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ST Services Stockton Terminal - 2 - 13 August 2007
<br /> The monitoring well closest to the source area, PS/MW-14, has historically contained free
<br /> product and therefore is not usually sampled. PS/MW-14 did not contain free product during
<br /> the first and second quarters, so Ash Creek sampled it. The highest concentrations, however,
<br /> generally occurred in ST/MW-1 during the second quarter. During the two quarters, maximum
<br /> TPHg and TPHd concentrations included 220,000 micrograms per liter (Ng/L) and 20,000
<br /> pg/L, respectively. Maximum BTEX concentrations were 23,000 pg/L, 17,000 pg/L, 3,100 pg/L,
<br /> and 17,000 pg/L, respectively. MTBE, TBA and TAME were detected at 240,000 pg/L, 160
<br /> pg/L and 1,200 pg/L, respectively. ST Services did not sample monitoring wells OW-8A and
<br /> OW-8B during the first quarter 2007, as required by Monitoring and Reporting Program (MRP)
<br /> No. R5-2004-0822.
<br /> Table 3 in the QMRs shows the results of monitoring for monitored natural attenuation (MNA)
<br /> parameters, which is being conducted to evaluate MNA as a potential remedial technology for
<br /> the Site. From January to April 2007, the parameters trends generally show low dissolved
<br /> oxygen (DO), oxidation reduction potential (ORP) and nitrate concentrations. Alkalinity, total
<br /> organic carbon (TOC) orhto phosphate, ferrous iron (Fe II), and potassium were all detected at
<br /> elevated concentrations. Sulfate concentrations were lower in the interior of the plume
<br /> compared to the margins.
<br /> NuStar prepared the Addendum in response to Regional Water Board staff's 15 May 2007
<br /> letter which requested that an evaluation of groundwater pump-and-treat technology be
<br /> included in an addendum to the 5 January 2006 Cleanup Plan. NuStar used MODFLOW-2000
<br /> to simulate groundwater cleanup by extracting groundwater from the sand and silt layers
<br /> underlying the site, and conducted sensitivity analyses by varying the conductivity and gradient
<br /> in the silt and sand layers separately. NuStar concluded that cleanup of the silt using pump
<br /> and treat technology would take about 4,500 years.
<br /> To compare the time required to clean up the site by pump-and-treat to that needed by MNA,
<br /> NuStar performed simple linear regression (SLR) on TPHg and BTEX concentrations
<br /> observed in MW-15, which is on the periphery of the Site. Based on the SLR projections to a
<br /> TPHg cleanup goal of 1,000 pg/L, NuStar calculated that cleanup could be achieved in 14 to
<br /> 50 years with MNA, so recommends MNA for cleanup of the Site.
<br /> Our comments are presented below.
<br /> 1. Regional Water Board staff do concur with NuStar's rationale to justify the selection of
<br /> MNA as the remedial method for site cleanup. The requirement for attainment of cleanup
<br /> standards is not limited to wells at the periphery of the plume but includes wells in the
<br /> plume interior including UP/MW-1 , where constituents of concern (CDCs) are generally
<br /> highest. Regional Water Board staff used SLR, the methodology employed by NuStar, to
<br /> predict whether the COC concentrations present in UP/MW-1 will attain water quality
<br /> objectives (WQOs) in about 40 years (see attachment). With no active remediation, the
<br /> SLR model projects the concentrations for TPHg, TPHd, benzene and MTBE in UP/MW-1
<br /> will increase to about 400,000 ug/L, 13,471 ug/L, 32,733 ug/L and 1,505,416 ug/L,
<br /> respectively. These results show that MNA will not attain the WQOs of 5 ug/L for TPHg,
<br /> 100 ug/L for TPHd, 1 ug/L for benzene, and 5 ug/L for MTBE in UP/MW-1 in the 14 to 50
<br /> years NuStar predicts. These results show that active remediation is necessary to clean up
<br /> groundwater pollution at this site.
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