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<br /> • Ozone Microsparge System Annual Status Report-Former Tosco(76)Station No 6981, Stockton,California
<br /> March 3,2003
<br /> Of the three wells used to evaluate the treatment system, well MW-7 has extensive historical
<br /> sampling data prior to the system activation This well was installed in 1992, and has been sampled
<br /> quarterly since that time Historical sampling data for MW-7 is attached as Appendix A
<br /> TPHg '
<br /> Figures 3, 4, and 5 illustrate the TPHg concentrations from quarterly sampling and monitoring
<br /> events of December 2001 through December 2002 The TPH level reported in MW-16 on
<br /> December 12, 2001 was 5,600 ppb The TPHg concentration has fluctuated over the last year, but
<br /> since .lune 6, 2002 it has not exceeded 790 ppb The last sampling results, on December 30, 2002,
<br /> reported the TPHg concentration in MW-16 as <50 ppb This reported concentration demonstrates
<br /> a 99% decrease since the system was placed into operation The TPHg concentration reduction in
<br /> VW-7 from 29,000 ppb on December 12, 2001, to 24,000 ppb on December 30, 2002 represents a
<br /> 17% decrease over the same time period The TPHg level reported in MW-7 on December 12,
<br /> 2001, prior to the operation of the system was <50 parts per billion (ppb) The TPHg concentration
<br /> has remained below 50 ppb with the exception of spikes of 130 ppb and 360 ppb on May 21, 2002,
<br /> and September 30, 2002, respectively(Figure 3) Historically, TPHg concentrations from the initial
<br /> groundwater sampling events (4116192 in MW-7 and 8121/99 in MW-16 and VW-7) to the most
<br /> recent sampling event (12/30102) have decreased by over 99% in wells MW-7 and MW-16, and by
<br /> 37% in well VW-7 Additionally, the maximum TPHg concentrations have decreased from 15,000
<br /> ppb to <50 ppb in well MW-7, from 5,600 ppb to <50 ppb in well MW-16, and from 37,900 ppb to
<br /> 24,000 ppb in well VW-7
<br /> Benzene
<br /> Figures 3, 4, and 5 illustrate the benzene concentrations from quarterly sampling and monitoring
<br /> events of December 2001 through December 2002 The benzene concentration reported in MW-16
<br /> on December 12, 2001 was 160 ppb The last sampling results, on December 30, 2002, reported
<br /> the benzene concentration in MW-16 as 0 53 ppb This is a decrease of 99% The benzene
<br /> concentration reduction in VW-7 from 710 ppb on December 12, 2001 to 170 ppb on December 30,
<br /> 2002 showed a 76% decrease over the same time period The benzene concentration in MW-7
<br /> actually had an abnormal spike of 49 on September 30, 2002, higher then the <0 50 ppb
<br /> concentration it exhibited during much of the year During the December 30, 2002 sampling event,
<br /> the benzene concentration in MW-7 returned to <0 50 ppb Historically, benzene concentrations
<br /> from the initial groundwater sampling events (4116192 in MW-7 and 8121199 in MW-16 and VW-7)
<br /> to the most recent sampling event (12130102) have decreased by over 99% in wells MW-7 and
<br /> MW-16, and by 94% in well VW-7 Additionally, the maximum benzene concentrations have
<br /> decreased from 4,800 ppb to <0 50 ppb in well MW-7, from 290 ppb to 0 53 ppb in well MW-16,
<br /> and from 2,900 ppb to 170 ppb in well VW-7
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