360 Degree Point Velocity Probe: Measuring Groundwater Flow Direction and Magnitude

Groundwater flow directions and magnitudes are typically estimated using 1-D Darcy’s Law calculations and are subject to large uncertainties.  The point velocity probe (PVP) is a device that can overcome some of these uncertainties by measuring velocity direction and magnitude. However, initial designs were limited by the presence of blind spots on the probes that required users to have some knowledge of flow direction prior to installation.  The 360° PVP design utilizes three tracer injection ports, offset by 120°, which enables flow measurements to be made without any prior knowledge of flow direction.  Results from laboratory tests conducted in a saturated, sand-packed tank indicate each injection port has a detectable range limit between 10 and 90° to the flow direction (α angle), measureable to an accuracy of 15° assuming the true flow direction in each test was perfectly known.  The actual flow direction at the face of the probe is subject to uncertainty due to heterogeneity in the sand packing.  Theoretical stagnation points occur where α = 0° and 180^o.  Measurements too close to these points suffer biases in flow direction up to +/- 10°.  When flow is directly into an injection port, the tracer bead is split around the probe. A qualitative assesment of flow direction can be achieved by taking an average of the two resulting directions.  The apparent velocites measured are likely positively biased when flow is into the injection port and negatively biased when flow is leaving the probe at a detector.  Measurement precision of flow magnitude is ≤ +/- 8.5% for all tests conducted where tank flow conditions did not change from one test to the next.  All of the flow velocities fall within the expected range determined using the 1-D Darcy’s Law calculation, assuming the porosity of the sand is within 0.25 and 0.45.