Calculating Aqueous Environmental Quality Standards to Protect Human Health: Derivation of a Predicted No-Effect Concentration

Efforts to establish water quality standards for the protection of human health involve a calculation of the amount of fish that can be consumed without exceeding a predetermined safe dose. This is often accompanied by a calculation of the concentration of the substance of concern (i.e., metal) in water that should not be exceeded, to prevent excessive accumulation of the metal in the food organism. Such an exercise was considered in Europe to identify a priority substance for the Water Framework Directive and was applied to selenium in prioritization efforts by the Joint Research Centre (JRC; Van Vlaardingen and Verbruggen 2007). The output from the above calculation gives concern because it can result in water concentrations that approach or are below background. To demonstrate, simulated PNEC calculations are derived for copper, chromium, iron, selenium, and zinc (Table 1). Because the percentage of diet and daily consumption values are fixed, the final water concentration is driven by the TDI value and the BAF. The TDI for most substances has been determined by regulatory bodies, and hence the BAF becomes the driver for the output. The BAF values we selected span a range of 500 to 10 000 and are fairly typical for these metals, although larger values have been reported. However, it is well accepted that BAFs are inversely related to exposure concentration and vary by several orders of magnitude depending on exposure (McGeer et al. 2003; DeForest et al. 2007). Selecting one BAF to represent all water bodies and all species will not accurately reflect the breadth of the distribution of BAFs for natural waters. This would necessitate the selection of a conservative BAF to ensure that fish are safe to eat regardless of location. The use of Equation 1 as a universal approach for deriving a national or state PNEC value or water quality standard for regulation would necessarily result in a low value, to be protective of all water bodies. The approach just described does not consider that many metals (those selected) are essential elements for fish and human health. Essential metals like copper and selenium are homeostatically regulated by fish to meet their metabolic needs. Hence, only minor changes in fish tissue concentrations result from large changes in the water concentrations. A further limitation of this approach is that fish obtain only a portion of their metal nutritional requirement via water exposure. For essential elements, the diet is also important. For selenium, the diet is the major uptake route. Hence, a backward calculation from fish tissue to a water concentration has serious limitations. In the example calculations with the selected BAFs, it can be seen that use of the described modeling approach for deriving PNEC values would result in values that are below the current US Environmental Protection Agency water quality criteria (WQC) with the exception of copper and a BAF of 500 (Table 1). For these calculations, the maximum TDI was used. If a lower value such as a reference dose were to be used, the PNEC values would become even smaller. Alternatively, fish advisories are commonly used to advise the public on consumption of fish with elevated levels of a metal. The advisories are specific to a fish species and site and thus incorporate the bioavailability of the metal at a given site. We advocate that this is a better approach to protecting the public. The WQC derived using one BAF and the previous approach will lead to PNEC values and WQC that will either under- or overprotect human health. The authors acknowledge they work for the metals industry and received support from the Copper Development Association in the preparation of this manuscript. Data, associated metadata, and calculation tools are also available from the corresponding author ([email protected]).