Devlin J F
Dept. of Geology, University of Kansas, Lindley Hall rm 120, 1475 Jayhawk Blvd., Lawrence, KS 66049, USA.
J Contam Hydrol. 2016 Sep;192:140-145. doi: 10.1016/j.jconhyd.2016.07.004. Epub 2016 Jul 15.
Point velocity probes (PVPs) are dedicated, relatively low-cost instruments for measuring groundwater speed and direction in non-cohesive, unconsolidated porous media aquifers. They have been used to evaluate groundwater velocity in groundwater treatment zones, glacial outwash aquifers, and within streambanks to assist with the assessment of groundwater-surfaced water exchanges. Empirical evidence of acceptable levels of uncertainty for these applications has come from both laboratory and field trials. This work extends previous assessments of the method by examining the inherent uncertainties arising from the equations used to interpret PVP datasets. PVPs operate by sensing tracer movement on the probe surface, producing apparent velocities from two detectors. Sensitivity equations were developed for the estimation of groundwater speed, v∞, and flow direction, α, as a function of the apparent velocities of water on the probe surface and the α angle itself. The resulting estimations of measurement uncertainty, which are inherent limitations of the method, apply to idealized, homogeneous porous media, which on the local scale of a PVP measurement may be approached. This work does not address experimental sources of error that may arise from the presence of cohesive sediments that prevent collapse around the probe, the effects of centimeter-scale aquifer heterogeneities, or other complications related to borehole integrity or operator error, which could greatly exceed the inherent sources of error. However, the findings reported here have been shown to be in agreement with the previous empirical work. On this basis, properly installed and functioning PVPs should be expected to produce estimates of groundwater speed with uncertainties less than ±15%, with the most accurate values of groundwater speed expected when horizontal flow is incident on the probe surface at about 50° from the active injection port. Directions can be measured with uncertainties less than 15° with the most accurate measurements occurring when the flow angles are relatively low - on the order of 20°. At still lower flow angles, quantitation may suffer due to experimental limitations related to tracer delivery. However, useful qualitative assessments of α may still be possible under these conditions.
点流速探头（PVP）是用于测量非粘性、未固结多孔介质含水层中地下水速度和方向的专用、成本相对较低的仪器。它们已被用于评估地下水处理区、冰川冰水沉积含水层以及河岸内的地下水速度，以协助评估地下水与地表水的交换。这些应用中可接受的不确定性水平的经验证据来自实验室和现场试验。这项工作通过研究用于解释PVP数据集的方程所产生的固有不确定性，扩展了对该方法的先前评估。PVP通过感测探头表面示踪剂的移动来操作，从两个探测器产生表观速度。开发了灵敏度方程，用于根据探头表面水的表观速度和α角本身来估计地下水速度v∞和流向α。测量不确定性的最终估计值是该方法的固有局限性，适用于理想化的均质多孔介质，在PVP测量的局部尺度上可能接近这种介质。这项工作没有涉及可能因粘性沉积物的存在而导致的实验误差源，粘性沉积物会阻止探头周围的坍塌、厘米级含水层非均质性的影响，或与钻孔完整性或操作员误差相关的其他复杂情况，这些误差可能大大超过固有误差源。然而，这里报告的结果已被证明与先前的经验工作一致。在此基础上，正确安装并正常运行的PVP应该能够产生地下水速度估计值，其不确定性小于±15%，当水平流以约50°的角度从活性注入端口入射到探头表面时，预计能得到最准确的地下水速度值。流向的测量不确定性小于15°，当流动角度相对较低（约20°）时，测量最准确。在更低的流动角度下，由于与示踪剂输送相关的实验限制，定量可能会受到影响。然而，在这些条件下，仍然可以对α进行有用的定性评估。
