Submarine Groundwater Discharge From Sediments and Sand Boils Quantified by the Mean Residence Time of a Tracer Injection

Michael Schlüter,Philipp Maier

doi:10.3389/feart.2021.710000

Abstract

To quantify submarine groundwater discharge, we developed an inexpensive automated seepage meter that applies a tracer injection and the computation of the mean residence time. The SGD-MRT is designed to measure a wide range of discharge rates from about 30 to 800 cm³/min and allows minimizing backpressures caused by pipe friction or flow sensors. By modifying the inner volume of the flow-through unit, the range of measurement is adjustable to lower or higher discharge rates. For process control and data acquisition, an Arduino controller board is used. In addition, components like temperature, conductivity, and pressure sensors or pumps extend the scope of the seepage meter. During field tests in the Wadden Sea, covering tidal cycles, discharge rates of more than 700 cm³/min were released from sand boils. Based on the measured discharge rates and numerical integration of the time series data, a water volume of about 400 dm3 with a seawater content of less than 12% was released from the sand boil within 7 h.

Highlights

The release of fresh or brackish waters from sediments, karst channels, or pockmarks into bottom waters has been reported for numerous coastal regions, river beds, or lakes (Zektzer et al, 1973; Lee, 1977; Bokuniewicz, 1980; Cherkauer and McBride, 1988; Moore, 1996; Burnett et al, 2003; Rosenberry, 2008; Judd and Hovland 2009; Povinec et al, 2012)
The Submarine groundwater discharge (SGD)-mean residence time (MRT) seepage meter is of modular design and applies components like conductivity probes, pressure or temperature sensors, or inexpensive positive displacement pumps applied for model making
The components of the SGD-MRT are mounted inside a rack built from standard strut profiles, which allows a flexible and extensible design of the seepage meter

Summary

Introduction

The release of fresh or brackish waters from sediments, karst channels, or pockmarks into bottom waters has been reported for numerous coastal regions, river beds, or lakes (Zektzer et al, 1973; Lee, 1977; Bokuniewicz, 1980; Cherkauer and McBride, 1988; Moore, 1996; Burnett et al, 2003; Rosenberry, 2008; Judd and Hovland 2009; Povinec et al, 2012). The relevance of SGD for the transport and release of nutrients, trace elements or gases such as methane or radon from sediments into the groundwater of coastal regions or lakes has already been emphasized by Johannes (1980), Zimmermann et al (1985), Shaw and Prepas (1990), Simmons (1992) and Bugna et al (1996). Overviews of different types of flow meters for quantifying fluid discharge from sediments are provided by Taniguchi et al (2019), Rosenberry et al (2020), and Duque et al (2020)

Methods

Results

Discussion

Conclusion