The draft Metal Mining Effluent Regulation (MMER) requires that all Canadian metal mines produce effluent that is non-acutely lethal to rainbow trout when tested in accordance with Environment Canada test methods. Mine operations will also be required to monitor the acute lethality of effluent to Daphnia magna. If a rainbow trout test produces mortality of more than 50% of the test organisms in 100% effluent, the sample is considered to “fail” the acute lethality test. In the event of a toxicity failure, the draft MMER requires that the mine implement a plan to investigate the cause of acute lethality. The Toxicity Reduction Evaluation (TRE) developed by the U.S. EPA (1989) is a commonly used step-wise approach designed to assist industrial dischargers to identify the causes of, and eliminate final effluent acute lethality.

The purpose of this Guidance Document is to provide TRE guidance specifically focused on challenges faced by the Canadian metal mining sector in order to assist mining facilities in meeting the acute lethality requirements for both rainbow trout and Daphnia magna. It is intended to provide mine managers with an effective tool for implementing an appropriate strategy for resolving acute lethality issues, provide laboratories with a useful guide for conducting TRE studies with metal mining effluents, and ultimately, increase the likelihood of achieving and maintaining a consistently non-acutely lethal metal-mining effluent. It is not intended to replace the existing U.S. EPA documents, but rather to provide supplementary guidance for application with Canadian regulatory test species and metal-mining effluents.

A TRE is a site-specific study designed to identify the substance(s) responsible for acute lethality, isolate the source, evaluate the effectiveness of control options, and confirm the reduction in acute lethality of the final effluent. Although the approach to any TRE may have similar components, the sequence of events or steps will be site-specific and depend on the nature of the toxicant, as well as the results and findings from each phase of work.

The initial step in the TRE process, which should begin prior to experiencing the first failure, is the development of an “Acute Lethality Response Plan”. This plan will increase the speed and efficiency with which the acute lethality failures can be addressed, by facilitating the data acquisition phase (with respect to mine facilities/operations), and assist in the decision making process. The “Acute Lethality Response Plan” may include (but is not limited to):

• Description of facility processes and operations.
• Description of effluent treatment facility and process.
• Line diagrams showing the major areas of operation and the main inputs to the treatment plant (if one exists).
• Documentation of facility operations/condition during collection of samples for routine acute lethality testing.
• Characterization (for chemistry and toxicity) of process streams over time to provide baseline data to be used for comparisons to samples collected during a toxicity episode.
• Results from acute lethality tests and chemical analysis for routinely monitored parameters.
• Up-to-date list of Material Safety Data Sheets (MSDS) for chemicals used in the process and effluent treatment (with available toxicity data for rainbow trout and Daphnia magna).
• Selection of a response team, which may include consultants (i.e., aquatic toxicologists, engineers experienced in the TRE process, if not already available within the facility) and mine personnel (i.e., management, operations, support personnel for sampling). Good communication and exchange of complete information among all team members is critical to the success of a TRE by speeding the response to the failure and increasing the likelihood of TRE success.

Immediately after the initial acute lethality failure is experienced, a review of the acute lethality test data should be conducted, to ensure that all Environment Canada test conditions were met. Water quality parameters (e.g., dissolved oxygen, pH, conductivity) measured during the test could also provide useful clues as to the cause of acute lethality. If samples continue to demonstrate the presence of acute lethality, a review of the information gathered as part of the Acute Lethality Response Plan, and an evaluation of remedial actions to optimize facility operations (including housekeeping practices, treatment plant optimization, and chemical optimization) should be initiated. If these activities are unsuccessful in resolving toxicity, subsequent stages in the TRE could involve a variety of approaches.

Establishing the degree (i.e., magnitude) and persistency (i.e., how toxicity changes over time) of acute lethality will be important, since these can influence subsequent TRE activities. The actual number of samples required to assess these factors will be site-specific and depend largely on effluent variability. Failure to understand the variability in effluent acute lethality and individual toxicants could lead to selection of treatment options or controls that do not consistently reduce acute lethality to compliance levels (U.S. EPA, 1999).

Although common toxicants associated with metal mining effluents have been identified (including ammonia, metals, pH, dissolved salts, and cyanide), effluent characteristics and toxicants will be unique to individual mines and operations. Therefore, the choice and combination of subsequent TRE approaches will depend upon several factors including the degree and persistency of acute lethality, availability and quality of historical toxicity and chemistry data, the type of operation/process, and the nature of the toxicant(s). Furthermore, the approach to a TRE study will unfold as information about the toxic event becomes available.

Three fundamental TRE components include:

1. Toxicity Identification Evaluations (TIEs)
2. Source Investigations (SIs)
3. Toxicity Treatability Evaluations (TTEs)

An effective TRE must determine the appropriate combination of these approaches and alternative strategies to eliminate acute lethality. However, regardless of the TRE strategy selected, good communication and co-ordination between the mine operators, toxicology, chemistry and engineering groups participating in the TRE is critical to the success of a study.

The objective of the Toxicity Identification Evaluation (TIE) is to identify the specific substances responsible for acute lethality. The TIE process is divided into three phases, which usually occur sequentially, but may be conducted simultaneously when patterns of toxicity begin to emerge during Phase I.

• Phase I involves characterization of the toxicants through a variety of effluent treatments (U.S. EPA 1991a).
• Phase II involves identification of the suspected toxicant(s) (U.S. EPA, 1993a).
• Confirmation of the suspected toxicants occurs in Phase III (U.S. EPA, 1993b).

The TIE approach will be most effective if acute lethality is consistent and persistent (i.e., does not degrade over time). In this case, characterization of the toxicant(s) (Phase I TIE) should be conducted. If successful, it may be necessary to identify (Phase II) and confirm (Phase III) the specific substance responsible for acute lethality prior to conducting a TTE or SI. Alternatively, characterization of the effluent may provide sufficient information without specifically identifying the substance(s) responsible (i.e., slight adjustment of pH eliminates toxicity). The information generated during the Phase I TIE could be used to modify the existing treatment system, or implement new treatment methods (TTE approach).

The TIE approach will be less effective and more difficult to complete if acute lethality is transient or non- persistent. Random toxicity events may require the analysis of more samples and, in some cases, may even necessitate abandoning TIE work on individual toxic samples (Ausely et al., 1998). Alternative approaches, in combination with TTE and SI evaluations, may be more successful under these conditions.

After completion of the Phase I characterization of an effluent, the TRE can proceed to:

1. TTE to evaluate various treatment methods for removal of the toxicant,
2. SI to identify the source of the toxicant, or
3. Phase II and III TIE to identify and confirm the specific substance responsible for acute lethality prior to conducting a TTE or SI.

TTEs and SIs can be conducted with or without identification of the specific toxicant(s), but will be more effective if a specific substance can be targeted for treatment. In the case that the TTE or SI approach is selected, confirmation testing (Phase III) will still be required to ensure that the method selected consistently removes acute lethality.

SIs and TTEs may be used as strategies in combination with, or as alternatives to, a TIE. Source Investigations determine whether the toxicants may be isolated in one or more waste streams. The approach to a SI may include identification of discharge locations and inputs to the effluent treatment plant (ETP), characterization of each discharge in terms of flows, acute lethality and chemical composition, and use of a mass balance approach to identify those streams representing the largest contribution to acute lethality and chemical loading. Once a specific process stream has been identified as the source of toxicity, a TTE could be conducted to reduce or eliminate the substance(s).

A toxicity treatability evaluation (TTE) involves the systematic evaluation of various treatment technologies, combinations of technologies, or management options (i.e., process or operational changes) to assess the ability of these technologies (or operational/process changes) to reduce levels of contaminants that are causing acute lethality. Once removal of acute lethality has been demonstrated at the bench-scale level, a decision can be made apply the technique at a larger pilot-scale or directly at the existing treatment facility.

In all TRE strategies, repeated testing and evaluations must be conducted. However, the number of samples to be treated and analyzed will depend on a variety of factors, including effluent variability, number of toxicants, conclusions drawn from data, cost of remedial action, regulatory deadlines and success of each phase (U.S. EPA, 1991).