1999-08-23_HYDROLOGY - M1999051 - Laserfiche WebLink

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mercury; • cyanide; and, • fuel and lubricants. QUALITATIVE ENVIRONMENTAL RISK ASSESSMENT FOR MINE DEVELOPMENT 25 Descriptors of Qualitative Hazard Assessment. Descriptors based on a five point qualitative scale (negligible, very low, low, medium and high) are developed to describe the relative magnitude of each of the seven hazards. The levels of the descriptors are defined according to the concentrations or volumes in effluent or discharge that constitute a hazard due to toxicity, physical amounts or other possible characteristics that could impact the environment. It must be noted that the levels for the descriptors for each hazard are specific to those environmental conditions at a specific project. The range of values for the hazard descriptors is generally determined by one of the following methods: • toxicity information from site specific or other published literature; • established criteria for environmental protection, estimates based on best engineering or biological judgement; • by including safety factors ranging from 10 to 1,000 (depending on the uncertainty of the effects of the hazard) to amounts or concentrations known to cause high impacts in order to obtain values having negligible impacts. For some of the hazards, the negligible value is set by: • judging the maximum amount of oncentration of material that would have no impact on the local ecosystems; • the effluent guideline if known or established; • the background concentrations or conditions to which the local ecosystems are adapted; and, • release of materials from the site at or below these levels would not be expected to impact the environment more than baseline conditions. The values set for the high level descriptors are amounts or concentrations which are known to cause severe or lethal impacts to the specific ecosystem. For some of the hazards present on the site, the amount or concentration that would cause a severe or lethal impact will not be present. This possible upper limit from the project site is used where it could be determined. The intermediate range of values is either based on experimental or known values for a range of effects on receptors such as aquatic organisms, or is set by interpolating linearly on a log scale. These values also take into consideration the nature of the hazard as it relates to local environmental conditions such as topography, stream flow and water quality. Where adequate information is not available, the range of values is set using professional judgement. A summary of the descriptors developed for the seven contaminants and physical hazards are given in Table 2. As examples of the logic followed in developing the hazard descriptors in Table 2, consider the following: Sediments. Sediments are defined as soils or solids carried by water either as mass flow, or as suspended materials during erosion and runoff from exposed surfaces into drainages. Therefore, sediments can be rated either according to volume or concentration. VOLUME. Sediments are produced from any exposed or loose soil surface during construction, stockpiling, or moving of materials; and from roads and ditches. The maximum amount of material available for sediment in runoff is dependent on the surface area exposed, the depth of erodible surface at the time of storm event and subsequent surface runoff. The maximum credible amount of sediment produced and moved at any one time as a single event is estimated to be about 12,000 cubic meters (120 hectares eroded to 1 cm average depth) from the mine area. The negligible and high values assigned for available sediment volume are based on professional judgment taking into account the exposed area, the rate of flow, gradient and characteristics of the surface, topography and local streams. The negligible volume is taken as 20 cubic meters because it would be quickly dispersed by water flow or could be easily cleaned up. The high volume is taken as 12,000 cubic meters. This amount, if released, could silt in a large stream 4 meters wide to one -half meter deep for 6 kilometers, resulting in long -term effects. CONCENTRATION. For surface runoff from disturbed areas at a sufficient flow velocity, the maximum concentration of suspended solids is estimated at about 50,000 mg /1, or 5 percent. The background conditions for suspended sediments is about 3 mg /l. The negligible concentration for suspended sediments is set at less than 5 mg /1, and the high value of 50,000 mg /1 is an estimate judged by the instantaneous or short -term impacts on aquatic biota and the more tolerant and mobile fish by interfering with gaseous exchange and respiration. No long- term hazards are expected from sediment releases in flowing streams. The intermediate ranges are on a linear log scale. ACID ROCK DRAINAGE (ARD). ARD is the contaminated drainage that occurs as a result of natural oxidation of sulfide minerals contained in rock which is exposed to air and water. Acid water may also carry dissolved heavy metals. Potentially acid - producing rock at the mine are the types of ore and waste rock that contain sulfides and have insufficient neutralizing capability. The walls and floor of the open -pit mine, stockpiled ore at the mill, and the waste rock stockpiles or final disposal sites in the impoundments or the mine pit are the potential areas or sources of ARD production. Hazards from ARD are the acid itself, which can impact aquatic ecosystems, or the dissolved metals which may be toxic. For ARD, the descriptors are based on pH as an indicator of the relative hazard since pH is a measure of the acidity and the potential to impact aquatic ecosystems. The impacts of metals are presented in Table 2 in terms of copper concentrations. The background mean concentrations of acid measured as pH in surface waters at the site and vicinity ranged from 4.7 to 6.7, with extremes