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4 <br /> certainty and accuracy of sophisticated model predic- <br /> tions can drop markedly; in that case, selection of a <br /> less-sophisticated approach is indicated. <br /> For this assessment, a ground-water transport model was <br /> selected on the basis of the quantity and quality of <br /> available input data compared to model requirements. A <br /> simpler model was selected because some site-specific <br /> data were absent or insufficient to justify more complex <br /> models. Even so, some model inputs had to be inferred <br /> from the limited hydrogeological information. Inferred <br /> model values were selected conservatively; that is, so <br /> that the ground-water concentration would, be over- <br /> estimated rather than underestimated. <br /> 3.2.2 .2 Ground water <br /> Transport of contaminants from the site to the nearest known <br /> producing water webs was modeled with the RESSQ computer program <br /> (Javandel et al. 1984) . Required input to RESSQ is as follows: <br /> relative coordinates, volumetric flow rate, and dia- <br /> meter(s) <br /> ia- <br /> meters) of contaminant source (s) and production well (s) <br /> concentration of contaminant at the source <br /> aquifer thickness, porosity, and pore water velocity. <br /> Model output includes the arrival times of streamlines from <br /> source to production wells and the contaminant concentration <br /> history for water produced. <br /> At the Stockton site, the volume flow rate of the contaminant <br /> source was-estimated-as the product of the local ground-water <br /> (pore) veloi-ity (v) and the cross-sectional area of the <br /> contaminated aquifer zone. The velocity is based on the geo- <br /> 19 <br />