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

Abstract

Radium isotopes and radon are routinely used as tracers to quantify groundwater and porewater fluxes into coastal and freshwater systems. However, uncertainties associated with the determination of the tracer flux are often poorly addressed and often neglect all the potential errors associated with the conceptualization of the system (i.e., conceptual uncertainties). In this study, we assess the magnitude of some of the key uncertainties related to the determination of the radium and radon inputs supplied by groundwater and porewater fluxes into a waterbody (La Palme Lagoon, France). This uncertainty assessment is addressed through a single model ensemble approach, where a tracer mass balance is run multiple times with variable sets of assumptions and approaches for the key parameters determined through a sensitivity test. In particular, conceptual uncertainties linked to tracer concentration, diffusive fluxes, radon evasion to the atmosphere, and change of tracer inventory over time were considered. The magnitude of porewater fluxes is further constrained using a comparison of independent methods: (1) 224Ra and (2) 222Rn mass balances in overlying waters, (3) a model of 222Rn deficit in sediments, and (4) a fluid‐salt numerical transport model. We demonstrate that conceptual uncertainties are commonly a major source of uncertainty on the estimation of groundwater or porewater fluxes and they need to be taken into account when using tracer mass balances. In the absence of a general framework for assessing these uncertainties, this study provides a practical approach to evaluate key uncertainties associated to radon and radium mass balances.

The Role of Methane Transport From the Active Layer in Sustaining Methane Emissions and Food Chains in Subarctic Ponds

“…To obtain a representative k 600 value, we used the hourly data on wind speed during the 24‐h period prior sampling. This time period corresponds to the maximum residence time of Rn within a pond, that can be approximated as if the role of evaporation is assumed to be negligible (Rodellas et al., 2018, 2020), where λ is the radioactive constant of Rn (0.181 days − 1); k is the gas transfer velocity (m day −1 ) and h is the pond mean depth (m). The estimated residence time of Rn was within the range of the residence time of CH 4 (1.4–2.8 days) for ponds from the same area computed by dividing the amount of CH 4 stored by the pond mean CH 4 surface flux from flux chambers (Kuhn et al., 2018).…”

“…1 / / k h if the role of evaporation is assumed to be negligible (Rodellas et al, 2018(Rodellas et al, , 2020, where λ is the radioactive constant of Rn (0.181 days − 1); k is the gas transfer velocity (m day −1 ) and h is the pond mean depth (m). The estimated residence time of Rn was within the range of the residence time of CH 4 (1.4-2.8 days) for ponds from the same area computed by dividing the amount of CH 4 stored by the pond mean CH 4 surface flux from flux chambers (Kuhn et al, 2018).…”

A State-Of-The-Art Perspective on the Characterization of Subterranean Estuaries at the Regional Scale

“…the natural radionuclides of radon ( 222 Rn; t ½ 3.8 days; Rn in following) and radium ( 224 Ra; t 1/2 3.7 days; 223 Ra; t 1/2 11.4 days; 228 Ra; t 1/2 5.75 years; 226 Ra; t 1/2 1,600 years, Ra in the following), which are generally enriched in STE groundwater compared to surface waters. They can provide qualitative and quantitative information on sources and types of water sources to the STE, estimates of seawater residence times in the STE, as well as an overall quantification of the water flux out of the STE (i.e., SGD) into the coastal sea and open ocean (Taniguchi et al, 2019), while the results often include large uncertainties (Rodellas et al, 2021).…”

The role of methane transport from the active layer in sustaining methane emissions and food chains in subarctic ponds

“…the role of evaporation is assumed to be negligible (Rodellas et al, 2018(Rodellas et al, , 2020, where λ is the radioactive constant of Rn (0.181 days − 1); k is the gas transfer velocity (m day −1 ) and h is the pond mean depth (m). The estimated residence time of Rn was within the range of the residence time of CH 4 (1.4-2.8 days) for ponds from the same area computed by dividing the amount of CH 4 stored by the pond mean CH 4 surface flux from flux chambers (Kuhn et al, 2018).…”