| Summary: | Abandoned exploration boreholes are commonly found around mine sites in a fractured
crystalline rock environment. If the abandoned boreholes have not been properly
decommissioned they have the potential to create connections through the rock fractures
and influence ground water flow and contaminant transport. A fully three-dimensional
discrete fracture model is used to investigate the impact of abandoned boreholes on
contaminant transport from a waste-rock pile overlying a fractured rock mass. Dissolved
contaminants travel through the fractured rock mass under the influence of a subhorizontal
regional hydraulic gradient towards a downstream compliance boundary. A
number of different fracture geometries are investigated to gain an understanding of the
field situations in which abandoned boreholes can be expected to have an impact. The
effect of fracture density, transmissivity contrasts, and borehole diameter and location are
studied. The simulation results show that vertical abandoned boreholes are most likely to
have an impact when large, sub-horizontal, high-transmissivity features are present in the
network. Low fracture density, aperture variability, relatively high horizontal
transmissivity, and the presence of major features in the fracture network all lead to
abandoned boreholes having a greater overall influence. If an abandoned borehole is
transversely offset from the central flow line passing through the source zone the
contaminant plume can migrate towards the borehole in a direction not predicted by the
average regional hydraulic gradient. In field-scale fracture networks smaller borehole
diameters leads to shorter breakthrough times and higher contaminant concentrations at
the downstream boundary due to the interplay between the fracture network and borehole
void space. The presence of abandoned boreholes can be expected to have important
implications in the design of monitoring networks to detect ground water contamination
when these fracture network and abandoned borehole properties exist.
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