1999-08-23_HYDROLOGY - M1999051 - Laserfiche WebLink

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QUALITATIVE ENVIRONMENTAL RISK ASSESSMENT FOR MINE DEVELOPMENT of 4.2 and 7.9. Minimum pH values of 1.6 to 2.8 were measured during a short peak acid production period and subsequent wash out from large -scale field tests on waste rock samples. These values increased to a pH of 2.7 to 4.0 at the end of the first year of the test. The negligible value for pH is the range 6.5 - 9.0 based on criteria for protection of aquatic life, although the acidity of the natural waters on the site results in a lower pH. The pH value chosen to describe high acidity is based on the characteristics of the receiving waters which are poorly buffered, and already somewhat acidic, and therefore, sensitive to the acids present in ARD. It was judged that a pH of 3.4 and less will pose a high hazard based on the potential for this level impacting and killing all the aquatic biota and fish in a long reach of a stream. This value was derived from experience on pH levels in ARD which has led to destruction of aquatic life in streams. The natural waters have a low extreme pH of 4.2 and this value was taken as the lower range for the very low hazard descriptor. The rest of the scale was then developed around these values. Exposure Assessment An exposure assessment for each failure mode takes into account the potential for, the magnitude, and duration of a hazard release, as well as the transport along pathways through the environment to an ecosystem at risk. The attenuation, or reduction in contaminant concentration, from the point of release to the ecosystem at risk must be accounted for in the exposure assessment. At the example project, two ecosystems can be exposed to the release of hazards. These are the aquatic and terrestrial ecosystems. Aquatic ecosystems in the project area can be influenced by all releases because of the high precipitation and associated runoff. Terrestrial ecosystems will be influenced by physical hazards due to slope instability. Although some of the other hazards can also influence terrestrial ecosystems, e.g. acidity in ARD, such exposures will be limited to small areas. The exposure assessments in each of the failure scenario risk assessments, as those given in, Appendix A, include consideration of: ► Possible failure mechanism ► Possible initiating events ► Potential for release ► Magnitude of release ► Duration of release ► Pathways ► Ecosystem at risk ► Exposure description and Qualitative Assessment Descriptors for Exposure Assessment. Developing descriptors for exposure assessments takes into account the characteristics of the ecosystem at risk, e.g., terrestrial ecosystems can tolerate higher volumes of sediment releases than aquatic ecosystems. Similar differences exist for the exposure of different ecosystems to chemicals. Descriptors for each hazard, based on possible consequences to the terrestrial and aquatic ecosystems, are presented in Table 2. In order to develop the descriptions for the qualitative hazard assessment, it was assumed that a release will occur and that the ecosystems will be exposed to the hazard in undiluted form. The descriptors proposed for the qualitative exposure assessment consider the consequences to the ecosystems. In this case, attenuation and dilution along the exposure pathway are considered. Therefore, the descriptors for qualitative exposure assessment consider the concentration or volume of a hazard reaching the ecosystem at risk. The descriptors summarized in Table 2 are then also used for the qualitative exposure assessment. For example, consider the case of cyanide release from the mill; although the hazard is high, removal, dilution, and degradation contribute so that the exposure is negligible. Consequence Assessment Consequences are the impacts or damages to the environment because of exposure to contaminants or physical hazards during project phases. Only impacts to the environment, i.e., the natural ecosystems and biological populations in the vicinity of the project, are considered in this qualitative consequence assessment. Exposure to contaminants by biological populations can result in minor, sub - lethal, or lethal effects, depending on the dose received by the population at risk.. Physical hazards generally are measured through actual destruction of the ecosystem or by habitat degradation and reduction in biological productivity or biomass. Secondary impacts to ecosystems can also be tracked. For example, poor water quality can reduce the numbers and kinds of aquatic organisms on which fish are dependent. Descriptors for Oualitative Consequence Assessment. Descriptors for the consequence assessment are developed considering the consequences of the exposure to the terrestrial and aquatic ecosystems. The aquatic ecosystem is divided for this purpose into two parts: aquatic biota and fish populations. Levels of consequence are related to actual doses received or physical impacts on the ecosystem. The basis for assessing consequences is site specific baseline information and experience at other mining projects. Table 3 gives a qualitative consequence description for each of the ecosystems and specific organisms, such as fish and aquatic biota, based on different levels of impact. The descriptors for the consequence levels are set using percentage changes in the biological parameters of growth, reproduction and productivity. Negligible consequence are the levels at which no impacts are discernable. The high consequence levels are total destruction for a long or indefinite period of a large area or a long reach of a major stream or body of water. Qualitative Risk Assessment and Characterization The qualitative risk assessment combines the information developed for the hazard, exposure and consequence assessments into a qualitative description of the risk posed by specific failure scenarios. In the process of evaluating the risk, the hazard, failure mechanism, relative risk, magnitude and duration of a release, as well as the pathways of a release and the consequences on the ecosystem at risk are considered. 27