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<br />r'- <br />"1" <br />"'- <br />,. <br /> <br />i.-":' <br /> <br />r'l <br />{ '-- <br /> <br />. ~'--, <br />;_,.J . <br /> <br />{ <br /> <br /><... <br /> <br />18 <br /> <br />grading between dikes will be required to level the pond area <br />regardless of the dike configuration. The near surface materials <br />at all three areas can easily be graded with conventional <br />construction equipment. <br /> <br />CJ <br /> <br />7.2 Seepage Considerations <br /> <br />7.2.1 General <br /> <br />As described in Section 5, the near surface soils and even the <br />weathered Mancos shale at the site are relatively permeable. <br />Therefore, seepage losses out of the proposed evaporation ponds <br />will be a major design consideration~ <br /> <br />For purposes of this report, preliminary estimates of seepage <br />were made assuming no seepage control measures were used. Two <br />components of seepage were considered; seepage under the dikes <br />(lateral seepage) and vertical seepage out the bottom of the <br />ponds. Simple flow-nets were used to estimate lateral seepage <br />and Darcy's Law, Q = kiA, was used to estimate vertical seepage. <br /> <br />The seepage estimates are intended only to indicate the magnitude <br />of seepage. Because of limited field data with respect to <br />permeability at depth (the deepest field tests were at 60 feet) <br />and no data on the effects of saline water on permeability over <br />long periods of time, more sophisticated modeling of partially <br />saturated flow, or time dependent analyses were not possible nor <br />warranted for this preliminary study. The key issue at this <br />stage is whether or not the soil/bedrock on-site is sufficiently <br />impermeable to make seepage control unnecessary. <br /> <br />( <br /> <br />7.2.2 Seepage Estimates <br /> <br />The key variable in estimating seepage is the permeability of the <br />materials through which seepage will occur. For simplification, <br />the materials at the site were divided into three categories; <br />soil overburden, weathered shale, and shale. As the logs in <br />Appendix A show, the average stratigraphy incorporating these <br />categories of materials is essentially the same for the East and <br />West site areas and slightly different for the Middle area. The <br />average depths and permeabilities used for the analyses are <br />summarized in Table 7-1. <br /> <br />c <br />