Solute transport measurement by ion-selective electrodes in fractured tuff

Coated-wire ion-selective electrodes were used to monitor the breakthrough of chloride in a tuff test block with a natural fracture. Minute samples of tracer solution were collected with filter paper. Preliminary results indicate the technique worked well for studying transport behavior in fractured rock at near-saturated flow conditions. It also appears to be a promising technique at unsaturated conditions. Breakthrough curves in the fracture and matrix, and a spatial concentration distribution map of the fracture, suggest the existence of preferential flow paths in the fracture segment, and substantial diffusion into the matrix. Estimates of average travel velocity, dispersion coefficient and longitudinal dispersivity in the fracture, are obtained using temporal moments analysis. Estimates of transport parameters in the matrix are hampered by insufficient data and the complex solute concentration history. A case study of the test block using a boundary integral method computer model helped to interpret the data.hydrology collectio