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<br />Standley Lake Off-Site Area. The Standley Lake off-site area includes sub-basins WOM 1, <br />SL 1 to SL 6 and SLR. These sub-basins are described below. <br /> <br />SOILS <br /> <br />WOM 1 - This sub-basin includes the drainage of the Woman Creek basin west of the South <br />Boulder Diversion Canal, starting at the base of the foothills at the mouth of Coal Creek <br />Canyon. The basin outlets into basin W 6 in the plant Buffer Zone and partially into the <br />South Boulder Diversion Canal as inadvertent drainage during high runoff. However, for <br />purposes of hydrological analyses, all flow is routed to W 6. <br /> <br />Drainage and flood control analyses are closely related to soil cover because of the <br />important function that infiltration and depression storage play in the rainfall-runoff <br />relationship. A soil infiltration map is given in Figure I1I-4. Soil type delineation is <br />presented in Figure I1I-5. Soil development on the Rocky Flats Alluvium may belong either <br />to the Nederland soil series or the Valmont soil series. A comparison of these soils is <br />shown in Table I1I-6. <br /> <br />SL 1 to SL 6 -These basins include all of the Standley Lake drainage from east of Indiana <br />Street, south of the Woman Creek basin, and west of the South Boulder Diversion Canal. <br />They include all of the Upper Big Dry Creek basin. SL 1 receives runoff from the Woman <br />Creek basin and outlets into Standley Lake. Basins SL 2 through SL 6 encompass the <br />Upper Big Dry Creek basin. These basins lie almost entirely east, south and west of the <br />Rocky Flats Plant site Buffer Zone. <br /> <br />Detailed studies by the U.S. Geological Survey (USGS) in 1970-75 indicate a high <br />infiltration rate of the Rocky Flats Alluvium, in the range of 3.5 to 7.5 inches per hour. For <br />the period of the USGS study, the measured runoff was only 1.4 percent of the rainfall, an <br />exceedingly low ratio (USGS, 1976). However, the precipitation events analyzed during the <br />study period were generally frontal storms with low intensities. <br /> <br /> <br />SLR - This basin encompasses the surface area of Standley Lake Reservoir. <br /> <br />For the same storms, the analysis shows a lag time between rainfall and runoff for the basins <br />of three to four hours, exceedingly long for such small basins. <br /> <br />Rock Creek <br /> <br />The USGS analyses demonstrate that for low intensity rainfall, the drainage basins of Rocky <br />Flats have high infiltration, are slow in response, and have little runoff. Such information <br />is useful when analyzing high intensity storms because of the low intensity precipitation <br />typically occurring before and after the high intensity "bursts" of rainfall. <br /> <br />The Rock Creek drainage basin is in the northwest portion of the Rocky Flats Plant Buffer <br />Zone. It includes sub-basins RC 1 and RC 2 which are not affected by Rocky Flats Plant <br />site development. Descriptions of these sub-basins follow. <br /> <br />GEOLOGY <br /> <br />RC 1 - This basin encompasses the Rock Creek drainage lying in the Buffer Zone of the <br />plant site northwest of the developed plant site. <br /> <br />The geologic framework of the study area directly affects the movement and quality of <br />groundwater and surface water. The following geologic description is extracted from the <br />USGS Open File Report 76-268 entitled, "Hydrology of a Nuclear-Processing Plant Site, <br />Rocky Flats, Jefferson County, Colorado" (USGS, 1976) and EG&G's Geologic <br />Characterization Report, (EG&G, 1990). Figures I1I-6, I1I-7 and I1I-8 show the geology of <br />the Rocky Flats Plant site. The general geologic structure of the area is reflected in the <br />north-striking beds dipping to the east or southeast. In the western part of the area, the beds <br />are north-striking, and nearly vertical to overturned. In the eastern part, beds strike <br />northeast and dip less than H degrees to the southeast. <br /> <br />RC 2 - This is the portion of the Rock Creek drainage basin lying west of the South Boulder <br />Diversion Canal. It outlets into the Canal. However, for hydrological analyses it is routed <br />to RC 1. <br />