Aquatic fate of fish tainting compounds in the Athabasca River Open Access
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Canada, Alberta, Fort McMurray
This report investigates the relationship between certain hydrocarbons that are present naturally, or could be introduced due to surface oil sands mining and upgrading activities, and their potential bioaccumulation and tainting of the commercial fishery in the Athabasca River. This includes defining the contaminants of concern, reviewing their bioavailability and bioconcentration properties and consideration of their persistence in the aquatic environment of the Athabasca River. A water management approach for setting ambient surface water objectives and effluent standards for fish tainting compounds is discussed within the context of basin-wide water resource planning. The scientific literature regarding petroleum related compounds that could cause off-flavour in fish is often confused and contradictory. This results from inconsistent testing protocols and lack of analytical precision. However, it is generally concluded that low molecular weight compounds, i.e., dibenzothiophenes, naphthenic acids, mercaptans and methylated naphthalenes are petroleum related compounds which can taint fish. Phenolics are also of concern. Detailed chemical characterization of wastewaters originating from oil sands extraction is lacking. Those data which are available indicate that the general composition of the wastewaters is the same as the raw bitumen, but relative concentrations are drastically altered. The oil in natural bitumen deposits is deficient in water-soluble components, saturated hydrocarbons (n-paraffins) and low molecular weight aromatic compounds, while being enriched in asphaltenic and nitrogen-, sulfur-, and oxygen-containing compounds (NSO compounds). The process effluents (e.g., upgrading wastewaters and tailings pond discharge) resemble more the synthetic crude than the parent bitumen, being enriched in aromatic and alipathic compounds, including those which have the potential to taint fish flesh. Cross comparison between compounds known to taint fish and those which could exist in oil sand wastewaters results in identification of the following major compound groups: 1. alkylated naphthalenes; 2. alkylated benzothiophenes; 3. alkylated dibenzothiophenes; 4. naphthenic acids; and 5. phenols. This list should be revised once the wastewater characterization is more complete. The uptake and bioaccumulation of tainting compounds is dependent upon the fish species of interest and the physico-chemical characteristics of the compound. Of the three potential uptake pathways, transfer across the gills will probably be more significant than epidermal adsorption. There is also some potential for uptake through the alimentary canal. The rate of uptake is dependent upon the compound, the exposure time, water temperature and fish species. Mechanisms may vary depending upon whether the fish feeds on plankton and bottom organisms, or is a predator. Lipophilic compounds tend to bioaccumulate more so than water soluble compounds, and the octanol-water partition coefficient of the compound can be used to define the approximate uptake potential. Fish metabolism will result in degradation of the compound once absorbed, and the rate of depuration is a key factor in defining a hydrocarbon's tainting potential. The aquatic fate of wastewater hydrocarbons must be considered in any management scheme. Hydrocarbons with tainting potential are not conservative and therefore would be subject to physical, chemical and biological processing if discharged to the Athabasca River. Abiotic factors would include surface spreading, photolysis, volatilization, dissolution, emulsification, adsorption and sedimentation. Since many of the compounds of concern are low molecular weight aromatics, photo-oxidation and volatilization would be major processing pathways. In the Athabasca River this would be tempered by ice-cover for much of the year. The larger molecular weight hydrocarbons would tend to adsorb to suspended or bottom sediments. NSO compounds would be the most soluble and tend to stay longer in solution. The natural microbiological community in the Athabasca River should be adapted to hydrocarbon degradation. The most readily degradable wastewater hydrocarbons are the n-alkanes and the low molecular weight aromatics. In the Athabasca River biotic degradation may be restricted much of the year by low water temperatures and minimal concentrations of nitrogen and phosphorus. There is some indication that the most active hydrocarbon degrading flora is associated with sediments. In addition to anthropogenic sources of compounds with a tainting potential, there is a possibility that tainting could occur secondarily due to microbial processing of wastewater discharges to the river or in the alimentary canal of the fish. This potential is poorly understood and requires considerable research to identify its significance. There are numerous computer models which simulate the aquatic fate of organic chemicals, and this is an active development area. Models range in complexity from relatively simple to extremely detailed, and require varying amounts of input data. These data can be derived from the scientific literature, laboratory studies or field experimentation. A relatively simple model should be used first to screen compounds of potential concern. Special considerations that must be incorporated in any modelling of the Athabasca River include: 1. Low river water temperatures for much of the year; 2. Prediction of very low concentration levels; 3. Ice cover conditions; 4. Mixing zone characteristics; and 5. Extremely variable sediment regime. The present petroleum effluent guidelines would probably not provide adequate protection against fish tainting. Site-specific surface water objectives and effluent standards should be developed, using either the whole-effluent or chemical-specific approach. The whole-effluent approach involves the direct measurement of effluent tainting with test organisms under laboratory or controlled field experiments. Once the threshold concentration of the effluent for tainting has been defined, this information would be used to calculate the required river dilution which would permit appropriate protection of the fishery. The chemical-specific approach to setting of receiving water objectives and effluent standards is the classical method and involves managing the specific chemical attributes of the wastewater. The chemical characteristics of the wastewater are evaluated and screened for parameters of concern. Desired concentrations of each parameter are defined for the receiving water based upon published criteria or site-specific field or laboratory experiments. After the receiving water objectives are set the effluent standards can be defined based upon probable future streamflow conditions, upstream quality, other point and non-point source loadings and in-stream assimilation. With respect to effluents with potential fish tainting hazard, there is currently no clear rationale for selecting either approach. This report discusses the merits of each and gives direction as to the kinds of data which must be collected prior to definition of the appropriate wasteload allocation procedure.
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