EVALUATION OF WATER QUALITY TRANSLATORS FOR MERCURY: CHARTING THE COMPLEX COURSE FROM A FISH DINNER TO A PERMIT LIMIT

Implementation of the water quality criterion for methylmercury, which is expressed as a fish tissue concentration, requires either fish tissue sampling or conversion of the criterion to a concentration in water, especially for the development of water quality based effluent limits (WQBELs). The translation from the concentration of methylmercury in fish (the human health endpoint of concern) to the concentration of total mercury in water can be viewed as a single-step or multi-step process. In the single-step process, total mercury in fish is directly translated to total mercury in water. In the multi-step process, methylmercury in fish is converted to dissolved methylmercury in water and then to total mercury in water.Three options for mercury translation include: (1) derivation of a site-specific bioaccumulation factor (BAF); (2) use of a bioaccumulation model; or (3) use of national default translators. National default translators are likely to be inaccurate on a site-specific basis, given the very high degree of variability observed in mercury methylation and bioaccumulation rates. Models have the potential to account for environmental factors contributing to this variability, but at present the available models are limited to reservoirs and lakes in a few geographic regions. In developing site-specific translators, some of the important issues to be addressed include definition of the study area, spatial variation in fish caught, species of fish, fish consumption rate, habitat differences that may affect bioaccumulation, and any site-specific bioaccumulation mechanisms. Underlying the use of any type of mercury translator is the premise that mercury levels in fish will respond in a linear manner to reductions in mercury loading. Since mercury speciation is concentration-dependent, the validity of this assumption merits further research.For mercury and other bioaccumulative chemicals, chemical data and fish consumption advisory information can be applicable data sources for listing decisions under Section 303(d) of the Clean Water Act. Three types of chemical data may be used, each of which has its advantages and disadvantages: concentrations in fish tissue, concentrations in the water column, and concentrations in sediment. Local fish consumption advisories for mercury can also be used, but statewide advisories should not be used to list all the state's water bodies as impaired in the absence of waterbody-specific data. Listing a waterbody as impaired requires development of a Total Maximum Daily Load (TMDL).A comparison of several existing mercury TMDLs shows differences in the following aspects: TMDL triggers; overall approach; source assessment; numeric targets; linkage between the loadings and the targets; and allocation and implementation approaches. The examples illustrate different means that have been used to express the mercury TMDL target. The target may be based upon the concentration in fish tissue, the concentration in the water column, or the concentration in sediment. Of the three, the use of the tissue concentration is an integrated measure that avoids the use of assumptions necessary in the other two approaches; however, new approaches are required to apply this target in calculation of WQBELs, as permit limits are traditionally expressed in terms of aqueous total metal concentrations or mass loadings. The use of a water concentration as the TMDL target has the advantage of being compatible with the current methods for conducting wasteload allocations and writing permits. The disadvantage is that the approach is oversimplified and can lead to unnecessarily restrictive permit limits. The use of sediment concentrations as the target has the advantage of being less subject to short-term fluctuations than water column concentrations but is difficult to implement in the permitting process.Factors that can be considered in allocation of mercury loadings among point and nonpoint sources include: existing relative source contributions, cost-effectiveness, technical and programmatic feasibility, and the likelihood of implementation. Where point sources are allocated load reductions, effluent limitations may be approached through several mechanisms, including reasonable potential analysis, pollution minimization plans, and the development of WQBELs. In the past, the reasonable potential analysis assumed mercury was not present in a discharge when the required minimum detection level was achieved. However, newer, more sensitive analytical methods for mercury are pushing the detection limits downward. For point sources that are a relatively minor component of the overall loading (a typical scenario), the imposition of pollution prevention measures as part of the permit conditions, such as the development of a source identification and reduction program, may be sufficient to satisfy a TMDL.As states adopt the fish tissue-based water quality criterion for mercury into their water quality standards, they will have to decide upon the approach to use to derive permit limits. The approach will depend upon the form of the fish tissue-based water quality standard used by the state. Translators can be used to convert the tissue-based concentration to a water column concentration, which can then be used in a Reasonable Potential Analysis (RPA) and the calculation of numeric effluent limits in the traditional manner. An alternative is to implement the fish tissue-based standard directly into permits without using a translator to convert to a water column concentration. This approach rests on the conclusion that Reasonable Potential exists if two conditions are met: (1) there are quantifiable levels of mercury in the discharge and (2) the mercury levels in fish tissue in the receiving water exceed the fish tissue-based water quality standard. The determination of mercury residues in local fish is required to implement this approach.Options for implementation of a mercury TMDL may include actions taken throughout the water body (other than traditional permit limits) or even regionally-based approaches. Within a particular water body, a mercury TMDL may include provisions for monitoring, examination of on-going pollution prevention programs, evaluation of potential technology improvements to enhance treatment plant performance, and performing studies to evaluate localized impacts and bioavailability. Regional approaches are attractive because atmospheric deposition is the major source of mercury in most watersheds, and airsheds are much larger than watersheds. Rather than focusing on individual point source discharges, regional approaches to mercury management are likely to be necessary to achieve significant reductions in fish tissue mercury concentrations.