The Effect of Point Velocity Probe Size on Groundwater Velocity Estimation in Noncohesive Sediments

Abstract

The point velocity probe (PVP) was developed to make in situ measurements of groundwater velocity in unconsolidated sediments and has been tested both in the laboratory and in the field. The purpose of this study was to evaluate the PVP under controlled laboratory conditions to determine if varying the PVP diameter size and the relative sediment size -fine-grained sand, medium-grained sand and gravel- affected the accuracy of the measurements of direction and magnitude. Four PVP sizes (11-cm, 6-cm, 4-cm, and 2-cm) were designed with the same relative angles between the injection port and detector sets (45° and 75°). Results for tests in medium-grained and fine-grained sands demonstrate that PVP diameter size was found to affect both the measurements for velocity and direction. As the diameter of the PVP was decreased, the accuracy of groundwater direction (&#945;) decreased, but the accuracy of the speed measurement was minimally affected. In gravel, reasonable estimates of groundwater speed were obtained, but flow direction (&#945;) was more problematic and could not be reliably determined with accuracies < 50° of the expected value. Measurements made with the smallest PVP were inferior to those obtained with a larger probe. The problem with measurements made with the PVP in gravel is likely due to the relative size of the probe injection ports and the tracer path to the detectors where pore size affect the measurement and the tracer does not behave in the expected manner as it does in sand. It is concluded that PVPs can be useful in measuring groundwater velocity in gravel, if appropriately large probe sizes are employed.