DSi as a Tracer for Submarine Groundwater Discharge

Abstract

Submarine groundwater discharge (SGD) is an important source of nutrients and metals to the coastal ocean, affects coastal ecosystems, and is gaining recognition as a relevant water resource. SGD is usually quantified using geochemical tracers such as radon or radium. However, a few studies have also used dissolved silicon (DSi) as a tracer for SGD, as DSi is usually enriched in groundwater when compared to surface waters. In this study, we discuss the potential of DSi as a tracer in SGD studies based on a literature review and two case studies from contrasting environments. In the first case study, DSi is used to calculate SGD fluxes in a tropical volcanic-carbonate karstic region (southern Java, Indonesia), where SGD is dominated by terrestrial groundwater discharge. The second case study discusses DSi as a tracer for marine SGD (i.e., recirculated seawater) in the tidal flat area of Spiekeroog (southern North Sea), where SGD is dominantly driven by tidal pumping through beach sands. Our results indicate that DSi is a useful tracer for SGD in various lithologies (e.g., karstic, volcanic, complex) to quantify terrestrial and marine SGD fluxes. DSi can also be used to trace groundwater transport processes in the sediment and the coastal aquifer. Care has to be taken that all sources and sinks of DSi are known and can be quantified or neglected. One major limitation is that DSi is used by siliceous phytoplankton and therefore limits its applicability to times of the year when primary production of siliceous phytoplankton is low. In general, DSi is a powerful tracer for SGD in many environments. We recommend that DSi should be used to complement other conventionally used tracers, such as radon or radium, to help account for their own shortcomings.

Estimating submarine groundwater discharge in Jeju volcanic island (Korea) during a typhoon (Kong-rey) using humic-fluorescent dissolved organic matter-Si mass balance

“…Ra isotopes and 222 Rn have served as the most powerful tools for gauging the magnitude and mechanism of SGD because Ra isotopes and 222 Rn are chemically conservative in seawater and enriched in groundwater 2 , 39 – 41 . Although it has received less attention than radioisotope tracers, DSi is also a useful tracer for determining SGD when DSi is highly enriched in fresh groundwater and it shows a conservative behavior in coastal aquifers 28 , 29 . In this study, the concentrations of DSi in fresh groundwater showed the highest values in fresh groundwater samples and exhibited good negative linear correlations with salinity.…”

“…Several studies have attempted to measure SGD using hydrological processes 26 , 27 , seepage measurements 4 , 9 , and geochemical tracers such as Ra isotopes 2 , 3 and 222 Rn 5 . In addition, dissolved silicon (DSi) can be used as a geochemical tracer for determining SGD when it shows a conservative behavior in coastal aquifers 28 , 29 .…”

Seasonal Dynamics of Microbial Diversity at a Sandy High Energy Beach Reveal a Resilient Core Community

“…Additionally, they exhibited higher concentrations of silicate, which is enriched in the island's freshwater lens. As silicate serves as a proxy for fresh water influence (Oehler et al, 2019), it is indicative for the exfiltration of brackish porewater at the beach. The site specific orientation within the PCA is best defined for the October campaign.…”

Complex Eyed Pockmarks and Submarine Groundwater Discharge Revealed by Acoustic Data and Sediment Cores in Eckernförde Bay, SW Baltic Sea

“…Numerous methods have been used to detect and quantify SGD (Burnett et al, 2006) including optical systems (Karpen et al, 2004); geochemical water column investigations of tracers like the natural radionuclides radium and radon (Burnett & Dulaiova, 2003; Moore, 1996; Moore et al, 2008; Scholten et al, 2013), dissolved silicon (Oehler et al, 2019), methane, and chloride (Dulaiova et al, 2010; Schlüter et al, 2004); remote sensing (Shaban et al, 2005; Tamborski et al, 2015; Wilson & Rocha, 2012); and direct investigations using cores and different types of seepage meters (Bugna et al, 1996; Burnett & Dulaiova, 2003; Cable et al, 1997; Sauter et al, 2001). Only a few geophysical methods, like geoelectric (Stieglitz, 2005; Viso et al, 2010), controlled source electromagnetic (Gustafson et al, 2019; Müller et al, 2011), or autonomous underwater vehicle investigations (Sauter et al, 2003) have been used to explore SGD.…”