Measurement of unsaturated rock water potentials in situ

Abstract

Characterizing water movement in unsaturated fractured rock is important for evaluating the isolation properties of waste repositories. By determining water potential gradients in situ, the direction and magnitude of groundwater flow can be established. This study evaluates four methods, originally designed for soil water potential measurements, for their ability to measure ranges of rock water potentials in situ. The downhole tensiometer, measuring potentials from 0 to -0.85 bars, worked well in laboratory simulations, but was not tested in the field. The osmotic tensiometer, with a measurement range of 0 to -3 bars, was successful in the laboratory but field experiments were inconclusive. Thermocouple psychrometers were not able to give precise water potential measurements, however, temporal trends revealed a range of water potentials from -2 to -50 bars. The absorber method, involving the equilibration of rock water potentials from -2 to -100 bars with filter papers, was also not able to give precise measurements. The four methods need further modifications to improve the accuracy of measurement of rock water potentials, especially for determining the hydraulic head gradient for flow calculations.