# Constraining the temporal variations of Ra isotopes and Rn in the groundwater end-member: Implications for derived SGD estimates

Marc Cerda-Domenech, Valenti Rodellas, Albert Folch, Jordi Garcia-Orellana

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Marc Cerda-Domenech, Valenti Rodellas, Albert Folch, Jordi Garcia-Orellana. Constraining the temporal variations of Ra isotopes and Rn in the groundwater end-member: Implications for derived SGD estimates. Science of the Total Environment, 2017, 595, pp.849-857. 10.1016/j.scitotenv.2017.03.005. hal-01765635

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1 Constraining the temporal variations of Ra isotopes and Rn in the
2 groundwater end-member: Implications for derived SGD estimates

3 Marc Cerdà-Domènech<sup>a,*</sup>, Valentí Rodellas<sup>b,c</sup>, Albert Folch<sup>d,e</sup>, Jordi Garcia-Orellana<sup>b,f</sup>

4

5 (a) GRC Geociències Marines, Dep. de Dinàmica de la Terra i de l'Oceà, Universitat de
6 Barcelona, 08028 Barcelona, Spain.

7 (b) Institut de Ciència i Tecnologia Ambientals (ICTA), Universitat Autònoma de Barcelona,
8 08193 Bellaterra, Spain.

9 (c) Center de Recherche et d'Enseignement de Géosciences de l'Environment (CEREGE), Aix-
10 Marseille Université, 13545 Aix-en-Provence, France.

11 (d) Department of Civil and Environmental Engineering, Universitat Politècnica de Catalunya
12 (UPC), c/Jordi Girona 1-3, Barcelona, Spain

13 (e) Associated Unit: Hydrogeology Group (UPC-CSIC), Spain

14 (f) Departament de Física, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain.

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16 *Corresponding author:

17 Marc Cerdà-Domènech

18 GRC Geociències Marines, Departament de Dinàmica de la Terra i de l'Oceà

19 Facultat de Ciències de la Terra, Universitat de Barcelona

20 c/ Martí Franquès s/n, 08028, Barcelona, Spain

21 Tel.: (+34) 93 402 13 69 Fax: (+34) 93 402 13 40

22 E-mail: cerda.domenech@ub.edu

23
24 **KEYWORDS**: submarine groundwater discharge, radium, radon, salinity interface, end-member

25 Highlights:

26 1. Ra isotopes and <sup>222</sup>Rn were measured for nearly two years in a coastal piezometer.

27 2. High Ra variability was registered, especially during intense rainfall events.

28 3. <sup>222</sup>Rn displayed minor variations due to the no influence of groundwater salinity on

29 radon emanation.

30 4. The hydrodynamics of coastal aquifers must be considered as essential to understand

31 SGD fluxes.

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35 ABSTRACT

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Submarine groundwater discharge (SGD) has been recognized as an important supplier of chemical compounds to the ocean that may influence coastal geochemical cycles. Radium isotopes ($^{223}$Ra, $^{224}$Ra, $^{226}$Ra, $^{228}$Ra) and radon ($^{222}$Rn) have been widely applied as tracers of SGD.

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Their application requires the appropriate characterization of both the concentrations of tracers in the discharging groundwater and their distribution in the coastal water column. This study evaluates the temporal evolution of Ra isotopes and $^{222}$Rn concentrations in a dynamic subterranean estuary of a microtidal Mediterranean coastal aquifer that experiences large displacements of the fresh-saltwater interface as a necessary initial step in evaluating the influence of SGD in coastal waters. We show that changes in groundwater salinities due to the seaward displacement of the fresh-saltwater interface produced large variations in Ra activities in groundwater (by a factor of ~19, ~14, ~6, and ~11 for $^{223}$Ra, $^{224}$Ra, $^{226}$Ra and $^{228}$Ra, respectively), most importantly during rainfall events. In contrast, the $^{222}$Rn activities in groundwater oscillated by a factor of 3 during these rainy periods. The large temporal variability in Ra activities hampers characterization of the SGD end-member when using Ra isotopes as tracers, and thus presents a challenge for obtaining accurate SGD estimates. This study emphasizes the need to understand the hydrodynamics of coastal aquifers to appropriately constrain the Ra isotopes and $^{222}$Rn concentrations in groundwater and when applying both tracers in dynamic microtidal coastal systems.

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Submarine Groundwater Discharge (SGD) includes both meteoric fresh groundwater flowing into the sea and seawater recirculation through coastal aquifers (Burnett et al., 2003). Both flows mix in coastal aquifers, so-called subterranean estuaries (STEs), where biogeochemical reactions occur as a consequence of interactions between terrestrially derived groundwater, recirculated seawater and the geological matrix (Moore, 1999). This dynamic mixing zone influences the transfer of several chemical constituents to the coastal ocean (Tovar-Sánchez et al., 2014; Sadat-Noori et al., 2016). SGD is now recognized as a relevant source of nutrients, trace metals and other compounds and contaminants to the coastal sea (Kim et al., 2003; Johnson et al., 2008; Gonneea et al., 2014; Pavlidou et al., 2014; Rodellas et al., 2015b; Trezzi et al., 2016)

Radium (Ra) isotopes ($^{223}$Ra, $^{224}$Ra, $^{226}$Ra, $^{228}$Ra) and radon ($^{222}$Rn) have been widely used as tracers to quantify SGD (e.g., Charette et al., 2001; Moore, 2003; Burnett et al., 2006, Burnett et al., 2008; Lee et al., 2012). Ra isotopes and $^{222}$Rn, which are continuously produced in geological materials by the decay of their uranium and thorium parents, are appropriate SGD tracers, mainly because they behave conservatively once released to the sea and because they are significantly enriched in SGD fluids relative to seawater (Burnett et al., 2006). The approach used to estimate SGD flows using Ra isotopes or $^{222}$Rn is based on characterizing the flux of Ra/Rn supplied by SGD ($J_{Ra}$ or $J_{Rn}$) using the Ra/Rn distribution in coastal waters and the Ra/Rn concentration in groundwater inflowing to the sea, the so-called SGD end-member ($Ra_{GW}$ or $Rn_{GW}$) (Burnett and Dulaiova, 2003; Moore, 1996a).

Most SGD studies have been devoted primarily to estimating the fluxes of Ra and Rn ($J_{Ra}$ or $J_{Rn}$) (e.g., Kim et al., 2005; Beck et al., 2008; Smoak et al., 2012), whereas the Ra and Rn concentrations in the groundwater end-member have been largely overlooked, even though they are a critical component of the tracer-derived estimates (Cho and Kim, 2016; Burnett et al., 2007; Gonneea et al., 2013, 2008). Ra activities in subterranean estuaries can vary significantly (one or two orders of magnitude) over space and time at a given study site (Ollivier et al., 2007; Beck et
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al., 2008). Thus, the lack of constraint on this term remains a significant source of uncertainty in Ra-derived SGD estimates (Charette, 2007; Gonneea et al., 2013).

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Several factors influence Ra activities in SGD, including the presence of Mn-Fe oxides (Gonneea et al., 2008), the pH of the subterranean estuary (Beck and Cochran, 2013), the ionic strength (Elsinger and Moore, 1980; Webster et al., 1995), the properties of the geological matrix (Swarzenski, 2007) and the residence time within the STE (Rodellas et al., 2017). Among them, the ionic strength of the solution (i.e., its salinity) has long been recognized as the main factor influencing the Ra activities in the SGD end-member, and Ra desorption increases significantly with salinity (Cho and Kim, 2016; Elsinger and Moore, 1980; Webster et al., 1995). As a consequence, Ra activities in SGD may vary substantially depending on the position of the freshwater-saltwater interface within the subterranean estuary. The position of this interface may oscillate, due to variations in the hydraulic gradient of the fresh groundwater (caused by, e.g., recharge and abstraction) and marine driving forces (e.g., tides, wave and storms) (Gonneea et al., 2013; Heiss and Michael, 2014). These temporal oscillations of Ra activity in STEs, along with the spatial heterogeneity of coastal aquifers, make it difficult to characterize the RaGW end-member, which is needed to provide accurate estimates of SGD (Michael et al., 2011).

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Unlike Ra, Rn is an inert gas. Thus, its chemical behavior is not influenced by the physicochemical characteristics of the coastal aquifer (e.g., groundwater salinity, temperature or redox conditions). Radon activities in coastal aquifers are thus primarily controlled by the content of ²²⁶Ra in the aquifer solids and dissolved in groundwater (Dulaiova et al., 2008). However, the physical and geochemical processes occurring in coastal aquifers (e.g., manganese cycling or ionic exchange) can affect the ²²⁶Ra distribution, thus driving changes in the ²²²Rn concentration of groundwater (Dulaiova et al., 2008).

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In contrast to many coastal areas of the world, the Mediterranean Sea is characterized by a microtidal regime, with tidal amplitudes commonly lower than 0.2 m. Therefore, tidal forces do not exert a significant water pumping influence in the coastal aquifers there. As a consequence, the location and movement of the salinity interface in subterranean estuaries in the Mediterranean region, as well as in other microtidal sea regions (e.g., the Caribbean Sea or the Baltic Sea), are

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111 mainly regulated by hydrogeological factors, such as aquifer properties and the aquifer water
112 budget. Thus, understanding the hydrogeological characteristics (i.e. recharge, discharge, etc.) of
113 coastal aquifers in microtidal regions and their impacts on the Ra and Rn concentrations in
114 groundwater is particularly important for improving tracer-derived estimates of SGD.

115 This study evaluates the temporal evolution of Ra isotopes and <sup>222</sup>Rn concentrations in a
116 Mediterranean microtidal coastal aquifer (Argentona, Catalonia, Eastern Iberian Peninsula) over
117 almost 2 years. During this period, two piezometers (PZ) were continuously monitored to measure
118 variations in groundwater levels, salinity and the Ra and <sup>222</sup>Rn concentrations in the groundwater.

119 The aim of this study is to assess hydrogeological dynamics affecting the variability in the
120 concentrations of Ra isotopes and <sup>222</sup>Rn in groundwater from this coastal microtidal aquifer. This
121 variability may have critical implications for the quantification of Ra/Rn-derived SGD fluxes and
122 therefore their implications for coastal biogeochemical cycles.

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## 2. STUDY AREA AND METHODS

### 2.1 Study area: The Argentona alluvial aquifer and the oceanographic setting

126 The Argentona alluvial aquifer is located between the Catalan Littoral Mountain Range and the
127 Mediterranean Sea (Barcelona, NW Mediterranean) and extends in the SW-NE direction (Figure
128 1). It has an area of approximately 35 km² that is mainly devoted to agricultural uses. The
129 climatology of this region is characterized by Western Mediterranean conditions, and the region
130 experiences high temperatures in summer (mean ~23 °C) and mild temperatures in winter (mean
131 ~10 °C). The annual precipitation of the area ranges from 500 to 800 mm, and is mainly related to
132 intense and short storms during the fall and spring season. Surface water flows in the Argentona
133 main stream are restricted to the rainy seasons, particularly during major rainfall events (e.g., > 50
134 mm in a few hours).

135 The Argentona aquifer is associated with a tectonic fault and sinking blocks where stream valleys
136 have developed. The geology of the alluvial aquifer, which is unconfined, is dominated by detrital
137 Quaternary sediments that are the product of chemical weathering of the granitic basement along
138 fissures (Montoto, 1967). These sediments constitute layers of unconsolidated sands, gravels and
139 clays deposited in an alluvial fan system during basin formation. In the nearshore area, the aquifer
140 is mainly composed of marine sands (Catalan Water Agency, 2010).

141 A water budget for the aquifer conducted by the Catalan Water Agency (ACA), which used a
142 numerical flow model of the aquifer system and covered the period 2006-2009, estimated an
143 aquifer recharge rate between 3.6·10⁶ to 5.4·10⁶ m³ y⁻¹. Major outputs from the aquifer include
144 urban, agricultural and industrial extraction ((1.4 - 1.9)·10⁶ m³ y⁻¹) and fresh groundwater
145 discharge to the sea ((1.2 - 2.4)·10⁶ m³ y⁻¹).

146 2.2 Groundwater monitoring and sample collection

147 During a period of 2 years (from January 2013 to December 2014), hydrogeological conditions
148 (groundwater level and salinity), Ra isotopes and ²²²Rn activities were monitored in the Argentona
149 coastal aquifer using two entirely screened piezometers. The first piezometer, PZ1-, was drilled to
150 a depth of 35 m by the Catalan Water Agency in 2007. It is located 380 m from the shoreline and
151 has a groundwater level approximately 6 - 7 m below the ground surface. The second piezometer,
152 PZ0, which was only monitored for 7 months (May 2014 to December 2014), was drilled
153 approximately 8 m from the shoreline to a depth of 2 m. Three CTD-Diver® data loggers were
154 installed in the piezometers to measure temporal salinity, temperature and groundwater table
155 variations. The first instrument was installed in PZ1 10 m below the ground surface (3 - 4 m
156 below the water level) to evaluate the freshwater conditions (hereinafter referred to as PZ1-S). The
157 second instrument was installed in PZ1 at a depth of 34 m to monitor variations in the mixing
158 zone between the freshwater and saltwater (hereinafter referred to as PZ1-B) (Figure 1). (3) The
159 third instrument was installed in PZ0 at a depth of 1.8 m below the ground surface to evaluate
160 conditions in the groundwater close to the coastal seepage face. Variations in air pressure were
161 compensated for using air pressure measurements that were made simultaneously using a Baro-
162 Diver® that was also installed at the top of the PZ1 piezometer.

163 Groundwater samples were collected at the same depths where the CTDs were installed, samples
164 from PZ1-S (n=15) were collected 10 m below the ground surface, samples from PZ1-B (n=15)
were collected 34 m below the ground surface, and samples from PZ0 (n=8) were collected 1.5 m
below the ground surface). A submersible pump was used to collect samples with volumes of 60 L
(PZ1-S), 25 L (PZ1-B) and 10 L (PZ0) for Ra isotopes. Samples with volumes of 250-mL were
collected for <sup>222</sup>Rn analyses from PZ1-S and PZ1-B. Radon-222 was not analyzed in PZ0 due to
the low water level (less than 20 cm), which increased the exchange of <sup>222</sup>Rn with the air and did
not permit us to obtain a representative sample. During each sampling survey, salinity and
temperature were measured *in situ* using a YSI-556 handheld probe.

Water samples for Ra isotopes were filtered through MnO₂-impregnated acrylic fibers (hereinafter
referred to as Mn fibers) at flow rates < 1 L min⁻¹ to quantitatively sorb Ra onto the Mn fibers
(Moore and Reid, 1973). For the <sup>222</sup>Rn samples, groundwater was pumped directly through a tube
that reached to the bottom of each 250-mL bottle. The groundwater was allowed to overflow for
several volumes of the sample bottle to reduce Rn losses to the atmosphere.

## 2.3 Analytical methods

Once in the laboratory, the Mn fibers containing the Ra isotopes were rinsed with Ra-free water
(Sun and Torgersen, 1998), partially dried and placed in a Radium Delayed Coincidence Counter
(RaDeCC) to quantify the short-lived Ra isotopes, <sup>223</sup>Ra and <sup>224</sup>Ra (Garcia-Solsona et al., 2008;
Moore and Arnold, 1996). After these measurements, the Mn fibers were ashed (820°C, 16 hours),
ground and aged for 3 weeks in counting vials to allow <sup>222</sup>Rn to come into secular equilibrium
with <sup>226</sup>Ra. Samples were measured using a well-type HPGe gamma detector. <sup>226</sup>Ra and <sup>228</sup>Ra
were measured via <sup>214</sup>Pb at the 351.9 keV photopeak and <sup>228</sup>Ac at the 911 keV photopeak,
respectively. <sup>222</sup>Rn activities in groundwater samples were determined using a portable <sup>222</sup>Rn-in-
air alpha spectrometer RAD7 (Durridge Inc.) coupled with a RAD-H₂O attachment that allows the
direct determination of <sup>222</sup>Rn from 250-mL bottles (Burnett et al., 2001).
## 3. RESULTS

### 3.1 Hydrogeological features and dynamics

Results of the continuous monitoring of the groundwater table and salinity at PZ1 are shown in Figure 2. The fresh groundwater part of the coastal aquifer (PZ1-S) showed relatively constant salinities (~0.4) with minimum values of 0.05 occurring during some rainfall events (Figure 2b).

In contrast, salinities at the bottom level (PZ1-B) varied significantly during the studied period and ranged from 0.3 to 17 (Figure 2c). Two general patterns in salinity can be identified at the bottom. (1) Salinities were stable during dry periods (sometimes showing minor and constant salinity increases due to reductions in recharge), with maximum values of 15 - 17 (e.g., January – February 2013 and July – September 2013). On the other hand, (2) large fluctuations occur during rainfall events, and sudden decreases in groundwater salinity were followed by gradual increases after rain/recharge episodes. This behavior is exemplified by a particular rainfall event (RFE-8; Figure 2d, 2e). After the precipitation fell, the groundwater table increased by 60 cm and the salinity at the bottom of PZ1 decreased sharply (over 23 hours) from 13 to 0.2. This fast piezometric response to precipitation can be attributed to the hydrogeological features of the alluvial aquifer, which has a high hydraulic conductivity and a small non-saturated zone in the lower part of the basin (Catalan Water Agency, 2010), but it is also accentuated due to the use of a fully screened piezometer (Shalev et al., 2009). Under stable conditions, an equilibrium exists between the aquifer, which is governed by the laws of groundwater motion, and the entire screened piezometer, which is governed by the hydrostatic law (Rushton, 1980). However, under changing conditions, this equilibrium is broken, and the position of the salinity interface in the piezometer may not correspond to its actual position in the aquifer (Rushton, 1980; Tellam et al., 1986; Carrera et al., 2010). For instance, during heavy rainfall events, the higher hydraulic level of fresh groundwater occurring in the upper part of the entirely screened piezometer pushes the saltwater downward faster than it occurs in the aquifer. As a result, after these episodes, salinity decreased faster in the piezometer than in the aquifer itself.

Rainfall-derived aquifer recharge produced an increase in the hydraulic gradient between the coastal aquifer and the sea, leading to seaward displacement of the salinity interface and a
215 significant decrease in groundwater salinity in the bottom section of the piezometer (Figure 2c, 2e). Once the infiltration of rainfall water had finished, the hydraulic gradient decreased gradually, 216 resulting in landward displacement of the salinity interface and a consequent increase in 217 groundwater salinity in the bottom section of the piezometer (Figure 2c, 2e). While the seaward 218 displacement of the salinity interface occurred shortly (a few hours to days) after the precipitation 219 episode, several weeks or even months were needed to recover the salinity values observed prior 220 to the rain event(s). For instance, salinity at PZ1-B decreased from 13 to 0.3 in less than a day 221 (~23 hours) associated with RFE-8, whereas several months (~6 months) were needed to recover 222 the initial salinity of 13-15 (Figure 2e). 223

224 Unlike aquifers dominated by tidal cycles, the impact of microtides on the hydraulic gradient, and 225 consequently the position of the salinity interface, can be negligible in PZ1. As shown in Figure 2f, the tidal range in the area was on the order of ~20 cm (data compiled from the Barcelona 226 mareograph and obtained from the State Ports Authority; www.puertos.es), a pattern commonly 227 observed in the NW Mediterranean Sea. These small tidal oscillations induced only small 228 variations in the groundwater table and in the groundwater salinities at the bottom of the 229 piezometer (tidally derived groundwater salinity variations of ~1). 230

231 Regarding PZ0, the piezometer closest to the shoreline but located in the same aquifer unit, the 232 recorded data on groundwater table and salinity followed a similar pattern as those observed in 233 PZ1 (Figure 3). Namely, stable salinities were observed during dry periods, with maximum values 234 of 3 (e.g., May 2014 and August – November 2014), and fluctuations of salinity were observed 235 during rainfall events, with salinity decreasing to reach minimum values of 0.1. Although the 236 piezometers, PZ1 and PZ0, are located 380 and 8 m from the shoreline, respectively, CTDs 237 installed in both piezometers registered almost the same variations in the groundwater table and 238 salinity (e.g., RFE-12, 13, 14 and 16), indicating similar responses to recharge/rain events, as 239 expected. However, various significant increases in groundwater salinities in PZ0 (up to salinities 240 higher than 18) that did not correspond with the fluctuations recorded in PZ1 were also observed. 241 These salinity increases in PZ0 coincided with increases in the significant wave height ($H_0$) in the 242 coastal sea, mainly related to the eastern storms characteristic of the spring and autumn seasons in
the northwestern Mediterranean Sea. The increase in the frequency and height of waves at the shoreline modifies the equilibrium of the hydraulic gradient, moving the salinity interface landward and increasing the salinity in PZ0 (Moore and Wilson, 2005). The influence of these storms on PZ0 lasted for short periods (less than 6 days), and PZ0 rapidly recovered the groundwater salinities observed before storm events, due to the contrary effect of the downward groundwater flow of the aquifer. Given the limited influence of these kind of storm events, the groundwater recharge-discharge cycle appears to be the main driver controlling the position of the salinity interface in the studied NW Mediterranean coastal aquifer.

3.2 Ra isotopes and <sup>222</sup>Rn characterization in groundwater

The activities of Ra isotopes and <sup>222</sup>Rn measured in groundwater samples from PZ1-S, PZ1-B and PZ0, together with their corresponding salinities, are presented in Table 1. The activities of Ra isotopes in shallow groundwater (PZ1-S) samples showed relatively constant values that ranged from 0.4 to 3.8 dpm 100 L<sup>-1</sup> for <sup>223</sup>Ra, 40 to 90 dpm 100 L<sup>-1</sup> for <sup>224</sup>Ra, 9 to 20 dpm 100 L<sup>-1</sup> for <sup>226</sup>Ra and 60 to 90 dpm 100 L<sup>-1</sup> for <sup>228</sup>Ra. The activity of Ra in PZ1-B presented a larger variability, and these values ranged from 2 to 80 dpm 100 L<sup>-1</sup> for <sup>223</sup>Ra, 70 to 2100 dpm 100 L<sup>-1</sup> for <sup>224</sup>Ra, 30 to 440 dpm 100 L<sup>-1</sup> for <sup>226</sup>Ra and 70 to 1700 dpm 100 L<sup>-1</sup> for <sup>228</sup>Ra. The activities of Ra isotopes were particularly low during rainfall events, when groundwater salinities in the piezometer were minimal, whereas the largest activities were measured during dry periods that corresponded to the highest groundwater salinities. On the other hand, Ra activities in PZ0 showed a similar variability as those seen in PZ1-B, and they ranged from 3 to 10 dpm 100 L<sup>-1</sup> for <sup>223</sup>Ra, 260 to 830 dpm 100 L<sup>-1</sup> for <sup>224</sup>Ra, 40 to 70 dpm 100 L<sup>-1</sup> for <sup>226</sup>Ra and from 200 to 760 dpm 100 L<sup>-1</sup> for <sup>228</sup>Ra. Ra activities in groundwater samples were higher than values commonly found in NW Mediterranean coastal aquifers (e.g., Rodellas et al., 2015a), most likely due to the high radium content of the granitic basement (which are rich in <sup>232</sup>Th decay chain radionuclides) and the relatively high salinities of the groundwater in the aquifer that favor radium desorption. The activities of Ra isotopes measured in the two piezometers showed strong positive correlations with salinity (R<sup>2</sup>=0.91, 0.97, 0.87 and 0.95 for <sup>223</sup>Ra, <sup>224</sup>Ra, <sup>226</sup>Ra and <sup>228</sup>Ra, respectively; p<0.001), suggesting that the groundwater sampled in PZ0, which is near the discharge area, has similar
characteristics as the groundwater in PZ1 (Figure 4a). Unlike Ra isotopes activities, the activities of <sup>222</sup>Rn in groundwater samples reflected relatively constant concentrations in the water samples from PZ1-S and PZ1-B throughout the studied period, with values ranged from 40·10<sup>3</sup> to 120·10<sup>3</sup> dpm 100 L<sup>-1</sup> (Figure 4b).

A detailed analysis of the rainfall event RFE-8 suggested that Ra activities in groundwater from PZ1-B followed a pattern linked to groundwater recharge cycles (Figure 2; Table 1). Thus, just after the precipitation event, Ra activities diminished by a factor of ~19, ~14, ~6, and ~11 for <sup>223</sup>Ra, <sup>224</sup>Ra, <sup>226</sup>Ra and <sup>228</sup>Ra, respectively, in a short period of time (Table 1). A few days after the event, and coupled with an increase of groundwater salinity, Ra activities gradually increased toward the activities measured before the rainfall event. The activity of <sup>222</sup>Rn followed a similar pattern to those of the Ra isotopes, although it only decreased by a factor of ~2 (from 81·10<sup>3</sup> to 39·10<sup>3</sup> dpm 100 L<sup>-1</sup>) after the precipitation event.

## 4. DISCUSSION

### 4.1 Variability in the activity of <sup>222</sup>Rn and the activities of Ra isotopes

The activities of Ra isotopes measured in groundwater from PZ1 and PZ0 spanned a wide range. In particular, <sup>224</sup>Ra in PZ1-B varied by a factor of 31 during the 2 monitored years, whereas <sup>222</sup>Rn activities were relatively constant throughout the sampling period and varied by less than a factor of 3. This contrasting pattern between Ra isotopes and <sup>222</sup>Rn is likely a consequence of differences in their geochemical behavior in the subterranean estuary. Radium desorption is an exchange process that is highly dependent on the ionic strength (salinity) of the solution; it commonly represents the main factor determining the activities of Ra isotopes dissolved in groundwater (Cho and Kim, 2016; Gonneea et al., 2008). Consequently, the activities of Ra isotopes in fresh groundwater from the shallow depths of the subterranean estuary are considerably lower than those in brackish groundwater from greater depths. Ra activities at the salinity interface are likely explained by binary mixing between low-Ra, low-salinity groundwater and high-Ra, high-salinity groundwater. Indeed, this behavior explains the strong linear correlations between the concentrations of Ra isotopes and groundwater salinities observed over the entire period of the
study (Figure 4a). Unlike Ra isotopes, groundwater salinity has little influence on the behavior of $^{222}$Rn in the aquifer because it is a noble gas. Moreover, $^{222}$Rn supported by $^{226}$Ra dissolved in groundwater usually represents a minor fraction of the total $^{222}$Rn activity in groundwater. Therefore, similar $^{222}$Rn activities are expected in fresh, brackish and salty groundwater, thus explaining the lack of significant correlations between $^{222}$Rn activities and salinity in groundwater from the studied site (Figure 4b). However, it should be noted that, after rainfall events, $^{222}$Rn activities decreased considerably, together with salinity. This coupled $^{222}$Rn-salinity decrease is attributed to dilution that is due to the vertical infiltration of rainwater into the highly permeable coastal aquifer, since rainwater has a salinity close to 0 and contains negligible amounts of $^{222}$Rn.

After the rainwater had interacted with the geological matrix, the activity of $^{222}$Rn in the meteoric fresh water started to increase, due to its production within the aquifer solids. Concentrations in equilibrium with the aquifer matrix were achieved within 15 - 20 days after rainwater infiltration into the aquifer.

## 4.2 Temporal variability of the salinity interface: Implications for Ra- and Rn-derived SGD estimates

The position and dynamics of the salinity interface are influenced to a large degree by multiple forcing mechanisms, which include marine (e.g., wave setup and swash infiltration, as well as tidal pumping) and terrestrial drivers (e.g., recharge patterns) (Li et al., 2008; Henderson et al., 2010; Santos et al., 2012; Abarca et al., 2013). Whereas tides are physical forces that are relevant for determining freshwater-seawater mixing in most coastal areas (Kim and Hwang, 2002; Michael et al., 2003; Charette, 2007; Bokuniewicz et al., 2015), they play a minor role in microtidal environments such as the Mediterranean Sea, the Baltic Sea and the Caribbean Sea (Szymczycha et al., 2012; Gonneea et al., 2014; Rodellas et al., 2014;). In such systems, the dynamics of the salinity interface are mainly governed by the inland hydraulic gradient, which mainly reflects the terrestrial groundwater recharge rate (Dausman and Langevin, 2005).

Microtidal coastal aquifers are thus highly influenced by precipitation regimes, and they become highly dynamic systems when precipitation events are temporally variable. This pattern is commonly observed in the Mediterranean basin, where 4 - 6 short (lasting hours to a few days)
and intense (a maximum precipitation rate of 40 mm·day⁻¹ occurred during the studied period, but it can be much higher) rainfall events represent more than 70% of the annual precipitation (e.g., RFE-1, 8, 13 and 15, Figure 2) (Gasith and Resh, 1999). According to the change in hydraulic gradient observed in PZ0 (the closest to the sea) over a period of nearly 8 months, it can be estimated that 50% of the discharge is produced during and/or within a few days after rain events.
Thus, short and intense rainfall events are likely to produce the highest rates of fresh SGD to the coastal sea, which would account for a significant proportion of the freshwater that seeps into the sea yearly. For the rest of the year, fresh SGD flows are expected to be significantly lower and more constant. Thus, estimating SGD fluxes in microtidal systems during periods of intense precipitation are particularly relevant from both a hydrogeological and a biogeochemical perspective.

The quantification of SGD flows to the coastal sea using Ra isotopes and ²²²Rn requires accurately characterizing their activities in groundwater discharging to the coastal sea (i.e., the SGD end-member). Indeed, Ra- and ²²²Rn-derived SGD fluxes can only be resolved at levels where the tracer concentration in the end-member is well constrained. Thus, the selection of an appropriate end-member is commonly the main source of uncertainty in final SGD estimates (Cho and Kim, 2016; Burnett et al., 2007; Dulaiova et al., 2008; Garcia-Orellana et al., 2010; Gonneea et al., 2013). Given that Ra isotopes are highly influenced by the ionic strength of the dissolved phase, the process of freshwater-seawater mixing dynamics are key processes in understanding the distribution of Ra in a coastal aquifer and constraining the Ra concentration in the SGD end-member (Charette et al., 2001; Gonneea et al., 2013; Michael et al., 2011). Indeed, several studies have reported that the activities of Ra isotopes in the SGD end-member vary over a wide range (up to orders of magnitude) both in space and time, and are often linked to changes in the ionic strength of the dissolved phase (Cho and Kim, 2016). Thus, accurately evaluating the complex and dynamic mixing of fresh groundwater with seawater (i.e., the position of the salinity interface) becomes critical for obtaining reliable Ra-derived SGD estimates.

In highly dynamic microtidal systems, achieving an appropriate understanding of the hydrodynamics of coastal aquifers requires significant monitoring and characterization efforts that
are often unavailable when SGD studies are conducted. To overcome this limited hydrogeological understanding, studies that use Ra isotopes to quantify SGD commonly rely on collecting a relatively large number of samples, as well as using different approaches to characterize the SGD end-member (potential approaches include, e.g., averaging the Ra activities from all the samples collected (Gonneea et al., 2014), taking the maximum measured Ra activities to obtain conservative estimates (Moore, 1996b), or using ranges of Ra activities covering the samples collected (Kim et al., 2005). Whereas these approaches may work in many of the tidal systems studied, using them in coastal aquifers characterized by highly dynamic freshwater-seawater mixing zones (with large salinity and thus large Ra variations) may introduce large uncertainties into the final SGD quantification. Constraining Ra activities in the SGD end-member during (or just after) intense rainfall events may be particularly challenging, given the large temporal variations in salinity and Ra in coastal groundwater, making it difficult to obtain reliable SGD estimates. As detailed above, it should be noted that inputs of fresh groundwater to coastal seas are expected to be much higher during these rainfall periods, and thus, the SGD evaluations during these periods have a major hydrological and biogeochemical interest. Conversely, Ra activities in the SGD end-member can be more easily constrained during dry periods because they are associated with reduced variability in coastal groundwater, thus allowing appropriate estimates of Ra-derived SGD fluxes to be made for the rest of the year.

Unlike Ra isotopes, the activity of <sup>222</sup>Rn in coastal groundwater appears to be relatively constant throughout the year due to the relatively small influence of groundwater salinity on <sup>222</sup>Rn behavior, even during intense rainy events (<sup>222</sup>Rn activities only varied by a factor 2 during these events). These small variations allow us to better constraint the activity of <sup>222</sup>Rn in the SGD end-member and, thus, better estimate SGD flows during rainy periods. This observed pattern suggests that <sup>222</sup>Rn may be a more appropriate SGD tracer than Ra isotopes in systems with highly dynamic salinity interfaces, particularly during the wet season, when groundwater salinities (and thus Ra activities) may change significantly over short temporal scales. However, whereas the Rn end-member can be relatively well constrained, Rn-derived SGD estimates are highly sensitive to Rn mixing losses and especially the loss of Rn to the atmosphere by degassing. These two loss terms
are difficult to determine, particularly under conditions with strong winds and high waves, which
often occur concurrently with eastern storm events.

## 5. CONCLUSIONS

Understanding the behavior of tracers used to determine submarine groundwater discharge (SGD) fluxes is crucial for accurately quantifying groundwater- and chemical- driven fluxes to the ocean in order to better understand coastal biogeochemical cycles. The results obtained in this work reveal that activities of Ra isotopes, which are commonly used tracers to quantify SGD, show a large variability in microtidal coastal groundwater systems throughout the year and particularly during intense rainfall events (e.g., activity changes by a factor of ~19, ~14, ~6, and ~11 for $^{223}$Ra, $^{224}$Ra, $^{226}$Ra and $^{228}$Ra during a particular rainfall event, RFE-8). This large variability, which is linked to the displacement of the salinity interface seawards, complicates the accurate estimation of the Ra-SGD end-member, resulting in Ra-derived SGD estimates with large uncertainties. In contrast, $^{222}$Rn activities present only minor variations (by a factor of 3), as a consequence of the relatively small influence of groundwater salinity on $^{222}$Rn activities. In microtidal systems, high SGD-driven fluxes to the sea of both freshwater and dissolved chemical compounds are expected to occur during the rainy season and/or intense rainfall events. Thus, scientific efforts should be particularly directed toward understanding SGD fluxes during this season, when the influence of SGD on hydrological budgets is expected to be significant. The observed differences between the rainy season and the dry period indicate that appropriate SGD quantification over longer (monthly or yearly) time scales cannot be carried out properly without considering the temporal variability in Ra activities.

In addition to the difficulties in estimating SGD flows during rainfall events (which are mainly due to the difficulty of characterizing the SGD end-member), the extreme variability in the concentrations of chemical compounds (e.g., nutrients, trace metals, contaminants, and other species) in the groundwater poses an additional challenge in constraining a representative groundwater end-member. The fluxes of dissolved compounds transported by SGD have usually been calculated by multiplying the SGD flux derived using Ra isotopes or Rn by the concentration
410 of the chemical compounds in the inflowing SGD. As in the case of the Ra end-member, it is
411 particularly critical to identify a representative concentration of the studied dissolved chemical
412 compound in the discharging groundwater during intense rainfall events. A poor understanding of
413 the behavior of dissolved chemicals in the coastal aquifer during rainfall events may lead to large
414 uncertainties in SGD-driven chemical fluxes. Since the SGD-driven fluxes during the wet periods
415 may account for a significant fraction of the chemical fluxes to the coastal areas, it is expected that
416 the impact of SGD on coastal biogeochemical cycles is particularly relevant during these events.

417 Characterizing the behavior of Ra/Rn and dissolved chemical compounds within subterranean
418 estuaries during rainfall events in microtidal systems remains a challenge, which must be
419 considered in future studies of SGD-driven nutrient fluxes to coastal areas in order to understand
420 the relative impact of SGD on coastal biogeochemical cycles.

421

422 ACKNOWLEDGMENTS

423 This work was funded by projects CGL2013-48869-C2-1 and CGL2013-48869-C2-2-R of the
424 Spanish Government. M.C.D acknowledges financial support from the Ministerio de Economía y
425 Competitividad (PhD fellowship, A-2014-9185) through the NUREIEV project (CTM2013-
426 44598-R) coordinated by GRC Geociències Marines of the Departament de Dinàmica de la Terra i
de l'Oceà (Universitat de Barcelona). The authors wish to thank the Generalitat de Catalunya for
its support of MERS (2014 SGR-1356) and Agència Catalana de l'Aigua (ACA). V.R. also
428 acknowledges financial support from the European Union's FP7 program (Marie Curie Actions
429 PCOFUND-GA-2013-609102) through the PRESTIGE program coordinated by Campus France.

430 This study is a contribution to the ANR @RAction chair (ANR-14-ACHN-0007-01) and Labex
431 OT-Med (ANR-11-LABEX-0061), which was funded by the "Investissements d'Avenir" program
432 through the A*MIDEX project (ANR-11-IDEX-0001-02) of the French National Research
433 Agency (ANR).
434

435
<figure><img src="image_2.png" /></figure>

437

438

439
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Location of the Argentona stream in the NW Mediterranean Sea, including the location of the two monitored piezometers (PZ).</figcaption></figure>

440

441

442

443
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Temporal evolution of the hydrogeological conditions and the Ra and Rn activities in the coastal aquifer: (a) Daily cumulative precipitation in the area and measured groundwater table elevations in the coastal aquifer; (b) and (c) groundwater salinities in the shallow (PZ1-S) and bottom (PZ1-B) parts, respectively, of piezometer PZ1; (d) groundwater table elevation and precipitation records from rainfall episode 8 (RFE-8); (e) Variations in Rn and Ra activities and salinity in PZ1-B</figcaption></figure>

444

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447

448

449
during RFE-8; (f) Sea level and salinity oscillations in PZ1-B. Vertical gray lines indicate rainfall events (RFEs) that significantly affected groundwater table elevation.

452

<figure><img src="image_5.png" /></figure>

453

**Figure 3.** Temporal evolution of hydrogeological conditions in PZ0: (a) Significant wave heights of coastal seawater ($H_0$) derived from the SIMAR dataset (2115138) oceanographic model (State Ports Authority; www.puertos.es); (b) daily cumulative precipitation in the area and measured groundwater table elevations in the coastal aquifer; (c) groundwater salinities in PZ0. Vertical gray lines and orange dotted lines indicate the major rainfall events (RFEs) and eastern storm events, respectively, that significantly affected groundwater table elevation or salinity in PZ0.

454

455

456

457

458

459

460

461

462

463

464

465
466

467

468

<figure><img src="image_6.png" /></figure>

469

470 **Figure 4.** Ra isotopes and <sup>222</sup>Rn activities vs salinity in groundwater from the Argentona aquifer

471 (PZ1-S, PZ1-B and PZ0). Dashed lines represent the best linear fits to the data.

472

473

474
<table><thead><tr><th></th><th>Date</th><th>Sal</th><th>222Rn<br>(10<sup>3</sup> dpm 100L<sup>-1</sup>)</th><th>223Ra</th><th>224Ra</th><th>226Ra</th><th>228Ra</th><th>224Ra/228Ra<br>AR</th><th>226Ra/228Ra<br>AR</th></tr></thead><tbody><tr><td colspan="10"><strong>PZ1-S (shallow)</strong></td></tr><tr><td>ARG3-PZ1-S</td><td>20/12/12</td><td>0.51</td><td>90 ± 5</td><td>2.0 ± 0.3</td><td>85 ± 5</td><td>18 ± 1</td><td>85 ± 4</td><td>0.99 ± 0.08</td><td>0.21 ± 0.01</td></tr><tr><td>ARG4-PZ1-S</td><td>29/1/13</td><td>0.42</td><td>101 ± 7</td><td>0.8 ± 0.2</td><td>53 ± 3</td><td>9 ± 1</td><td>70 ± 3</td><td>0.76 ± 0.04</td><td>0.13 ± 0.02</td></tr><tr><td>ARG6-PZ1-S</td><td>24/4/13</td><td>0.39</td><td>86 ± 8</td><td>0.8 ± 0.2</td><td>56 ± 4</td><td>14 ± 1</td><td>65 ± 4</td><td>0.87 ± 0.08</td><td>0.22 ± 0.02</td></tr><tr><td>ARG7-PZ1-S</td><td>28/5/13</td><td>0.36</td><td>90 ± 8</td><td>0.9 ± 0.1</td><td>50 ± 3</td><td>10 ± 1</td><td>56 ± 1</td><td>0.90 ± 0.05</td><td>0.18 ± 0.02</td></tr><tr><td>ARG8-PZ1-S</td><td>28/6/13</td><td>0.35</td><td>85 ± 3</td><td>0.4 ± 0.2</td><td>44 ± 3</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td></tr><tr><td>ARG10-PZ1-S</td><td>23/9/13</td><td>0.39</td><td>80 ± 9</td><td>1.5 ± 0.2</td><td>66 ± 3</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td></tr><tr><td>ARG11-PZ1-S</td><td>30/10/13</td><td>0.40</td><td>76 ± 12</td><td>1.2 ± 0.2</td><td>68 ± 3</td><td>13 ± 1</td><td>65 ± 5</td><td>1.05 ± 0.10</td><td>0.20 ± 0.02</td></tr><tr><td>ARG12-PZ1-S</td><td>21/11/13</td><td>0.37</td><td>45 ± 3</td><td>0.8 ± 0.2</td><td>59 ± 3</td><td>12 ± 1</td><td>62 ± 5</td><td>0.95 ± 0.09</td><td>0.19 ± 0.02</td></tr><tr><td>ARG13-PZ1-S</td><td>24/11/13</td><td>0.37</td><td>57 ± 6</td><td>0.9 ± 0.2</td><td>56 ± 3</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td></tr><tr><td>ARG15-PZ1-S</td><td>26/11/13</td><td>0.39</td><td>62 ± 3</td><td>0.6 ± 0.2</td><td>61 ± 3</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td></tr><tr><td>ARG16-PZ1-S</td><td>28/11/13</td><td>0.39</td><td>57 ± 8</td><td>0.5 ± 0.2</td><td>60 ± 3</td><td>13 ± 1</td><td>68 ± 5</td><td>0.88 ± 0.08</td><td>0.19 ± 0.02</td></tr><tr><td>ARG17-PZ1-S</td><td>1/12/13</td><td>0.39</td><td>52 ± 4</td><td>0.8 ± 0.2</td><td>59 ± 3</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td></tr><tr><td>ARG18-PZ1-S</td><td>9/12/13</td><td>0.38</td><td>n.a</td><td>0.4 ± 0.2</td><td>56 ± 3</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td></tr><tr><td>ARG19-PZ1-S</td><td>9/2/14</td><td>0.36</td><td>n.a</td><td>0.5 ± 0.2</td><td>43 ± 2</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td></tr><tr><td>ARG20-PZ1-S</td><td>13/4/14</td><td>0.39</td><td>n.a</td><td>3.8 ± 0.5</td><td>62 ± 3</td><td>12 ± 1</td><td>60 ± 5</td><td>1.04 ± 0.10</td><td>0.20 ± 0.02</td></tr><tr><td colspan="10"><strong>PZ1-B (bottom)</strong></td></tr><tr><td>ARG3-PZ1-B</td><td>20/12/12</td><td>8.68</td><td>94 ± 12</td><td>42 ± 5</td><td>1704 ± 123</td><td>193 ± 11</td><td>1108 ± 50</td><td>1.54 ± 0.13</td><td>0.17 ± 0.01</td></tr><tr><td>ARG4-PZ1-B</td><td>29/1/13</td><td>6.43</td><td>103 ± 5</td><td>21 ± 2</td><td>1179 ± 90</td><td>106 ± 8</td><td>658 ± 47</td><td>1.79 ± 0.18</td><td>0.16 ± 0.02</td></tr><tr><td>ARG6-PZ1-B</td><td>24/4/13</td><td>11.14</td><td>95 ± 13</td><td>55 ± 4</td><td>2074 ± 108</td><td>251 ± 11</td><td>1596 ± 68</td><td>1.30 ± 0.09</td><td>0.16 ± 0.01</td></tr><tr><td>ARG7-PZ1-B</td><td>28/5/13</td><td>3.47</td><td>81 ± 14</td><td>20 ± 2</td><td>737 ± 40</td><td>26 ± 2</td><td>574 ± 27</td><td>1.28 ± 0.09</td><td>0.05 ± 0.01</td></tr><tr><td>ARG8-PZ1-B</td><td>28/6/13</td><td>5.46</td><td>92 ± 4</td><td>22 ± 1</td><td>1102 ± 50</td><td>161 ± 7</td><td>762 ± 30</td><td>1.45 ± 0.09</td><td>0.21 ± 0.01</td></tr><tr><td>ARG10-PZ1-B</td><td>23/9/13</td><td>4.32</td><td>83 ± 5</td><td>33 ± 2</td><td>917 ± 50</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td></tr><tr><td>ARG11-PZ1-B</td><td>30/10/13</td><td>4.55</td><td>81 ± 6</td><td>30 ± 2</td><td>956 ± 50</td><td>178 ± 8</td><td>764 ± 30</td><td>1.25 ± 0.08</td><td>0.23 ± 0.01</td></tr><tr><td>ARG12-PZ1-B</td><td>21/11/13</td><td>0.39</td><td>39 ± 2</td><td>1.6 ± 0.5</td><td>67 ± 4</td><td>29 ± 6</td><td>68 ± 5</td><td>0.99 ± 0.08</td><td>0.42 ± 0.09</td></tr><tr><td>ARG13-PZ1-B</td><td>24/11/13</td><td>0.74</td><td>53 ± 11</td><td>3.6 ± 0.5</td><td>118 ± 7</td><td>34 ± 2</td><td>125 ± 7</td><td>0.94 ± 0.08</td><td>0.28 ± 0.02</td></tr><tr><td>ARG15-PZ1-B</td><td>26/11/13</td><td>0.98</td><td>66 ± 7</td><td>3 ± 1</td><td>160 ± 8</td><td>33 ± 2</td><td>139 ± 7</td><td>1.15 ± 0.08</td><td>0.24 ± 0.02</td></tr><tr><td>ARG16-PZ1-B</td><td>28/11/13</td><td>3.57</td><td>61 ± 2</td><td>17 ± 2</td><td>582 ± 40</td><td>79 ± 6</td><td>461 ± 30</td><td>1.26 ± 0.12</td><td>0.17 ± 0.02</td></tr><tr><td>ARG17-PZ1-B</td><td>1/12/13</td><td>5.60</td><td>74 ± 10</td><td>33 ± 4</td><td>856 ± 100</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td></tr><tr><td>ARG18-PZ1-B</td><td>9/12/13</td><td>8.00</td><td>82 ± 5</td><td>61 ± 7</td><td>1516 ± 80</td><td>244 ± 30</td><td>1344 ± 137</td><td>1.14 ± 0.13</td><td>0.18 ± 0.03</td></tr><tr><td>ARG19-PZ1-B</td><td>9/2/14</td><td>10.36</td><td>n.a</td><td>23 ± 2</td><td>1513 ± 70</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td></tr><tr><td>ARG20-PZ1-B</td><td>13/4/14</td><td>11.42</td><td>n.a</td><td>82 ± 6</td><td>1834 ± 121</td><td>440 ± 18</td><td>1715 ± 70</td><td>1.07 ± 0.08</td><td>0.22 ± 0.01</td></tr><tr><td colspan="10"><strong>PZ0</strong></td></tr><tr><td>ARG21-PZ0</td><td>24/4/14</td><td>0.75</td><td>n.a</td><td>5 ± 0.4</td><td>257 ± 11</td><td>47 ± 4</td><td>222 ± 12</td><td>1.16 ± 0.08</td><td>0.21 ± 0.02</td></tr><tr><td>ARG22-PZ0</td><td>30/4/14</td><td>2.99</td><td>n.a</td><td>9 ± 2</td><td>762 ± 33</td><td>62 ± 7</td><td>755 ± 95</td><td>1.01 ± 0.13</td><td>0.08 ± 0.01</td></tr><tr><td>ARG23-PZ0</td><td>13/5/14</td><td>3.85</td><td>n.a</td><td>9 ± 2</td><td>828 ± 35</td><td>40 ± 7</td><td>741 ± 117</td><td>1.12 ± 0.18</td><td>0.05 ± 0.01</td></tr><tr><td>ARG24-PZ0</td><td>16/5/14</td><td>3.51</td><td>n.a</td><td>12 ± 2</td><td>802 ± 37</td><td>67 ± 4</td><td>480 ± 67</td><td>1.67 ± 0.25</td><td>0.14 ± 0.02</td></tr><tr><td>ARG25-PZ0</td><td>3/6/14</td><td>1.20</td><td>n.a</td><td>3 ± 1</td><td>313 ± 15</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td></tr></tbody></table>
<table><tr><td>ARG26-PZ0</td><td>7/6/14</td><td>1.70</td><td>n.a</td><td>4 ± 1</td><td>392 ± 19</td><td>38 ± 6</td><td>232 ± 34</td><td>1.69 ± 0.26</td><td>0.16 ± 0.04</td><td>476</td></tr><tr><td>ARG27-PZ0</td><td>11/6/14</td><td>1.70</td><td>n.a</td><td>4 ± 1</td><td>357 ± 16</td><td>55 ± 11</td><td>343 ± 51</td><td>1.04 ± 0.16</td><td>0.16 ± 0.04</td><td></td></tr><tr><td>ARG28-PZ0</td><td>26/6/14</td><td>2.30</td><td>n.a</td><td>8 ± 2</td><td>517 ± 23</td><td>67 ± 12</td><td>200 ± 44</td><td>2.59 ± 0.58</td><td>0.34 ± 0.10</td><td>477</td></tr></table>

n.a: not analyzed

478

479

<caption>**Table 1.** Salinity, Ra isotopes and <sup>222</sup>Rn activities and Activity Ratios (ARs) in the shallow and bottom parts of piezometer PZ1 (n.a: not analyzed).</caption>
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