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Hydrogeological decision analysis : monitoring networks for fractured geologic media

University of British Columbia

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.

Thesis/Dissertation

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2009-02-20

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http://hdl.handle.net/2429/4862

Geological Sciences

University of British Columbia

UBCV

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