in site conditions, field.and laboratory techniques, and quality assurance and quality control (QA/QC) procedures, but also on human factors. In our experience on numerous sites, contouring chemical data has not provided reliable results in estimating contaminant mass and comparing historical trends to assess plume stability, particularly on a quarterly basis. Contouring chemical constituents in groundwater is best used as a graphical representation of the spatial distribution of constituents, rather than as a basis for estimating mass of hydrocarbon constituents. As stated by Evan Nyer et al, in a technical paper published in the Spring 1996 issue of Groundwater Monitoring & Remediation': "Once contours are put onto a map, they are taken as fact, and trying to convince someone otherwise is like trying to put toothpaste back into the tube. Obviously this can have serious f- implications at sites where litigation over responsibility is ongoing." In addition, the California Regional Water Quality Control Board, San Francisco Bay Region stated in Page 4 of the Supplemental Instructions to State Water Board, December 8, 1995 Interim Guidance on Required Cleanup at Low Risk Fuel Leak Sites: "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." 1575 TREAT BOULEVARD.SUITE 201 •WALNUT CREEK.CA 94598 • (510) 295-1650 • (510) 29S-1823 CONTRACTORS LICENSE NO.A-652544