Review
of use of an Elevated Water Table as a Method to Control and Reduce
Acidic Drainage from Tailings
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Report
2.17.1
March 1996
EXECUTIVE
SUMMARY
Methods such
as subaqueous disposal are available to control and reduce acidic
drainage from tailings and can be applied to many, but not all tailings
management facilities. These available methods have various drawbacks
and in particular high cost. There is a need for alternate methods
which can reduce the cost of closure and perhaps eliminate some
of the drawbacks associated with the available methods. This report
reviews one such alternate method that is increasingly being considered
for the closure of sulphide tailings impoundments - the application
of elevated water table concepts.
The concept
of using an elevated water table within tailings is of great interest
as available data suggests that the rate of oxidation of sulphide
tailings, in conditions where the water table approaches the surface
of the tailings, is very low and similar to the rate of oxidation
of sulphide tailings maintained under water cover. There are three
basic approaches that can be applied to raise the elevation of the
water table and the associated capillary zone within tailings:
1) Modifying
the water balance of the tailings;
2) Enhancing
the water retention ability of the tailings; and
3) Constructing
groundwater flow barriers within the tailings.
Modification
of the water balance involves increasing water input into, or decreasing
water losses from, the tailings. Water retention can be improved
by enhancing the physical characteristics of the tailings prior
to placement - thickened tailings which provide a very high level
of saturation and considerable capillary zone height represent one
means of achieving this objective. The concept of a groundwater
flow barrier involves the installation of a barrier within tailings
to reduce the horizontal and often preferential downgradient flow
of pore water within the tailings. The suitability of these approaches
is dependent upon site specific conditions.
Not all tailings
management facilities are well suited for the application of elevated
water tables. Tailings stacks were historically designed to minimize
the water level within the tailings and maintain as small a surface
pond as possible. The application of elevated water table concepts
to raised stacks presents a considerable challenge. The application
of the concept to other tailings impoundments (e.g. valley dam impoundments)
is possible and may be economically beneficial in comparison to
other closure options.
Elevated water
tables have been an intrinsic component of closure strategies for
tailings management facilities but have only recently become proposed
or applied as a principal basis of closure plans. As such, experience
in the application and performance assessment of elevated water
tables is recent and being accumulated. The developing state of
knowledge is evidenced by the relatively limited data available
regarding the application of elevated water tables to sulphide tailings
impoundments. Nine sites, however, provide relevant but preliminary
data:
Five
sites (the Elura mine in Australia, the Greens Creek mine in Alaska,
the Cluff Lake mine in Saskatchewan, Les Mines Selbaie in Québec,
and the Kidd Creek metallurgical site in Ontario) involve the
ongoing or discontinued disposal of thickened tailings. These
sites represent a broad range of site conditions, and tailings
disposal and closure strategies.
Three
sites (the Falconbridge New Tailings site, the Dona Lake mine
site, and the Stanrock mine site - all in Ontario) involve the
modification of the water balance of the tailings. At the Falconbridge
New Tailings site, research has been carried out to investigate
the important relationships between oxygen consumption rates within
tailings, the degree of saturation, and depth to the tailings
water table. At the Dona Lake site, a closure plan is being implemented
which relies in part on the elevation of the water table within
the coarse tailings beach area. The proposed approach for the
historic Stanrock tailings stack is based on replacing spigotted
tailings dams with engineered water retaining structures - this
would ultimately allow the water table within the tailings to
rise and inhibit acid production from much of the tailings. Chemical
treatment of acidic drainage from the Stanrock tailings would
continue for an interim period following the construction of the
new containment structures.
The
Sturgeon Lake site in Ontario involves a combination of a modified
water balance and a groundwater flow barrier.
Chemical treatment
of acidic drainage may be required, in some instances, after the
application of an elevated water table concept. Key sources of acid
production could include sulphide solids in the near surface and
not sufficiently saturated zone, and fluctuations in the water table
elevation. The effect of water table fluctuation could be mitigated
in some cases by the use of a cover to reduce evaporative losses,
increase infiltration, or to elevate the water table to the cover.
The additional and interim use of chemical treatment may be cost
beneficial when compared to other options.
Numerical modelling
techniques are being used to predict the performance of elevated
water table concepts in controlling or reducing acidic drainage.
These techniques draw upon soil science and civil engineering methods
of predicting water movement in soil, and can be complemented by
numerical models for the prediction of acidic production in tailings.
These models are important as they provide a means of preliminarily
assessing the performance of a closure option, and of addressing
the effect of changes in ambient conditions (e.g. drought conditions).
Preliminary
and conceptual cost comparisons were carried out based on the closure
of three types of tailings impoundments: a tailings stack; a valley
impoundment underlain by a pervious zone; and a valley impoundment
underlain by an impervious zone. Closure options included the perpetual
collection and treatment of acidic drainage, the use of an engineered
cover, and applications of elevated water table concepts. The first
order estimates of closure costs indicate that elevated water table
concepts, when suitable, can provide significant closure cost saving
in comparison to collection and treatment, and the use of an engineered
cap.
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