Abstract

Preferential flow in fractured Cretaceous chalk has been investigated using infrared thermography (IRT) in a quarry in Denmark. Fracture mapping along the vertical quarry walls shows numerous horizontal fractures and four sets of vertical fractures respectively striking 25°, 60°, 145°, and 175°. Water flows out of the exposed walls mainly through fractures and to a minor extent by seepage through the matrix. To clarify which of the five fracture systems are hydraulically active, the exposure has been investigated using IRT. By making use of the contrast between the constant temperature of groundwater and the temperature of the exposed wall on a cold winter and a hot summer day, zones of groundwater discharge have been delineated. The results of the IRT suggest that the vertical shear fractures are the main hydraulically active conduits. Furthermore, the horizontal fractures are more hydraulically active in areas where they intersect a vertical stained fracture or in association with some flint layers. Stains of Fe3+ and Mn were observed primarily in fractures with a strike of approximately 25° and 175°, suggesting that the water flows preferentially in these fracture sets. The possible influence of the regional setting on the flow pattern is considered. The results suggest that the combined use of detailed fracture analysis and IRT is a valuable method for providing the input data required for both modeling and monitoring of fractured media.

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