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Geosyntec° <br /> consultants <br /> C Tech, developer of EVS®, recommends that the "nugget" (i.e., value of the semi- <br /> variogram model at distance = 0) in the semi-variogram model be equal to zero, forcing <br /> the kriging algorithm to match input data exactly. While this method has the benefit of <br /> honoring the source data, the downside is that EVS®'s semi-variogram model is likely <br /> not to be the"best-fit" in terms of minimizing the modeling error. <br /> The interpolated results of 3D kriging are rectangular grids containing nodes at regularly <br /> spaced intervals in each dimension. The chemical concentrations being interpolated are <br /> stored for each grid node location and used for subsequent display and analysis. Once a <br /> 3D grid is calculated, it is used to create a faces file representing 3D isoconcentration <br /> shells.Final grid values were contoured and displayed at the MCLs of 6 µg/L for cis-1,2- <br /> DCE and 0.5 µg/L for VC. The island-wide models are shown at a five times vertical <br /> exaggeration and the area specific models at a three times vertical exaggeration. <br /> 3.2 Results <br /> Island-wide, Site 3, Site 4, Central Area,and Landfill Area 3D PDFs of the EVS®model <br /> isoconcentration contours for cis-1,2-DCE and VC are shown in Figures 11 through 20. <br /> Subject to the foregoing limitations,these figures provide a reasonable approximation of <br /> the current distribution of COCs in groundwater across RRI. <br /> 3.3 Limitations <br /> The depictions of plume geometry presented in Figures 11 through 20 represent an <br /> estimate of the distribution of the COCs at RRI; however, as with any approach used to <br /> interpolate data,there are uncertainties and limitations to the approach that may result in <br /> alternative interpretations of the distribution of COCs in groundwater.These uncertainties <br /> and limitations are as follows: <br /> The exact subsurface distribution of the COCs cannot be completely ascertained <br /> given temporal changes in groundwater flow directions and COC concentrations, <br /> as well as the data gaps and other limitations described herein. The spatial and <br /> temporal spread of the chemical data may not encompass the entire distribution <br /> of chemicals in the groundwater. Additional assumptions are necessary as to <br /> chemical concentrations in areas that may not be completely represented by data <br /> from project wells. <br /> RRI Groundwater Monitoring Work Plan 18 1 June 2018 <br />