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Mr. Stephan Defibaugh -2 - 12 May 2009 KMEP Stockton Terminal • 0 Objectives at the SFPP, L.P Stockton Terminal, Stockton, California (MNA Estimate). The update, however, is limited to select wells. Groundwater sampling summarized in the Annual Report shows that non-aqueous phase hydrocarbons (NAPH) have been absent from on-site monitoring wells since 1996. The highest concentrations of contaminants of concern (CDCs) are generally detected in manifold area well SP/M-9 and co-mingled area well SP/M-10. During the fourth quarter of 2008, SP/M-10 contained the highest concentrations of total petroleum hydrocarbons as gasoline (TPHg), benzene, toluene, ethylbenzene, and xylenes (BTEX) at 17,000 micrograms per liter (ug/L), 440 pg/L, 910 pg/L, 780 pg/L, and 33,000 pg/L, respectively. The highest concentrations of methy tertiary butyl ether (MTBE) and tertiary butyl alcohol (TBA) were detected in SP/M-9 at 3,100 pg/L and 90,000 pg/L, respectively. In the manifold area well SP/M-9, the Mann-Kendall shows that the MTBE concentration is decreasing. The MNA Estimate predicted that MTBE in this well will attain the water quality objective (WQO) of 5 ug/I by 2010. TPHg and benzene concentrations in this well are currently 11,000 pg/L, and 3,700 pg/L respectively. MTBE concentrations in manifold are well SP/M-1 observed in May and November 2008 were 990 pg/L and 1,200 pg/L, respectively. The MNA Estimate predicted that this constituent would attain the WQO in this well by biodegradation in 2009. However, the Annual Report did not contain a Mann-Kendall update for MTBE in this well. Table 2 shows that these concentrations have been relatively stable since May 2005. 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 and SP/M-9, and co-mingled area well SP/M-10. The oxygen reduction potentials (ORP) in these wells were generally at or below-100 millivolts, which indicates that the interior of the plume is anaerobic. However, the Annual Report did not contain updated ORP data. Sulfate concentrations are up to five orders of magnitude higher in the distal wells than the manifold and co-mingled wells. Nitrate (NO3) and total orthophosphate were generally absent or depleted in all wells. The MNA parameters strongly suggest that biodegradation is a principal mechanism for petroleum hydrocarbon attenuation in the plume. Our comment is presented below. 1. The discrepancy between KMEP's estimate for SP/M-3 to achieve the WQO for MTBE by 2008, as predicted by its regression model, and the observed MTBE concentration of 7.7 pg/L prompted Central Valley Water Board staff to evaluate KMEP's projected date in accordance with the US EPA's guidance document Calculation and Use of First Order Rate Constants For Monitored Natural Attenuation Rate Studies. As stated in our 8 May 2008 letter, this guidance document (Guidance) is available on the internet at http://www.epa.gov/ahaazvuc/download/issue/540S02500.pdf. Our calculations show that the first order rate constant (kpoint) for MTBE in SP/M-3 is 0.677 Ng/year. Using this kp.int, we project the attainment date for MTBE in SP/M-3 to be 2009.