Following preliminary studies that indicated the potential for using geophysical
methods for mapping acid mine drainage (AMD), the Mine Environmental Neutral
Drainage (MEND) program sponsored a study to demonstrate the applications of
geophysical methods to the AMD problem. Funding was provided by INCO Limited and
the federal and provincial governments through the Northern OntarioDevelopmont
Agreement.

The resource exploration industries have developed sophisticated instrumentation for
mapping the electrical properties of the subsurface. In recent years, this equipment has
been adapted for engineering and environmental applications, including determination
of ground water quality.

Any increase in total dissolved solids (ADS) in groundwater results in an increase in
electrical conductivity. Using commercially available equipment, the lateral and vertical
variation of conductivity can be mapped, although resolution decreases with increasing
depth.

Where the conductivity due to man-made contaminants such as AMD exceeds the
natural variations in background conductivity, geophysical surveys can be used to map
the subsurface distribution of AMD in three dimensions.

Other geophysical methods can be used to map the distribution of disseminated
sulphides and zones of active oxidation or reduction reactions. The induced
polarization method (IP) was developed to assist in mapping disseminated sulphides
such as those found in porphyry copper deposits. This method can be used to map the
lateral and vertical distribution of sulphldes in tailings as part of programs to assess the
acid-generating potential of tailings areas. The selfpotential (SP) method measures the
passive voltage distribution and can indicate zones of active oxidation and reduction.

Under suitable conditions, the combination of electrical conductivity measurements, IP,
and SP can be used to map:

i) sulphides – the source of AMD,

ii) zones of active oxidation, and

iii) the distribution of the reaction resultant AMD.

High-quality airborne, ground and borehole geophysical data were acquired over areas
with well-documented AMD problems. The geophysical data were correlated with
hydrological and chemical data from ongoing ground water investigations to establish
the utility of the methods.

The results confirmed that electromagnetic (EM) methods that remotely measure the
conductivity of the subsurface are very useful for location and detailed threedimensional
mapping of AMD in groundwater. Airborne surveys can provide rapid
reconnaissance scale surveys, while ground surveys and borehole surveys provide
progressively more detail.

In addition to the EM work, combined IP and electrical conductivity surveys were tested
on a large revegetated tailings area. The IP method detected disseminated sulphides
as expected, and the combination of IP and conductivity surveys can beused for
simultaneous three-dimenslonal mapping of sulphides and ground water quality in
tailings. United SP surveying showed strong SP anomalies over the tailings however,
additional SP data and information on the distribution of the oxidation found on the
tailings is required to properly assess this method.

All of these methods require measurement of the electrical properties of the subsurface
and are adversely affected by electrical noise from power lines and surface and buried
metal. The response of AMD may be masked by naturallyconductive ground water,
bedrock lithologies, or clay soils.