Use of an oxygen-regulated ultrasonic seepage meter for direct measurement of the physics and chemistry of submarine groundwater discharge

Abstract

An oxygen-and-light-regulated seepage meter (RSM) was developed to directly measure fluxes of redox-sensitive elements and compounds across the sediment-water interface in estuaries and other water bodies. Submarine groundwater discharge (SGD) is measured manually by the bag-method, or in high temporal (1 s) resolution by an ultrasonic flow sensor. Results from field tests in Guinea Creek, Rehoboth Bay, DE indicate the RSM can effectively maintain ambient estuarine redox and light conditions, and accurately measure chemical fluxes across the sediment-water interface. By calculating an endmember-based advective chemical flux from the aquifer, the biogeochemical flux of an element or compound to or from the estuarine sediments due to biogeochemical processing can be calculated. During three seepage meter deployments in June and August, simultaneous recharge of brackish surface water and discharge of fresh or nearly fresh groundwater was measured during higher stages of the tide against upward groundwater advection. Mechanisms including (sub)tidal pumping, convection, and bioirrigation could be possible explanations for the observed exchange.