Golder Associates Ltd. in association with SENES Consulting was retained by
Public Works and Government Services Canada to carry out a study to characterize the Whistle Mine Waste Rock Pile at the Whistle Mine site near Sudbury Ontario. The intent of the Whistle waste rock project was to develop and illustrate technology for evaluating waste rock piles and to provide a database of
information for characterizing the waste rock.

During the study quality data on the acid mine drainage characteristics of the
Whistle Mine waste rock pile were collected and compiled. The field investigation
involved collection of waste rock samples, drilling of five boreholes and
excavation of three test pits. Wells, gas sampling ports and thermistors were
installed in the boreholes, and lysimeters were installed in the test pits. Water
quality data was collected from seepage locations, wells, and lysimeters.
Measurements of oxygen and carbon dioxide and measurements of temperature
distribution were collected. Solid samples collected on the site were analysed for
chemical properties. Laboratory testing of solid samples included elemental
analysis, static tests, kinetic tests, wash tests and bacterial analysis.

The database collected is quite extensive and should be useful for supporting other investigations; however, it should be noted that the Whistle waste rock pile is different from the other waste rock piles at MEND study sites, and at other mines, in several ways:

• the waste rock is coarse and angular, and comprised of very large hard
  rocks;
• the pile was very difficult to drill and sample due to high porosity and large
  void spaces;
• the pile contains zones of rock fines, which are interspersed between the
  large rocks;
• air penetrates to the bottom of the pile, therefore the supply of oxygen is not limiting the oxidation of sulphide; and,
• the interior of the pile is cold, as noted from the temperature data collected,
  temperature of the collected seepage, and from field observations of venting of cold air from the base and top of the pile.

These differences should be kept in mind when attempting to apply the data and
information obtained from the Whistle waste rock study to the interpretation of
waste rock piles at other sites.

The following is a summary of some of the key conclusions and recommendations.

• Tests for acid generation potential indicate that the pile is acid generating and oxidation of sulphide minerals is not limited by oxygen flux into the pile.
• Concentrations and loading rates for column tests and seepage indicate that the water from the column test is still somewhat buffered. The loading rates from the column tests for sulphate and nickel respectively are about 39 and 1.8 times greater than those measured in seepage from the waste rock pile. The smaller difference in nickel loading is due to neutral conditions in the column. If the column test had been continued until the leachate became acidic, the nickel loading would have increased significantly.
• The differences between leachate produced from the column test and from the waste rock pile have illustrated that caution should be used in applying the results of short-term kinetic tests to longer term behaviour of waste rock. Practice has shown that it can take a year or more for a sample to become acidic in a column test. This acidity greatly increases metal loading rates. Another caution regarding column tests relates to loading rates from the columns. The absolute rate from columns can be much greater than that observed in the field (as illustrated above) due to high proportion of finer, more reactive material in the column, and due to accelerated rainfall cycles.
• Grain size distribution and structural composition of the rock was used to estimate the water content, and porosity of the pile. This data was then combined with a hydrologic assessment to assess the flow through the waste rock. The results indicate that infiltration into the waste rock will take anywhere from days to months to reach the seepage collection point. Most of the flow will occur under heavy infiltration events such as heavy rainfall or spring runoff. The water balance indicates that the most significant loadings will occur during the spring runoff period.
• A unique opportunity to expand the Whistle waste rock database will become available when the waste rock pile is relocated to the pit as part of site closure activities. When this occurs, it is recommended that further study be conducted while the pile is being relocated.
• Additional study on bacterial catalysed oxidation at low temperatures is required to determine the potential implications of low temperature, bacterially catalysed oxidation for arctic and subarctic tailings, and waste rock deposition.
• Additional investigation into the relationship between pile structure and flow characteristics should be completed. This, coupled with a detailed evaluation of existing literature, and research in the fields of unsaturated flow and geotechnical engineering would be of benefit in understanding flow through unsaturated waste rock.
• Based on our experience at the Whistle Mine site the following data is of critical importance for an assessment of any waste rock dump:
  • structural parameters of the dump (grain size distribution, porosity, water content, distribution of material and dump construction);
  • acid generation potential of the waste rock (based on Acid:Base Accounting,  kinetic test work, detailed mineralogical characterization, seepage characterization, and analysis of temperature and gas composition data); and,
  • a detailed site water balance (based on detailed precipitation data, estimates of flow rates through the pile, estimates of evaporation, estimates of water retention in the pile, and measurement of the surface area of the pile).