Coupling End‐Member Mixing Analysis and Isotope Mass Balancing (222‐Rn) for Differentiation of Fresh and Recirculated Submarine Groundwater Discharge Into Knysna Estuary, South Africa

Abstract

Quantification of submarine groundwater discharge (SGD) is essential for evaluating the vulnerability of coastal water bodies to groundwater pollution and for understanding water body material cycles response due to potential discharge of nutrients, organic compounds, or heavy metals. Here we present an environmental tracer‐based methodology for quantifying SGD into Knysna Estuary, South Africa. Both components of SGD, (1) fresh, terrestrial (FSGD) and (2) saline, recirculated (RSGD), were differentiated. We conducted an end‐member mixing analysis for radon (222Rn) and salinity time series of estuary water over two tidal cycles to determine fractions of seawater, riverwater, FSGD, and RSGD. The mixing analysis was treated as a constrained optimization problem for finding the end‐member mixing ratio that is producing the best fit to observations at every time step. Results revealed highest FSGD and RSGD fractions in the estuary during peak low tide. Over a 24 h time series, the portions of FSGD and RSGD in the estuary water were 0.2% and 0.8% near the estuary mouth and the FSGD/RSGD ratio was 1:3.3. We determined a median FSGD of 41,000 m³ d−1 (1.4 m³ d−1 per m shoreline) and a median RSGD of 135,000 m³ d−1 (4.5 m³ d−1 per m shoreline) which suggests that SGD exceeds river discharge by a factor of 1.0–2.1. By comparison to other sources, this implies that SGD is responsible for 28–73% of total DIN fluxes into Knysna Estuary.

Scrutinizing Relationships between Submarine Groundwater Discharge and Upstream Areas Using Thermal Remote Sensing: A Case Study in the Northern Persian Gulf

“…SGD is an important pathway from the terrestrial to the marine environment that plays a significant role in hydrological and ecological processes such as: nutrient cycling, geochemical mass balances, and primary productivity in the coastal waters [1,2]. The importance of SGD as a source of nutrients, carbon and trace metals to coastal waters in water resources management and marine ecology has become increasingly recognized [3][4][5][6]. SGD has important impacts on variables such as water quality and phytoplankton dynamics which in turn relate to issue such as algal blooms and eutrophication [7].…”

Investigating River Water/Groundwater Interaction along a Rivulet Section by 222Rn Mass Balancing

“…A sensitive parameter that is in general (and independently from the river size) less unequivocal to define is the local radon groundwater endmember [41][42][43]. Site-specific values can be spatially variable due to small-scale changes in the local aquifer geology.…”

Radon (222Rn) as tracer for submarine groundwater discharge investigation—limitations of the approach at shallow wind-exposed coastal settings

“…The naturally occurring radionuclide radon ( 222 Rn, t ½ = 3.8 days) is widely used as tracer for SGD investigations (e.g. Burnett et al, 2006 ; IAEA-TECDOC-1595, 2008 ; Burnett et al, 2008 ; Stieglitz et al, 2010 ; Schubert et al, 2014 ; Petermann et al, 2018 ). Radon is continuously produced in any mineral matrix (thus, in any aquifer) by the decay of radium ( 226 Ra).…”