This report describes the geochemical processes occurring in acid-generating waste rock piles
and evaluates available geochemical computer models with respect to their abilities to simulate |
the geochemical processes to predict the quality of acid rock drainage (ARD).

The geochemical processes which most control ARD quality are precipitation and dissolution,
chemical diffusion, and surface reactions. Precipitation and dissolution control neutralization of
acidic solutions and fixation of metals within solids. Chemical diffusion and surface reactions
control the rates at which all precipitation and dissolution reactions occur. Quantitative reaction
rates are required to accurately predict water quality from waste rock piles. Reaction rates
determine which secondary minerals will form after dissolved ionic species and complexes are
liberated in aqueous solutions in contact with the waste rock. Reaction rate equations need to
incorporate the important influence of rate-modifying mechanisms such as bacterial catalysis.
Galvanic effects may also have to be incorporated; however, insufficient data disallow a proper
evaluation of their importance.

Drying and wetting are the most important physical processes that affect water chemistry, by
concentrating aqueous solutions to the point of precipitating solids and subsequently dissolving
and removing these solids from solution contact. All processes mentioned above also occur
during natural weathering of massive sulphide rock formations.

Field data required to describe geochemical processes in waste rock include detailed water
analyses, rock mineralogy and trace element chemistry, exposed surface area, temperature,
oxygen availability and water infiltration rates. Thermodynamic data are required to predict
precipitation and dissolution reactions in waste rock. However, thermodynamic data for
important secondary minerals in ARD are missing. Reaction rate equations and rate constants
necessary to perform kinetic calculations are also insufficient. Data from laboratory and field
tests can be used to compensate for the inadequacies of presently available geochemical
databases.

Existing geochemical models are divided into five classes: equilibrium thermodynamic models,
mass transfer models, coupled mass transport-mass transfer models, supporting models, and
empirical and engineering models. Each class of geochemical models can be viewed as
addressing a different type of ARD prediction objective. Equilibrium geochemical
thermodynamic models address the identification of the soluble and mobile metal species and
maximum metal concentrations. Mass transfer models more specifically address maximum
metal concentrations and their evolution with time. Mass transfer-flow models address the
prediction of concentration and load versus time. The engineering models are more directed
towards examining decommissioning options. More developments are required in each model
category to adequately achieve each prediction objective.

It is recommended to define thermodynamic equilibrium constants for important secondary
minerals and to obtain kinetic rate data for dissolution and precipitation of important minerals.
It is suggested that a model developed to predict water quality from waste rock piles should
have a geochemical component that takes into account reaction kinetics. In the short term,
model development efforts should be based on applications and improvements of some existing
mass transfer models using data from well-defined laboratory and field test cases