This report describes the results of laboratory diffusion lysimeter studies that were conducted as a
part of a broad research program titled “Development of Wet Barriers on Pyritic Uranium Tailings for
Controlling Acid Generation”. The research was undertaken jointly by CANMET, Elliot Lake
Laboratory and Rio Algom Limited, under the Canadian Mine Environmental Neutral Drainage
(MEND) Program.

Purpose:
Diffusion lysimeter studies were undertaken to determine the surface oxidation, leaching and mass
release characteristics of underwater deposited pyritic uranium tailings for two different cases: 1)
un-oxidized tailings that have been kept underwater for more than 12 years and 2) weathered
(partially oxidized) tailings. The studies, conducted for shallow water covered tailings having a well
mixed (circulated) water column without surface or porewater flows, determined the mass transfer
and diffusion related transfer parameters (transfer flux and coefficients) for metals and radionuclides
from underwater deposited tailings to the upper lying water column.
The results would provide suitable inputs for modeling the long-term surface water chemistry of water
covered tailings.

Methods:
The studies were conducted using aquarium type PlexiglasTM lysimeters for un-oxidized tailings,
obtained from the Quirke mill and kept underwater in the laboratory since 1982, and weathered
tailings, obtained from the Quirke waste management area and deposited underwater in the
laboratory in 1990 . A shallow water cover, approximately 0.2 m in depth, was provided using distilled
water. The water column above the tailings was continuously mixed and maintained at a constant
height in each lysimeter. The mass transfer parameters for various ionic species such as Ca, Mg, Fe,
Mn, SO4-2 and Ra-226 etc. were determined, in the absence of any surface and/or
porewater flows, by measuring their concentrations in the surface as well as pore
waters as a function of time.

Duration of the Study:
The experiments, conducted at the CANMET, Elliot Lake laboratory, were started in
November 1993 and completed in August 1994.

Results:

Un-oxidized Tailings:

• Un-oxidized tailings, deposited underwater in 1982, oxidized very slowly
  and a narrow oxidized and iron hydroxide precipitate zone, 2-3 cm in
  thickness, was formed at the surface of the tailings at the water-tailings
  interface. Mobilization and release of iron to the surface water, under
  oxidizing conditions, resulted in its precipitation and covering of tailings with
  a layer of ferric hydroxide.
• The oxidized zone at the tailings surface released low acidity (5 – 20 mg
  CaCO3/l), low concentrations of Mn and Pb, and high concentrations of Ca,
  Mg and Ra-226 to the surface water. Gypsum dissolution contributed to
  increased release of Ca and to a certain extent that of Mg.
• Dissolution of Ra from the tailings surface, and most significantly under
  acidic conditions, was a dominant factor in increased Ra mobilization and
  its release to the surface water. Solubility of gypsum, when present, and
  hence the sulphate ion concentration controlled the release of Ra and its
  surface water concentration that decreased with increasing sulphate
  concentration. Diffusion of Ra from the tailings porewater was low (less
  than 10%) compared to mass dissolution from the surface of the tailings.
• Concentration of other metals in the surface water were low as a result of
  iron hydrolysis and precipitation.
• Tailings porewater contained mainly dissolved gypsum, Mg, and Ra, and
  low concentrations of Fe, Mn and Pb. Long-term exposure of the tailings to
  natural and fluorescent room light in the lysimeter resulted in the formation
  of an algae layer on the surface of the tailings which slowly contributed to
  oxygenation of the tailings substrate and decrease in the previously
  established anoxic conditions as well as sulphate reduction.

Weathered Tailings:

• Similar to un-oxidized tailings, further oxidation of weathered and partially
  oxidized tailings underwater was very slow and limited to near surface zone
  of the tailings.
• The weathered tailings released high acidity (~ 1700 mg CaCO3/l) and high
  metal concentrations (e.g. Fe ~ 550 mg/l and Al ~ 110 mg/l) to the
  porewater but their diffusion to the surface water was moderate to low
  (acidity ~ 10 – 20 mg CaCO3/l, Fe ~ 0.5 mg/l and Al ~ 2.0 mg/l).
• Iron was dissolved from weathered tailings but its transfer to the surface
  water was also low, as it hydrolyzed and precipitated forming an iron
  hydroxide sink layer at the tailings surface. Mass dissolution of gypsum and
  Ra from the tailings surface resulted in increased concentrations of Ca, Mg
  and Ra (10,000 – 32,000 mBq/l) in the surface water. Transfer of other
  metals was low.
• For weathered tailings under high acidic conditions, the transfer flux of Ra
  was significantly higher (approximately 50 times) and those of Ca, SO4 and
  Mg were significantly lower, (e.g. decrease in transfer coefficients by 2 to 3
  orders of magnitude), than those for un-oxidized tailings.

In both cases, the slow oxidation of underwater deposited tailings at the surface as well
as the dissolution and release of metals and radionuclides from the oxidized surface
have been important factors in determining the surface water quality. The data suggest
a need to examine the use of diffusion or oxygen barriers above the tailings surface for
further controlling the surface oxidation as well as the release of metals and
radionuclides to the surface water column.