Review
of water Cover Sites and Research Projects
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Report
2.18.1
September 1997
SUMMARY
In
1996, the Geotechnical Research Centre at The University of Western
Ontario initiated a project to review water cover research and applications.
The objectives of the project were to document the design and performance
of water covers used to prevent acid generation in sulphide-bearing
mine waste and compile results from water cover research. Water
cover sites in Canada, Norway and Sweden were reviewed. Research
programs reviewed were from Canada, Norway and the United States.
They include MEND-sponsored work at the CANMET Elliot Lake Laboratory
and Noranda Technology Centre, Pointe-Claire, Québec, and
research undertaken by former United States Bureau of Mines, Norwegian
Water Research Institute and Norwegian Hydrotechnical Laboratory.
The
Province of Ontario, Canada Centre for Mineral and Energy Technology
(CANMET) and Brunswick Mining and Smelting Corporation provided
funding for the project.
The
survey of water cover sites showed that the selection of the minimum
depth of water is based on maintaining saturation of tailings in
the event of a drought and preventing tailings resuspension. Predictions
of resuspension are based on empirical correlations with bed shear
stress, bed water velocity, or the wave height to water depth ratio.
These correlations were generally obtained from experiments on particles
other than tailings. Estimates of bed shear stress, velocity, or
wave height are calculated from empirical equations relating these
parameters to wind speed and pond fetch length, using linear wave
theory. There was no conclusive verification of resuspension prediction
at any of the sites we reviewed.
Methods
presently used to predict the quality of pond and effluent waters
do not consider many biological, geochemical and physical phenomena
that may significantly affect water cover performance. In general,
not enough long-term field data are available to verify water quality
predictions, however, at one site field data have already shown
the predictions to be unconservative.
The
character of the reviewed sites, in terms of site hydrology, the
initial composition of the tailings, the degree of oxidation of
the tailings prior to flooding, the physical layout of the site,
and the treatment of the tailings before or after flooding, varies
considerably. Due to the dissimilarity of the sites examined, no
correlation between water depth and design performance could be
made. Observations common to many sites include a dramatic increase
in pH during the summer, possibly due to microfauna activity, CO2
degassing and increase in alkalinity from surface runoff, and the
formation of a thin coating of orange iron oxyhydroxides and organic
matter at the tailings-water interface.
In
column and lysimeter experiments for both oxidized and unoxidized
tailings, flooding produces a thin surface coating of brownish-orange
iron oxyhydroxides which in previously unoxidized tailings marks
the extent of vertical oxidation into the tailings. The finite thickness
of the oxidation zone apparently occurs because of the attainment
of steady-state conditions between pyrite oxidation and sulphate
reduction to sulphide. With time, the growth of organic deposits
at the surface occurs, which further impedes upward metal flux to
the water cover. Column experiments on oxidized tailings show that
flooding causes the dissolution of oxidation products and the subsequent
release of metals into the water cover, often resulting in metal
concentrations higher than permissible levels. The dissolution of
some oxidation products, or other minerals, may be slow and contribute
to high metal concentrations in the water cover for many years.
The use of a sand or a peat protective cover at the tailings-water
interface reduces metal fluxes into the water cover to negligible
levels. However, metal flux into the water cover may resume when
the absorptive capacity of the protective cover is reached. The
use of a peat layer creates highly anoxic and reducing conditions
in the tailings. Peat also produces acidity that can significantly
increase the mobility of some metals in the tailings pore water.
The
addition of lime to previously unoxidized tailings prior to flooding
inhibits the acitivity of iron-oxidizing bacteria by keeping pH
high. Lime addition to oxidized tailings increases pH and limits
the mobility of most metals in the water cover and pore waters.
Mixing the top portion of tailings with lime was found to be more
effective than simply adding the lime to the surface. Traditional
methods used to estimate the required amount of neutralizing material,
such as acid-base accounting, were inaccurate when applied to long-term
(37-week) experiments on flooded tailings.
Laboratory
test results show that coarser tailings under a water cover initially
generate higher amounts of acid drainage than finer tailings. This
could be due to the initially faster rate of vertical oxygen transport
in coarser tailings.
The
effectiveness of a Biologically Supported Water Cover (BSWC) has
been demonstrated in the laboratory. A BSWC involves colonizing
flooded tailings with plants to facilitate organic buildup for limiting
resuspension and metal flux through biological adsorption. Preliminary
results suggest that its effectiveness in the field is dependent
on the depth of water and wind conditions.
It
is apparent from laboratory and field studies that flooding tailings
is the most successful method presently known for preventing and
controlling ARD. However, in many cases the water cover discharge
will require treatment to meet regulatory standards, and it is not
currently possible to accurately predict the required amount and
length of treatment. State-of-the-art predictive contaminant models
ignore many important phenomena in water covers, including secondary
mineral dissolution and metal release independent of oxidation and
resuspension. Some of the work reviewed hypothesized that flooding
will eventually establish a diagenetic environment for some tailings
minerals with accumulation of organic matter. It is, however, uncertain
how long this will take to develop and whether it will occur under
all conditions. Furthermore, some metals may continue to be released
to the water cover in such an environment. Although water covers
are a promising technology, fundamental understanding of many phenomena
influencing their performance is unknown, and the minimum depth
of water required cannot presently be confidently determined. The
present prediction tools do not consider other important tailings
properties such as thixotropy and cohesion.
We
recommend that additional research and field work be initiated to
address the uncertainties and issues raised in this review. The
key areas for such work may include verification of resuspension
predictions at a few selected sites, fundamental understanding of
tailings properties and behaviour, the nature and rate of accumulation
of the organic matter-iron hydroxide sediment at the tailings-water
interface, and the contribution of resuspension to tailings oxidation
and long-term water quality.
ACKNOWLEDGEMENTS
The authors
would like to thank the following individuals who provided assistance
and information during the review:
T.P. Lim and
Nand Davé, CANMET Elliot Lake Laboratory
Al Vivyurka,
Rio Algom, Ltd.
Bernard Aubé,
Michael Li, Serge Payant, Luc St-Arnaud, and Pascale St-Germain,
Noranda Technology Centre
Gilles Trembley,
MEND Secretariat
Gail Amyot,
Cambior Inc.
William White
and Robert Lambeth, formerly of the United States Bureau of Mines
Steve Torok,
US Environmental Protection Agency, Alaska
Rolf Tore Arnesen,
Norwegian Water Research Institute
Per Broman,
formerly of Boliden Minerals AB, Sweden
Michael Aziz
and Jim Robertson, Equity Silver Ltd.
Arve Slørdahl,
Chr. Salvesen and Chr. Thams, Lokken Verk, Norway
Tore Dahl and
Einar Tesaker, Norwegian Hydrotechnical Laboratory, Trondheim Norway
Dan Bazinet,
Ontario Ministry of Environment and Energy
Leonard Surges,
Noranda Mining and Exploration Inc.
Elizabeth Milliken
and Ajay Verma, The University of Western Ontario
Philp Arkoh-Forson
and Christine Anagnostis (for the use of their dictionaries)
We apologize
to those omitted.
The
Ontario Ministry of Environment and Energy, Ontario Ministry of
Northern Development and Mines, CANMET and Brunswick Mining and
Smelting Corporation provided funding for the project under the
MEND (Mine Environment Neutral Draining) program.
Français
| Contact Us
| Help | Search
| Canada Site
Home | What's
New | CANMET-MMSL
| MMS Site
| NRCan Site
|