Evaluating subsurface flow using tracers, piezometers and geology

Two fracture sets are of utmost importance: one parallel to bedding (dipping ESE) observed throughout the borehole, and another dipping WSW, present only in the upper 113 m, likely caused by valley incision and unloading. Piezometer and tracer test data reveal that these fractures also control hydraulic flow paths. Shallow flow occurs along fractures parallel to valley slopes, which remain open due to near-surface stress. At greater depth, flow is governed by bedding-parallel faults dipping into the flanks due to the anticlinal structure. Piezometers upstream of the grout curtain show stronger connectivity on the left valley flank; this is reduced downstream, indicating the curtain's effectiveness in lowering hydraulic pressure. The Branch Fault beneath the right abutment shows a low hydraulic response and is therefore interpreted not to serve as a major flow path.