| Summary: | In this dissertation, a decision analysis framework is developed to assist in the
design of monitoring networks at hazardous waste sites located above a fractured
geologic unit. The decision analysis framework is based upon risk-cost-benefit analysis,
performed from the perspective of the owner/operator of the landfill facility. The costs
considered are those that are directly associated with the construction and operation of the
monitoring network (actual costs). The risks considered are those that are associated with
the detection of migrating contaminants and consequent costs of remediation, and the
failure of the facility and the costs resulting from failure (expected costs). The benefits
are considered to be the same regardless of the monitoring strategy adopted, and are
neglected.
The fractured rock formation underlying the hypothetical landfill site is modelled
in vertical section using a two-dimensional discrete fracture model. This model uses a
particle tracking method to simulate the transport of a non-reactive solute through the
fractured rock unit. Three fracture geometries are investigated, each with different
hydrogeological behaviour. For each of these geometries, four monitoring schemes are
considered: 1) monitoring the fractures that carry the highest volumetric flows, 2)
monitoring the fractures that have the largest apparent apertures, 3) monitoring the areas
of highest fracture density, and 4) placing the monitoring locations at predetermined
depths. The effects of the distance of the monitoring network from the contaminant
source, and the number of monitoring locations installed at each monitoring well site, are
investigated for each of the four monitoring strategies in each of the three fracture
geometries. The base case analysis is performed using a pseudo-three-dimensional
approach that is adopted in an attempt to achieve consistency between the expected costs
of remediation and failure, which assume a three-dimensional domain, and the costs of
monitoring, which are calculated on the basis of each individual monitoring well site.
The best monitoring alternative in two of the three geometries investigated, and
the highest probabilities of detection in all three fracture geometries occur when the
fractures carrying the highest flows are monitored. However, the monitoring strategy that
provides the highest probability of detection is not necessarily the best alternative.
In the geometries modelled, the probability of detection is influenced by the
amount of vertical spreading the contaminant plume undergoes near the contaminant
source as a result of the toruousity of the preferred flow paths through the fracture
network. The increase in the probabilities of detection brought about by the installation
of a “backup” monitoring network is insufficient to justify such an installation. However,
the decision analysis developed in this study does not evaluate other functions that are
potentially filled by a “backup” monitoring system.
The combination of monitoring options that provide the best monitoring
alternative is insensitive to changes in the detection threshold and changes in the discount
rate over the ranges investigated. The length of time between samples, and variations in
the characteristics of the pseudo-three-dimensional analysis have only a small influence
over the combination of monitoring options that provide the best monitoring
alternative.
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