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MEMORANDUM <br />August 19, 1999 -5- <br />• High <br />Table 1, column 4 presents the results of the consequence assessment. <br />8.0 RISK CHARACTERIZATION <br />993 -2057 <br />The risk characterization is determined by consideration of the previous three assessments. <br />Hazard assessments may range from negligible to high for the potential hazards <br />(subsidence, groundwater contamination, soils contamination, and surface water <br />contamination), while exposure and consequences may range from negligible to high. For <br />a risk to be characterized as negligible either the likelihood or the magnitude, duration or <br />consequence must be negligible. Thus, even if the hazard may range from negligible to <br />high, negligible likelihood, magnitude, duration, or pathways of release can reduce the <br />overall risk to negligible. <br />As shown on Table 1, column 5 the risk characterization indicates that all project <br />environmental risks are negligible with the exception of cavity coalescence, operator error, <br />spills, and vehicle accidents. This risk characterization is driven by operation design that <br />supports early detection monitoring and mitigation of potentially adverse situation. A <br />discussion of the reasoning leading to the overall risk assessment is given in the following <br />sections. <br />8.1 Cavity Mechanisms <br />Potential failure mechanisms associated with the cavity have to do primarily with loss of <br />control of the solutioning process: where control of solutioning is maintained the <br />calculations carried out by Agapito and Associates (1996) confirm stability and preclude <br />pathways to the aquifers or significant subsidence. <br />If control of solutioning were to be lost the potential consequences would be either <br />reduction in the roof web or reduction in the pillars between the cavities. Reduction in the <br />roof web could have potential stability implications in terms of collapse of the roof, <br />wellbore damage and connection to the overlying aquifers. The hazard associated with this <br />is considered to be negligible to high, since in the extreme case of loss of control the B <br />Groove aquifer could be affected. However the likelihood of this event occurring is <br />considered to be negligible because of the use of a light fluid cushion to control upward <br />growth. This cushion can be used to control any excessive upward solutioning. The <br />presence of the cushion is known due to fluid volume monitoring, and any loss of integrity <br />in the pipe or the roof will be immediately detected. It is also worth noting that the internal <br />liquid pressure of the process liquid and the presence of insoluble rubble will enhance the <br />roof stability. Since the likelihood of this failure is negligible the overall risk is also judged <br />to be negligible. It should also be noted that any contact between the process solution and <br />\ \GAl DEN1\ PROJECTS \WP\99\2057\2057TM1.DOC Golder Associates <br />