MEND Project 3.12.2

Sponsored by: Canadian Centre for Mineral and Energy Technology and Ontario Ministry of Northern Development and Mines

February 1993

EXECUTIVE SUMMARY

For the Mine Environmental Neutral Drainage (END) Program, an environmental survey of the Panel wetlands area in Elliot Lake, Ontario was conducted in 1991. The survey was undertaken by the Surface Environmental Research group of the Elliot Lake Laboratory, Mineral Sciences Laboratories, CANMET, Energy, Mines and Resources Canada, in collaboration with Rio Algom Limited, Elliot Lake, Ontario.

The broad objectives of the survey were to:

• Determine the surface and groundwater hydrogeochemistry of the Panel wetlands tailings basin;
• Establish whether the existing natural wetlands and water cover on acid generating tailings were providing any treatment to acid mine drainage, and to evaluate their controls on acid generation and migration of contaminants associated with the oxidation of pyrite;
• Characterize wetlands tailings/sediment substrate for metals, sulphide and sulphate sulphur speciation, and its oxidizing/reducing microbe populations, and to determine their roles in various geochemical and biological interactions; and
• Characterize wetlands vegetation for metals and radionuclide uptake.

The survey results were as follows:

• The Panel wetlands study site was a small basin located in a bedrock valley containing partially submerged pyritic uranium tailings. It had a total area of 14.5 ha, and contained approximately 236,000 tonnes of tailings spread over an area of 12.9 ha. Approximately 88% of the tailings area was underwater, leaving an area of 1.6 ha in the western part of the basin where the tailings were exposed. The average thickness of the tailings in the basin was 0.92 m.
• The tailings were deposited in the basin as a result of a tailings spill upstream in the late 1950s which completely filled the western and central part of the basin, and spread a thin layer of tailings to the eastern part of the basin. Underneath the tailings, the basin contained a layer of peat 0.3 to 4 m thick, underlain by sand and gravel deposits.
• The eastern part of the basin contained ponded water, 0.4 to 1.4 m deep. A shallow water layer, 0.1 to 0.5 m deep, extended throughout the west/central part of the basin which supported a dense vegetative cover consisting of cattails, marshland grasses, sedges, sphagnum and other acidophilic mosses. The deep ponded water contained submergent vegetation such as pondweeds.
• A beaver dam at the east end of the basin regulated the water level and its discharge flow. The basin contained a total water volume of approximately 24,000 m3, with an average depth of 0.2 m.
• The site had a total catchment and drainage area of approximately 49 ha. On an annual basis the site received a total net precipitation of approximately 138,920 m3 (calculated from mean annual precipitation data) corresponding to an average flow of 4.4 L/s or 9 L/s per km2. The drainage water volume was estimated to be six times the volume of the water contained in the basin which corresponded to a dilution factor of 6.
• At the extreme west end of the site, there was an acidic tailings pond impounded by a clay core cross-valley dam. No visible seepage of acidic water from this pond was observed towards the basin
• The surface water flow from the site was towards the east. It was irregular and intermittent, and was measured at 16.5 L/m in the fall.
• In general, the groundwater flow was also from west to east. Sub-surface water from exposed tailings areas in the western section of the basin discharged into the central water body. In this section the water table was 0.2 to 2.0 m below surface, and the groundwater flow was upwards and towards the east. The measured horizontal and vertical gradients ranged from 0.003 to 0.015 and from 0 to -0.15, respectively. Upward vertical gradients were highest in the fall. Sub-surface discharge from the western part of the basin to the central pond was estimated at 380 to 1800 m3/a which was less than 2 to 8% of the total surface water volume of the basin.
• In the eastern part of the basin, the groundwater flow was also towards the east but in a downward direction. The measured horizontal and vertical gradients ranged from 0.0002 to 0.001, and from -0.72 to 0.14, respectively.
• The surface water in the basin was slightly to moderately acidic in the exposed and vegetated western parts of the basin with low to medium concentrations of dissolved solids (600 to 2000 mg/L), iron (1 to 80 mg/L), calcium (150-500 mg/L), and sulphate (50 to 1000 mg/L).
• In the central and eastern parts of the basin where a permanent water body existed, the surface water was near neutral to moderately alkaline, pH (6.2 to 9.8), with low concentrations of dissolved solids (100 to 300 mg/L), iron (0.002 to 0.4 mg/L), calcium (30 to 50 mg/L), and sulphate (50 to 100 mg/L).
• There was no strong seasonal dependence of the surface water quality except pH in the central and eastern parts of the basin which increased from 7.5 to 9.8 in the summer
• The groundwater at shallow depths in the basin was mostly tailings derived porewater with slightly acidic to near neutral pH (5.7 to 7.8), low to moderate acidity (10 to 200 mg CaCO3/L), low to high alkalinity (0 to 1400 mg CaCO3/L), low to moderate iron (0.5 to 70 mg/L), high Ca (400 to 800 mg/L), high sulphate (800 to 1500 mg/L), and high Ra-226 (280 to 10,900 mBq/L). There was no strong seasonal dependence except for dissolved iron concentration which was variable.
• The soil substrate in the basin mostly consisted of tailings except near the far east end where the original peat sediments existed. The paste pH of the substrate varied from near neutral to highly acidic, 7.5 to 2.1.
• Thiobacillus ferro-oxidans (AT) and sulphate reducing bacteria (SRB) were present in all soil substrate and sediment samples from exposed and shallow water cover sites in the western part of the basin. At deep water sites near the centre and towards the east, the bacterial counts for TF reduced drastically to insignificant numbers (0 to 100). Sulphate reducing bacteria populations at these sites exceeded those for Thiobacillus ferro-oxidans. There were no clear trends in SRB distribution profiles along the length of the basin.
• The sulphur speciation data also showed that both sulphide and sulphate concentration were variable within and between sites without clear trends. Sulphate reduction was clearly evidenced by a strong smell of H2S in the groundwater at deep water central and eastern locations.
• Tailings were oxidizing in the exposed and shallow water covered and vegetated part of the basin. In exposed areas, oxidation was taking place near the surface in the unsaturated zone and in the vicinity of the water table. In vegetated areas, oxidation of both organic matter and tailings was taking place from surface to the root zone of the substrate. Because of fine grained tailings and their high degree of moisture saturation, the overall oxidation rates were low, producing low pH (3.4 to 5.5) surface drainage and sub-surface water.
• In the vegetation zone, no improvement in the surface water quality was noted as it drained from exposed areas towards the ponded water. Both the surface and groundwater data indicated that the vegetation was oxidizing the substrate rather than providing the treatment. Some iron was precipitated and removed as ferric hydroxide in the vegetation zone.
• The acidic surface drainage from exposed tailings and vegetated areas was diluted by a factor of 6 to 10 as it drained and mixed with the ponded water. The groundwater from western and central sections of the basin was also discharging in the ponded water where it neutralized the acidic surface water and precipitated dissolved iron, aluminum and manganese as hydroxides when the waters mixed.
• The pH of the ponded water increased from 7.5 to 9.5 in the summer which was attributed to the bacterial reduction of nitrates in the organic sediments, and to the photosynthetic process of some submergent vegetation (pondweeds) producing ammonia and hydroxyl ions, respectively. This phenomenon needs to be further investigated.
• From water quality data for surface drainage from exposed and shallow water cover vegetated areas, and pond water near the discharge end, the annual rates of total iron production, as a result of pyrite oxidation, and iron discharged from the system were calculated as 183.7 kg/y and 9 kg/y, respectively. These values corresponded to an annual pyrite oxidation and iron discharge rates of 1.11 and 0.04 mg Fe per kg of tailings in the basin per year.
• The existing wetland system, because of its various physical, chemical and biological controls, retained or recycled approximately 96% of the total iron produced as a result of pyrite oxidation. It was estimated that at these rates it would take approximately 31.7 X 103 y for all the pyrite to oxidize, and 926 x 103 y for all the mobilized iron to leave the system, assuming that the rate did not change with time.
• For calcium and Ra-226, the corresponding times for their complete removal from the system were calculated as approximately 708 y and 40 x 103 y, respectively.
• For cattails and grasses, the observed metal uptake levels were similar to those observed at other pyritic uranium tailings, base metals and gold tailings sites. High concentration of iron, aluminum, calcium, and other heavy metals were observed in pondweeds and sphagnum moss, but their contribution to the total bio-mass production and metals retention and removal load was very small compared to cattails and grasses which were the most abundant species.
• In all vegetation, the observed metal concentrations were below plant toxicity levels with little or no significant accumulation warranting concerns related to wind dispersion or animal forage. No symptoms of plant toxicity were observed.
• Ra-226 concentrations in the wetland vegetation (30 to 3800 mBq/g) were significantly elevated in all the species compared to background levels of 10 to 20 mBq/g in terrestrial vegetation for local and distant controls

It can be concluded that the wetland/water system in the Panel wetlands basin was effectively controlling the acidic drainage from partially submerged pyritic uranium tailings. The system would continue to function as long as the water cover was maintained. Its performance could be further improved if all the tailings were completely submerged.

It is recommended that the sediment oxidation-reduction dynamics and the photosynthetic process of submerged plants in the eastern part of the basin should further be investigated in order to understand the seasonal behaviour of the observed high pHs in the summer.

Last Modified: 2003-11-26		Important Notices

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