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1 1 • <br />1 <br />August 19, 1999 <br />materials spilled. Traffic controls on the lease area will include speed limits, yield and <br />stop signs, and warning signs. Road maintenance will include snow plowing, dust control, <br />and grading. The exposure assessment indicated that the likelihood of a spill due to a traffic <br />accident is very low even with driver training, road maintenance, and adequate signage. <br />Magnitude and duration of exposure are very low due to relatively small volumes of <br />reagents and fuels as well as tanker spill prevention controls. Pathways are a medium risk <br />depending on where the spill occurs, but would likely have direct access to soil and <br />vegetation. The environment at risk are soils and vegetation along the roads. The overall <br />consequence assessment is that high environmental consequences could occur due to the <br />high hazard to soils and the pathways for impacts to the environment. However, the overall <br />risk characterization is described as very low due to the very low likelihood of occurrence <br />and very low magnitude and duration of spillage. Proposed mitigation in addition to the <br />operational controls identified in the operating plan are presented on Table 1 and discussed <br />in Section 9. <br />8.7 Evaporation and Drainage Ponds <br />\ \GAI DEN1\ PROJECTS \WP\99 \2057\2057TM1.DOC Golder Associates <br />MEMORANDUM <br />- 993 -2057 <br />Potential failure mechanisms for the evaporation and drainage ponds include leaks, <br />overtopping, and direct contact by wildlife. The first failure mechanism includes leaks due <br />to poor construction and puncture during operations. The hazard assessment indicates that <br />subsidence and surface water contamination are a negligible hazard, while groundwater and <br />soils contamination are a very low hazard. Initiating events and likelihood for leaks related <br />to poor construction is negligible due to construction quality assurance, liner testing prior <br />to use, and the leak detection system. Puncture during operations is also negligible due to <br />the limited life of facilities (5 years) and control of access to the ponds through fencing. <br />Magnitude and duration of exposure are expected to be negligible due to leak detection <br />monitoring. Pathways have very low risk with direct flow through soils with the potential <br />to reach the alluvial monitoring well. The environment at risk is a medium risk due to <br />potential impacts to the soils and alluvial aquifer. The consequence assessment indicts that <br />there is a potential very low to hazard and potential pathway to impacts of soil and <br />groundwater. However, the overall risk characterization is negligible due to negligible <br />likelihood as well as magnitude and duration of exposure. <br />The second potential failure mechanism is overtopping of the ponds due to extreme storm <br />events or plant vessel failure. The hazard assessment (Table 2) indicated that there was <br />very low risk for groundwater, surface water, and soils contamination, with negligible risk <br />for subsidence. Likelihood of occurrence for both the extreme storm events and plant <br />vessel failure is negligible. Extreme storm event (2 feet of freeboard given maximum <br />operating levels resulting in 10 -acre feet of storage) containment will be part of the design <br />criteria for pond sizing. Plant vessels will be designed to exceed normal operation <br />pressures and will be tested prior to operations. Magnitude of the exposure for extreme <br />storm overtopping is expected to be very low due to the specific gravity differences <br />between the rainwater and soda solutions resulting in the clean floating on top. Magnitude <br />