EXECUTIVE SUMMARY
This evaluation reviewed the relationship between water quality and biological effects in freshwater ecosystems.
The current definition of dissolved metals as the metals component of a water sample that passes through a 0.45 µm filter is inadequate. Specifically, recent research has shown that substantial amounts of metal bound to colloids can pass through this pore size, and thereby can be analyzed as part of the dissolved fraction. The toxicity of these colloid-bound metals is not known, and research is needed to determine their bioavailability and toxicity. We recommend that a technical workshop be convened that addresses what constitutes the dissolved metal fraction, the potential implications of this definition on toxicological effects in the receiving environment, and evaluates various field techniques (e.g., filter type and apparatus) that potentially will minimize contamination of field collected water samples at mine sites. This workshop could also set the basis for future research priorities.
A myriad of abiotic and biotic factors influence the bioavailability and toxicity of metals to aquatic organisms. Furthermore, the relationship between factors that affect toxicity are not always linear. For example, low pH can either increase or decrease toxicity of metals in freshwater ecosystems. Although the dissolved metal fraction, specifically the free metal ions, best predict toxicity in well-defined synthetic media” this relationship appears to break down in natural waters. This is especially true when one evaluates the influence of organic matter on metal toxicity. We recommend that research be conducted that addresses the relationship of dissolved and total metals and toxicity to aquatic organisms using natural receiving water.
There are a number of modeling approaches that may be of use to monitoring metals in Canadian receiving environments. For example, the U.S. EPA has developed metal translators and conversion factors that convert dissolved metal levels to total recoverable metal levels. In addition, there are a number of chemical equilibrium models that have the potential to be valuable tools in metal monitoring. However, for these models to be useful in routine biological monitoring they need to develop some predictive capacity. For example, can output from one of these models that is based on the chemical characteristics of a mine effluent and the receiving environment be used to predict the toxicity of that effluent? This approach would require using these models in conjunction with toxicity testing to develop a predictive approach to biological monitoring.
In Canada, a frequently cited goal is to analyze trace metals in receiving waters at detection limits equal to or lower than 1/10 of the corresponding CCME (or provincial) water quality guidelines. We recommend that a risk based approach be used to establish detection limits on a site-specific basis. Such an approach would incorporate the chemistry of the effluent and receiving environment (the use of equilibrium models could be used here) to determine the relative risk of metals of concern in relation to their biological effects and required detection limits. For analytes of concern, detection limits should be based on analytical requirements needed to ensure that reported non-detects are significantly different than appropriate effect concentrations (i.e., generic criteria if applicable or site-specific criteria).
Current technologies analyze total metal levels, whereas the free metal ion may be the most important species in terms of toxicity. There are analytical techniques that characterize free metal ion concentrations in water, such as anodic stripping voltammetry and ion selective electrodes. While these approaches can measure the free metal species, they have a number of problems that hinder their general use for monitoring metals in Canadian receiving environments. For example, anodic stripping voltammetry is time consuming, has a fairly high level of interference and requires a high level of operator expertise. Although ion selective electrodes are susceptible to interference problems in more saline waters, this technique offers the most promise as an analytical tool for monitoring metals in the environment. Further development of alternative analytical techniques is required before they are useful for routine monitoring programs.
AETE
