1999-08-23_HYDROLOGY - M1999051 - Laserfiche WebLink

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20 RISK ASSESSMENT /MANAGEMENT ► Hazard assessment ► Exposure assessment ► Consequence assessment ► Risk characterization Each of these steps is discussed below. The five point scale proposed for a qualitative risk assessment consists of the following descriptors: ► negligible ► very low ► low ► medium ► high Before embarking on the four step approach, it is necessary to identify potential failure modes and mechanisms. These are identified based on a knowledge of the site and the proposed design elements, as well as experience on past failures at mine sites. This step if the risk assessment is very similar to design engineering where each failure mode is identified and a mitigative design developed. Hazard Assessment In the first step hazards associated with a specific situation are identified; e.g., an investigation is made of mine waste and tailings to be deposited into an impoundment, or the waters to be discharged from a treatment plant. The constituents of concern are identified and described as to the hazards they may pose to the environment. The final results of the hazard assessment is the selection and description of those hazards which are present for each potential failure mode. Exposure Assessment The next step in the qualitative risk assessment is to perform an exposure assessment. For each potential failure mode a substance which poses a hazard (such as sediment) can be transported from its point of release to where it impacts the environment. Transport can take place through various media, such as over land, through surface water, groundwater and air. Exposure to the environmental systems can be through physical means (e.g. sediment directly covering land and vegetation or released into streams) or chemical means such as heavy metals released into streams. The elements of the exposure assessment which must be considered include failure mechanism, initiating events, relative risk of release, magnitude of release, duration of release, pathways and ecosystem at risk. The final result of the qualitative exposure assessment is the description of the relative magnitude of exposure associated with each potential failure mode using the five point scale. The hazard assessment and exposure assessment steps address the questions: ► what can happen? ► how likely is it to happen? Consequence Assessment The third step is evaluating the consequences or impacts of a hazard following release and exposure of a local ecosystem, e.g. the consequences to fish of the release of heavy metals into a stream. Consequences are also described using a five point scale. The consequence assessment addresses the question: ► if it does happen, what are the consequences? Risk Characterization The fourth, and last step, is to characterize the risk associated with each failure mode based on the hazard assessment, exposure assessment and consequence assessment. Risk is also characterized using the five point scale. Figure 1 presents a schematic of the proposed qualitative environmental risk assessment approach. EXAMPLE APPLICATION OF QUALITATIVE RISK ASSESSMENT Introduction In applying a qualitative risk assessment to a specific mining project a number of assumptions must be made, for example, a project site can be divided on the basis of geographic location and activity into different areas such as: ► Mine - mine, mine waste disposal, water management system. ► Mill and Tailings - mill, tailings disposal, water reservoir. ► Transportation, Storage and Handling Facilities - dock, roads, storage at mine and mill. Each of these areas can further be divided into components such as the waste rock stockpile in the mine area and the individual tailings impoundments. Such division is useful in identifying individual failure modes. Failure modes can be identified for each component during four phases of project development. The four phases used in this example are: ► Construction ► Operations ► Reclamation ► Closure Project Description Figure 2 shows the main components of the example project. The project is located on a coastal plain. The major environmental factors associated with the project are described below. Climate. Frequent frontal systems combine to produce abundant precipitation throughout the region, especially along west - facing mountain slopes. Winters are mild, and snow on the lower elevations is a minor component of the total precipitation. Average precipitation increases with increasing elevation from approximately 1700 to 2200 millimeters per year. Average monthly temperatures range from about 1 °C in January to approximately 14 °C in July and August. Phvsioaraphic Units. The mine area is located along the boundary of two major physiographic units. The lowlands to the east are generally characterized by extensive areas of little to no