3 Alternative methods for measuring groundwater velocity

Darcy’s Law provides a reliable and convenient basis for determining groundwater flow, specific discharge (or Darcy flux) and seepage velocities (hereafter referred to as simply ‘velocity’ for brevity). However, in response to the complexities discussed above, there are good reasons to develop alternative methods to measure flow, velocity, and flux. To determine groundwater velocities without reference to Darcy’s Law is the goal of the so-called direct velocity measurement methods. In fact, there are no methods currently available that are capable of actually measuring groundwater flow over the entire range of flow rates that might apply. The most versatile and accommodating approaches use tracer detection to infer groundwater movement rates. Tracers are substances that dissolve readily in groundwater and that are transported without interferences from chemical or biological processes that might transform them, or temporarily remove them from the water stream (i.e., sorption). Examples include such chemicals as chloride (Cl), bromide (Br), tritium (3H), fluorescent dyes, freon compounds (chlorofluorocarbons), and sometimes heated water (e.g. Davis et al., 1980). The techniques vary primarily in 3 specifics: 1) the choice of tracer; 2) the detection method; and 3) the means of access to groundwater i.e., through a well or via direct contact with the aquifer material. Before using a groundwater tracer, it is important to consider the potential impact on groundwater quality and to obtain permission from groundwater oversight agencies, if applicable. The following sections explain the techniques available for various scales of measurement, including inter-well techniques, in-well techniques, and techniques that require direct contact with the aquifer.


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