New perspectives on the use of 224Ra/228Ra and 222Rn/226Ra activity ratios in groundwater studies

Extreme precipitation events induce high fluxes of groundwater and associated nutrients to coastal ocean

“…The characteristics of coastal alluvial aquifers linked to the presence of ephemeral streams in the Maresme County may favor the concurrent occurrence of different water-flow processes. Indeed, previous works at the study site of Argentona have already shown that different SGD components coexist (Diego-Feliu et al, 2021;Martínez-Pérez et al, 2022;Palacios et al, 2019). Meteoric groundwater flowing seawards and recirculated seawater mix in multiple aquifer levels which are separated by semi-confined thin layers of silt and clays.…”

“…Groundwater samples for Ra isotopes (10-25 L) and nutrients were collected periodically from 2015 to 2020 in several piezometers at the experimental site of the Medistraes project, located in the coastal alluvial aquifer of the Argentona ephemeral stream (corresponding to T1). The experimental site consists of 16 piezometers located at 30-100 m from the coastline with 2 m screened intervals at depths ranging from 15 to 25 m (see Diego-Feliu et al, 2021;Palacios et al, 2019, for more details about the ex-perimental site; Fig. 1).…”

“…3). Although many processes may explain the activities found in coastal waters, the observed trends for 224 Ra/ 228 Ra AR and 228 Ra activities can indicate that the relative contribution of the brackish component of SGD, which is characterized by 224 Ra/ 228 Ra ARs close to the equilibrium value (1.0 to 2.2; Diego-Feliu et al, 2021), increased during the occurrence of the EPE. This is also coherent with the increase in groundwater level (Fig.…”

“…The different spatiotemporal scales of both SGD components are especially important when using Ra isotopes as tracers of SGD. In fact, these isotopes are instrumental for differentiating SGD pathways, since their enrichment rates strongly depend on the transit time of groundwater through the coastal aquifer (e.g., Diego-Feliu et al, 2021;Michael et al, 2011). Coastal seawater samples collected during the three samplings performed in Maresme County were enriched in both 224 Ra and 228 Ra relative to offshore waters (Fig.…”

“…The activity ratio (AR) of 224 Ra/ 228 Ra can similarly be used to evaluate the temporal scale of SGD pathways (Diego-Feliu et al, 2021). Thus, the 224 Ra/ 228 Ra AR found in coastal seawater samples after the EPE of October 2019 decreased from a baseline value of 6 to approximately 4.…”