Water quality impact predictions related to sulphide mine wastes involves ongoing
research and technological development. Investigations of existing sulphide tailings have
shown the types of water quality problems that can result from sulphide mineral
oxidation and leaching of acidic and/or metal-bearing drainage into the environment.
The rate of sulphide oxidation is the primary controller of the quality of drainage in
many types of mine waste. Geochemical reactions can alter the characteristics of water
generated in zones of oxidation but these are secondary after acid generation and
dissolution of metals. Although water quality issues have been studies extensively for
sulphide tailings, for example, there have been few attempts to quantify rates of
oxidation in-situ. The Oxygen Consumption Method, that is a recent technological
development, has been shown to provide a quantitative assessment of in-situ oxidation
rates in tailings. The measurements clearly and rapidly show the reactivity of tailings.
The results provide immediate feedback on the oxidation status of a tailings
impoundment and can be used to calibrate models that require reaction rates as input.
The method is relatively new and measurements were, until recently, available at only
a few field sites where research was conducted during method development.
This study was conducted to initiate a data base of measurements at a variety of
tailings impoundments. Sites were selected to represent different conditions such as
sulphide minerals (pyrite or pyrrhotite), sulphide content, acidic versus neutral
conditions, age since deposition and sites with oxygen barrier covers over existing
tailings. This report presents the results of that study and presents an interpretation of
the data including several types of information that can be extracted from the data.
In situ sulphide tailings oxidation rates were measured at six field sites using the
Oxygen Consumption Method. Results were examined to evaluate the rate of in situ
oxidation at each site and to examine possible trends in oxygen consumption rates
with other physical site characteristics including age of tailings, moisture content,
sulphide content, and the effectiveness of different cover scenarios.
Overall, the results indicated that the Oxygen Consumption Method can provide useful
information at a wide variety of tailings facilities. Measured rates were observed to be
related to sulphide content, moisture content and climatic conditions. Significant
reduction of rates was noted for tailings that were covered by oxygen barrier layers (or
soil covers) with variable effectiveness exhibited by different types of cover materials
and method of construction.
Oxygen consumption rates ranging from less than 1 mol O2 m-2 a-1 to over 5000 mol
O2 m-2 a-1 were measured during this investigation. A strong trend of increasing
oxygen consumption rate with increasing sulphide content was observed. This is
attributed to the influence of diffusion depths that increase significantly in the
near-surface zone of active oxidation. A trend of lower oxygen consumption rates was
observed for cooler, wetter climatic conditions than for dryer, warm conditions. Oxygen
consumption rates measured in early May were approximately 25% of rates measured
during the warmer and dryer summer months.
A trend of decreasing oxygen consumption rates with tailings exposure time was also
observed. This is attributed to the downward migration of the zone of oxidation that
increases the diffusion path length with increased moisture content at depth over time.
Higher moisture at depth further acts to lower the effective gas diffusion coefficient (De)
within the zone of active oxidation. Oxygen consumption rates measured on highly
saturated tailings were notably lower than for similar tailings where a developed
vadose zone existed. Oxygen consumption rates measured on freshly deposited, fully
saturated tailings and on tailings that were highly saturated were non-trivial and
represent potentially significant loadings to receiving waters.
Measurements of oxygen consumption rates to evaluate the effectiveness of various
cover scenarios at full-scale, test plot and in situ column scale revealed that the
addition of a layer of non-reactive material can substantially reduce the flux of oxygen
to the tailings. Simple covers constructed of local materials such as gravel or till were
observed to reduce the oxygen consumption rate as much as one order-of-magnitude.
A more complex, multi-layer engineered cover reduced the oxygen consumption rate
by as much as two orders-of-magnitude. However, improper design can compromise
the efficiency of multi-layer covers. Many of the covers investigated were sensitive to
potential drying out which could greatly reduce their effectiveness.
Results from the oxygen consumption rate measurements can also be used to address
issues such potential required neutralization, potential pore water loadings, rate of
depletion of sulphide inventory, and depletion rates for neutralization potential. These
quantities and the timing involved can be used to optimize management of reactive
tailings. An example of originally neutral tailings that will eventually generate acidic
leachate was assessed to determine the timing available for rehabilitation activity and
budget timing.
This study clearly showed that the oxygen consumption method provides a rapid and
cost effective assessment technique that can provide useful and critical data for wide
variety of sulphide tailings sites. The data can be used for waste management planning
and evaluation of covers from test-plot to full-scale. The data can also be used to
calibrate predictive models without the complications of decoupling highly variable
hydrology from geochemistry.
