Laboratory validation of a point velocity probe for measuring horizontal flow from any direction.

The point-velocity probe (PVP) quantifies groundwater speed and flow direction, i.e., velocity, at the centimeter scale. The first probe designs required that the flow direction be known a priori, within about 100° in order to position the probe during installation. This study introduces and assesses a '360° PVP' that measures flow from any direction without foreknowledge of the groundwater velocity. In tests conducted in a Nested Storage Tank (NeST) aquifer simulator packed with sand, PVP-measured velocities matched expected velocities within ±9° in direction and ±15% in magnitude, on average, consistent with previously reported PVP performances in laboratory studies. In tests involving 17 repacked NeSTs, the measured and expected velocities were within ±30° and ±30% on average, illustrating the sensitivity of flow to porous medium packing, and the probes' ability to sense these changes; the porosity was found to vary considerably between packings i.e., n=0.34±0.2. For flow directions between 0° and 80° of an injection port, the experimental error on velocity magnitude was within the ranges reported above. At higher flow angles, experimental sources of error contributed to greater uncertainties. Fortunately, in these cases there were always alternative injection ports (with lower angles to flow) that could be used to circumvent any biases. At low experimental flow angles (<10°) the calculated values tended to overestimate the actual flow angles. Fortunately, these cases were identifiable by the detection of tracer at detectors on either side of the active injection port. In several tests designed with an expected flow direction of 0°, averaging the calculated directions from each side of the injection port resulted in improved matches to the expected flow direction.