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Contaminant transport through abandoned boreholes in fractured rock Burns, Sean Raymond Patrick

Abstract

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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