API PUBL 4751:2005 pdf download.Evaluation of Water Quality Translators for Mercury
For many facilities, the major issue in developing mercury translators is incorporating the fish tissue mercury criterion into a discharge permit limit. Permits may be reopened and reassessed as part of a TMDL implementation. At least 45 states have fish consumption advisories due to mercury and over 1,000 water bodies are listed as being impaired due to mercury, thus triggering TMDLs. Several methods are generally used in developing mercury TMDLs: (1) the concentration in fish tissue; (2) the concentration in the water column; or (3) the concentration in sediment. Of the three methods, using a fish tissue concentration as the TMDL target is the most direct measure of the desired endpoint, protection of human health. Determining how to allocate loadings among point and nonpoint sources is the next major part of a TMDL. EPA (2002a, 2004a) offers three approaches to allocation of loadings, depending upon the relative contributions of point source and nonpoint source loadings. First, where point source loadings dominate, the TMDL should specify reductions in these loadings, alone or together with nonpoint source loadings, to attain water quality standards (WQS). Second, where point source loadings are small, reductions in nonpoint sources are expected to achieve the TMDL. The third scenario also involves relatively small contributions from point sources, but reductions in nonpoint sources are not expected to be sufficient to attain WQS. The approaches used to implement a mercury TMDL will vary depending upon the allocations, as described above. Where there are waste load allocations (WLAs) to point sources, the traditional method to implement a TMDL is likely to be used (i.e., through the NPDES permit). The implementation of effluent limitations for mercury and other pollutant parameters can be approached through several mechanisms including reasonable potential and development of WQBELs.
2.2.1 Method 1630
EPA Method 1630 is the main method for quantifying methylmercury in fish tissue and aqueous samples (EPA 2001a), although it has not yet been promulgated. The specific steps in Method 1630 include: 1) distillation to remove the methylmercury from the sample; 2) ethylation to a methylethylmercury form and collection on a graphitic carbon adsorbent trap; and 3) thermal desorption through a gas chromatography column to separate the mercury species before reduction to elemental mercury in a decomposition furnace. The elemental mercury is detected using CVAFS. Method 1630 has a method detection limit (MDL) of 0.02 ng/L (or ng/g) but has the ability to detect as low as 0.009 ng/L. Also included in this Method is Appendix A for the determination of dimethylmercury. The method for dimethylmercury varies from the original Method 1630 by eliminating the distillation step and purging the entire sample onto the graphitic carbon absorbent trap. The elemental mercury is separated from the dimethylmercury by using gas chromatography. Two mercury peaks, representing the elemental and dimethylmercury species, are separated in time and are detected by CVAFS. Appendix A for the determination of dimethylmercury has a MDL of 2 pg/L. This is a performance based method and actual criteria data are determined before and during analysis. Precision is based on replicate analysis of actual samples and has a limit of 31% RSD. Accuracy is determined using NIST standards for OPR and MS/MSD. OPR samples must fall within 67 – 133% and MS/MSD samples between 65 – 135%. Several studies have been performed citing the Appendix A method that may be eventually used for validation. In the interim, Appendix A is still considered draft. Unlike EPA Method 1630, the method for dimethylmercury does not specifically discuss how to analyze solids such as fish tissue.