The Mactaquac Generating Station is a large (660 MW) hydroelectric facility on the Saint John River, approximately 19 km upstream from Fredericton, New Brunswick. An alkali-aggregate reaction within the dam's concrete structures is causing their differential expansion. This has prompted the dam's operator, NB Power, to be proactive in monitoring for any signs of concentrated seepage that could arise where the dam's clay till core abuts a concrete diversion sluiceway. Seepage is a leading cause of dam failures. An emerging method to non-invasively investigate an embankment's interior is Electrical Resistivity Imaging (ERI). ERI is sensitive to changes in water saturation, temperature and ionic content, all of which can be indicators for localized seepage. In this work, a 100 electrode areal array was installed to focus on the interface between the embankment and its concrete abutment. Numerous hurdles and challenges were solved throughout this work. The installation of a durable and effective array was achieved by drilling into the slope and injecting bentonite into the drill holes before driving in long (0.91 m) electrodes to keep contact resistances low. Limitations posed directly by the relatively short survey line lengths on the back of the dam were mitigated by implementing a pole-dipole survey geometry which yields relatively large depths of exploration and maintained sensitivity near the ends of the lines. An enigmatic current regulation issue, which affected repeatability, was solved by customization of measurement array geometries and by averaging many surveys over time with a smart-averaging processing routine designed to reject outlier measurements. Many necessary changes were made to electrode array configuration over the course of the study and data quality has steadily improved. Early results are encouraging, yielding repeatable resistivity models for the embankment that are consistent with its internal structure. Time lapse surveys were successful in observing localized resistivity changes in the embankment, interpreted as seasonal temperature change. Also observed were small resistivity change anomalies within and just outside the core of the embankment, which are inferred to be consequences of changes in saturation and water temperature as well as road salt presence in the winter. Localized seasonal resistivity anomalies have been identified that are suggestive of two paths for elevated seepage: i) water passing through the concrete abutment and entering the rockfill on the downstream of the core, and ii) a possibility of elevated seepage through the core adjacent to the abutment. Further seasonal monitoring as well as incorporating data form electrodes installed across the dam crest will be needed to confidently assess whether seepage through the core is anomalous next to the abutment.
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