SITE INFORMATION AND CORRESPONDENCE

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Mr. Stephan Defibaugh - 3- • 9 May 2008 KMEP Stockton Terminal 2. The calculated WQO attainment dates presented in the MNA Estimate imply a precision that may not be attainable by the regression method that KMEP used. An acceptable procedure for calculating the uncertainty in the projected dates is through the use of the confidence intervals on the slope of the rate constant (kPo;n0. The Guidance contains instructions for describing the uncertainty. 3. We concur with the recommendation to supplement the MNA Estimate using the Mann- Kendall statistical analysis. This methodology will prove particularly useful for data displaying increasing trends where KMEP has determined that the existing trends do not support calculation of a time estimate. The Mann-Kendall test is computed by summing ranked data. Therefore, in order for Regional Water Board staff to concur with use of this methodology, KMEP must ensure calculations are performed using a consistent range of dates for all the data points. If this requirement proves to be problematic, the bulk attenuation rate constant calculation (k) described in the Guidance could serve as an acceptable alternative to the Mann-Kendall method. 4. The MNA data indicate that the center of the plume is anoxic. However, the nutrients required to sustain biodegradation have been depleted, and KMEP has not identified an alternate metabolic pathway. This indicates that attenuation of the plume may depend on non-destructive mechanisms such as dispersion, sorption, and dilution. Reliance on these processes may greatly extend the time required to attain WQOs. We concur with the recommendation to use a modeling program to simulate biodegradation at the Site. The Guidance contains recommended procedures for calculating a biodegradation rate constant (A) that will be needed to run the models and lists several analytical and deterministic models that could be used to model the Site. Regional Water Board staff believe that use of a deterministic program would more accurately model site conditions than analytical models, such as NAS and BIOSCREEN. 5. We are concerned that the use of the bimodal trend concept is applied inconsistently to the SP/M-1 data. The so-called bimodality of the MTBE data is attributed to the remedial excavation performed in the manifold area in 2006. However, this argument cannot be applied to the benzene and TPHg data in this well, because the point at which trend lines change from increasing to decreasing is identified as occurring in 1998. In addition, we do not concur that three data points constitute sufficient data to reliably project the 2008 WQO attainment date for MTBE in this well. The high solubility and low organic carbon partition coefficient (K..) of MTBE indicate that high concentrations of this constituent could attenuate more rapidly that benzene and TPHg. However, this model will require data from more observations before the projections can be considered to be accurate. The Guidance recommends a minimum of six data points. 6. The x axes of the time-concentration graphs in the appendices show a code instead of actual dates. Please change the numeric codes to actual dates to match the graphs shown in Table 2. 7. A review of the State Water Resource Control Board's (State Water Board's) GeoTracker database indicates that none of the required electronic data deliverables (EDFs) have been submitted. The GeoTracker Global Identification number for this site is 2050053. Please ensure that you have access to this site as the Responsible Party and to allow for review