Dynamic Seepage Meter: Theory with Application Examples

Most single-point methods of measuring seepage fluxes across the surface water-groundwater interface in lakes, streams, and estuaries (e.g., volumetric, head-based, and thermal) have one trait in common: they produce seepage rate values, averaged over substantial periods of time, thereby limiting resolution of the intra-day dynamics. Recently, Solomon et al. (Water Resources Research, 2019, in review) presented a new instrument and modification to a previously tested concept (Solder et al., Groundwater, 2016). This instrument has an open-bottom permeameter (OBP) design, which is commonly used for investigating hydraulic conductivity of the interface with falling or rising head tests, but historically not used for flux estimates. The novel dynamic seepage meter (DSM) evaluates the transient water level in the OBP-based instrument with submillimeter accuracy, exceeding the performance of traditional pressure transducers. The initial dynamics of the water level response over fractions of an hour holds the necessary information to infer the natural seepage rate in both gaining and losing conditions. The tests can be repeated frequently in an automatic regime. If a single test lasts long enough, hydraulic conductivity, in addition to the seepage rate can also be accurately determined. Here, a detailed hydrodynamic theory of the flow systems inside and outside the OBP is presented and the accuracy of measured water fluxes is investigated with emphasis on interpretation of the data with ambient noise. The results of this study will facilitate rapid, accurate, and massive data collection in diverse field conditions. (Research was supported by the NSF grant EAR 1744719.)