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ARCADIS Ms. Amy Terrell <br /> January 29, 2014 <br /> Objective and Goal <br /> The objective of the NSZD study was to compare the physical LNAPL removal rate to <br /> the NSZD removal rate. The objective was met by conducting quarterly LNAPL <br /> removal activities and by conducting two separate CO2 flux sampling events at two <br /> locations within the footprint of the LNAPL plume and two background locations <br /> outside of the footprint of the LNAPL plume. The goal was to assess if hand bailing <br /> LNAPL from wells is inconsequential compared to the natural degradation rate of <br /> LNAPL. <br /> Site-Specific LNAPL Observations <br /> The following key observations were summarized in the LNAPL Mobility Report <br /> (ARCADIS 2010): <br /> • LNAPL bail-down tests conducted at MW-06A, TW-01, and TW-04 during <br /> November 2009 indicated that it took two weeks for LNAPL to recover to <br /> approximately 90 percent of the initial thickness. <br /> • LNAPL observations in the monitoring well network at the site and LNAPL <br /> mobility calculations summarized in the LNAPL Mobility Report indicate that the <br /> LNAPL plume is stable and not migrating. <br /> Natural Source Zone Depletion Description <br /> As described in the Response to Comments (ARCADIS 2012), NSZD is a <br /> combination of processes that reduce the mass of LNAPL in the subsurface through <br /> time. Petroleum LNAPLs are complex mixtures of hydrocarbon compounds and it is <br /> well documented that LNAPLs degrade after being released to the <br /> subsurface. LNAPL degradation results in a change in LNAPL properties over time <br /> as LNAPL constituents are depleted from the LNAPL mixture at variable rates. <br /> The two primary mechanisms responsible for LNAPL depletion are volatilization and <br /> dissolution. The significance of both mechanisms is related to the LNAPL properties <br /> (e.g. volatility and solubility of the LNAPL) and the site setting. Site conditions control <br /> movement of soil gas and groundwater within the impact area. For example, a lower <br /> rate of LNAPL dissolution would be expected in a low permeability unit comprised of <br /> silts and clays due to limited groundwater flux (versus a highly transmissive sand). <br /> Page: <br /> G:\Pmjects\UPRR_Roseville\RC000709_Tracy Mnt Facildyl9_DELIVERABLESU014.01 NSZD Rpffracy ca-Maint Facility_NSM Rpt(01-29-14).Eoc 2/9 <br />