Présentations Colloques

Oral Presentation
Novakowski Kent
Effects of test scale on hydraulic measurements in a crystalline rock setting
Concern about the scale of representation for the results of hydraulic tests conducted in crystalline rock aquifers has been a long-standing issue in the hydrogeological characterization of these settings. In this study we evaluate the effects of measurement scale on bulk values of transmissivity, storativity, vertical hydraulic conductivity, vertical specific storage and specific yield in a sparsely-fractured granite gneiss having a very thin overburden cover (ie. the water table resides in the bedrock). Scale effects are identified through the comparison of results from constant head tests conducted contiguously with depth, open-well slug tests, open-well pulse interference tests and long-term, open-well pumping tests. Scale artefacts relating to measurement or analysis methods are reduced by testing the same well array at each measurement scale. The wells were constructed using air-percussion methods to a depth of approximately 30 m and are arranged in a triangular format with separation of approximately 10 m between each pair. The slug and pulse interference test results were interpreted using new analytical models developed to accommodate horizontal fracturing in this type of setting. The long-term pumping tests were interpreted using an analytical model based on porous-media equivalency. All transient tests were interpreted using formal parameter estimation methods. The results show that the geometric mean values for the bulk horizontal properties (transmissivity, and storativity) vary over less than an order of magnitude from local-scale tests to long-term pumping tests. Although interpretation of the pumping tests for vertical properties and specific yield proved inconclusive, averaging the results of open-borehole pulse interference tests provided a good estimate of the value of specific yield that was obtained from the results of the constant head tests. Based on our observations, we conclude that scale effects are relatively minimal in this setting, and that pulse interference tests may be a less time-intensive and more reliable alternative to pumping tests in the determination of vertical hydraulic properties and specific yield.