Hydrol. Earth Syst. Sci., 26, 4619–4635, 2022 https://doi.org/10.5194/hess-26-4619-2022 © Author(s) 2022. This work is distributed under the Creative Commons Attribution 4.0 License. Extreme precipitation events induce high fluxes of groundwater and associated nutrients to coastal ocean Marc Diego-Feliu1 , Valentí Rodellas1 , Aaron Alorda-Kleinglass1 , Maarten Saaltink2,3 , Albert Folch2,3 , and Jordi Garcia-Orellana1,4, 1 Institut de Ciència i Tecnologia Ambientals, UAB, Bellaterra, 08193, Spain 2 Department of Civil and Environmental Engineering, UPC, Barcelona, 08034, Spain 3 Hydrogeology Group, UPC-CSIC, Barcelona, 08034, Spain 4 Departament de Física, UAB, Bellaterra, 08193, Spain  deceased, 5 July 2022 Correspondence: Marc Diego-Feliu (marc.diego@uab.cat) and Valentí Rodellas (valenti.rodellas@uab.cat) Received: 19 November 2021 – Discussion started: 1 February 2022 Revised: 13 June 2022 – Accepted: 14 August 2022 – Published: 20 September 2022 Abstract. Current studies of submarine groundwater dis- solutes from land to ocean (Santos et al., 2021). The sig- charge (SGD) are commonly conducted under aquifer base- nificance of this process at local, regional, and global scale flow conditions, neglecting the influence of episodic events stems mainly from its role in modulating the water and chem- that can significantly increase the supply of nutrients and ical budgets of oceans, controlling coastal ecosystems, and water. This limits our understanding of the social, biogeo- contributing to the well-being of coastal societies (Alorda- chemical, and ecological impacts of SGD. In this study, we Kleinglass et al., 2021; Lecher et al., 2015; Luijendijk et al., evaluated the influence of an extreme precipitation event 2020). In the last 3 decades, there have been many studies (EPE) on the magnitude of SGD. To do so, three seawa- focusing on quantifying SGD and associated solute fluxes in ter sampling campaigns were performed at a Mediterranean multiple sites across the globe, including coves, bays, estu- ephemeral stream-dominated basin after an EPE (∼ 90 mm in aries, and entire basins (e.g., Beck et al., 2008; Kwon et al., few hours) and in baseflow conditions. Results indicate that 2014; Tamborski et al., 2020). However, most of the SGD in- the groundwater flows after the EPE were 1 order of mag- vestigations are conducted under baseflow conditions, that nitude higher than those in baseflow conditions. The SGD is, in the absence of any meteorological, hydrological, or induced by EPEs, which only take place a few days per year, oceanographical events (e.g., storms, monsoons, sea-level represented up to one third of the annual discharge of ground- anomalies) which might significantly impact the magnitude water and associated nutrients at the study site. This work of SGD. Only a few articles have focused on the evaluation accentuates the need to account for episodic increases in the of the temporal variations in SGD induced by episodic events supply of water and nutrients when aiming to provide reli- (Adyasari et al., 2021; Gonneea et al., 2013; Hu et al., 2006; able annual SGD estimates, particularly in the current con- Rodellas et al., 2020; Sugimoto et al., 2016). Extrapolating text of climate change, since the occurrence of such events is SGD estimates derived under baseflow conditions to obtain expected to increase worldwide. annual fluxes neglects the role of such events, which may represent an important contributor to overall SGD fluxes. Extreme precipitation events (EPEs) represent one of the main natural hazards producing severe societal and economic 1 Introduction costs in urban, agricultural, and mountainous areas (Booij, 2002; Camarasa-Belmonte and Soriano-García, 2012; Schu- Submarine groundwater discharge (SGD) – the flow of macher, 2017). The meteorological causes of these events groundwater from the coastal aquifers to the coastal ocean – include the formation of cyclones, fronts, monsoons, iso- is one of the primary processes regulating the transfer of Published by Copernicus Publications on behalf of the European Geosciences Union. 4620 M. Diego-Feliu et al.: Extreme precipitation events induce high fluxes of groundwater lated thunderstorms, upslope flow precipitation, and others, 2 Methods and vary from region to region (Kunkel et al., 2012). The social and environmental consequences also vary geograph- 2.1 Study site ically and depend on diverse aspects such as topography, soil characteristics, geological setting, land-surface use and Maresme County is a coastal region located to the NE of characteristics, and human-induced changes to the landscape the city of Barcelona (Spain, western Mediterranean Sea) and coastal areas (Schumacher, 2017). Due to the disparate and extends from the Catalan littoral mountain range to the causes and consequences associated with them, there are dif- Mediterranean Sea (Fig. 1). The county has a population ferent ways of defining an “extreme” precipitation event: density of ca. 1100 habitants per square kilometer, and is some authors define such events as those exceeding an ar- highly anthropized, with 15 % agricultural and 30 % urban bitrary threshold of rainfall accumulated within 24 h (e.g., land use (Rufí-Salís et al., 2019). The geomorphology of the Lionello et al., 2006; Meenu et al., 2020); others use the area is structurally associated with the fracturing and sink- 90th, 95th, or 99th all-day or wet-day percentile (Pendergrass ing of blocks (NW–SE direction), which developed in a set and Knutti, 2018). Whilst episodic increases in surface runoff of stream valleys (Catalan Water Agency, 2010). The ge- linked to EPEs are often well characterized (e.g., Camarasa- ology of these valleys is dominated by Quaternary detrital Belmonte and Tilford, 2002; Moore, 2007; Rajurkar et al., sediments that constitute layers of gravels, sands, and clays, 2004), little is known about the influence of EPEs on SGD- which result from chemical weathering and the dragging driven water flows and associated solute fluxes to the coastal of granite materials through torrential courses. These Qua- ocean. Extreme precipitation events (EPEs) may indeed pro- ternary deposits constitute different aquifers corresponding mote aquifer recharge through the infiltration of rainwater with each stream valley (Fig. 1). Simultaneously, each stream (Ramos et al., 2020; Yu et al., 2017), although its effects on valley forms an ephemeral stream. In Maresme County, there piezometric levels (quantitatively and temporally) depend on are around 40 ephemeral streams, representing a density of several factors, such as soil composition, geological charac- 1 ephemeral stream per kilometer of coastline. Annual pre- teristics, the hydraulic parameters of the aquifer, and others. cipitation in the area ranges from 350 to 930 mm yr−1 (2015– Infiltrated water displaces groundwater stored in the aquifer 2020) and is mainly governed by EPEs that take place dur- towards the sea, and in some cases, may also enhance mixing ing the autumn and spring seasons. Here, precipitation events processes in the coastal aquifer (Anwar et al., 2014; Palacios with > 75 mm d−1 (corresponding to the 99th wet-day per- et al., 2019; Robinson et al., 2018). The significance of EPEs centile) are regarded as EPEs (Pendergrass and Knutti, 2018) on SGD may be exacerbated in areas subject to an arid or and have a recurrence of ca. 13 months. In this area, most of semi-arid climate, with scarce and unevenly distributed pre- these streams are hydraulically disconnected from their allu- cipitation, and where extreme events may be the only form vial aquifers; therefore, surface runoff takes place only after of recharge for the aquifer (Taylor et al., 2013). Understand- the most significant rainfall events, which are characterized ing the role of EPEs on SGD-driven water flows and asso- by its short duration and great intensity. The nature of floods ciated solute fluxes is thus fundamental in order to (1) ac- associated with EPEs are well known and have been de- curately constrain annual fluxes of any dissolved compound scribed in detail, especially in gray literature (Cisteró and Ca- transported by SGD, (2) integrate episodic-induced SGD in marós, 2014; Riba, 1997). The flood events consist of differ- global estimates, (3) evaluate the environmental impacts of ent stages which take place in a few hours (2–6 h), depending these episodic events on coastal ecosystems, and (4) foresee on the intensity and duration of the EPE: (1) some minutes the role of SGD in the future climate-change scenario. after the precipitation has started a thin layer (some cm) of This study evaluates the significance of SGD and the asso- “dirty” water (from the surroundings) flow towards the sea; ciated nutrient fluxes induced by EPEs in a Mediterranean (2) after some minutes to hours, a cleaner water mass which coastal zone, the implications of neglecting EPEs on an- carries heavier materials overcomes the first one; (3) the nual estimates, and its importance in the context of climate flood level increases progressively towards a maximum dis- change, using Ra isotopes as tracers of SGD. The study charge rate which remains constant for a short period of time; was conducted at a typical Mediterranean ephemeral stream- and (4) the water level decreases gradually until completely dominated basin. These areas are characterized by rapid re- disappearing (Cisteró and Camarós, 2014). Water velocities sponse to precipitation due to their geomorphological fea- in one of the heaviest precipitation events occurred in the Ar- tures (small catchment areas, sharp slopes, coastal alluvial gentona ephemeral stream (180 mm in 24 h) was calculated aquifers, and sporadic surficial torrential courses; Camarasa- to be in the order of 2.7 to 3.8 m s−1 (Martín-Vide, 1985). The Belmonte and Segura-Beltrán, 2001), and thus represent an phreatic level in the lower part of the ephemeral streams is ideal setting for gauging the influence of such events. shallow (2–3 m below the ground level) and the materials are highly permeable (Martínez-Pérez et al., 2022). This facili- tates the rapid aquifer recharge after an EPE, since infiltrated rainwater may easily reach the water table, and diminishes the role of interflow circulating through the vadose zone. The Hydrol. Earth Syst. Sci., 26, 4619–4635, 2022 https://doi.org/10.5194/hess-26-4619-2022 M. Diego-Feliu et al.: Extreme precipitation events induce high fluxes of groundwater 4621 Figure 1. Study site map. (a) Location map of Maresme County, (b) geological description, bathymetry, and sampling stations, (c) location map of the Argentona site piezometers, and (d) schematic diagram of the piezometers and their screening sections within the perpendicular transect of the experimental site of Argentona. T1, T2, and T3 are the offshore transects associated with the ephemeral streams of Argentona, Cabrera de Mar, and Vilassar de Mar, respectively. nature of tides in the Maresme region is semidiurnal, with an ducted on 11 March 2020; it was not affected by any rainfall amplitude in the order of a few centimeters (∼ 20–40 cm). event, and was therefore considered to have been conducted The bathymetry of the Maresme continental shelf is domi- under baseflow conditions (accumulated rainfall of 18 mm in nated by steps and ridges, with moderate slopes (1.1◦ ), and the prior 40 d). Seawater samples were collected at different an average shelf width of 13 km (Durán et al., 2014). stations from three perpendicular transects to the coastline corresponding to the ephemeral streams of Argentona, Cabr- 2.2 Field methods era de Mar, and Vilassar de Mar (Transects T1, T2, and T3, respectively; Fig. 1). Each transect consisted of 7 offshore Three samplings were conducted in the southern section of stations distributed along the first 1000 m. The central tran- Maresme County during 2019 and 2020. The first two sam- sect (T2) had three additional stations at 1500, 2000, and plings (hereinafter P1 and P2, chronologically) were per- 4000 m from the coastline. Coastal seawaters were collected formed shortly after an EPE with an accumulated precipita- directly from the shore by filling 25 L water containers. At tion rate of ∼ 90 mm in 1 d, corresponding to the 99.6th wet- each station, surface and deep (only selected stations) sea- day percentile (Fig. 2a). The EPE took place on 22 Octo- water samples were collected by placing a submersible pump ber 2019; P1 and P2 were conducted on 25 and 29 Octo- at ∼ 0.5 m depth and ∼ 1 m above the seabed, respectively. A ber 2019 (∼ 4 and 8 d after the rainfall event, respectively). single runoff water sample was taken at the initial stage of the The third sampling (named BF, after “baseflow”) was con- https://doi.org/10.5194/hess-26-4619-2022 Hydrol. Earth Syst. Sci., 26, 4619–4635, 2022 4622 M. Diego-Feliu et al.: Extreme precipitation events induce high fluxes of groundwater perimental site; Fig. 1). Each piezometer was purged with a submersible pump to remove at least 3 times the volume of stagnant water before sampling. Continuous in situ ground- water level, conductivity, and temperature time series were monitored at a shallow piezometric well (N3-15; 15 m depth, 2 m screened interval, from 11 to 13 m, 80 m from the coast- line) using a CTD diver. Salinity and temperature of ground- water and seawater samples were measured in situ with two handheld probes (HANNA HI98192 and WTW COND 330I). Rainfall data were obtained from a meteorological sta- tion from the Meteorological Catalan Service (Servei Mete- orològic de Catalunya; SMC) at the municipality of Cabrils (see Fig. 1). 2.3 Analytical methods Samples collected for Ra isotopes in both seawater and groundwater were weighted and filtered through MnO2 - impregnated acrylic fibers, at a controlled flow rate (< 1 L min−1 ) to ensure the quantitative adsorption of Ra onto the fibers (Moore and Reid, 1973). Fibers were then washed with Ra-free deionized water and partially dried to a fiber–water ratio of 1 : 1 (Sun and Torgersen, 1998). Each fiber was measured twice with the radium delayed coinci- dence counter (RaDeCC; Moore and Arnold, 1996). Short- lived Ra isotopes (223 Ra, T1/2 = 11.4 d; 224 Ra, T1/2 = 3.66 d) were quantified using the first RaDeCC measurement. The Figure 2. Temporal evolution of meteorological, oceanographical, and hydrogeological data in Maresme County: (a) specific conduc- activities of 223 Ra are not reported in this study because the tivity measured at the shallow piezometer N3-15 and accumulated high 224 Ra activities prevented the proper quantification of 223 Ra with the RaDeCC system due to the cross-talk effect precipitation, (b) groundwater level, and (c) significant wave height and mean sea level (MSL). Groundwater level is displayed as the (Diego-Feliu et al., 2020). The second measurement, per- variation of groundwater level relative to the values of March 2020. formed 1 month after sampling, was used for quantifying the The data from the buoy and CTD diver was smoothed by using a unsupported activity of 224 Ra (excess 224 Ra; 224 Raex ), by ac- low-pass filter (12 h averaged). Red lines indicate the groundwater counting for the activity of its parent, 228 Th, in the fiber. and seawater samplings performed at the study site (P1, P2, and BF) The quantification of 224 Ra was made following the guide- and gray bands indicate the EPEs that took place during the moni- lines and limits proposed by Diego-Feliu et al. (2020) in or- toring period (10 d after the event are included in the band). der to avoid interferences inherent to the detection system, while 224 Ra uncertainties were estimated following Garcia- Solsona et al. (2008). The MnO2 fibers were subsequently flood, when the amount of water flowing represented only a incinerated, grounded, and transferred to gamma counting thin, “dirty” layer of water (some cm). vials. After radioactive equilibration (∼ 21 d), the activities Samples were collected for Ra isotopes (223 Ra, 224 Ra, of long-lived Ra isotopes (226 Ra, T1/2 = 1600 years; 228 Ra, 226 Ra, and 228 Ra; 25–120 L for each sample), which are T1/2 = 5.75 years) were measured using a HPGe gamma widely applied tracers of SGD (Garcia-Orellana et al., 2021), spectrometer. The photopeaks of 214 Pb (352 keV) and 228 Ac and nutrient analysis. Depth profiles of salinity and temper- (911 keV) were used to quantify the activities of 226 Ra and ature were performed at each station by using a YSI 600XL 228 Ra, respectively. probe. Groundwater samples for Ra isotopes (10–25 L) and Samples for the analysis of silicate (SiO2 ), phosphate nutrients were collected periodically from 2015 to 2020 in (PO3− − − 4 ), nitrite (NO2 ), nitrate (NO3 ), and ammonia (NH4 ) + several piezometers at the experimental site of the Medis- were collected in 10 mL high-density polyethylene (HDPE) traes project, located in the coastal alluvial aquifer of the Ar- vials after filtration through nylon syringe filters (pore size: gentona ephemeral stream (corresponding to T1). The exper- 0.45 µm). Vials were immediately stored in a portable fridge imental site consists of 16 piezometers located at 30–100 m and subsequently frozen at the laboratory until analysis. The from the coastline with 2 m screened intervals at depths rang- analysis was performed using a colorimetric method with ing from 15 to 25 m (see Diego-Feliu et al., 2021; Folch et al., an Auto-analyzer AA3 HR (Seal Analytica). The detection 2020; Palacios et al., 2019, for more details about the ex- limits of the method were 0.016, 0.020, 0.003, 0.006, and Hydrol. Earth Syst. Sci., 26, 4619–4635, 2022 https://doi.org/10.5194/hess-26-4619-2022 M. Diego-Feliu et al.: Extreme precipitation events induce high fluxes of groundwater 4623 0.003 µM for SiO42− , PO3− − − + 4 , NO2 , NO3 , and NH4 , respec- used for a calibrated regional groundwater numerical model tively. of the southern section of Maresme County. The model is not publicly available as it has been developed for a specific work of the Spanish railway public company. According to the soil 3 Results mass balance, surface runoff associated with this EPE was about 1 hm3 . 3.1 Meteorological and hydrological context 3.2 Radium and nutrient concentrations The temporal evolution of groundwater level, conductivity, significant wave height, mean sea level (MSL), and accu- The activities of Ra isotopes in groundwater samples mea- mulated precipitation from October 2019 to April 2020 are sured during the 2015–2020 period in the Argentona site shown in Fig. 2. Three major precipitation events took place ranged from 10 to 940, 10 to 550, and 1 to 50 Bq m−3 for 224 Ra, 228 Ra, and 226 Ra, respectively (see Fig. S1 in the in October 2019 (∼ 90 mm), December 2019 (∼ 100 mm), and January 2020 (∼ 160 mm), which had a direct impact on Supplement). The activities of Ra increased with groundwa- groundwater level and conductivity. These events are con- ter salinity, presenting some variations that are mostly asso- sidered EPEs following the threshold value derived from ciated with differences in the geological matrix (Beck and the 99th wet-day percentile (∼ 75 mm). After each EPE, the Cochran, 2013; Webster et al., 1995). The seawater activi- groundwater table from the shallow piezometer (N3-15) rose ties of 224 Ra and 228 Ra isotopes generally decreased with between 60 and 130 cm, gradually recovering the previous increasing distance offshore for all transects and seawater values 7–10 d after the event. The magnitude in the increase samplings (Fig. 3). Maximum activities were found in the of the groundwater level (60, 70, and 130 cm, respectively) first sampling after the rainfall event (P1) for 224 Ra (median: correlated with the amount of accumulated rainfall corre- 14.4 Bq m−3 , interquartile range (IQR): 11.7–19.9 Bq m−3 ) sponding to each EPE. Precipitation events were followed by and 228 Ra (5.8 Bq m−3 , IQR 4.9–7.1 Bq m−3 ). The sea- a drastic reduction of groundwater conductivity at the shal- water activities of 224 Ra and 228 Ra in subsequent sam- low piezometer, reaching minimum values of ∼ 1 mS cm−1 , plings (P2 and BF) were 3 to 5 times lower than those and a subsequent gradual increase before the next precipi- of P1 (4.0 Bq m−3 , IQR 2.4–10.2 Bq m−3 for 224 Ra; and tation event (Fig. 2). As inferred by electrical resistivity to- 1.4 Bq m−3 , IQR 1.1–1.9 Bq m−3 for 228 Ra). Seawater activ- mography profiles shown in Palacios et al. (2019), conductiv- ities of 226 Ra were low, and comparable to open seawater ac- ity variations in the piezometric wells of the study site were tivities in all samplings (1.8 Bq m−3 , IQR 1.4–2.0 Bq m−3 ), not derived from dilution with low-conductivity rainwater, revealing the lack of major inputs for this Ra isotope. Ac- but associated with the movement of the mixing zone due to tivities of Ra isotopes in the runoff sample collected dur- EPEs. Significant wave-height fluctuations occurred mainly ing the EPE were 110 ± 10 Bq m−3 for 224 Ra, 90 ± 5 Bq m−3 in response to changes in wind and atmospheric pressure dur- for 228 Ra, and 18 ± 1 Bq m−3 for 226 Ra. Nutrient concentra- ing EPEs, increasing rapidly from 2 to 5 m above the base- tions in groundwater from the experimental site of the Argen- line value of approximately 0.5 m. Similarly, the MSL pre- tona ephemeral stream (2015–2020 period) ranged from 10 sented oscillations linked to the EPEs, which are usually as- to 1070 µM for NO− − 3 , 0.1 to 3.9 µM for NO2 , 0.3 to 40.1 µM 3− sociated with atmospheric fronts and strong winds, and also for NH+ 4 , 0.8 to 10.1 µM for PO4 , and 50 to 230 µM for to seasonal meteorology, with MSL values that are higher SiO4 (Fig. S2). Nitrate (NO3 ) and silica (SiO2− 2− − 4 ) presented from October to December than from January to April. Some a similar pattern, with maximum concentrations in low salin- hours after the precipitation started, runoff occurred in the ity samples (Sal < 10) and a downward trend with increas- ephemeral stream of Argentona, showing a similar pattern ing groundwater salinity. Conversely, the concentrations of 3− to the runoff reported by previous studies (Cisteró and Ca- NO− + 2 , PO4 , and NH4 were relatively low for most of the marós, 2014; Martín-Vide, 1985; Riba, 1997). Estimates for groundwater samples, except for those collected at the shal- 3− runoff discharge or velocity are difficult because in the mid- low piezometers for NO− 2 , intermediate for PO4 , and deep + 19th century, a set of galleries and dams were constructed for NH4 (see Fig. S2). at the upper part of the Argentona ephemeral stream (mu- nicipality of Dosrius) to collect groundwater and surficial water from this area and transporting it to the municipali- ties of Barcelona and Mataró. The effect that these struc- tures may have regarding surface runoff is uncertain. How- ever, a soil mass balance (based on type of soil, land use, geology, precipitation, slope, etc.) of the lower part of the Ar- gentona ephemeral stream has been used to provide a semi- quantitative estimate for surface runoff during the rainfall event of 22 October 2019. The soil mass balance has been https://doi.org/10.5194/hess-26-4619-2022 Hydrol. Earth Syst. Sci., 26, 4619–4635, 2022 4624 M. Diego-Feliu et al.: Extreme precipitation events induce high fluxes of groundwater seawards and recirculated seawater mix in multiple aquifer levels which are separated by semi-confined thin layers of silt and clays. The different aquifer units and mixing zones may promote the combined discharge of fresh and saline ground- water (brackish) at the coastline or far beyond, depending on the continuity of the confining layers (Folch et al., 2020; Martínez-Pérez et al., 2022). A seawater recirculation cell has been observed in the upper part of the aquifer (i.e., upper saline plume; Robinson et al., 2018), where seawater infil- trates the most surficial layers due to wave setup and/or sea- level variations associated with EPEs or storm surges (Pala- cios et al., 2019). Offshore exchange of seawater due to the movement of the freshwater–saltwater interface may also oc- cur in response to the increased infiltration of rainwater in- land associated with EPEs. Interflow may occur after an EPE, however it may probably reach the water table easily due to the thin vadose zone (2–3 m) and the high permeability of the surficial materials (Martínez-Pérez et al., 2022). Porewater exchange may also occur due to its almost ubiquitous char- acter and the disparate mechanisms driving the water flow (Santos et al., 2012). In this work, the water-flow processes described above Figure 3. Radium isotopes activities (224 Ra, 228 Ra, and 226 Ra) have conveniently been clustered into two main SGD com- in coastal seawater samples collected during the samplings per- ponents: brackish and saline SGD. Brackish SGD is defined formed in October 2019 (P1 and P2) and March 2020 (BF) for the here as the combined discharge of meteoric groundwater and three transects perpendicular to the coastline corresponding to the density-driven (long-term) recirculation of seawater through ephemeral streams of Argentona (T1), Cabrera de Mar (T2), and the saltwater wedge, regardless of the degree of mixing be- Vilassar de Mar (T3). tween the two water masses and the aquifer unit considered. It should be noted that this SGD component (1) does not rep- resent a net water input to the coastal ocean, (2) excludes 4 Discussion water-flow processes that only involve the recirculation of seawater through permeable sediments, and (3) also includes 4.1 Groundwater and nutrient fluxes calculation the contribution that interflow may have on groundwater dis- charge after the occurrence of an EPE. On the other hand, 4.1.1 Pathways of submarine groundwater discharge beach-face recirculation of seawater through the upper saline (SGD) plume, porewater exchange, and offshore exchange of sea- water due to the movement of the freshwater–saltwater in- Submarine groundwater discharge (SGD) incorporates a set terface are ascribed here to the saline SGD component. This of water-flow processes involving the discharge of fresh SGD component (1) represents a zero net water input to the groundwater and the circulation of seawater through perme- coastal ocean for timescales longer than that of the process able sediments (Garcia-Orellana et al., 2021; Michael et al., driving its oscillations, and (2) comprises a set of water- 2011; Santos et al., 2012). The driving forces and pathways flow processes with disparate spatiotemporal scales (min- of these processes likely determine the extent of the chemi- utes to days). Besides the obvious difference between these cal reactions occurring in the subterranean estuary (Moore, two components in terms of water composition and origin, 1999). Therefore, considering all the different SGD path- it should also be noted that brackish SGD may be mediated ways that occur concurrently in a specific study site is fun- by medium- to long-term spatiotemporal scale processes and damental for deriving reliable estimates of SGD and associ- conversely, saline SGD is likely to be governed by short- to ated nutrient fluxes (Garcia-Orellana et al., 2021). The char- medium-term spatiotemporal scale flow paths. acteristics of coastal alluvial aquifers linked to the presence The different spatiotemporal scales of both SGD compo- of ephemeral streams in the Maresme County may favor the nents are especially important when using Ra isotopes as concurrent occurrence of different water-flow processes. In- tracers of SGD. In fact, these isotopes are instrumental for deed, previous works at the study site of Argentona have al- differentiating SGD pathways, since their enrichment rates ready shown that different SGD components coexist (Diego- strongly depend on the transit time of groundwater through Feliu et al., 2021; Folch et al., 2020; Martínez-Pérez et al., the coastal aquifer (e.g., Diego-Feliu et al., 2021; Michael 2022; Palacios et al., 2019). Meteoric groundwater flowing et al., 2011). Coastal seawater samples collected during the Hydrol. Earth Syst. Sci., 26, 4619–4635, 2022 https://doi.org/10.5194/hess-26-4619-2022 M. Diego-Feliu et al.: Extreme precipitation events induce high fluxes of groundwater 4625 three samplings performed in Maresme County were en- riched in both 224 Ra and 228 Ra relative to offshore waters (Fig. 3), suggesting the occurrence of a land-based Ra source. While the 224 Ra enrichment may result from any ground- water discharge, regardless of spatiotemporal scale, due to its short half-life (224 Ra is enriched in all SGD pathways), coastal waters enriched in 228 Ra may be indicative of long- scale SGD pathways (e.g., terrestrial groundwater discharge; Rodellas et al., 2017; Tamborski et al., 2017a). 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. This decrease Figure 4. Coastal-normalized flow of brackish and saline submarine is simultaneously followed by an increase in absolute 228 Ra groundwater discharge (SGD; dark and light blue, respectively) for activities, which are 2 times higher than those in baseflow the three samplings at Maresme County. conditions (Fig. 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 rel- SGD. This is consistent with the variation on the 224 Ra/228 Ra ative contribution of the brackish component of SGD, which AR in coastal seawater after the EPE (see Sect. 4.1.1) and co- is characterized by 224 Ra/228 Ra ARs close to the equilibrium herent with Darcy’s flow calculations (Appendix B). These value (1.0 to 2.2; Diego-Feliu et al., 2021), increased during estimates of the relative contribution of the brackish com- the occurrence of the EPE. This is also coherent with the in- ponent are generally much larger than estimates of fresh crease in groundwater level (Fig. 2), and therefore hydraulic groundwater discharge for the Mediterranean Sea (1 %– forcing, after the EPE of October 2019. 25 %, Rodellas et al., 2015), global estimates (10 %, Kwon et al., 2014; 0.06 %, Luijendijk et al., 2020), and local stud- 4.1.2 Submarine groundwater discharge (SGB) ies (5 %–55 %; Alorda-Kleinglass et al., 2019; Beck et al., 2008; Kiro et al., 2014; Knee et al., 2016; Rodellas et al., The SGD associated with brackish and saline water 2017; Tamborski et al., 2017a). This difference most likely flows were determined by means of a steady-state mass emphasizes that while the studies presented above are mainly balance of 224 Ra and 228 Ra. The assumptions, consider- focused on distinguishing fresh and saline SGD, here we are ations, and models used to estimate SGD are discussed targeting brackish (encompassing meteoric groundwater and in detail in Appendix A. The sampling influenced by recirculated seawater) and saline SGD, as previously dis- the EPE of October 2019 (P1) presented the highest to- cussed. It should also be noted that estimates presented in tal SGD (brackish and saline) (510 × 103 m3 km−1 d−1 , this study should be taking as semi-quantitative in view of IQR: 320–890 × 103 m3 km−1 d−1 ), 1 order of the biases, limitations, and uncertainties discussed in detail magnitude higher than those flows from subse- in Appendix A (e.g., endmember selection, steady-state as- quent samplings: 40 × 103 m3 km−1 d−1 (IQR: 30– sumption, lack of consideration for runoff). However, these 90 × 103 m3 km−1 d−1 ) for P2 and 70 × 103 m3 km−1 d−1 limitations are inherent to almost any SGD study and es- (IQR: 40–110 × 103 m3 km−1 d−1 ) for BF (Fig. 4). Thus, pecially those using Ra isotopes as tracers (Garcia-Orellana the effect of this EPE on the discharge of groundwater et al., 2021; Rodellas et al., 2021), and may not invalidate only lasts for a few days, since the SGD estimates of the the implications derived from this study, which is trying to second sampling (P2; 8 d after the event) are comparable to ascribe the relative significance of EPEs in water and nutri- those from baseflow conditions. Besides the similarities in ent fluxes to the coastal ocean. SGD estimates from these two samplings (P2 and BF), they also present similar groundwater levels, conductivities, and 4.1.3 SGD-driven nutrient fluxes Darcy’s flow estimates (Figs. 2 and B1). Thus, the temporal extent of the EPE effects on SGD is consistent with the The SGD-driven nutrient fluxes were estimated by consider- recovery of groundwater level, which commonly occurs ing the Ra-derived flows of brackish and saline SGD and the from 7 to 10 d after the rainfall ceases (Fig. 2). respective nutrient concentration in groundwater from both In baseflow conditions, the brackish component of SGD fractions (see Appendix A). Note that results are not ex- (including fresh groundwater and density-driven seawater pressed here in terms of net nutrient inputs, since the fluxes discharge) represented 60 % of the total SGD (Fig. 4). The of nutrients from the coastal ocean to the coastal aquifer are relative contribution of this SGD component increased after not considered. However, the influence that these fluxes have the rainfall event of October 2019 by up to 75 % of the total on the calculations may be negligible because concentra- https://doi.org/10.5194/hess-26-4619-2022 Hydrol. Earth Syst. Sci., 26, 4619–4635, 2022 4626 M. Diego-Feliu et al.: Extreme precipitation events induce high fluxes of groundwater episodes (Fig. 5). Fluxes after P1 were 9 times higher for DIN and 7 times higher for DIP and DSi than those in BF. The predominant pathway of the discharge for DIN, DIP, and DSi to the coastal ocean was the brackish component of SGD. This pathway represented ∼ 60 % of the total in- puts of DIP and DSi in baseflow conditions and up to ∼ 75 % after an EPE (Fig. 5). Nitrogen inputs during EPEs and in baseflow conditions were chiefly governed by the discharge of brackish SGD (∼ 99 % of the total DIN inputs; Fig. 5). The significant difference between the supply of nitrogen through brackish and saline SGD relies on the high concen- trations of nitrate (∼ 1000 µM) in coastal aquifer freshwater (see Fig. S2), which exceed the maximum groundwater con- centration for drinking water set by the World Health Orga- nization (WHO, 2011). Contrastingly, saline SGD is a rele- vant source of nitrite and ammonia, representing ∼ 70 % and ∼ 40 % of the total fluxes, respectively. It should be noted that nutrient fluxes were estimated by multiplying the volu- metric water flux of brackish and saline SGD by the mini- mum nutrient concentration from a set of onshore samples, selected following the criteria used for the Ra endmembers, Figure 5. SGD-derived nutrient fluxes of dissolved inorganic ni- trogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved as explained in the appendices (see Appendix A.2.4). Since silicate (DSi) at Maresme County. Panel (a) shows the relative con- it was not possible to directly collect the discharging ground- tribution of brackish (dark blue) and saline (light blue) SGD dur- water, by using onshore samples we are implicitly assuming ing EPE (inner pie chart) and in baseflow conditions (outer pie that no nutrient transformation occurred between the sam- chart). Panel (b) represents the median nutrient fluxes at Maresme pling point and the discharge point within the subterranean County during the EPE of October 2019 (P1) and in baseflow con- estuary (Cook et al., 2018). This assumption is perhaps one ditions (BF) (fluxes normalized by the coastline length). Error bars of the main sources of uncertainty in the reported nutrient indicate the interquartile range (25 % and 75 % percentile). fluxes as it has already been shown by many authors (Sawyer et al., 2014; Weinstein et al., 2011; Wong et al., 2020). It should also be noted that these SGD-derived nutrient esti- tions of nutrients in seawater are orders of magnitude lower mates may be biased due to the groundwater endmember se- than those in groundwater (see Fig. S2). Total SGD-driven lection, since nutrient concentrations in discharging ground- fluxes in baseflow conditions for dissolved inorganic nitro- waters may vary during EPEs due to dilution, increasing lix- gen (DIN), dissolved inorganic phosphorus (DIP), and dis- iviation of fertilizers, or enhancement of biogeochemical re- solved silicate (DSi) derived from median SGD estimates in actions in the mixing zone of coastal aquifers (Spiteri et al., the study site were 16.2, 0.06, and 5.4 × 103 mol km−1 d−1 , 2008a). Although all the assumptions made for the quantifi- respectively (Fig. 5). The median fluxes, normalized by the cation of nutrient fluxes may result in high degrees of un- study site area, were lower compared with median SGD- certainty, the results presented in this study enable the as- derived nutrient fluxes estimated worldwide for DIP and sessment of EPE significance as a major driving force for DSi, but significantly higher for DIN (2.7 times higher; San- transporting nutrients to the coastal ocean. tos et al., 2021). The DIN : DIP ratio was 390 : 1, much higher than the Redfield ratio of 16 : 1, but comparable with 4.2 Implications of EPE on SGD and associated SGD-derived input in the Mediterranean Sea (80 : 1–430 : 1; nutrient fluxes Rodellas et al., 2015) and in studies worldwide (259 ± 1090 : 1; Santos et al., 2021). The high loads of N and the dispro- 4.2.1 Episodic events portionate ratio DIN : DIP relative to the Redfield ratio in the study site may result from the lixiviation of nitrogen from Although several studies have focused on understanding the agricultural activities (representing ∼ 15 % of the total land seasonal dynamics of SGD (Charette, 2007; Gwak et al., use; Rufí-Salís et al., 2019), and the attenuation of P along 2014; Michael et al., 2005; Rodellas et al., 2017), limited re- groundwater flow paths due to adsorption onto Mn/Fe oxides search has been conducted on SGD driven by episodic events present in the coastal aquifer (Robinson et al., 2018; Spiteri (Adyasari et al., 2021; Wilson et al., 2011). This is mainly et al., 2007, 2008b). because of the inherent difficulties related to monitoring and After an EPE, the supply of all nutrients increased due to sampling during and after these extreme events. Some studies the higher brackish and saline SGD associated with these have already shown that SGD may vary in direct or delayed Hydrol. Earth Syst. Sci., 26, 4619–4635, 2022 https://doi.org/10.5194/hess-26-4619-2022 M. Diego-Feliu et al.: Extreme precipitation events induce high fluxes of groundwater 4627 response to meteorological and oceanic episodic events such and other dissolved compounds (Luijendijk et al., 2020; San- as sea-level anomalies (Gonneea et al., 2013), waves (Bakht- tos et al., 2021). However, most SGD studies are conducted yar et al., 2012; Rodellas et al., 2020; Sawyer et al., 2013), in periods with stable meteorological conditions to evaluate hurricanes (Hu et al., 2006), typhoons (Cho et al., 2021; baseflow SGD and associated nutrient, metal, or contami- Sugimoto et al., 2016), and temperature inversion (Moore nant fluxes. Based on the results obtained from monitoring and Wilson, 2005). Regarding SGD induced by precipita- the EPE occurring at Maresme County in October 2019, the tion events, Sugimoto et al. (2016) reported high values of discharge of groundwater associated to this single event ac- SGD 11 d after a precipitation event in Obama Bay (Japan); counted for 13 % (IQR: 5 %–40 %) and 8 % (IQR: 5 %–18 %) Gwak et al. (2014) suggested that SGD in the II-Gwang wa- of the annual brackish and saline fraction of SGD, respec- tershed (South Korea) was partially triggered by intensive tively. Moreover, the nutrient inputs resulting from this event precipitation events; and Uddameri et al. (2014) indicated (lasting only 8 d; ∼ 2 % of the year) represented 13 % (IQR: that precipitation events associated with the Emili hurricane 5 %–40 %) for DIN, and 11 % (IQR: 5–30) for DIP and DSi, contributed to the SGD in Baffin Bay (USA). At our study of the yearly supply of nutrients at the study site. The in- site, the significant increase (1 order of magnitude; Fig. 4) in crease in SGD-driven nutrient fluxes during these events may both the brackish and saline SGD after an EPE, may be me- be mediated, on the one hand by the total SGD increase, diated by different processes: (1) increase of fresh ground- but also because this increase is more significant for brack- water discharge due to aquifer infiltration of rainfall, sub- ish SGD, which presents higher concentrations of nutrients sequent increase of hydraulic gradient, and displacement of (see Supplement; Fig. S2), relative to saline SGD. These re- stored water towards the sea (Anwar et al., 2014; Palacios sults suggest that annual estimates based on samplings con- et al., 2019; Santos et al., 2012; Yu et al., 2017), (2) increase ducted in baseflow conditions may systematically underesti- in the exchange of seawater and density-driven discharge due mate SGD and associated nutrients, particularly at study sites to movements of the fresh–saltwater mixing zone, and (3) in- affected by EPEs or other episodic events that can signif- crease of shoreface circulation of seawater and porewater ex- icantly impact SGD. Periodic and seasonal samplings may change due to the effect of sea-level rise and waves associ- be taken as snapshots, and are only representative of the ated with the EPE (Fig. 2). time periods with similar environmental conditions. A better The higher total SGD driven by EPEs, especially the characterization of the hydrological and meteorological con- increase of brackish relative to saline SGD during these text is necessary in pursuit of more reliable annual estimates, episodic events (Fig. 4), induces the transport of large which may include seasonal and episodic-related variations. amounts of nutrients from the freshwater fraction of coastal In this scenario, alternative methods such as groundwater aquifers into the coastal ocean. These fluxes, which are sub- level monitoring, Darcy’s law calculations, electrical resistiv- stantially higher than those in baseflow conditions (Fig. 5), ity tomography characterization, among other methods, may may represent a significant periodic and episode-related nu- be instrumental in the design of proper sampling strategies trient input of particular relevance in sites where surface and to capture seasonal and episodic variations (e.g., Folch water renewal is limited (e.g., coastal lagoons and/or semi- et al., 2020; Palacios et al., 2019). enclosed bays), EPEs occur frequently (e.g., Mediterranean region), and the response to EPEs is fast due to the geo- 4.2.3 Climate change logical and geomorphological characteristics of the coastline (e.g., alluvial aquifers in ephemeral stream-dominated areas, such as Maresme County). However, in other areas, the re- Climate change and its associated social and environmental sponse to EPEs may be much slower. For instance, aquifers impacts have become one of the most pressing scientific chal- with a high thickness of vadose zones, confined aquifers with lenges for the 21st century. This requires the acquisition of a recharge areas far from the coastal zones, or systems with holistic and integrative knowledge of systems and processes soils with water deficits (among other factors), may smooth for modeling and predicting future scenarios. Fundamental or delay the effects of EPEs. The biological and ecological research relating environmental key variables (e.g., tempera- implications of these events – which may include eutrophi- ture, sea-level rise, precipitation) with social processes (e.g., cation, formation of red and green tides, and mass fish death land demand, coastal overpopulation, groundwater squeeze) (Hu et al., 2006; Lee et al., 2010; Montiel et al., 2019; Valiela becomes crucial to this aim. Research on SGD is no ex- et al., 1990; Zhao et al., 2021) – are far from being under- ception to this trend, and in recent decades, several studies stood, and require further attention. have evaluated the key factors contributing to groundwater flows discharging into the ocean. Some examples of SGD re- 4.2.2 Annual SGD estimates search linked to climate change include understanding sea- level and/or tidal controls on SGD (Gonneea et al., 2013; A proper understanding of the temporal patterns of SGD in Wilson et al., 2015); the influence of land-use changes (Rufí- local to regional-scale studies is essential for deriving an- Salís et al., 2019); the relationship to increasing seawater in- nual estimates to predict reliable ocean budgets of nutrients trusion (Werner et al., 2013); and the fate and evolution of nu- https://doi.org/10.5194/hess-26-4619-2022 Hydrol. Earth Syst. Sci., 26, 4619–4635, 2022 4628 M. Diego-Feliu et al.: Extreme precipitation events induce high fluxes of groundwater trients in groundwater (Beusen et al., 2013; Van Meter et al., these episodic events mask the implications that they may 2018; Tait et al., 2014). have for coastal geochemical cycles, coastal ecosystems, nu- The precipitation–recharge relationship is one of the key trient budgets, and hydrological cycle estimates. We have as- parameters influencing the discharge of groundwater. Indeed, sessed the fluxes of SGD induced by an EPE in an ephemeral the amount of precipitation and the frequency and/or dis- stream-dominated basin of the western Mediterranean Sea. tribution of rainfall events, together with the hydrogeologi- The SGD induced by the EPE increased by 1 order of mag- cal characteristics of the receiving aquifers, strongly affects nitude and represented up to 13 % and 8 % of the total annual both the quantity and chemical quality of groundwater dis- discharge of brackish and saline SGD, respectively. Simi- charge to the ocean (Kundzewicz and Döll, 2009; Stigter larly, fluxes of nutrients driven by SGD during an EPE repre- et al., 2014). Climate models indicate substantial spatial vari- sented 11 %–13 % of the annual total SGD, and up to ∼ 30 % ation in future changes to average precipitation. While the in- during abnormally rainy seasons. This study highlights the creased specific humidity and transport of water vapor from relevance of these extreme events on the discharge of ground- tropical regions is likely to increase the amount of precipi- water and solutes to the coastal ocean, noting their impli- tation in high-latitude land masses, precipitation in subtropi- cations for annual SGD estimates and the possible conse- cal and semi-arid regions, like the Mediterranean Sea, is ex- quences for coastal biogeochemistry cycles. The results of pected to decrease. Simultaneously, EPEs in these regions are this study contribute to the understanding of the evolution likely to increase in intensity and frequency (IPCC, 2021). of SGD with respect to future climate-change scenarios, pre- Consequently, the yearly recharge of groundwater associated senting an opportunity for streamlining future research in or- with precipitation in the Mediterranean region may diminish, der to help managers and policy makers better estimate SGD also reducing the annual discharge of groundwater. In that and its related consequences. scenario, EPEs may become a major driving force of SGD, having a dominant significance in the annual fluxes of solutes to the coastal ocean. Appendix A: SGD and nutrient fluxes calculations The potential relevance of EPEs on SGD in future con- In this appendix, we develop the methodology used for deter- ditions can be qualitatively evaluated for Maresme County mining the SGD and associated nutrient fluxes to the coastal by considering the period from October 2019 to April 2020, ocean. This includes the definition of the conceptual model when three precipitation events of > 75 mm took place and the discussion of the model assumptions used in the cal- (Fig. 2). Since the historical recurrence of EPEs in the area is culations. around 13 months (based on the meteorological data from 2015 to 2020), this 7-month period can be regarded as a A1 Radium mass balance future-like year with increased recurrence of EPEs. Assum- ing that each one of the EPEs produces an increase in SGD The magnitude of SGD and its associated nutrient fluxes are comparable to the event monitored in this study, the rela- quantified in this study by using Ra isotopes, which is one tive contribution of SGD during EPEs would represent 30 % of the most commonly applied techniques (Garcia-Orellana (IQR: 15 %–70 %) and 22 % (15 %–40 %) of the annual SGD et al., 2021; Taniguchi et al., 2019). While single isotopes issued by the brackish and saline fraction, respectively. Sim- can be used for quantifying SGD driven by single pathways, ilarly, nutrient fluxes associated with EPEs for this period the combination of different isotopes is instrumental in dis- would represent 34 % (IQR: 15 %–70 %) for DIN and 30 % criminating SGD in sites with multiple pathways (Alorda- (IQR: 15 %–60 %) for DIP and DSi of the yearly nutrient Kleinglass et al., 2019; Charette, 2007; Rodellas et al., 2017; inputs supplied by SGD. Note that due to the assumptions Tamborski et al., 2017b). In this study, Ra isotopes are used made in the determination of groundwater and nutrient fluxes for discriminating and quantifying both the fluxes of brack- (e.g., steady state, endmember selection; see Appendix A), ish and saline SGD. Whilst brackish SGD, as defined in the estimates are seemingly conservative, especially consid- this study, is mainly associated with long-term SGD pro- ering that the monitored EPE is minor, relative to other EPEs cesses (meteoric groundwater discharge and density-driven occurring in 2019 and 2020 (Fig. 2). These estimates empha- seawater recirculation over the saltwater wedge), saline SGD size the need for integrating episodic events, such as EPEs, comprises short-term processes that only involve the cir- in future climate-change scenarios, in order to properly con- culation of seawater through permeable sediments or the strain the fluxes of groundwater and solutes to the coastal coastal aquifer (i.e., porewater exchange, shoreface circu- ocean driven by SGD. lation of seawater, seasonal exchange of seawater; Garcia- Orellana et al., 2021; Michael et al., 2011). A steady-state mass balance of short-lived 224 Ra (T1/2 = 3.66 d) and long- 5 Conclusions lived 228 Ra (T1/2 = 5.75 years) isotopes was constructed as follows: The EPEs are potential drivers of SGD and nutrients to the coastal ocean. The lack of studies assessing the impact of Hydrol. Earth Syst. Sci., 26, 4619–4635, 2022 https://doi.org/10.5194/hess-26-4619-2022 M. Diego-Feliu et al.: Extreme precipitation events induce high fluxes of groundwater 4629 rect response to an EPE (∼ 90 mm). However, considering FBSGD · 228 RaBSGD + FSSGD · 228 RaSSGD the flushing time of Ra isotopes in the coastal system (see Sect. 1.2.2), the Ra delivered by this punctual runoff may Iex−Ra228 = + IRa228 · λRa228 have decreased by > 90 % for the first sampling (P1) and TF by > 99 % for the second sampling (P2), due to decay and FBSGD · 224 RaBSGD + FSSGD · 224 RaSSGD mixing with offshore waters. Yet, the relative contribution Iex−Ra224 that runoff may have had on Ra inventories during P1 and = + IRa224 · λRa224 . (A1) P2 were calculated using the Ra activities of the runoff sam- TF ple collected during the EPE, and the calculated runoff dis- charge by using soil mass balance. Although the estimates The two terms on the right-hand side of Eq. (A1) represent may be uncertain, the results indicate that the relative signif- the Ra outputs and include the offshore exchange of Ra, due icance of runoff derived from the EPE in the Ra inventories to the mixing between coastal and open ocean waters, and are 2 % and 1 % for 224 Ra, and 1 % and 8 % for 228 Ra at Ra decay. The terms I and Iex refer to the mean inventories P1 and P2, respectively. Note that these values are low and [Bq m−1 ] of Ra and excess Ra (inventory of excess Ra con- comparable with the common uncertainties derived from the centrations in coastal waters relative to open ocean), respec- measurement of Ra isotopes. It should be also noted that the tively, TF is the Ra flushing time [d], and λ is the Ra decay calculated Ra inputs through surface runoff are likely over- constant [d−1 ]. The mean Ra inventories were determined by estimated since the surface water sample was collected at the averaging the activity (or excess activity) [Bq m−3 ] of Ra at beginning of the flood and it is representative only of the ini- each station, normalized by depth and distance to the shore. tial thin and “dirty” water (with more particles per mass of The normalization was made by integrating the rectangular water) flow and not of the “cleaner” water mass which repre- trapezoid area confined within the distance between two sub- sents most of the total runoff discharge. Therefore, Ra runoff sequent stations and their respective depth. The two terms on was discarded as a major source of Ra isotopes to the coastal the right-hand-side of Eq. (A1) account for Ra inputs to the ocean during the sampling periods. The decay of Ra and the study site, which are supplied via brackish and saline SGD exchange with offshore waters were considered as the major (total flux of Ra; FRa [Bq m−1 d−1 ]). The activities [Bq m−3 ] sinks of Ra. The decay was assessed via the Ra inventories of Ra in the discharging groundwater (RaBSGD and RaSSGD ; at the study site and the offshore exchange, by evaluating the Ra endmember) were used to convert the Ra flux of both flushing time of Ra (TF ). isotopes concurrently, to a coastline-normalized volumetric flow [m3 km−1 d−1 ] for each of the SGD pathways (brackish A2.2 Radium flushing time SGD, FBSGD and saline SGD, FSSGD ). The terms and values used for the Ra mass balance for each sampling are shown in The flushing time of Ra (TF ) is a parameter that describes the Table A1. transport of Ra in surface water bodies due to advection and A2 Model assumptions and considerations dispersion processes (Monsen et al., 2002). In Ra mass bal- ances, this parameter is fundamental for evaluating the ex- A2.1 Sources and sinks of Ra change of Ra between coastal and offshore waters. In this work, rather than evaluating Ra flushing times, we used The proposed model for quantifying the Ra flux to the sea 224 Ra/228 Ra of coastal and offshore waters (1000 m from assumes that brackish and saline groundwater discharge are coastline) to determine the water apparent age (TW ) (Moore, the only sources of Ra at the study site (Eq. A1). Diffu- 2000). The water apparent age is a good proxy for tempo- sive fluxes of Ra from sediments were considered negligible, ral scales of advective and mixing processes occurring at due to the presence of coarse-grained sands with low spe- the study site. Coastal waters in Maresme County presented cific surface area (Luek and Beck, 2014) and are assumed 224 Ra/228 Ra ARs ranging from 1.6 to 2.9 times higher than to represent low (10 %) inputs compared to the total Ra in- those of offshore waters, which led to seawater apparent ages puts (Beck et al., 2007; Garcia-Orellana et al., 2014, 2021). of 2.4 ± 0.9, 5.6 ± 1.9, and 5.0 ± 1.7 d for P1, P2, and BF, re- Atmospheric deposition was discarded as a major source of spectively. The lower seawater residence time of P1 is coher- Ra since its contribution in small-scale study sites is of- ent with the oceanographic conditions (e.g., higher winds, ten  1 % (Garcia-Orellana et al., 2021). Production of Ra waves and currents that enhanced advection and exchange from dissolved Th was implicitly included by reporting the with offshore waters) linked to the EPE occurring 4 d before activities of Ra isotopes as “excess” Ra activities (activities (see Fig. 2). not supported by their progenitors). The Ra inputs from sur- face water were also discarded for the sampling conducted A2.3 Steady-state conditions in March 2020 (BF) due to the total absence of runoff dur- ing the sampling period. In October 2019, 4 d before the first Steady-state conditions (i.e., tracer inventories that do not sampling conducted at the study site, runoff occurred in di- vary with time; da/(dt·V ) = 0) are often assumed in Ra mass https://doi.org/10.5194/hess-26-4619-2022 Hydrol. Earth Syst. Sci., 26, 4619–4635, 2022 4630 M. Diego-Feliu et al.: Extreme precipitation events induce high fluxes of groundwater Table A1. Definition of terms, values, and units used for the Ra mass balance for each sampling (P1, P2, and BF). Data in brackets represent the interquartile range (1st and 3rd quartile). Term Definition Values Units P1 P2 BF IRa224 224 Ra inventory 113 ± 15 26 ± 2 31 ± 9 × 103 Bq m−1 Iex−Ra224 224 Ra excess inventory 98 ± 15 23 ± 2 28 ± 9 × 103 Bq m−1 IRa228 228 Ra inventory 46 ± 3 11 ± 2 9±3 × 103 Bq m−1 Iex−Ra228 228 Ra excess inventory 41 ± 3 10 ± 2 8±3 × 103 Bq m−1 FRa224 a 224 Ra flux 62 ± 9 9±1 11 ± 4 × 103 Bq m−1 d−1 FRa228 a 228 Ra flux 16.8 ± 1.3 1.8 ± 0.4 1.6 ± 0.5 × 103 Bq m−1 d−1 FRa224 /FRa228 224 Ra/228 Ra flux ratio 3.7 ± 0.3 5.2 ± 1.2 7.6 ± 2.7 – TF b Ra flushing time 2.4 ± 0.9 5.6 ± 1.9 5.0 ± 1.7 d Unknowns FBSGD Brackish SGD flow 380 25 45 × 103 m3 km−1 d−1 (235–660) (10–55) (15–75) FSSGD Saline SGD flow 125 20 25 × 103 m3 km−1 d−1 (90–225) (15–30) (20–30) a Flux of Ra supplied by brackish and saline SGD, left-hand side of Eq. (A1); b Radium fluxing time determined as water apparent age following Moore (2000) balances (e.g., Alorda-Kleinglass et al., 2019; Beck et al., endmembers were constrained to groundwater samples with 2008; Rodellas et al., 2017). This assumption implies that salinities higher than 5; and (3) the 224 Ra/228 Ra ARs of both Ra inputs and outputs are balanced for a time period equiva- endmembers must satisfy the following equation: lent to the tracer residence time in the system (Rodellas et al., 224 Ra 224 Ra 2021). In Maresme County, the tracer residence time ranged BSGD FRa224 SSGD 228 Ra < < 228 Ra , (A2) from 1.6 to 2.6 d for 224 Ra and from 2.4 to 5.6 d for 228 Ra. BSGD FRa228 SSGD The tracer residence time can be estimated by dividing the where FRa224 /FRa228 is the ratio between the total fluxes radium inventory in surface waters by the sum of all losses of 224 Ra and 228 Ra to the coastal ocean for each of the three (i.e., radioactive decay and exchange with offshore waters) samplings (note that if condition 3 is not met, the water flows (Rodellas et al., 2021). The assumption of steady state may resulting from the concurrent mass balance may be negative). therefore not be valid due to the significant difference be- A brackish and saline SGD was determined for each of the tween Ra activities from P1 and P2 (Fig. 3), which were possible combinations between brackish and saline Ra end- carried out only 4 d apart. Note, however, that using a non- members that satisfied the above-mentioned conditions, and stationary Ra mass balance would have required the moni- we reported the final SGD fluxes as the median value and toring of activities in coastal waters of Ra isotopes over the the interquartile range. Conservative fluxes of nutrients were sampled period to understand its temporal patterns. More- computed by multiplying the minimum concentrations of nu- over, assuming steady state instead of a decrease of activities trients within the brackish and saline endmembers (discrimi- in coastal waters (−da/dt) (the pattern that was observed in nated by conditions 1 and 2), and the median (± interquartile the EPE from 2019; Fig. 3), results in conservative estimates range) of each SGD component (brackish and saline). of SGD induced by the EPE relative to that in baseflow con- ditions. Appendix B: Darcy’s law calculations A2.4 Endmember selection The relative significance of EPEs in annual SGD esti- Due to the large spatial variability of Ra isotopes in the mates derived from the Ra mass balance was compared groundwater activity at the experimental site of Argentona, with Darcy flux estimates (Q = −kh · i). For these calcu- constraining the Ra activity of the SGD endmember for lations, hydraulic conductivity (kh , m s−1 ) was assumed both the brackish and saline components is particularly diffi- to be in the order of 10−3 m s−1 (characteristic of clean cult. To overcome this limitation, endmembers were selected sands) in the range of local studies (Tybaud Goyetche and according to the following conditions: (1) the selection of Laura Del Val, personal communication, 2022). The hy- the brackish Ra endmembers (RaBSGD ) was constrained to draulic gradient was determined as the difference between groundwater samples with low salinities (Sal < 5); (2) saline MSL data and groundwater-level data acquired from a CTD Hydrol. Earth Syst. Sci., 26, 4619–4635, 2022 https://doi.org/10.5194/hess-26-4619-2022 M. Diego-Feliu et al.: Extreme precipitation events induce high fluxes of groundwater 4631 diver deployed in a piezometric well at the experimental site vious works and literature, the preparation of the first and sub- of the Argentona ephemeral stream (N3–15, 80 m from the sequent drafts, and the revision and responses to anonymous re- shoreline). Absolute Darcy flux results (Fig. B1) should only viewers. VR contributed to the study conceptualization, the sam- be taken as indicative. In fact, the relative variation of Darcy pling and laboratory work, the revision and editing of the differ- flux during EPEs can be used as a proxy for groundwater ent drafts, and the supervision and administration of the research project. AAK helped with the sampling and laboratory work and the discharge. Results indicate that the EPEs of October 2019, revision and editing of the different drafts. MS participated in the December 2019, and January 2020 represented 2 %, 4 %, development of modeling approaches and the revision and editing of and 6 %, respectively, of the annual groundwater discharge. the original draft. AF oversaw the study conceptualization, the de- The relative significance of EPEs derived from these calcula- velopment of the methodological approaches, the revision and edit- tions is slightly lower than those obtained from the Ra mass ing of the different drafts, the supervision and administration of the balance. This discrepancy can be associated with the differ- research project, and the acquisition of funding. JGO contributed to ent discharge processes that each method captures. While the study conceptualization, the development of the methodologi- Ra mass balance enables the quantification of processes with cal approaches, the revision and editing of the different drafts, the different spatiotemporal scales and different compositions of supervision and administration of the research project, and the ac- groundwater (e.g., terrestrial groundwater discharge, pore- quisition of funding. water exchange), Darcy’s law only captures the discharge of meteoric or brackish groundwater due to the hydraulic gradi- ent at the shallowest aquifer. Competing interests. The contact author has declared that none of the authors has any competing interests. Disclaimer. Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Acknowledgements. This work is dedicated to Jordi Garcia Orel- lana, a passionate professor, mentor, and friend who devoted his career to inspiring and encouraging many young researchers. The Figure B1. Temporal evolution of Darcy flux from October 2019 authors would also like to thank all colleagues from the Grup de Re- to April 2020 at the experimental site of the Argentona ephemeral cerca en Radioactivitat Ambiental de Barcelona – GRAB (Universi- stream. Red lines indicate the groundwater and seawater samplings tat Autònoma de Barcelona) and Maravillas Abad from ICM-CSIC performed at the study site (P1, P2, and BF) and gray bands indicate for the analysis of nutrients. The authors would like to acknowledge the major precipitation events of October 2019, December 2019, SIMMAR (Serveis Integrals de Manteniment del Maresme) and the and January 2020. Consell Comarcal del Maresme for the construction of the research site. Data availability. Rainfall data from 2015 to 2020 are registered Financial support. This research has been supported by the Agen- by the Meteorological Catalan Service (Servei Meteorològic de cia Estatal de Investigación (grant nos. PID2019-110212RB-C22, Catalunya, SMC) at the meteorological station located in the mu- CGL2016-77122-C2-1-R/2-R, and PID2019-110311RB-C21), the nicipality of Cabrils, Spain. These data are only available upon re- Generalitat de Catalunya (grant no. ACA210/18/00007), the Direc- quest through the SMC web page (https://www.meteo.cat/wpweb/ ció General de Recerca, Generalitat de Catalunya (grant nos. 2017 serveis/cataleg-de-serveis/dades-meteorologiques/; Servei Meteo- SGR-1588 and 2017 SGR 1485), the Agència de Gestió d’Ajuts rològic de Catalunya, 2020). Oceanographic data (mean sea level Universitaris i de Recerca (grant nos. 2017FI_B_00365 and 2019- and significant wave height) were obtained from the national buoy BP-00241), and the Ministerio de Economía y Competitividad network of the Spanish government. These data are available (grant nos. MDM2015-0552-17-1 and BES-2017-080740). through the following web page (https://portus.puertos.es/?locale= es#/; Puertos del Estado, 2020). Review statement. This paper was edited by Albrecht Weerts and reviewed by two anonymous referees. Supplement. The supplement related to this article is available on- line at: https://doi.org/10.5194/hess-26-4619-2022-supplement. Author contributions. 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