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Potential Effects of Contaminants on Fraser River Sockeye Salmon

Potential Effects of Contaminants on Fraser River Sockeye Salmon

Potential Effects of Contaminants on Fraser River Sockeye Salmon

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when multiple samples were reported in surface water at the same stati<strong>on</strong> <strong>on</strong> the samesampling date and time; the maximum value for each c<strong>on</strong>taminant <str<strong>on</strong>g>of</str<strong>on</strong>g> potential c<strong>on</strong>cernwas used as the exposure point c<strong>on</strong>centrati<strong>on</strong> in all cases except for dissolved oxygen,when the minimum value was used. Total metals c<strong>on</strong>centrati<strong>on</strong>s in surface waterdata were used in the assessment rather than dissolved metals to achieve greaterspatial and temporal coverage <str<strong>on</strong>g>of</str<strong>on</strong>g> the surface water quality data; implicati<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> thisprocedure are discussed in Chapter 7.The treatment <str<strong>on</strong>g>of</str<strong>on</strong>g> envir<strong>on</strong>mental data has the potential to influence the results <str<strong>on</strong>g>of</str<strong>on</strong>g> theassessment. In particular, the treatment <str<strong>on</strong>g>of</str<strong>on</strong>g> less than detecti<strong>on</strong> limit data can affect theresults <str<strong>on</strong>g>of</str<strong>on</strong>g> the exposure assessment, the hazard evaluati<strong>on</strong>, and the calculati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> theWater Quality Index. A number <str<strong>on</strong>g>of</str<strong>on</strong>g> investigators have evaluated the implicati<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g>applying various procedures for estimating the c<strong>on</strong>centrati<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> c<strong>on</strong>taminants <str<strong>on</strong>g>of</str<strong>on</strong>g>potential c<strong>on</strong>cern from less than detecti<strong>on</strong> limit data (Gaskin et al. 1990; Porter andWard 1991; El-Shaawari and Esterby 1992; Clarke and Brand<strong>on</strong> 1994). While there isno c<strong>on</strong>sensus <strong>on</strong> which data censoring methods should be used in variousapplicati<strong>on</strong>s, the simplest methods tend to be used most frequently, including deleti<strong>on</strong><str<strong>on</strong>g>of</str<strong>on</strong>g> n<strong>on</strong>-detect values or substituti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a c<strong>on</strong>stant, such as zero, the detecti<strong>on</strong> limit, or<strong>on</strong>e-half the detecti<strong>on</strong> limit (USACE 1995).To address the need for guidelines for statistical treatment <str<strong>on</strong>g>of</str<strong>on</strong>g> less than detecti<strong>on</strong> limitdata, the USACE (1995) c<strong>on</strong>ducted a simulati<strong>on</strong> study to assess the performance <str<strong>on</strong>g>of</str<strong>on</strong>g>ten methods for censoring data. The results <str<strong>on</strong>g>of</str<strong>on</strong>g> that investigati<strong>on</strong> indicated that nosingle data censoring methods works best in all situati<strong>on</strong>s. Accordingly, USACE(1995) recommended a variety <str<strong>on</strong>g>of</str<strong>on</strong>g> methods depending <strong>on</strong> the proporti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the data thatrequires censoring, the distributi<strong>on</strong> and variance <str<strong>on</strong>g>of</str<strong>on</strong>g> the data, and the type <str<strong>on</strong>g>of</str<strong>on</strong>g> datatransformati<strong>on</strong>. For data sets for which a low to moderate proporti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the datarequire censoring, substituti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the detecti<strong>on</strong> limit is generally the preferred methods(i.e., to optimize statistical power and c<strong>on</strong>trol type I error rates). However, as theproporti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the data that requires censoring and the coefficient <str<strong>on</strong>g>of</str<strong>on</strong>g> variati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the dataincreases, statistical power is better maintained by substituting <strong>on</strong>e-half the detecti<strong>on</strong>limit for the less than detecti<strong>on</strong> limit data, particularly for log-normally distributed andtransformed data. Substituti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> zero or other c<strong>on</strong>stants was also recommended fora variety <str<strong>on</strong>g>of</str<strong>on</strong>g> circumstances. Overall, it was c<strong>on</strong>cluded that simple substituti<strong>on</strong> methodswork best to maintain power and c<strong>on</strong>trol error rates in statistical comparis<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g>chemical c<strong>on</strong>centrati<strong>on</strong> data (USACE 1995).In this analysis, decisi<strong>on</strong>s regarding the treatment <str<strong>on</strong>g>of</str<strong>on</strong>g> less than detecti<strong>on</strong> limit data weretaken by c<strong>on</strong>sidering the recommendati<strong>on</strong>s that have emerged from previousinvestigati<strong>on</strong>s in the c<strong>on</strong>text <str<strong>on</strong>g>of</str<strong>on</strong>g> their potential effects <strong>on</strong> the results <str<strong>on</strong>g>of</str<strong>on</strong>g> this assessment.Including all <str<strong>on</strong>g>of</str<strong>on</strong>g> the surface water, sediment, and fish-tissue chemistry data that werecompiled in the project databases, more than 30% <str<strong>on</strong>g>of</str<strong>on</strong>g> the data required censoring priorto data analysis. To minimize the potential effects <str<strong>on</strong>g>of</str<strong>on</strong>g> the less than detecti<strong>on</strong> limit data<strong>on</strong> the results <str<strong>on</strong>g>of</str<strong>on</strong>g> the analysis, n<strong>on</strong>e <str<strong>on</strong>g>of</str<strong>on</strong>g> the less than detecti<strong>on</strong> limit data for which thedetecti<strong>on</strong> limits were greater than the corresp<strong>on</strong>ding toxicity screening value or toxicitythresholds for surface-water, sediment, or fish- tissue chemistry were used in theA-32

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