OVERVIEW OF APPROACHES TO MERCURY TMDLS: 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. Three options for mercury translation include: (1) derivation of a site-specific bioaccumulation factor; (2) use of a bioaccumulation model; or (3) use of national default translators. There are obstacles involved with each option. The major challenge to developing mercury translators is the enormous variability in the site-specific potential for mercury methylation and bioaccumulation.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 relatively minor point sources (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.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.