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• • ExxonMobil Global Services Colony Shale Oil Project ESR Reclamation Studies Weighing Lysimeter Performance 1990-2003 Project No. 353 Page 16 the available stored water in the topsoil zone is becoming depleted, and in spite of favorable air temperature and abundant sunshine, the actual evapo-transpriation begms to taper off as the water stored it the soil is depleted. The significance of the above seasonal process to design of spent shale piles is key to detemvning the potential for deep percolation into spent shale piles. As was proposed by Wymore, and by many other researchers over the years, the potential evapotranspiration in this region always greatly excceds the annual precipitation, and conceivably could result in consumption of all incipient precipitation. Because the ground is frozen in the winter and the precipitation is temporarily locked up in snow pack, the two key time periods when percolation can occur are the initial spring melt and the summer rainfalls. Precipitation in late fall, with most plants dormant, serves mainly to re-wet the shallow soils that were depleted of moisture iri the previous summer. Winter precipitation falls in the form of snow, generally on frozen ground, and does not percolate into the ground. The portion of winter precipitation that has not been evaporated (sublimed) during sunny winter days, remains as snow pack at spring melt. At this point, the portion that does not runoff in the spring but perwlates into the soil can become deep percolation, but only if it can move past the plant root zone before the plant demand draws it back up as evapotranspiration. In the summer months it is possible that the plant cover and the warm soil surface can consume all of the incipient precipitation, although intense thunderstorms can exceed the infiltration capacity of the soil cover, such that some of the summer rain nuns off. Alternatively, if there is a very wet period in the summer, there could be runoff in less intense stomvs. Nevertheless, it is cleaz that light rains are probably totally consumed, while heavier rains produce runoffand sometimes infiltrate into the soil. Both of these phenomena are visible on Fig 4.1, but after about 1996, by which time a dense plant community had become established, it does not appear that the uppermost region of the combusted shale (red TDR's on Fig. 4.1) received any percolation from summer storms unless the stomas were intense enough to cause measurable runoff. 1995 and 1997 are example of a late summer (September) series of storms during a relatively wet series of years that did produce runoff and started LACHF:L FELICE & Associates