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15,000 cubic yards of burn ash would have to be temporary relocated. The pyrolytic oil could then be excavated from the bottom of the canyon. Next both of the 180-foot slopes would have to be cut back from a 1.5:1 (horizontal to vertical) to at least 3:1 above the stains to safely excavate the material, or extensive shoring would be required in accordance with Title 8, California Code or Regulation, Section 1541, et seq. 3.3 Field Modifications 3.3.1 Clay Plug Design After reviewing the condition of the stain with the CH2M HILL site engineer, U.S. EPA on- scene representative, and Sukut Construction, the CIWMB engineer decided to place a"clay plug" throughout the stained area and a 60-mil co-extruded textured HDPE geomembrane over the clay fill. This option was the best available alternative considering the weather limitations, expenditure limits, technical reasons (e.g., slope stability, excavation limits, shoring requirements, and slope steepness), and that free product (oil) was no longer flowing from the stained areas. The CIWMB engineer first proposed placing granular material in a 2-foot by 2-foot trench at the bottom of the canyon and a composite geonet underdrain at the stained areas along the slope to drain the pyrolytic oil from the gravel lenses. A 60-mil co-extruded textured HDPE geomembrane would then cover the drainage layer. The cover (i.e., foundation, barrier, and vegetation layers) would be placed on top of the geomembrane. A 4-foot, Schedule 40 stain- less steel, slotted well casing would extend from a gravel pack at the low point of the drain- age system. However, with 2 days of continued excavation, the pyrolytic oil did not show signs of further seepage. Investigations showed only lighter stained material remained. After discussing the changes in the pyrolytic oil condition, the CIWMB engineer removed the granular drainage material and the Schedule 40 stainless steel well casing and replaced the first design with 4 to 5 feet of clay compacted to 90 percent modified compaction. The clay plug was placed to prevent the oil from migrating further down canyon. Unfortunately, the spillway elevation and resulting water level reached the clay plug, and, in the worst case scenario, the water level would be 2 to 5 feet above the oil stain. To mitigate this condition, the CIWMB engineer elected to place the 60-mil geomembrane over the clay plug. The geomembrane was placed into anchor trenches above and below the clay plug. A 2- foot barrier layer and a 1-foot vegetation layer covered the geomembrane. The geomembrane will prevent water from saturating the clay plug and allow the oil to migrate into the retention basin. Figure 7 shows the toe design. 3.3.2 Cover Design After discussions with the Central Valley RWQCB representative, the cap design was changed to incorporate evapotranspiration cover concepts. Originally, the cover design allowed for one V-ditch through the center of the canyon. Because the site statistically received fewer than 10 inches of rain per year, the upper fill area of the cover was designed to have 1 to 2 percent grade at the center to create a crown. Drainage was directed to two CIWMB 20