干旱区生态环境知识资源中心

A new methodology was developed for the in situ investigation of flow and transport processes through undisturbed, discrete fractures in the vadose zone. The experimental setup and technical means were designed for a scale of several meters, larger than any conceivable laboratory experiment. The setup consists of four components: (1) a 25 cm diameter horizontal borehole is core-drilled along a vertical, discrete fracture plane, exposing the fracture at the borehole ceiling; (2) compartmental ponds are installed along the traced fracture on the horizontal part of the outcrop, Each compartment (25 cm long) is connected to a separate feeding source, which can be tagged by a different tracer, and the water head is electronically controlled. Thus, each segment of the exposed fracture could be subjected to a different flux with a specific tracer under a constant hydraulic head; (3) a compartmental sampler, divided into 20 cm long cells, is installed beneath the compartmental ponds in the horizontal borehole, The effluent draining from a particular fracture segment is collected by individual cells of the sampler; (4) the percolating fluids accumulating in the various sampler cells are frequently drained by a collection system into individual sampling vessels.

This experimental setup was field-tested with untagged water in preliminary experiments in fractured chalk. It can provide unique and important information about flowpath distributions along a natural fracture, and about the chemical evolution of the solutions percolating through the fractures.