Control
of Acidic Drainage in Layered Waste Rock: Laboratory Studies and
Field Monitoring
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Report
2.37.3
September 1997
EXECUTIVE
SUMMARY
This study
represents another contribution in the MEND series on the prediction
and control of acidic drainage from waste-rock dumps. The Samatosum
Minesite in southeastern British Columbia implemented full-scale
layering of acid-generating and acid-neutralizing rock in its waste-rock
dump, based on a series of column tests.
This study
has reviewed existing information, reinterpreted old data, and obtained
new data for the layered waste-rock dump at Samatosum and the column
tests which simulated it. Most of the rock units at Samatosum had
significant proportions of net-acid-generating rock, except for
mafic pyroclastics (MAF). As a result, all rock was divided into
MAF and Potentially Acid Generating (PAG) rock. The PAG was encapsulated
by MAF layers within the dump, and the overall ratio of Neutralization
Potential (NP) to Total Acid Potential (TAP) within the dump was
3:1.
Column tests
containing 10-50 kg of rock simulated various layered and unlayered
sequences of PAG and MAF rock. These columns were operated up to
5.5 years to determine if the dump design was appropriate. However,
the two columns containing layers of PAG and MAF to simulate the
dump had NP:TAP ratios close to 1:1, instead of 3:1.
Because effluent
concentrations of metals like calcium and magnesium were not measured
during column testing, rates of NP consumption and depletion could
not be determined. However, simple geochemical relationships and
data from other mines indicated that the ratio of NP consumption
to sulphide depletion in the layered columns was likely greater
than 1:1. Additionally, not all measured NP at Samatosum is available
for neutralization: up to 10 t CaCO3/1000 t is unavailable.
Therefore, the columns were predicted to eventually release net
acidity if they had continued. This contradicted previous predictions,
primarily because previous work assumed that the rate of NP consumption
was lower and equivalent to the rate of alkalinity production. In
reality, alkalinity production typically represents only a portion
of total NP consumption.
Comparisons
of pre-test and post-test analyses on various grain-size ranges
from the columns indicated layering as small as 0.2 m did not alter
the reaction rates and the geochemical behaviour of the PAG and
MAF material. In other words, layering with MAF did not slow reaction
rates in the adjacent PAG layers. However, the effluent from the
layered columns did represent a composite from the two materials.
Significant
accumulation of sulphate from sulphide oxidation was noted in PAG,
where it originated, and in basal MAF layers within the columns.
Also, NP was significantly depleted in MAF layers, particularly
in the finest grain-size range. In fact, NP depletion in the finest
MAF particles caused some MAF to become net acid generating and
suggested that NP in the coarser particles may not be readily available.
This also supported the prediction of net acidity from the columns.
Monitoring
of drainage from the dump at Samatosum has revealed signs of impending
net acidity. In fact, pH at one station has fluctuated between neutral
and acidic values, with the acidic values becoming lower and more
persistent with time. However, unlike the columns, the dump has
an overall ratio of NP:TAP of 3:1, and thus is not predicted to
generate net acidity.
The reconciliation
of predictions with monitoring data is based on (1) the observations
in the columns that layering does not suppress reaction rates in
the PAG and (2) coarser rock can preferentially channel water through
a layer. Consequently, the appearance of net acidity in some dump
drainage is simply the result of physical conditions rather than
a failure of geochemical principles. This highlights the importance
of physical design and physical hydrogeology in any future design
and construction of a layered dump. If drainage does not pass through
all available neutralizing layers, acidic drainage may appear even
in the presence of excess neutralizing potential.
A final note
focusses on metal leaching. Although acidic pH may be prevented
with carefully designed and implemented layering, leaching of metals
is not so easily controlled. Because reaction rates in even small
layers of net-acid-generating Samatosum rock, on the order of 0.2
m, could proceed unattenuated by adjacent net-acid-neutralizing
rock, metal leaching can probably occur at accelerated rates in
layered dumps. If site-specific solubilities of secondary minerals
are relatively high, aqueous metal concentrations may then exceed
water-quality requirements even in near-neutral drainage. Therefore,
layering is not necessarily a control technique for metal leaching.
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