Table of Contents
Background: As we learned in Part 1, Bigass Mining (BAM) Company had just finished construction and had started mining and milling operations. The BAM tailings management facility (TMF) was designed as a non-discharge facility but was discharging. The TMF had odor, foam issues at the discharge point and the receiving water stream that happened to have trout, non-native, sometime had dead trout. The local community located a couple of kilometers downstream were upset with the odor, foam, and dead trout. Despite these issues the company was in compliance with in country and World Bank water quality limits. The Company financed the Project by borrowing money from several banks. The bank hired and utilized external exerts to monitor and audit the Company’s performance including environmental aspects. The auditors were “concerned” with the odor, foam, dead fish and potential delays or work stoppages that might affect the Company operations and revenue, as well as, image and reputation with the community and with NGO’s.
BAM Tailings Pond Chemistry
The BAM tailings pond water chemistry is dependent on three factors:
- The geochemistry of the tailings slurry;
- The residual reagents from the mill, still present in the tailings slurry; and,
- Dilution and geochemical reactions, including the effects of wind and sun, on tailings water over time.
The BAM tailings water chemistry changed rapidly in response to the chemistry of the tailings, primarily with respect to pH and dissolved copper/cyanide. The sodium cyanide used during the concentrator processing complexes with the dissolved metals and was found in the tailings water primarily as a dissolved copper-cyanide complex. The concentration of dissolved copper was proportional to the concentration of dissolved cyanide in the tailings slurry and pond water. This ratio changed over time as “less-oxidized” ores are processed.
Water treatment for cyanide destruction was implemented in the mill and during emergency discharge periods in the tailings pond to ensure compliance with respect to cyanide and dissolved copper concentrations. Lime addition was also used to enhance precipitation of copper from the tailings pond water. Aeration of the tailings pond water was used to enhance volatilization and oxidation of residual reagents, degradation of cyanide and to decrease alkaline pH. The process team implemented best practice in the concentrator to reduce reagent usage (sodium cyanide, frother, xanthates, etc) and improve the control of pond water chemistry.
BAM Flowsheet Description
The BAM concentrator used grinding and flotation processes. The flotation circuit was a conventional copper/zinc circuit producing primarily a bulk copper concentrate and a zinc concentrate. The bulk copper concentrate was further processed in subsequent flotation stages to recover molybdenum.
Reagents
The BAM milling reagents that are of particular interest with respect to water quality were:
- Lime, with respect to pH control;
- Sodium cyanide, with respect to concentrations of cyanide and metallo-cyanide complexes in tailings water;
- Frother, with respect to foam downstream;
- Frothers and xanthates, with respect to odour; and,
- Hydrogen peroxide for cyanide destruction
- Sodium hydrosulphide, recently introduced for molybdenum flotation.
The quantity of reagent used depends on the type of ore being milled, typically:
- Lime use depended on the presence of copper and degree of oxidation – typically copper was floated with a small addition of lime (pH 8.5 to 9.0).
- Sodium cyanide was used for several reasons in the concentrator:
o primarily, to complex aqueous metals and prevent activation of minerals (e.g. prevent zinc from reporting to the copper concentrate), and thus more cyanide is required when processing oxidized ores;
o to depress sulphides, such as pyrite, and increase the grade of the concentrate.
Ecotoxicology
Environmental toxicology is a multidisciplinary field of science concerned with the study of the harmful effects of various chemical, biological and physical agents on living organisms. Ecotoxicology is a subdiscipline of environmental toxicology concerned with studying the harmful effects of toxicants at the population and ecosystem levels. (Wikipedia)
As part of the baseline environmental studies, BAM had not completed aquatic toxicity testing to determine what effects discharge effluents would have on water quality, fish, and aquatic resources in the receiving streams impacted by the mine activities including pits, overburden storage area, waste disposal area, low-grade stockpiles and tailings.
Action Plan
The BAM management team including primarily the environmental and processing departments prepared a Request for Proposal to complete the ecotoxicology study including,
Study Objectives
- conduct an ecotoxicological assessment of tailings impoundment effluent as it interacts with the downstream environment; and
- assess current effluent and receiving water limits and parameters, and provide a sound basis for establishment, or re-establishment of site-specific criteria.
Task and Deliverables
Task: Ecotoxicological Effects of Tailings Impoundment Discharge to Receiving Water Aquatic Resources
Assess the ecotoxicological effects of tailings water discharge to aquatic resources of the watersheds affected by BAM mining operations. Consider the effects of water quality from different ore campaigns. This task is the first priority in this project and should be based on short term acute and chronic toxicity testing, historical monitoring information, information from other relevant studies conducted elsewhere, and other information considered relevant by the Consultant.
Deliverable:
Report detailing testing procedures, results, conclusions with respect to potentially adverse effects.
Mitigation Measures
If potentially adverse impacts or effects are identified, assist in development and provision of mitigation measures.
Deliverable:
Report outlining adverse effects, mitigation options, pros/cons, practicality, effectiveness, approximate cost of each option, and recommended option.
Task: Ecotoxicological Effects of Tailings Impoundment Discharge to Downstream Environment
Including water quality, sediment, fish, invertebrates, algae, and mammals. Consider the effects of water quality from different ore campaigns. It is recognized that some components of this task may use testing results of Task 1, which is a higher priority task. It is also recognized that fully comprehensive conclusions from this assessment may only be achievable through long term study. Due to information being required in the near future, conclusions of this assessment should specify the limitations and assumptions used and be based on shorter term acute and chronic toxicity testing, historical monitoring information, information from other relevant studies conducted elsewhere, and other information considered relevant by the Consultant.
Deliverable:
Report detailing testing procedures, results, limitations of study, assumptions, conclusions with respect to potentially adverse effects, recommendations for longer term data acquisition.
Mitigation Measures
If potentially adverse impacts or effects are identified, assist in development and provision of mitigation measures.
Deliverable:
Report outlining adverse effects, mitigation options, pros/cons, practicality, effectiveness, approximate cost of each option, and recommended option.
Task: Water Quality Limits and Parameters Assessment
Evaluate and assess the current receiving and effluent water quality compliance limits, standards and guidelines applicable to this project. Provide a basis and rationale for justifying or refuting established parameters and limits, particularly copper, molybdenum, total suspended solids, nitrate, and CN species. Conclusions to be based on results of previous Tasks, standards elsewhere in the world, information from other relevant studies conducted elsewhere, and other information considered relevant by the Consultant.
Deliverable:
Report systematically assessing various regulatory requirements, standards and guidelines, recommending changes, and providing sound justification for such changes.
The Request for Proposal was released to several reputable external consulting companies and the contract was awarded based on the combination of cost, quality, understanding of the scope, reputation, communication, and other factors.
Results
The main objectives of the study were to:
- Determine the acceptable range of effluent quality discharged from the tailings pond that will still protect rainbow trout present in the receiving waters.
- Review current effluent and receiving water quality limits to determine if they are appropriate, and if not, provide a scientific basis for revisions to the site-specific receiving water quality criteria.
The results that accomplished the objectives are presented in the following sections.
Data Review
The Consultant performed a comprehensive data review of the hydrological, water quality and biological data. The data review provided a thorough understanding of the hydrologic, water quality and biological environment that the mine operated within. The receiving waters before operations began as part of the baseline studies, and after commissioning of the mine nearly one year ago as part of their extensive monitoring program. An Environmental Effects Monitoring program was executed in the receiving waters including benthic macroinvertebrate and fish surveys. Aquatic toxicity testing using rainbow trout and Ceriodaphnia with synthetic tailings solutions was completed. The permit limits for tailings discharge and the possible linkages between hydrology and water quality were reviewed.
The hydrologic data was reviewed to determine stream discharge characteristics, options for dilution with tailings pond effluents, including seasonal variation in natural runoff and groundwater dilutions between the discharge point and the receiving water compliance point.
Standards Analysis
The hydrologic and water quality data review provided the necessary background information on the expected range of values for tailings effluent discharge and the receiving waters. The data was compared with recognized USEPA / Canadian ambient water quality criteria (metals and cyanide) for the protection of aquatic life (particularly rainbow trout). The existing water quality data was also compared with the latest ecotoxicology literature on rainbow trout. Particular attention was paid to the factors that affect the bio-availability of metals to aquatic organisms, including the concentration of total and dissolved metals, amount of dilution water available, pH, hardness, alkalinity, suspended solids, total and dissolved organic carbon, dissolved metal concentration, etc.
Ecotoxicology Testing
Aquatic toxicity testing using rainbow trout was performed including acute 96-hour and chronic 14-day survival and growth tests to adapt the generic ambient water quality criteria to site specific conditions. Laboratory aquatic toxicity testing using actual tailings solutions reflecting variations in the solution as a result of reagent addition, water treatment for cyanide, pH, metals, and defoamer addition. Other factors considered included seasonal variation in natural runoff and groundwater dilutions.
Findings and Recommendations
Avoiding getting into the weeds, or in this case the mud, total metals limits cannot be relied on to ensure protection of aquatic habitat. It is the dissolved fraction of the metals that is bioavailable, and potentially toxic to aquatic organisms. For example, total copper contains both dissolved and particulate Cu. The particulate fraction of Cu is not bioavailable and thus is not toxic to organisms. However, the dissolved portion is bioavailable and potentially toxic to aquatic organisms including fish, macroinvertebrates, etc.
The Consultant recommended that the Company adapt revised site-specific water quality limits (particularly Cu, Mo, Zn) for discharge and receiving waters. The rationale and basis for adapting the limits were presented to the in-country regulators, banks and review teams. The revised water quality criterion was adapted and implemented as part of the Company Standard Operating Procedures and Reporting.
Mitigation and Monitoring
As a result of revised water quality limits, the Company:
- instituted increased monitoring at the mill and within the TMF;
- joint operations and environmental personnel monitoring teams were established;
- enhanced water treatment schemes including pH, flocculation, hydrogen peroxide, defoamers, reagent reduction and aeration;
- internal reporting and communication was implemented on a daily basis between departments;
- external experts were provided with performance data and on site quarterly for review and opinion on performance;
- modeling and predictive system tools were put in place;
- compliance monitoring and performance was provided to regulators, community, upper management, banks and auditors monthly.
Subsequent semi-annual environmental effect monitoring in the receiving waters showed increases in the abundance and taxa of benthic invertebrates. More important, no dead fish downstream.
Lessons
Site specific discharge limits should be assessed and established as part of the environmental baseline monitoring program with the environmental impact assessment. If the host country does not require such work, the company should perform the work separately as part of the Company’s internal due diligence.
Environmental effects monitoring and ecotoxicology work should also be completed within the baseline studies. The work should be specific to the site water effluent quality (bench test, pilot test or actual) and actual receiving water quality.
Continuous monitoring should be scheduled as determined by risk analysis and probability of upset conditions.
Communication of performance and compliance between operations, environment, and social departments are key to success.
External experts, bug and bunny, should not only be included the baseline programs, but as part of an ongoing and scheduled audit program, particularly operations start-up.