Monitoring Report - Phase IV Composite Soil Cover - Acid Waste Rock Study Heath Steele Mines

The Heath Steele Waste Rock Study was initiated in the Spring of 1988 at the Heath Steele Mines (HSM) site in New Brunswick. The objectives of the study were to develop strategies for the long term management of acid generating waste rock, to evaluate the performance of a soil cover placed over an existing acid waste rock pile at HSM, and to assess the cover's effectiveness as a method for long term management of acid generating waste rock

The project was carried out under the Mine Environment Neutral Drainage (MEND) Program with funding provided by Brunswick Mining and Smelting Corporation, Noranda Technology Centre, New Brunswick Department of Natural Resources and Natural Resources Canada through the Canada/New Brunswick Mineral Development Agreement.

The first phase identified the following four acid generating waste rock piles at Heath Steele for further study:

Pile

Estimated Tonnes

AP^[1]

AC^[2]

18A

18B

7/12

3,250

19,500

235,700

14,700

158

211

1,5

0,5

0,8

0,4

Note: [1]: Acid production potential (kg/tonne) of the waste rock
[2]: Acid consuming potential (kg/tonne) of the waste rock

Pile 7/12 was moved onto a prepared base with an impermeable synthetic membrane during Phase I. A composite soil cover was then placed on the pile during Phase IV.

Six or seven instrument clusters were installed at each of the four selected piles during Phase II. Each consisted of a piezometer, a pore gas measurement system for different depths, and an equivalent series of temperature sensors. Oxygen and temperatures were measured for enrh sampling point at monthly intervals. Water volumes and qualities were monitored on Pile 7/12. Climatic data was also accumulated for the mine site.

In Phase m, the Noranda Technology Center reviewed and tested a range of cover options and recommended, for Pile 7/12, a composite cover consisting of a 30 cm base granular layer, a 60 cm low permeability saturated glacial till layer, a 30 cm overlying coarse-grained granular layer. and a 10 cm erosion protection layer. One of the criteria was to use locally available soil material for the low permeability layer. A cover was placed on Pile 7/12 in late summer of 1991.

In Phase IV', monitoring of oxygen and temperature continued monthly at all piles. After placement of the cover at Pile 7/12, infiltration rates and leachate quality were also measured at that pile, as was the moisture content of the various soil layers of the cover.

The data Collected over the 72 month period of the project demonstrated that in the uncovered piles, relatively uniform increases of temperature existed towards the center bottom of the piles, and an associated drop in oxygen concentrations due to consumption of oxygen by the reaction. This was probably due to the exothermic oxidation reaction. The oxygen profiles however varied more than those for temperature. Seasonal variations were observed in both temperature and oxygen. The project confirmed that uncovered acid waste rock piles provide an favorable environment for oxidation of the sulphide material and thus the generation of acid leachates.

The composite cover on Pile 7/12 resulted in a depletion of oxygen within the pile and a reduction of pile temperatures; the flow of leachate from the pile was reduced from approximately 3 m³ per year prior to the cover to only 0.1 m-l per year after placement of the cover; i.e. to less than 2 percent of total precipitation; and the glacial till layer within the cover maintained its moisture content at the level at which it was placed over the 36 months of the evaluation.

These results indicate that the composite soil cover was effective in reducing the oxidation reaction in the pile, and therefore, reducing the production rate of acidic leachate and at the same time lessening the impact caused by acidic waste rock on the environment. Performance of the cover depends on maintaining its integrity with respect to preventing damage by roots, burrowing animals or physical impacts. The glacial till layer must also remain saturated. Thus, low level of maintenance of a soil-covered pile would be required.

The total cost of constructing the cover on Pile 7/l2 in 1991 was $60,000 (Canadian currency), or about $31 per square meter of the waste rock surface. For larger waste rock piles, this unit cost could be less because of larger quantities involved. The cost for engineering and construction quality control is not included because of the research nature of the project.

The results from the analytical leachate testing tend to indicate a decrease or no improvement in water quality immediately after construction of the cover. However, there was a well defined and steady improvement of water quality after 1994, which tends to indicate that the quality of the leachate is still improving. The pH of the leachate increased steadily since the placement of the covers indicating that it had become less acidic. Although the concentrations and loadings were greatly reduced, they still exceeded the criteria set by the regulatory agencies, thus dictating treatment of the leachate before discharge to the environment.

The piles at Heath Steele are small compared to waste rock piles at many other sites, and different gas transfer mechanisms might apply elsewhere. However, data collected in the project indicates that the cover system is an effective method of reducing oxygen ingress and thus. the oxidation rate. Despite the promising performance to date, additional monitoring of the cover is still required before its long-term stability can be properly assessed as a full-scale final closure measure.