Acid Mine Drainage (AMD) research under the Mine Environment Neutral Drainage (MEND)
program has been ongoing since 1988 at the Heath Steele Mines site outside Newcastle, New
Brunswick. This research consists of two distinct projects. The initial study, noted as “Heath
Steele Waste Rock Study” (MEND Project 2.31.1), has been ongoing since 1988 and has as its
objective the evaluation of the nature of several waste rock piles and the examination of the
performance of a composite soil cover on top of pile 7/12 with respect to its effectiveness in
limiting the AMD. In 1990, this study recommended that further investigative work be carried
out at the site to determine the air permeability of the Heath Steele waste rock piles and to
assess the gas transfer mechanisms present in the piles, this initiated the “Assessment of Gas
Transfer–ANSTO Model at Heath Steele” project. The objectives of the project were to develop
an understanding of the processes governing the pyrite oxidation rates in acid waste rock piles
at Heath Steele using both the field data and the numerical model FIDHELM developed by the
Australian Nuclear Science & Technology Organisation (ANSTO), to quantify those processes
and to evaluate the effectiveness of the composite soil cover for acid waste rock management.

The project had two major components: field measurements and modeling using FIDHELM.
The field work that included the installation of monitoring probes permitted in situ
measurements of various parameters required by FIDHELM, namely the oxygen diffusion
coefficient, air permeability and thermal conductivity. The modeling consisted of running
FIDHELM utilizing the Heath Steele data collected in the field program. The gas transport
mechanisms dominating the oxidation rates in the Heath Steele waste rock piles were identified
using observed temperature and oxygen profiles in conjunction with FIDHELM simulations. A
user’s manual for FIDHELM was also presented with the report.

The air permeability, oxygen diffusion coefficient and thermal conductivity were measured
successfully and the values were found to be within the range measured in four other piles of
waste rock at three different mine sites and in climates ranging from tropical to arctic.
FIDHELM was shown to be a useful tool to assess the various gas transfer mechanisms in
pyritic waste rock piles. Numerical simulations using FIDHELM with the average measured
values of oxygen diffusion coefficient, thermal conductivity and gas permeability indicate that
gas transport was dominated by diffusion. FIDHELM results also indicate that particular details
in the oxygen profile measured at the Heath Steele waste rock piles can be explained by the
heterogeneity of the pile with respect to both gas permeability and IOR. The modeling of the
pile with the soil cover (pile 7/12) and the field data indicate that the composite soil cover is an
effective way to reduce the ingress of oxygen to acid generating waste rock. The data collected
to date from covered pile 7/12 is consistent with the simulations.

While the piles at Heath Steele are small compared to waste rock piles at many mine sites
around the world, and while the balance of gas transport mechanisms in these larger piles may
be different from those at work in the piles at Heath Steele, it is also clear that the cover system
has been effective in reducing oxygen ingress and, with this, in reducing the oxidation rate.

Since it is possible that advection driven by wind effects is a mechanism in some of the piles, it
is recommended that a program of work be set up to quantify this effect and to compare
measured values with the results of modeling with a version of FIDHELM modified to accept
space/time varying surface pressure. Continuous monitoring of temperature and pore gas
oxygen concentration profiles throughout the entire year is also recommended and may provide
better understanding of the chemical processes and gas transfer mechanisms present in waste
rock piles.

This may allow any temperature dependence of the IOR to be estimated. Monitoring and
simulations using FIDHELM should be used to assess the importance of pile inhomogeneity on
the overall pollutant load from a pile. Finally, the IOR and the sulphur content of the waste rock
from Heath Steele should be estimated using laboratory testing.