The Database for Acid Rock Drainage (DBARD) and a detailed study of the
tailings impoundment at Gibraltar – McLeese Operations, British Columbia have
provided the basis for this study evaluating the ability of Acid Rock Drainage
(ARD) prediction data to indicate on-site water quality.
Analysis of the database has shown that sites with similar geologic or geographic
settings cannot be compared directly, although experience from previous work at
other sites can aid new sites in developing waste characterization programs. The
static prediction data is dependent on geochemistry of the sample and the
prediction method and procedure. Improved methods of data interpretation can
account for these factors. Identification of “indicator” elements can be used to
calculate acid generating and neutralization potentials from site specific
correlations and be included in kinetic tests and on-site monitoring programs to
characterize oxidation and dissolution reactions. The water quality and
oxidation/neutralization rates for the DBARD kinetic tests are not predicted by the
static test data due to the short time frame of the tests, the time lag to ARD and/or
the inaccurate values of the static test data. Kinetic tests need to be initiated as
early as possible to allow stable conditions to be established and the maximum
amount of time for leaching. Various methods proposed to calculate neutralization
depletion rates were shown to provide similar rates. With longer term kinetic tests,
the assumption that calculated oxidation/neutralization rates can be extrapolated
into the future needs to be verified through additional test work.
Interpretation of the Gibraltar static prediction data for the tailings inferred a
potential for ARD. This potential is indicated after approximately 3 years of kinetic
testwork with increasing oxidation/neutralization reaction rates, although the pH is
neutral. Oxidation and neutralization reactions are also occurring on-site, although
water quality is predominantly affected by the chemistry and infiltration/migration
of the tailings pond supernatant. Water quality changes during supernatant
migration are attributed to physical processes, chemical reactions and/or mixing of
waters with different compositions. These factors all contribute to the ultimate
water quality on-site suggesting that extrapolation of laboratory data is more
qualitative than quantitative.
Recommendations for analyses, data manipulation, criteria for interpretation, and
reporting of ARD prediction data have been made for the different stages of waste
characterization: 1) mineralogical characterization, 2) static testing, and 3) kinetic
testing.
