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• March 2000 -58- 993-2099.150 <br />from the site data, these values were compared to the statistical data used previously and <br />were found to generally exceed the mean evaporation values used in the water balance. <br />During operations, annual average evaporation losses were estimated to be two percent of <br />solution applied by drip emitters, which is consistent with that used in Amendment No. 7 <br />(CC&V 1998). During the eight-year post operations period, losses were estimated to be <br />5 percent of the solution applied during the summer months (June through September). <br />This accounts for higher evaporation from wobblers which will be used during post <br />operations. A monthly distribution was developed for the solution application evaporation <br />losses by multiplying annual average evaporation by a monthly weighting factor. This <br />factor was determined by dividing montlily lake evaporation by the average annual lake <br />evaporation. Evaporative losses on the inactive portions of the VLF were modeled by <br />conservatively assuming that 25 percent of the monthly precipitation that falls on the <br />• inactive area was lost to evaporation, which is also consistent with that incorporated in the <br />water balance model presented in Amendment No. 7 (CC&V 1998). No evaporation <br />losses from precipitation falling within the active area under leach was assumed, which is <br />also consistent with that incorporated in the water balance presented in Amendments No. 6 <br />and No. 7. Past experience with modeling evaporative losses on bare granular surfaces <br />with the HELP computer model has shown that between 40 and 75 percent of precipitation <br />is evaporated. The 25 percent value was chosen to conservatively overestimate <br />precipitation gains to the system for PSSA sizing purposes. <br />The field capacity moisture content of the ore was modeled at 7.5 percent of dry weight, <br />based on data provided by CC&V. Data pertaining to the ore field capacity are presented <br />in Appendix B-7. The moisture content of the ore during active leaching will exceed the <br />static field capacity. The excess moisture is excluded in the water balance model, as the <br />volume of solution exceeding static field capacity will drain from the VLF when leaching <br />• ceases and the moisture content of the ore returns to static field capacity. <br />I:\94~2094~FINAL REPURT2p94MA1N2.0~C Golder Associates <br />