Improved Approach for the Investigation of Submarine Groundwater Discharge by Means of Radon Mapping and Radon Mass Balancing

Abstract

The paper presents an improved approach for investigating submarine groundwater discharge (SGD) based on radon mapping and radon mass balancing in the coastal sea. While the use of radon as an environmental tracer in SGD studies is well-established, we identified based on our longstanding experience six methodical shortcomings of the conventional approach and suggest corresponding developments. The shortcomings include: (1 and 2) inadequate consideration of both detection equipment response delay and influence of tidal stage; (3 and 4) incorrect quantification of radon losses, due to offshore mixing and degassing resulting in a potentially incorrect radon mass balance; (5) inaccurate determination of the terrestrial groundwater endmember, due to inhomogeneous radon distribution in the coastal aquifer; and (6) difficulties in distinguishing between discharged fresh groundwater and recirculated seawater. The improved approach is practically demonstrated in a step by step manner in a large-scale field study, which was carried out in False Bay (South Africa) and which consisted of two parts, namely (i) qualitative SGD localization along the entire False Bay coastline based on coastal radon distribution patterns and (ii) quantitative SGD investigation within a defined coastal area of interest (AOI) based on a radon mass balance (RMB). The plausibility of the AOI related results was evaluated by a hydrogeological model, used for qualitative SGD localization, and a hydrological model, applied for estimating groundwater recharge within the AOI catchment.

Submarine Groundwater Discharge: Updates on Its Measurement Techniques, Geophysical Drivers, Magnitudes, and Effects

“…For example, beyond the embayment scale, ocean-basin scale SGD estimates were recently performed using inventories of the relatively long-lived 228 Ra (Kwon et al, 2014). Uncertainties are often high for both radon and radium mass balance estimates, largely but not exclusively, because of the difficulty in constraining a value for the groundwater "endmember" (the concentration of the tracer in the discharging groundwater; Burnett et al, 2007;Schubert et al, 2019).…”

“…The latter issue was also tackled by Petermann and Schubert (2015), who introduced a methodology to correct radon data collected using an air-water exchanger (RAD-Aqua, Durridge) connected to a RAD-7 radon detector for its response delay when moving between high and low radon activity water masses. Schubert et al (2019) recently suggested additional improvements in the application of radon mass balance approaches that should improve corrections for radon losses from mixing and atmospheric evasion. Since the "early days" of SGD studies, there has been a natural progression in improving radon measurement methodologies from labor-intensive grab samples collected in the field and analyzed in the lab or on ships (Mathieu et al, 1988), to the use of automated systems such as the RAD-AQUA (Burnett et al, 2001), to completely automated detection systems that can work unattended for months to years.…”

Investigating Groundwater Discharge into a Major River under Low Flow Conditions Based on a Radon Mass Balance Supported by Tritium Data

“…Applicable tracers include the stable isotopes of water (e.g., [7][8][9][10][11][12][13]) as well as naturally occurring radioisotopes. Examples for the latter are 35-sulphur, 3-hydrogen (tritium), 222-radon, and the radium 224/223 ratio (e.g., [14][15][16][17][18][19]). Major ions and trace elements are also frequently applied as tracers (e.g., [20][21][22]).…”

Conceptual uncertainties in groundwater and porewater fluxes estimated by radon and radium mass balances

“…Representative average wind speeds (scaled to 10 m above the surface) are estimated in four different ways and then subsequently used to derive gas transfer velocities from the wind-k equation of MacIntyre et al (1995) (2) average wind speed during the 24 h before the end of the sampling (u = 2.0 m s −1 ); (3) average wind speed during the 48 h before the end of the sampling (u = 3.5 m s −1 ); (4) weighting the influence of degassing on radon concentration depending on their proximity to the sampling time (i.e., events closer to the sampling campaign are more influential). A weighting factor for the importance of degassing events depending on its proximity to the sampling campaign (w t ) is adapted from (Schubert et al 2019) based on the radon decay constant (λ) and the gas transfer velocity (k) as primary controls on radon inventories for La Palme Lagoon:…”