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Mr. Stephan Defibaugh • - 2 - 9 May 2008 KMEP Stockton Terminal KMEP interpreted the manifold area concentration trends in SP/M-1 and SP/M-9 as bimodal; i.e. showing an increasing trend for early time data followed by a decreasing trend in more recent data. KMEP estimates that benzene, MTBE, and TPHg in SP/M-1 will attain WQOs in 2009, 2016, and 2019, respectively. The TPHg and benzene concentrations in SP/M-9 have fluctuated up and down since 2003, and KMEP concludes that this is due to residual petroleum hydrocarbon mass in the soil beneath the active infrastructure in the manifold area. Therefore, KMEP states that this well will be slower to exhibit decreasing trends and did not estimate when these constituents will attain WQOs. KMEP recommends further monitoring of SP/M-9 to assess longer-term trends. For the commingled area wells, KMEP states there are bimodal trends of MTBE and TPHg concentrations in SP/M-3 and calculated that WQOs for these constituents would be achieved in 2008 and 2011, respectively. KMEP determined that the increasing benzene and TPHg trends observed in SP/M-10, and the increasing MTBE trend in SP/M-7 and SP/M-4 do not support calculation of a time estimate and recommends further monitoring to assess longer- term trends. In the distal well group, MTBE is the only parameter that currently exceeds the WQO. KMEP projects that the WQO of 5 ug/I will be achieved in SP/M-6 by 2011. The MNA parameter data show that some of the highest concentrations of total alkalinity, chemical oxygen demand (COD), biological oxygen demand (BOD), ferrous iron, and methane, as well as the lowest concentrations of dissolved oxygen (DO), are present in manifold area wells SP/M-1, SP/M-9, and SP/M-10. The oxygen reduction potentials (ORP) in these wells were generally at or below -100 millivolts indicating that the interior of the plume is anaerobic. The opposite is generally true for the distal and deep zone wells. Nitrate (NO3) is almost absent from the site. Sulfate concentrations are up to five orders of magnitude higher in the distal wells than the manifold and commingled wells. Total orthophosphate concentrations were generally higher at the manifold area wells than in the distal wells. With the exception of the 30 October 2007 concentration these levels were below 1 milligram per liter (mg/L). KMEP is proposing to update the estimates discussed in the MNA Estimate in subsequent annual monitoring reports. KMEP may use additional statistical methodologies, such as Mann- Kendall trend tests and/or MNA software programs, such as NAS and BIOSCREEN. Our comments on the MNA Estimate are presented below. 1. We are concerned that the simple linear regression (SLR) models provided in the MNA Estimate cannot be relied on to accurately estimate WQO attainment dates. These potentially erroneous predictions may be attributable to the use of concentration data that are not independent of external factors, such as water levels and seasonal fluctuations. External data that correlate with KMEP's petroleum hydrocarbon concentration data could potentially alter existing trends as observed in the time-concentration graphs generated for SP/M-3, SP/M-4, and SP/M-7. In order to obtain a more precise projection of the time that will be required to reach WQOs, KMEP needs to utilize the methodology for determining the point decay rate constant (kpoint) provided in the US EPA guidance document Calculation and Use of First Order Rate Constants For Monitored Natural Attenuation Rate Studies. This guidance document (Guidance) is available on the internet at http://www.epa.gov/ahaazvuc/download/issue/540S02500.pdf.