Theory and testing of a device for measuring point-scale groundwater velocities

A novel method of measuring groundwater velocity was introduced by Di Biasi (1999). The method consisted of a mini-tracer test conducted along the surface of a cylindrical probe. In principle, the method could enable the measurement of in situ groundwater velocity at a very small spatial scale in porous media, without the need for instrument calibration. Despite encouraging results from preliminary laboratory testing, the technique was unproven for field applications, and lacked a strong theoretical basis. The purpose of this project was to address these deficiencies by advancing the theory underlying the method and by exploring the limitations and operational procedures through a more comprehensive series of laboratory and field tests. A further requirement of the study was to modify the original design of the probe such that it was sufficiently robust for installation in the field.; The tip of the probe consists of a stainless steel cylinder, 15 cm in length and 3 cm O.D. Measurements are made by running a small scale tracer test between an injection port and two detectors, all of which are located along the circumference of the probe.; Direction and magnitude of the velocity vector at the point of measurement were determined based on the theory of ideal flow around a circular cylinder. The average linear groundwater velocity outside the influence of the probe was determined based on the measured apparent velocities at the two detectors.; Numerical modeling showed the scale of measurements to be within a 0.5 cm distance from the probe. Interpretation of the breakthrough curves using a 1-D parameter estimation code (PULSEPE) was successful in determining velocities between 1 and 320 cm/d. The duration of such tests was found to vary between 4 days and 10 minutes for the respective velocities. The accuracy of the results was independent of the orientation of the injection port and the detectors to the direction of flow. Zones of reduced hydraulic conductivity around the probe were found to cause a lower velocity zone, while K-enhanced zones formed higher velocity zones which could bias the measurements. This observation has considerable relevance to the method used to install the probe. To minimize this problem in field applications, installation of the probe by jetting was found to be effective.; Laboratory experiments confirmed the applicability of the continuum approach to flow around the probe. Consequently, the method can accurately measure point groundwater velocities based on the theory of ideal flow around a circular cylinder. Results of laboratory tests also showed that measurements of velocity within the capillary fringe can be made with similar accuracy to measurements below the water table.; Field experiments under controlled flow conditions were carried out at CFB Borden. The method was capable of measuring velocity values on a vertical scale of only a few centimetres. The measured variations in velocity were consistent with measured variations in hydraulic conductivity at the site. The experiments also showed the method to be the most accurate among three other direct methods of measuring groundwater velocity including the Geoflo RTM meter, point dilution and the colloidal borescope.