COMPLIANCE INFO_1998-2004 - Laserfiche WebLink

Home Browse Search

r San Joaquim County Public ealth Services January 12, 1998 Page 2 approximately 21,000 ug/12 would be expected. The highest benzene concentrations detected in wells where LPH has been documented are much lower and are as follows: MW -3 (850 ug/1 in a sample obtained of 5 November 1990) MW -4 (500 ug/1 in a sample obtained on 25 March 1992) and MW -6 (920 ug/1 in a sample obtained on 25 March 1992). Concentrations of aromatic petroleum hydrocarbons have declined substantially since the highest benzene concentrations were detected. The most recent data -- associated with samples obtained on 15 October 1987 -- shows that aromatic hydrocarbons were detected in only two of the three wells sampled. The highest benzene concentration detected at that time (55 ug/1) was reported in a sample obtained from MW -3. It seems reasonable to conclude that the LPH previously documented and removed from wells MW -3, MW -4, and MW -5 is consistent with weathered product documented when Mobil replaced the underground storage tanks during 1986. The data shows that LPH has been removed to the extent practicable. MTBE concentration data is now shown in Figure 3, replacing the dissolved oxygen measurements shown in past reports. You should note that estimated MTBE concentrations for samples analyzed during 1993 and 1994 are also shown on Table 1 - Summary of Results of Groundwater Sampling. Prior laboratory documentation is appended to the enclosed report; I have no other information regarding the suspected or confirmed presence of MTBE in groundwater other than the data summarized in this report. It is my understanding that PACE included MTBE in the gasoline standard used to determine the retention time of various gasoline components, however, MTBE was not included in the calibration standards used to calculate sample concentrations. This is why Pace refers to the MTBE concentrations for samples analyzed during 1993 and 1994 as estimates. You will note that MTBE was reported to be present in samples obtained prior to the sale of the site to Tosco in 1994, however, MTBE concentrations detected since the introduction of "Cleaner -Burning Gasoline" required by CARB since spring, 1996 are significantly higher. For example, respective MTBE concentrations of 27,000 ug/1 and 14,000 ug/1 were reported in a samples obtained from AMW-5 and MW -2 on 15 October 1996. Wells AMW-5 and MW -2 are located adjacent to the underground storage tanks, and concentrations of MTBE appeared to have increased substantially between the sampling conducted on 4 October 1995 and 4 January 1996. In contrast to the LPH, the MTBE appears to be a consequence of the operation of the facility after BP sold the site. benzene, and 3.5 % benzene was typical (Ground Water Monitoring Review, Spring 2990, V.X.,Nos (Kreamer and Stezenbach - Development of a Standard, Pure -Compound Base Gasoline Mixture for Use as a Reference in Field and Laboratory Experiments — pp. 135 to 145). z The solubility of pure benzene in water is appoximately 1780 mg/l. Gasoline containing 1.2 percent benzene mixed with water will yield an equilibrium benzene concentration in the water of about 21 mg/l. (1780 mg/l * 0.012 percent = 21.36 mg/1).