EXPLORING WAVE-INDUCED ARTIFACTS AND ASYMMETRY IN SEEPAGE METER MEASUREMENTS

Submarine groundwater discharge (SGD) is an important transport pathway for terrestrial solutes to coastal waters. Many methods exist to infer SGD rates, but only seepage meters directly measure flux and its small-scale spatial distribution through the seabed. The reliability of seepage meter measurements has been questioned because 1) short-timescale forces such as surface waves may interact with seepage meters to induce SGD, and 2) resistance through the meter may result in reduced discharge measurements. To address these questions we conducted an experimental investigation of seepage meter measurement artifacts.

Experiments in a 925-gallon tank with known flux across the sediment-water interface show that flux measured with seepage meters is less than the flux through the tank. This factor difference is constant across a range of both positive and negative head gradients. Thus, actual SGD may be calculated by multiplying seepage-meter-measured flux by the factor difference specific to the seepage meter design.

Seepage meters were also deployed in the sandy bottom of a wave flume to measure flux induced by wave action. In addition to net flux, unidirectional flux into and out of the seabed was measured using 1-way valves attached to collection bags. Arrays of pressure sensors were deployed at and below the bed in locations both inside and outside of a seepage meter. Pressure measurements indicated the extent to which seepage meters disturbed the pressure field and allowed Darcy calculation of wave-induced SGD in the presence or absence of seepage meters. Seepage meter measurements agreed well with both Darcy calculations and analytically-modeled SGD based on surface wave characteristics. Over a wave period, induced discharge and recharge cancelled and net measured SGD was minimal. Though net SGD was near zero, unidirectional fluxes were considerable. These fluxes can be important drivers of benthic mixing, reaction, and transport between groundwater and surface water.