# Role of colloids in the transfer and dispersion of trace elements into river waters through a former mining district

Marie Heydon, Lucia Perez Serrano, Eva Schreck, Carole Causserand, Oleg

Pokrovsky, Philippe Behra, Jérôme Viers

► To cite this version:

Marie Heydon, Lucia Perez Serrano, Eva Schreck, Carole Causserand, Oleg Pokrovsky, et al.. Role of colloids in the transfer and dispersion of trace elements into river waters through a former mining district. Applied Geochemistry, 2023, 155, pp.105736. 10.1016/j.apgeochem.2023.105736. hal-04174212

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# Role of colloids in the transfer and dispersion of trace elements into river waters through a former mining district

Marie Heydon<fnref n="1" />, Lucia Perez Serrano<fnref n="1" />, Eva Schreck<fnref n="1" />, Carole Causserand<fnref n="1" />, Oleg S. Pokrovsky<fnref n="1" />,<fnref n="2" />, Philippe Behra<fnref n="3" />, Jérôme Viers<fnref n="1" />

## Corresponding Author

Marie Heydon

E-mail address: marie.heydon@get.omp.eu

Postal address : Géoscience Environnement Toulouse, 14 Avenue Edouard Belin, 31400, Toulouse, France

<footnote n="1">¹ Géosciences Environnement Toulouse (GET), Université de Toulouse, CNRS, IRD 14 Avenue Edouard Belin, 31400, Toulouse, France</footnote>

<footnote n="2">² BIO-GEO-CLIM Laboratory, Tomsk State University, Tomsk 634050, Russia</footnote>

<footnote n="3">³ Laboratoire de Chimie Agro-industrielle, LCA, Université de Toulouse, INRAE, 31400 Toulouse, France</footnote>
# 1. Introduction

The remediation of mining districts on the surrounding ecosystems is a central environmental concern in many regions worldwide. The Orbiel Valley, located in the South of France, has been exploited for various elements since the Second Iron Age (Beyrie et al., 2011). Located in this valley, the mining district of Salsigne has been the most important gold (Au) mine in Europe (Trueb, 1996) and was also considered as a major Au deposit in the French Variscan basement (Lescuyer, 2005) in view of the fact that 104 t of Au has been extracted from this mining district. Salsigne was at the same time, also the largest arsenic (As) producer in the world during the 20th century with 397,000 t of As extracted (Demange et al., 2006). Mineral extraction was stopped in 2004 leaving about 12 million t of waste (Desaulty 2016).

Rehabilitation works including creating waste storage areas sites (Montredon, Artus and Combe du Saut) have been undertaken by the French government and a treatment plant was set up, which allowed to purify a fraction of the waters percolating from these storage areas (Girard, 2011).

Despite these remediation efforts, the release of contaminants into surface and ground waters, soil and atmosphere remains a key issue, even though the underlying factors and mechanisms of the active processes in this area remain poorly addressed. Nevertheless, this issue has often remained a source of tensions in the valley between a segment of the local population, environmental associations and the state services. For these reasons, it has become important to assess trace elements transfer and identify the underlying physicochemical processes.
Although As is volatile and can easily transfer into the atmosphere (O'Neill, 1995), it is often found in the hydrosphere (Masuda, 2018). This reservoir seems to be a key point in the biogeochemical cycle of As in controlling its fate and behavior through the transport of particulates and dissolved species. In this context, several research groups have recently initiated concerted hydrogeochemical studies to better understand As transfer to surface waters in the Orbiel Valley (Khaska et al., 2015, 2018; Delplace et al., 2022). Khaska et al. (2015) have highlighted the fact that As concentrations in the Orbiel River (about 36 µg/L and locally up to 120 µg/L) were much higher than the water geochemical background level (about 4 µg/L) following its passage through the erstwhile mining district. Moreover, using Sr isotopes, they have shown that a part of the As present in the river water mainly comes from lime arsenates. This end product, resulting from the neutralization of percolation waters by the treatment plant, was stored for a long time on the soil surface without any particular
precautionary measures. The impact of mining activity could also be accentuated by flash floods, such as the dramatic flood which occurred in October, 2018 (Ayphassorho et al., 2019). This episode has also rekindled fears of dispersion of the contaminants, particularly As, which could be transported either in the dissolved or in the particulate form. Indeed, Delplace et al. (2022) have shown that sediments transported by this flood could have very high As concentrations (515 µg/g downstream), which is more than 12 times the pedo-geochemical background.

Since waste storage areas are closely linked to the hydrographic network (Fig. 1), it seems important to better understand the physicochemical factors (dissolved organic carbon (DOC), alkalinity, pH, anions and cations in solution, etc.) governing the transport of potential contaminants that of As. Moreover, these toxic elements (including As) may have different environmental properties, such as aggregation, bioavailability, and toxicity, depending on their chemical configuration and their distribution among particulate, dissolved, colloidal, free, and labile forms (Lead et al., 1997; Town and Filella, 2002). In various former mining sites, it has been shown that these sediments and more generally, the particulate phase, contribute significantly to the dispersion of As-contaminated mine tailing (Marron 1992; Tansel and Rafiuddin, 2016), and that such sediments subsequently become a sizable source of As release (Larios et al., 2012; DeVore et al., 2019; Jacob-Tatapu et al., 2021). In contrast, other geochemical studies performed on various mining sites worldwide have demonstrated that As is essentially present in the dissolved phase: for example, in the river waters from the upper Rio Doce River basin in Quadrilátero Ferrífero in Brazil (Gontijo et al., 2016). High As concentrations in the dissolved phase were also observed in ground waters from the Hetao basin in Inner Mongolia (Guo et al., 2011) or in West Bengal, India (Majumder et al., 2014).

Generally, one can consider that material transport by the river occurs in the so-called “dissolved” form (< 0.2 or 0.45 µm) or in the particulate form (> 0.2 or 0.45 µm); both cut-offs have been used depending on the chosen protocol. Generally, it has been assumed that the differences in elemental concentrations between 0.22 µm and 0.45 µm filtrates are within the limit of uncertainty in analyses or due to natural variations (Pokrovsky et al., 2016).

However, even in the so-called “dissolved phase”, submicrometric organic or mineral particles subsist, and are usually termed “colloids”. In this study, we used the operational (empirical) definition of colloids as entities having sizes between 3 kDa and 0.45 µm, following recent works on riverine and soil solutes (Krickov et al., 2019; Raudina et al., 2021). Thus, the lower fraction is defined here as the “true dissolved” (or free) fraction and the upper fraction is the particulate fraction (Lead and Wilkinson, 2006). Characterization
techniques for colloids in natural waters are numerous and debated because of the instability
of suspended particles. Various parameters or processes, such as temperature or ionic strength
changes, concentration variation, redox or microbial degradation, can have an effect on the
particle size-distribution (Chen and Buffle, 1996). Several methods have been tested to
characterize colloidal suspensions, such as reverse osmosis, diffusive gradients in thin films,
tangential flow or cross-flow filtration, and field-flow fractionation (Serkiz and Perdue, 1990;
Zhang and Davison, 1995; Sigg et al., 2006; Baalousha and Lead, 2007; Guo and Santschi,
2007). Ultrafiltration, with a smaller cut-off at 3 kDa, which is assumed to correspond to the
truly dissolved phase, remains the most rapid and efficient method (Luan et al., 2022; Xu and
Guo, 2017).

Moreover, as shown in many previous studies, due to their large specific surface and high
reactivity, colloidal materials can play an important role in the transport of elements (Behra,
1986; Buffle et al., 1992; Sholkovitz, 1995; Dupré et al., 1999; Vasyukova et al., 2012). In
this regard, a precise characterization and fractionation of natural aquatic colloids is highly
important to better understand the geochemical and physical processes contributing to their
binding to other phases. Many studies have notably highlighted the importance of the
colloidal organic carbon (COC) content in the transport of trace elements, which is highly
dependent on the size of these colloids and therefore, on their origin (Buffle, 1988; Eyrolle et
al., 1996; Pokrovsky and Schott, 2002). Regarding As, COC seems to be an important carrier
due to the formation of ternary complexes like Fe- or Al-OM-As (Mikutta and Kretzschmar,
2011; Sharma et al., 2011).

The first objective of this work was to investigate the physicochemical parameters of the river
from upstream to downstream in order to characterize the role of colloids in metal and
metalloid transport in the waters of the Orbiel River and its main tributaries. The second and
more specific objective was to understand how the size distribution of this colloidal matter,
i.e., truly dissolved vs. dissolved, changed along the river in the mining district. To attain
these objectives, we performed a field campaign in April 2022, during the high-water season
in the Orbiel Valley, including the Orbiel River main stream and its tributaries. During this
high spatial resolution campaign, waters were sampled and (ultra)filtered on-site (4 size
fractions, from 0.22 µm to 3 kDa) and then analyzed in the laboratory to better explain the
variations occurring along the rivers and to have a better grasp of the potential role of
colloidal matter in As dispersion in this former mining district. Finally, special efforts were
dedicated to the fractionation of rare earth elements (REE) as a powerful tracer of colloidal
fate in natural waters (Elderfield et al., 1990). The REEs constitute a group of 14 elements
(La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) that have a 5d¹⁶s² electronic configuration in the ground state, with progressive filling of the 4f electronic sublayer from La to Lu. Their special electronic configuration makes them very similar but with significantly different physicochemical properties throughout the series. As a result, REEs may have a particular distribution (variation in the proportion of light, medium or heavy REEs) depending on the specific features of the solution in which they were dispersed (the presence of organic or inorganic ligands in solution, colloidal material, particle, etc.). The distribution pattern of REEs can thus provide crucial information on the mechanisms operating in solution (e.g., enrichment in light REEs during sorption on Fe(hydr)oxides (Sholkovitz, 1995; Gaillardet et al., 2014)). Indeed, REEs patterns and light (LREE, La to Nd) or heavy (HREE, Er to Lu) rare earth elements distribution can then become relevant not only for source tracing in mining environments, but they also provide information on the complexing power and affinities of colloids. REEs patterns are thus, of great value in monitoring the evolution of the colloidal pool under physicochemical parameters variations, and therefore deserve to be tested in the mining context of the Orbiel Valley.

## 2. Materials and methods

### 2.1. Study site context

#### Geological context

The Orbiel Valley is located on the southern slope of the Montagne Noire (Cassard et al., 1993). The Orbiel River drains through different geological terrains, which date from the Late Proterozoic to Early Devonian in the northern part, and from Eocene to Paleocene in the southern part (Berger et al., 1993; Debat et al., 1993). The axial zone of the Montagne Noire is essentially made up of metamorphic formations (schists, metapelites and quartzite) generally from the Cambrian-Ordovician age. Going south, we find formations of the Minervois nappe with detrital formations (sandstone) at the base, topped by carbonate or dolomitic formations. Further south, tertiary sedimentary formations appear in unconformity. The Salsigne district is characterized by numerous occurrences of various metals and metalloids (Pb, Zn, Cu, W, etc.) and of course, Au-As ores. Four phases of mineralization have been described (Lescuyer et al., 1993): a Cambrian episode contemporary to carbonate sedimentation, a Lower Devonian episode corresponding to small sedimentary occurrences of Fe, a Late Variscan episode, and a Mesozoic episode (Melleton and Girardeau, 2019).
132 Hydrological and climatological features

133 The Orbiel catchment area is 253 km² with a maximum flow path length of 34.8 km. At the
134 outlet of the catchment, the Orbiel River flows into the left bank of the Aude River. The
135 tributary rivers sampled during this study are the Rieutort, the Grésillou and the Russec (Fig.
136 1a). The climate is Mediterranean-influenced with high and/or flash rain precipitations during
137 winter or spring, occasional storm events during September or October, and strong and
138 frequent winds (Castel, 1997).

139 The mean annual flow of the Orbiel River is 1.5 m³/s at the Lastours station and 2.6 m³/s at
140 the Bouilhonnac station (Banque Hydro Website). During the flood that occurred during 14–
141 15 October 2018, it was estimated that the water discharge at the Bouilhonnac station reached
142 490 m³/s (Lebouc et al., 2019). Regular flood episodes are typical of a Mediterranean climate,
143 among which are the historic floods which date from 1901, 1930, 1940, 1992, 1996, 1999,
144 2005, 2001, 2014 and 2018 (Ayphassorho et al., 2019; Ferre, 2005). The depth of runoff
145 reached 354 mm/y (Banque Hydro Website). The average yearly temperature is estimated to
146 be around 13 °C, with predominantly warm temperatures during the summer months.

147 2.2. *Sampling strategy*

148 In a field campaign conducted over the 20th and 21st April 2022 during a rainy period
149 (rainfall event of 55 mm) of the high-water season, fourteen samples were taken and filtered.
150 The flow of the Orbiel was between 2 and 3 m³/s (Banque Hydro Website, Fig. 1b).
151 Extensive spatial sampling was performed so as to obtain a general picture of the main course
152 (Fig. 1). Samples were taken upstream (TOURB: "Tourbière", MIR-AM: "Miraval amont",
153 MIR: "Miraval-Cabardès", MAS: "Mas-Cabardès"), at midstream, (LST: "Lastours") and
154 finally downstream, near old tailings and mine installations. Waters drained by these areas
155 may potentially influence the river water composition further downstream. A station of
156 interest is the treatment plant at "Gué De Lassac" (GDL) where the stabilization of As by
157 lime precipitation is ongoing since more than 15 years. To estimate potential changes at
158 GDL, a sample was collected upstream of this location, at Limousis (LIM). An intermittent
159 torrent (MIR-TO) was also sampled between MIR-AM and MIR, in view of its contribution
160 to the Orbiel of about 3 %.

161 The influence of the tributaries was also considered; samples were collected in Rieutort (RT)
162 and Russec at Salitis (SLT) before their respective confluence with the Orbiel River. PES
163 (Pestril) is noted here as a special case, as it represents an important resurgence of karstic
origin which drains directly into the Orbiel River upstream of Lastours (Raynaud, 2012). The Grésillou was sampled before its confluence at Lastours (GRE), such as one of its small tributaries, “Ruisseau de Villanière” (RUV). The outlet point of the Orbiel River, at Conques-sur-Orbiel (CNQ), was also sampled to estimate the integral impact of metal ores on the whole catchment.

## 2.3. Filtration process

All the collected samples were filtered through 0.22 µm pore size in order to separate the “dissolved” and particulate phases, using a 250 mL sterile filtration unit equipped with PVDF membranes (JetBiofil®). Room temperature cascade ultrafiltration using progressively smaller cut-offs (0.025 µm, 100 kDa and 3 kDa) was then performed to quantify the colloidal fraction (Fig. 2). The 0.025 µm filtration was carried out using a 250 mL Nalgene® frontal filtration system with MCE MF-Millipore® membranes filters. The system was rinsed 3 times with MQ water and then with 100 mL of the sample. The 100 kDa and 3 kDa ultrafiltrations were performed using a centrifuge (Sigma® 2-7) and Amicon® Centrifugal filter unit (15 mL). Ultrafiltration cartridges were rinsed 3 times with MQ water and then with 6 mL of the sample. The centrifugation system maintained a constant gravitational field of 2340g through the whole process. Ten-minute centrifugation cycles were carried out and repeated 1 to 4 times to ensure that the samples were fully filtered. The strength of this protocol has already been demonstrated in previous methodological studies (Ilina et al., 2016; Oleinikova et al., 2017a; Vasyukova et al., 2010) on various riverine waters. More specifically, the strength of the protocol was verified from i) the lack of filter clogging and effective pore size change and ii) the absence of any sizable evolution of solute concentration over consecutive (ultra)filtrations through the same porosity. Results from (ultra)filtrations are then reported in two forms: first, the fractions denoted “< X µm” contain everything that passed through this filter and include all the smaller fractions, and the second, discrete sizes ranges such as “0.025 µm –100 kDa” that are calculated by subtracting the concentrations from the two respective fractions.

## 2.4. Physicochemical analyses at each filtration step

The temperature, pH and conductivity parameters were measured *in-situ* directly with a multiparameter probe (HACH SL1000, detail in Supplementary Information). The different steps during sampling, filtration, and treatment are listed in Fig. 2. At each filtration step,
sample aliquots were reserved for alkalinity and measurements of anions ($Cl^-$, $NO_3^-$ and $SO_4^{2-}$), for the analysis of major cations and trace elements, and for DOC analysis. Samples for cation, trace elements and DOC analyses were acidified for preservation. The frontal filtration system and the polypropylene (PP) bottles used for storage of solutions were cleaned in the laboratory clean room with 1 M HCl and MilliQ deionized water (18.2 MΩ.cm).

Concerning DOC results, there were samples that showed anomalous concentrations and these are not reported here. It is assumed that during the preparation of the samples for the determination of DOC, three samples were contaminated (GDL at 0.025 µm, GDL at 3 kDa and RT at 3 kDa). These samples were discarded to avoid ambiguity in interpretation of results.

## 2.5. Analytical methods

All physicochemical parameters of the samples were determined at Geosciences Environment Toulouse (GET laboratory), at the Observatoire Midi-Pyrénées (OMP). Anions ($Cl^-$, $NO_3^-$ and $SO_4^{2-}$) were analyzed by ion chromatography (ICS 2000 Dionex®). Two certified standard solutions (MISSIPPI-14 and Multi Element IC) were measured to verify the accuracy of analyses. Differences with certified values were within 5%. Alkalinity was determined by acid titration (Schott® Titroline Alpha plus) followed by a Gran-method treatment. DOC was measured using a Shimadzu® TOC CSN after acidification and sparging of the sample. The correctness of analyses was tested by measuring two standard solutions (BURTARP-14 and PERADE-17); variations of, respectively, 6.98 and 6.40 % were obtained; these values were acceptable, being under the recommended threshold of 10 %. The standard errors of analyses were 1.96 %, 1.48 % and 8.30 % at 2, 10 and 30 µg/L, respectively. Blanks were made on the Amicon® Centrifugal unit with MQ water (Blank$_{100}$ kDa = 1.037 ppm and Blank$_{3}$ kDa = 0.983 ppm) and were comparable with the MQ water itself (Blank$_{MQ}$ = 0.123 ppm). The DOC yields from 100 kDa and 3 kDa filtration cells were calculated as the difference between the Blank$_{100}$ or 3 kDa and the Blank$_{MQ}$ (0.914 and 0.860 ppm, respectively); these values were subtracted from the results. Major cations and trace elements concentrations were obtained by a quadrupole ICP-MS (iCAP Q from Thermo Scientific®- Kinetic Energy Discrimination mode using He). In and Re (2-3 µg/L) were used as internal standards. Two certified solutions: SLRS-6 (NRC-CNRC) and EPOND (MB56A.1.2N.L5) were also processed to check the validity of the analytical procedure.
227 **3. Results**

228 **3.1. Upstream–downstream concentrations of major elements and DOC**

229 In the Orbiel River, the SO₄²⁻ concentration (Table 1) increased downstream, with the highest 230 SO₄²⁻ content (32 mg/L) measured at the outlet of the river (CNQ). The concentration with 231 respect to the filtration cut-offs remained stable at 0.22 µm, 0.025 µm and 100 kDa. However, 232 the concentration decreased significantly at 3 kDa for all the samples except for that from the 233 peatland close to the Orbiel River source (TOURB). With respect to the tributaries, the RUV 234 presented a much higher concentration (4460 mg/L) compared to the other samples. For the 235 SLT, the measured SO₄²⁻ concentration (80 mg/L) was also high, even though the increase 236 was much lower than that of RUV. Concerning NO₃⁻ in the Orbiel River, we noted that even 237 though the concentration increased from upstream to downstream (LST), all the samples 238 measured had low concentrations (< 3 mg/L). We did not observe any differences in the NO₃⁻ 239 concentration between the filtrates and ultrafiltrates. Among the tributaries, the RUV showed 240 the highest NO₃⁻ concentration (25 mg/L). The Cl⁻ concentration (Table 1) increased 241 downstream, reaching its highest value at the outlet of the Orbiel River (8 mg/L). No 242 difference between the various filtrations was observed for the Cl⁻ concentrations. Among the 243 tributaries of Orbiel, RUV exhibited the highest Cl⁻ concentration (22 mg/L).

244 In the Orbiel River, the DOC concentration (Fig. 3a) decreased from upstream (TOURB, peat 245 bog zone, ~ 8 mg/L) to the LST station (~2 mg/L). The concentration then increased slightly 246 after LST to 3 mg/L and remained stable until CNQ. The DOC concentrations in the 247 tributaries ranged from 1 to 3 mg/L. The karstic spring (PES) exhibited the lowest 248 concentration (1 mg/L).

249 Both Ca and Mg (Fig. 3b) showed similar trends, with an increase in concentration towards 250 downstream. Ca and Mg concentrations at TOURB were about 1 and 0.3 mg/L, respectively. 251 At the outlet of the basin (CNQ), Ca and Mg concentrations increased strongly to 21 and 5 252 mg/L, respectively. Regarding the tributaries, MIR.TO presented values similar to the 253 peatland (TOURB). RT, (Orbiel tributary near MAS) had Mg concentration similar to the 254 MAS sample. The PES, RUV and SLT exhibited high concentrations of Ca (26, 30 and 49 255 mg/L respectively). Similarly, the Mg concentrations of these stations were higher than in the 256 Orbiel River (6, 24 and 16 mg/L respectively). However, GRE seems to present lower 257 concentrations for both elements.
The Ca/Mg ratio (Fig. S1) was between 3 and 4 upstream (from TOURB to MIR) and increased strongly at MAS, where the ratio was 6. Downstream (from LST to CNQ), the ratio was initially around 4. The variation in Ca/Mg ratios was greater for the tributaries than for the Orbiel River; RT presented a high ratio similar to MAS. In contrast, RUV exhibited the lowest ratio with a value of 1, due to Mg enrichment.

The calculated balance of inorganic ionic charge (BIC) was between -20 and 14 % for 0.22 µm filtered samples. For the other samples, the BIC was between -19 and 27 %, -19 and 22 % and, -20 and -10 % at 0.025 µm, 10 kDa and 3kDa, respectively. The median was -14%, which could be acceptable because the analyses did not take into account the contribution of humic carboxyl content, which can be up to 10 µeq/mg humic organic carbon (Oliver et al., 1983).

The Al and Fe concentrations (Fig. 3c) in the Orbiel River showed upstream to downstream variations similar to those described for the DOC (Fig. 3a) with both showing their highest levels of 218 and 196 µg/L, respectively, at TOURB. Their concentrations then decreased with minimum values of 21 µg/L for Al and of 22 µg/L for Fe at LST. Downstream of this location, the Al and Fe concentrations increased at GDL to 47 and 69 µg/L respectively, and decreased again going further down (CNQ). All tributaries, except the RT and the GRE, had Al and Fe concentrations that were between 71 and 99 % lower than in the Orbiel River before their confluence. At RT, the concentrations were slightly higher than at MAS, greater by 1.5 times for Al and 1.2 times for Fe. At GRE, the Al concentration was around 33 µg/L, i.e., 10 times higher than at LST. The Fe concentration was about 6 µg/L, i.e., lower than at LST, which was about 22 µg/L. At both PES and RUV, the Al and Fe concentrations reached a minimum of 1.69 and 0.20 µg/L, respectively.

The As concentration (Fig. 3d) in the Orbiel River was very low in the upstream bog zone (0.29 µg/L). Further downstream, it first increased slowly from TOURB to LIM, to increase more rapidly between LIM and CNQ. Most of the points in the upstream of the Orbiel River showed low As concentrations compared to the tributaries. On the contrary, the As content was much higher in the Orbiel tributaries of GRE and SLT with 83 and 57 µg/L, respectively. On the other hand, the Pb content (Fig. 3d) was significantly higher in the bog (0.60 µg/L) than in the rest of the samples (from 0.03 to 0.20 µg/L).

Variation of La and Ce concentrations are reported in Fig. 3e, where a similar trend to that described for Al and Fe is observed. Indeed, La and Ce concentrations decreased from
upstream to LST, where they reached a minimum, i.e., 0.06 and 0.05 µg/L, respectively, and then increased when going further downstream to GDL before decreasing again at CNQ. With respect to the tributaries, both La and Ce concentrations were low at PES, RUV and SLT, in contrast to the torrent (MIR-TO) or the RT.

## 3.2. Colloidal matter contents from upstream to downstream

To better understand the behavior of colloidal matter, the total dissolved (< 0.22 µm) concentrations of DOC and some elements (Al, Fe, As, and Pb) were separated into 4 fractions depending on their cut-off threshold (Fig. 4). Fractions 1–3 represent the colloidal fraction containing different molecular weight species, while fraction 4 (< 3 kDa) represented the “truly dissolved”, low molecular weight forms. The percentage of colloidal matter ([0.22 µm, 3 kDa]) is expressed as (([0.22 µm] - [3 kDa]) / [0.22 µm]) * 100 (in %) where [0.22 µm] is the concentration measured in the 0.22 µm filtrate and [3 kDa] is the concentration measured in the 3 kDa filtrate.

In Fig. 4a, it is seen that DOC is present both in “truly dissolved” organic carbon (TDOC) and in colloidal organic carbon (COC) fractions. Results also show that COC was higher in fraction 3 (smallest colloidal fraction in the 3 to 100 kDa range) and almost nonexistent (< 0.2 mg/L) in the largest fraction (fraction 1 from 0.025 to 0.22 µm). While the COC decreased from TOURB to LST, the TDOC fraction remained between 0.7 and 1.9 mg/L. Finally, from TOURB to MAS, the percentage of COC remained between 80 and 86 % but decreased to 52 % at LST. These results show that COC was abundant in the upstream part and decreased at the LST station, where half of the organic matter was present in the TDOC fraction.

The percentage of Fe in the colloidal fraction was rather constant (> 90 %) from upstream to downstream. Fe was mostly present in fraction 3 from TOURB to MAS and at the RT (Fig. 4b). At LST and GDL, Fe was equitably present in fractions 3 and 1. Both Al and Pb (Fig. 4c,d) were also mainly present in the colloidal fractions and especially in fraction 3 from TOURB to MAS. At LST and GDL, Al was mainly present in the “truly dissolved” fraction, while Pb was contained mostly in fractions 1 and 3. At RT, Al was mostly in fractions 3 and 4, and Pb was mostly present in colloidal fractions 1 and 3. On the other hand, about 40 % of As was present in the colloidal fraction at TOURB, which decreased to 5 % at GDL (Fig. 4e). The enrichment in As in the Orbiel River was mainly observed in fraction 4.
321

### 3.3. REE concentrations from upstream to downstream

322

323

324

325

326

327

328

329

Upper crust normalized REEs patterns, as described by Taylor and McLennan 1985, are shown for each of the studied fractions in Fig. 5. Note that all the patterns of the < 0.22 µm fraction (Fig. 5a) present two characteristics: a negative anomaly in Ce and an enrichment in medium rare earths elements (MREE, Sm to Ho) as compared to LREE and HREE. Some of the samples clearly seem to be more strongly enriched in HREE then in LREE, such as in TOURB and MIR.AM. This stronger enrichment in HREE is observed, particularly for TOURB and MIR.AM samples, as well as for MIR, RT, MAS, GDL and LIM. However, no such enrichment is observed at GRE, as well as at CNQ, LST, RUV, SLT and PES.

330

331

332

333

334

The < 0.025 µm and < 100 kDa fractions (Fig. 5b, c), which were analyzed only for seven samples, followed the same patterns as described for the < 0.22 µm fraction (Fig. 5a). In contrast, the < 3 kDa fraction exhibited a different pattern (Fig. 5d). Although Ce anomaly persisted, the enrichment in MREE was not observed. In this case, there was a net enrichment in HREE relative to LREE.

335

## 4. Discussion

336

### 4.1. Physicochemical characterization of the Orbiel River and its tributaries

337

338

339

340

341

342

343

To improve our *a priori* understanding of the principal ion hydrochemistry in the Orbiel Valley, a Piper diagram was constructed based on the results from physicochemical characterization to represent the chemical composition of water samples (< 0.22 µm) from the Orbiel Valley (Fig. 6). The Piper diagram constitutes two triangles and a rhombus that represent, respectively, the cationic and anionic composition, and the overall composition. Three geochemical composition types were then identified: (1) Ca or/and Mg-HCO₃, (2) Na-K-Cl and (3) hyper-SO₄²⁻.

344

345

346

347

348

349

It turns out that the Orbiel River samples (triangles) are aligned. The most upstream Orbiel samples (TOURB, MIR.AM) are of the Na-K-Cl type. Although MIR is Cl-Ca type, it is close to the upstream samples. The downstream samples (MAS, LST, LIM, GDL and CNQ) are Ca-HCO₃ type. Thus, when going from upstream to downstream, the Orbiel water undergoes a change in geochemical composition, which likely, is due to increase in the Ca concentration and carbonate alkalinity.

350

351

This change in composition can be easily explained by a change in lithology from upstream to downstream, since metapelite or clastic-abundant formations are predominant in the
upstream parts, whereas downstream areas are dominated by carbonated formations (Delplace et al., 2022). The increase in Ca and Mg concentrations and alkalinity likely reflects the dissolution of calcite (Eq. 1):

$$ \underset{\text{1}}{\mathrm{CaCO_3(s) + H_2O(l) + CO_2(g) \rightleftharpoons Ca^{2+}(aq) + 2HCO_3^-}(aq)} \quad \text{(Eq. 1)} $$

Additionally, tributaries may have an influence on the geochemical composition of the Orbiel River due to the different lithology. The Rieutort River, which is the main left bank tributary of the Orbiel River in the upstream portion of the watershed, does not influence the chemical composition of the river, since it drains the same geological formations as the upstream area of Orbiel. Going further downstream, the presence of karstic Pestril spring increases the carbonate signature of the waters of the Orbiel River. This resurgence (Ca-HCO₃ type) is one of the most Ca- and alkaline-enriched waters in our sample set due to its karst origin. As explained by the French hydrogeologist André Tarrisse (personal communication), the source of Pestril can be traced to a resurgence of hydrological losses occurring in the Clamoux Valley located to the east of the Orbiel (Fig. 1a), which could contribute to a third or more of the Orbiel River water discharge during the low water period. The right bank tributary GRE, is an intermittent river which did not have any flow during the sampling period and therefore had no influence on the chemical composition of the Orbiel River presented in this study. The composition of SLT is Ca-Mg-HCO₃ type, but with a significant SO₄²⁻ concentration (80 mg/L). For a great part of its length, the Russec flows over Paleocene limestones, which accounts for this Ca enrichment. The SO₄²⁻ concentration measured in the Russec River may originate from the oxidative dissolution of sulfide minerals such as pyrite (Eq. 2) (Bonnissel-Gissinger et al., 1998) and arsenopyrite, potentially found in the sediment or in areas where waste from past mining activities were stored:

$$ \underset{\text{376}}{\mathrm{FeS_2(s) + 3.5 O_2(g) + H_2O(l) \rightleftharpoons Fe^{2+}(aq) + 2SO_4^{2-}(aq) + 2H^+(aq)}} \quad \text{(Eq. 2)} $$

In the case of the Russec River, we identify two specific areas that can give rise to excess of not only SO₄²⁻ but also metal(loid)s: the site of the former Malabau mine (Fig. 1a), in the upstream part of the catchment, and the waste storage site of Montredon (Fig. 1a) (Géoderis, 2012). For the latter, percolation water on the western side of this storage may flow into the Russec watershed. The pH remains circumneutral in these waters due to the dissolution of limestones, that plays the role of a buffer (Sigg et al., 2014). This process may also be the
reason for the high SO₄²⁻ content (4461 mg/L) of the RUV water that originates from the Salsigne open pit (Fig. 1a).

The piper diagram (Fig. 6) shows an alignment of the GDL, CNQ and SLT stations, assuming that the Russec River (whose downstream point is SLT, and flows into the Orbiel between GDL and CNQ stations) contributes to the enrichment of Ca, Mg, SO₄²⁻ and HCO₃⁻. This hypothesis is then confirmed by the data showed in Table 1. The SLT water discharge was not measured but during our field campaign, we visually noted it to be a significant tributary of the Orbiel River.

## 4.2. The role of colloidal matter as a transfer vector of metal(loid) in river waters: identification of natural and anthropogenic factors

### 4.2.1. Organic matter evolution and OC correlation with Al, Fe and Pb

The DOC concentrations show a clear decrease in colloidal fraction from upstream to downstream; a similar trend is observed for the concentrations of Al, Fe and Pb. The relationships between Al-DOC and Fe-DOC in all fractions and their statistical analysis (Fig. 7, Fig. S2) showed that Al and Fe are closely associated with colloidal organic carbon (COC) in fraction 3. Many studies have already shown that both these elements are strongly associated with organic matter in natural waters, particularly in boreal and tropical regions (Buffle et al., 1992; Stumm, 1993; Eyrolle et al., 1996; Dupré et al., 1999; Buerge-Weirich et al., 2002; Pokrovsky et al., 2014; Sigg et al., 2014). This association stems from complexation of the two metals with the carboxylic and phenolic functional groups present in organic matter, their adsorption onto clay particles, or association with Fe hydr(oxides) stabilized by organic matter (Lead and Wilkinson, 2006; Ilina et al., 2016). The comparison of the trends in Al-DOC and Fe-DOC with those obtained by Dupré et al. (1999) (Fig. S3) suggests that Al could be complexed with colloidal organic matter over a range of different colloid sizes. Furthermore, the stoichiometry (Table S1) highlighted a dominance of OC in the colloidal fraction, with similar amounts of Fe, Ca and Al, or the prevalence of Ca and SO₄²⁻ over other metals in MIR and MAS. Going downstream, we observed a decrease in DOC concentration, caused by a decrease in COC fraction, with TDOC fraction remaining constant all along the river course. A reason for this observation could be a decrease only in large colloidal size allochthonous (terrestrial) OC. With the available data, it is difficult to precisely determine the reason for the disintegration of large colloidal matter. Nevertheless, we can mention three processes that could be responsible for this phenomenon: i)
biodegradation of organic colloids by heterotrophic bacteria (Oleinikova et al., 2018); ii) photoreduction (Oleinikova et al., 2017b; Waite, 1990); and iii) the destabilization of colloids by a shielding effect linked to an increase in ionic strength, particularly of Ca and Mg ions (O'Melia, 1989). We find that it is from the village of Lastours (LST station) that the percentage of colloidal matter appears much lower. It is also in this area that the ionic strength of the river waters increases significantly due to the contribution from the karstic source Pestril about 500 m upstream of the LST station. The presence of very low molecular weight organic matter is maintained by the in-stream production of small molecular size (< 3 kDa) autochthonous exometabolites and biotically-generated organic molecules (Bagard et al., 2011, Krickov et al., 2019).

For Pb, the highest concentrations (0.60 µg/L) were found in the upstream part, in the bog area. We note that the former mining district does not appear to be a source of Pb for the Orbiel River because concentrations do not increase in the river when crossing the former mining district. This result highlights the potential role of peatland in Pb accumulation upstream which is further supported by the fact that the Pb accumulation is one order of magnitude higher than the riverine world average concentration (0.079 µg/L; Gaillardet et al., 2014). Such a high Pb concentration in the peatbog area (TOURB) can be explained by Pb accumulation in peat and mosses via atmospheric deposition (De Vleeschouwer et al., 2007). Indeed, the peat, which is naturally rich in organic matter, is known to have a strong affinity to Pb (Vile et al., 1999). As this peat bog has been considered to be of ombrotrophic type, essentially fed by water inputs (rain, lateral flow) and atmospheric particles (Martínez-Cortizas et al., 1997), this very high Pb content probably reflects the accumulation of Pb from far-range atmospheric fallouts (gasoline) until the end of the 1990s (Dunlap et al., 1999; Shotyk et al., 2005). Alternatively, it could be a deposition of Pb-enriched atmospheric aerosols and/or particles historically coming from the old mine of La Loubatière, located to the west of the studied site (Burnol and Prouhet, 1995). In this case, the correlation between DOC and Pb in the colloidal fraction (Fig. 7) could be due to Pb release from the peatland area in the form of organic complexes (Town and Filella, 2002), which remain associated with the DOC in the Orbiel River.

### 4.2.2. Size fractionation and evolution of As along the river course

Although the measured As concentrations were low when compared to other mining sites (e.g., Nieto et al., 2007), a significant increase in As concentration from LST (3 µg/L) to CNQ (20 µg/L) was observed. At the level of LST, the Orbiel River enters the formerly
mining district of Salsigne, where once extraction and treatment sites existed, and where now
waste storage areas are located (Artus, Montredon, La Combe du Saut, see Fig. 1 for sites
localization). Outside of flood periods and in the absence of intense surface leaching, the As
present in the Orbiel River may come from tributaries (Gresillou, Russec, which drain former
mining areas or from groundwater (alluvial plain of the Orbiel River) impacted by
percolating waters from the above-mentioned waste storage areas. Previous studies have
shown that lime arsenate waste generated following the treatment of percolating water
(Girard, 2011), which was stored in the valley without any precautionary measures until a
few years ago, could be a significant additional source for As in the Orbiel River (Khaska et
al., 2015).

We noted an increase in As concentrations from upstream to downstream, but As was present essentially in the truly dissolved form, which is consistent with previously published works (Majumder et al., 2014; Gontijo et al., 2016). However, it should be noted that some studies have reported that significant amount of As can be present in colloidal material in some cases (Ritter et al., 2006), but this seems to be mainly observed in environments very close to mines tailing primary leaching (Cai et al., 2017; Gomez-Gonzalez et al., 2016; Neubauer et al., 2013). In particular, it has been shown that As can be associated with organic matter, and Fe or Al, in the form of a ternary complex (Sharma et al. 2011; Neubauer et al. 2013). Indeed, assuming that in the oxygenated (100 % O₂ saturation) waters of the Orbiel River basin, As was mainly present as As(V), this metalloid is negatively charged (HAsO₄²⁻ and H₂AsO₄⁻) in the range of pH of natural waters, thus allowing its sorption on the surface of Fe or Al oxy-hydr)oxides. Moreover, Buschmann et al. (2006) and Fakour and Lin (2014) showed that the binding of As(V) with humic acid is favoured at low As/DOC ratio (in the order of 1 nmol As/mg DOC). Here, the As/DOC ratio is much higher downstream with values of 23, 54 and 92 nmol As/mg DOC, at LST, GDL and CNQ, respectively. Upstream, the ratio was lower but it was also shown that in this context, Al can be a strong competitor to binding sites if an excess of this metal was present, which is the case in our study. Furthermore, Li et al. (2019) have demonstrated that the correlation between As and DOM is mainly observed when DOM is of the humic type, whereas microbial degradation of small molecular size contributes to the reductive dissolution of Fe oxides promoting the release of As.

Finally, the fact that As was essentially present in the truly dissolved fraction suggests that As was in aqueous form or was associated with compounds of very small molecular weight;
indicating high As mobility in the Orbiel hydrosystem (Guo et al., 2011). In this regard, the presence of truly dissolved As can be of high concern because of its greater bioavailability and hence, potential toxicity to humans and the environment (Ren et al., 2017).

### 4.2.3. Distribution and fractionation of REE along the river course

The positive correlation between REE and DOC in rivers worldwide is due to the strong affinity of REE to organic ligands (Dupré et al., 1999, Pourret et al., 2010). The REE pattern obtained in the 0.22 µm fraction of the Orbiel River waters exhibited an enrichment in MREE and HREE with respect to LREE; both these enrichments tended to decrease from upstream to downstream. The Ce anomaly (Table 1) is 0.74 upstream, in the bog area, and tends to decrease downstream (0.45 at CNQ), indicating that the negative Ce anomaly increases downstream. This chemical fractionation of Ce in relation to neighboring REEs is linked to the fact that unlike other REEs, Ce can change its valence state (+III to +IV) under highly oxidizing conditions and can be oxidized in the form of cerianite ($CeO_2$) on the surface of Fe and Mn oxy(hydr)oxides or clays. In our case, it seems unlikely that there are processes in our river that induce this chemical fractionation. We therefore suggest that an increase in negative Ce anomaly from upstream to downstream is linked to a source effect, i.e., linked to the contribution from tributaries. We note that waters from the Russec and RUV have high negative Ce anomalies. However, this anomaly does not appear to be a tracer to the influence of mining waste, similar to the Pestril karstic spring, which, while not being impacted by mining activities, has one of the highest negative Ce anomalies. The enrichment in MREE was probably due to the preferential complexation of these elements with organic substances. Similar enrichment patterns in MREE have been observed for REE complexation with humic acids (Pourret et al., 2007), fulvic acids, and low-molecular weight acetic acid (Wood, 1993). According to Pourret et al. (2007), in the pH range of 7-8.5, REEs occur predominantly as humic organic complexes (> 60 %) and this preferential complexation of MREE with organic matter could be revealed by the ultrafiltration experiments. Indeed, we observed that the degree of MREE enrichment was negligible in the truly dissolved fraction (< 3 kDa). Note, however, that MREE enrichment can also be due to the dissolution of certain mineral phases, such as phosphates (Hannigan et al. 2001) or to the presence of colloidal Fe- and Mn-oxy(hydr)oxides (Johannesson et al., 1996).

The < 3 kDa fraction of the Orbiel River and tributaries exhibited a pronounced enrichment in HREE. This is consistent with results obtained in other rivers worldwide (Elderfield et al., 1990; Sholkovitz, 1992). This HREE enrichment may be explained by a higher affinity of
HREE to low molecular weight organic acids potentially found in the < 3 kDa fraction. Low molecular weight fulvic acids present in the < 3 kDa fraction are characterized by an increased affinity to metals as compared to higher molecular weight organic acids (Wang and Dei, 2003). This trend is thus expected, given that the stability of REE-organic complexes increases with atomic number (Pourret et al., 2007).

Finally, we found that the REE pattern was unchanged when going from upstream to downstream. As a result, it cannot be used as efficient tool from the point of view of source-tracking to understand the contribution of solutes from mining waste.
521 **Conclusion**

522 This study provides new insights on the physicochemical factors that determine the behavior
523 of chemical species in a mine region drained by rivers. We demonstrate that the influence of
524 lithology on the concentration of major and trace elements is dominant in the headwaters,
525 except for Pb. The latter was assumed to originate from atmospheric or from old-mining Pb
526 depositions in the peat bog. Downstream, past and/or present anthropogenic control of the
527 river waters' chemical composition (especially As and SO₄²⁻) was evidenced. Results also
528 showed that the presence of organic colloidal matter plays a key role in transporting trace
529 elements, Pb, and REE in this area. In contrast, As was mainly carried in truly dissolved and
530 potentially bioavailable (low molecular weight) form. Further studies would require a
531 thorough characterization of mining waste tailings and transportable sediments, which is
532 essential to gain a better understanding of anthropogenic contamination and its dispersion
533 through the Orbiel Valley, especially in the light of high flood episodes that may give rise to
534 contrasting patterns of contaminants and colloids behavior.
Acknowledgments

This work was carried out under the framework of the “DiagnOSE” project, partly funded by
the Region Occitanie Pyrénées – Méditerranée, France, and managed by Toulouse INP. We
thank Camille Duquenoy for her assistance with ICP-MS analyses. Oleg Pokrovsky was
partially supported by the TSU Program Priority-2030. Finally, this study could not have
been conducted without precious help from local communities, especially the council and the
mayor of Lastours town, who kindly allowed us to use one of the meeting rooms for all the
filtration and chemistry experiments during our field campaign. We also thank several other
people in the valley who have helped us in many ways in our study. We acknowledge both
the editor and the anonymous reviewers for their helpful and constructive comments and
corrections.
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**Figure 1.** Overall picture of the Orbiel Valley. a) Location of sampling points (in bold the samples selected for filtration at different cut-offs), tailings and old mining facilities in the Orbiel Valley (Orbiel River and main tributaries). b) Daily discharge (m³/s) measured at Lastours (Banque Hydro Website) from September 2021 to September 2022; the arrow represents the studied sampling campaign.

**Figure 2.** Schematic representation of the (ultra)filtration process.

**Figure 3.** Concentrations of DOC and major and trace elements of interest in the Orbiel River (continuous line) and its tributaries (triangles) in the samples filtered at 0.22 µm:
(a) DOC concentrations (mg/L); (b) Ca and Mg concentrations (mg/L); (c) Al and Fe concentrations (µg/L); (d) As and Pb concentrations (µg/L) and (e) La and Ce concentrations (µg/L). The lower horizontal axis shows the main Orbiel course samples, from upstream (TOURB) to the outlet point (CNQ). On the upper horizontal axis, tributaries are represented from upstream (MIR.TO) to downstream (SLT).

**Figure 4.** Concentrations of DOC, major (Al, Fe) and trace (As, Pb) elements of the Orbiel river and its tributary (RT) in progressive filtration and ultrafiltration aqueous fractions (fraction 1: 0.22-0.025 µm; fraction 2: 0.025 µm-100 kDa; fraction 3: 100-3 kDa; fraction 4 < 3 kDa).

Fraction 4 corresponds to the so-called “truly dissolved” fraction; the sum of the 4 fractions corresponds to the concentration at the 0.22 µm cut-off threshold. (a) DOC concentrations (mg/L); (b) Fe concentrations (µg/L); (c) Al concentrations (µg/L); (d) Pb concentrations (µg/L) and (e) As concentration (µg/L).

The lower horizontal axis shows the main Orbiel course samples, from upstream (TOURB) to downstream (GDL or LST) and one of his its tributary. The horizontal axe is up to scale.
**Figure 5.** REE patterns normalized to Upper Crust (Taylor and McLennan 1985) calculated for each filtration cut-off (0.22 µm, 0.025 µm, 100 kDa and 3 kDa). The solid lines represent the Orbiel River samples and the dotted lines those of the tributaries. Vertical bars represent the cut-offs between the LREEs, MREEs and HREEs.

**Figure 6.** Piper diagram calculated for water samples (< 0.22 µm) from the Orbiel Valley. Triangles represent the samples taken from the Orbiel River, whereas the circles represent the samples from the tributaries.

**Figure 7.** Pearson's correlation matrix for each fraction. SREE is the sum of the REE concentrations. As, Fe, Al, and SREE concentrations are expressed in µg/L. DOC and sulfate concentrations are expressed in mg/L. The number displayed is the Pearson's coefficient. The stars indicate the p-value range ("**" is p-value <0.001," **" is p-value ∈ [0.001, 0.01], "*" is p-value ∈ [0.01, 0.05], "." is p-value ∈ [0.05, 0.1] and " "is p-value >0.1.
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Overall picture of the Orbiel Valley. a) Location of sampling points (in bold the samples selected for filtration at different cut-offs), tailings and old mining facilities in the Orbiel Valley (Orbiel River and main tributaries). b) Daily discharge (m³/s) measured at Lastours (Banque Hydro Website) from September 2021 to September 2022; the arrow represents the studied sampling campaign.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Schematic representation of the (ultra)filtration process.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Concentrations of DOC and major and trace elements of interest in the Orbiel River (continuous line) and its tributaries (triangles) in the samples filtered at 0.22 µm: (a) DOC concentrations (mg/L); (b) Ca and Mg concentrations (mg/L); (c) Al and Fe concentrations (µg/L); (d) As and Pb concentrations (µg/L) and (e) La and Ce concentrations (µg/L). The lower</figcaption></figure>
horizontal axis shows the main Orbiel course samples, from upstream (TOURB) to the outlet point (CNQ). On the upper horizontal axis, tributaries are represented from upstream (MIR.TO) to downstream (SLT).

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

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

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>
**Figure 4.** Concentrations of DOC, major (Al, Fe) and trace (As, Pb) elements of the Orbiel river and its tributary (RT) in progressive filtration and ultrafiltration aqueous fractions (fraction 1: 0.22-0.025 µm; fraction 2: 0.025 µm-100 kDa; fraction 3: 100-3 kDa; fraction 4 < 3 kDa).

Fraction 4 corresponds to the so-called “truly dissolved” fraction; the sum of the 4 fractions corresponds to the concentration at the 0.22 µm cut-off threshold. (a) DOC concentrations (mg/L); (b) Fe concentrations (µg/L); (c) Al concentrations (µg/L); (d) Pb concentrations (µg/L) and (e) As concentration (µg/L).

The lower horizontal axis shows the main Orbiel course samples, from upstream (TOURB) to downstream (GDL or LST) and one of its tributary. The horizontal axe is up to scale.

<figure><img src="image_10.png" /></figure>

**Figure 5.** REE patterns normalized to Upper Crust (Taylor and McLennan 1985) calculated for each filtration cut-off (0.22 µm, 0.025 µm, 100 kDa and 3 kDa). The solid lines represent the Orbiel River samples and the dotted lines those of the tributaries. Vertical bars represent the cut-offs between the LREEs, MREEs and HREEs.
<figure><img src="image_11.png" /><figcaption>**Figure 6.** Piper diagram calculated for water samples (< 0.22 µm) from the Orbiel Valley. Triangles represent the samples taken from the Orbiel River, whereas the circles represent the samples from the tributaries.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure 7.** Pearson's correlation matrix for each fraction. SREE is the sum of the REE concentrations. As, Fe, Al, and SREE concentrations are expressed in µg/L. DOC and sulfate concentrations are expressed in mg/L. The number displayed is the Pearson's coefficient. The stars indicate the p-value range ("***" is p-value <0.001," **" is p-value ∈ [0.001, 0.01], "*" is p-value ∈ [0.01, 0.05], "." is p-value ∈ [0.05, 0.1] and " " is p-value >0.1.</figcaption></figure>
**Table 1.** Compilation of physico-chemical parameters, alkalinity, DOC, traces elements and REE concentrations for all the filtration cut-offs. Samples are described according to their filter fraction and sampling location. Ce anomaly is calculated as follows: $Ce \text{ anom.} = 3(Ce)_n/(2(La)_n+(Nd)_n)$ (Sholkovitz 1995) where n refers to the shale normalized ratios (Taylor and McLennan 1985).
<table><thead><tr><th rowspan="2">Sample</th><th rowspan="2">Cut-off threshold</th><th rowspan="2">GPS coordinates (X, Y)</th><th rowspan="2">pH</th><th rowspan="2">T°</th><th rowspan="2">Conductivity (25 °C)</th><th colspan="10">mg/L</th></tr><tr><th>°C</th><th>(µS/cm)</th><th>Alkalinity</th><th>DOC</th><th>SO<sub>4</sub><sup>2-</sup></th><th>NO<sub>3</sub><sup>-</sup></th><th>Cl<sup>-</sup></th><th>Na<sup>+</sup></th><th>Mg<sup>2+</sup></th><th>K<sup>+</sup></th><th>Ca<sup>2+</sup></th></tr></thead><tbody><tr><td rowspan="10">Orbiel River</td><td>0.22 µm</td><td></td><td></td><td></td><td></td><td>3.7</td><td>8.57</td><td>0.86</td><td>0.8</td><td>3.32</td><td>2.87</td><td>0.36</td><td>0.26</td><td>1.01</td></tr><tr><td>0.025 µm</td><td>43.4376550, 2.3059630</td><td>6.43</td><td>8.6</td><td>28.7</td><td>3.1</td><td>8.33</td><td>0.87</td><td>0.71</td><td>3.36</td><td>3.07</td><td>0.39</td><td>0.28</td><td>1.12</td></tr><tr><td>100 kDa</td><td></td><td></td><td></td><td>3.7</td><td>7.65</td><td>0.87</td><td>0.69</td><td>3.36</td><td>3.06</td><td>0.39</td><td>0.28</td><td>1.11</td></tr><tr><td>3 kDa</td><td></td><td></td><td></td><td>3.7</td><td>1.70</td><td>0.83</td><td>0.71</td><td>3.46</td><td>2.61</td><td>0.24</td><td>0.22</td><td>0.57</td></tr><tr><td>0.22 µm</td><td></td><td></td><td></td><td>6.7</td><td>5.56</td><td>2.13</td><td>1.68</td><td>4.72</td><td>3.42</td><td>0.51</td><td>0.37</td><td>1.80</td></tr><tr><td>0.025 µm</td><td>43.408117, 2.328650</td><td>6.82</td><td>9.4</td><td>42.3</td><td>6.1</td><td>5.53</td><td>2.14</td><td>1.69</td><td>4.73</td><td>3.45</td><td>0.51</td><td>0.37</td><td>1.81</td></tr><tr><td>100 kDa</td><td></td><td></td><td></td><td>6.7</td><td>4.74</td><td>2.13</td><td>1.64</td><td>4.72</td><td>3.43</td><td>0.52</td><td>0.37</td><td>1.89</td></tr><tr><td>3 kDa</td><td></td><td></td><td></td><td>6.7</td><td>0.78</td><td>1.82</td><td>1.66</td><td>4.79</td><td>3.17</td><td>0.44</td><td>0.33</td><td>1.44</td></tr><tr><td>0.22 µm</td><td></td><td></td><td></td><td>10.4</td><td>5.32</td><td>3.5</td><td>1.64</td><td>5.06</td><td>3.64</td><td>0.69</td><td>0.48</td><td>2.77</td></tr><tr><td rowspan="3">MIR</td><td>0.025 µm</td><td>43.382225, 2.345610</td><td>7.40</td><td>13.5</td><td>49.5</td><td>9.8</td><td>5.80</td><td>3.51</td><td>1.72</td><td>5.09</td><td>3.67</td><td>0.70</td><td>0.49</td><td>2.83</td></tr><tr><td>100 kDa</td><td></td><td></td><td></td><td>9.8</td><td>4.47</td><td>3.53</td><td>1.7</td><td>5.08</td><td>3.65</td><td>0.69</td><td>0.49</td><td>2.74</td></tr><tr><td>3 kDa</td><td></td><td></td><td></td><td>9.8</td><td>0.96</td><td>2.24</td><td>1.64</td><td>5.12</td><td>3.24</td><td>0.60</td><td>0.42</td><td>2.28</td></tr><tr><td rowspan="4">MAS</td><td>0.22 µm</td><td></td><td></td><td></td><td></td><td>20.1</td><td>3.88</td><td>6.66</td><td>1.65</td><td>5.73</td><td>4.15</td><td>1.03</td><td>0.69</td><td>5.86</td></tr><tr><td>0.025 µm</td><td>43.367090, 2.373015</td><td>7.45</td><td>13</td><td>76.8</td><td>20.1</td><td>3.81</td><td>6.67</td><td>1.7</td><td>5.77</td><td>4.16</td><td>1.03</td><td>0.70</td><td>5.77</td></tr><tr><td>100 kDa</td><td></td><td></td><td></td><td>20.1</td><td>2.72</td><td>6.67</td><td>1.68</td><td>5.76</td><td>4.12</td><td>1.02</td><td>0.69</td><td>5.59</td></tr><tr><td>3 kDa</td><td></td><td></td><td></td><td>20.1</td><td>0.70</td><td>4.66</td><td>1.71</td><td>5.86</td><td>3.96</td><td>0.97</td><td>0.65</td><td>5.04</td></tr><tr><td rowspan="4">LST</td><td>0.22 µm</td><td></td><td></td><td></td><td></td><td>43.3</td><td>1.88</td><td>12.73</td><td>2.29</td><td>6.8</td><td>4.71</td><td>2.76</td><td>0.75</td><td>11.74</td></tr><tr><td>0.025 µm</td><td>43.333952, 2.378199</td><td>7.27</td><td>11.1</td><td>133</td><td>43.3</td><td>2.64</td><td>12.97</td><td>2.42</td><td>6.85</td><td>4.69</td><td>2.69</td><td>0.75</td><td>11.66</td></tr><tr><td>100 kDa</td><td></td><td></td><td></td><td>42.7</td><td>1.88</td><td>12.96</td><td>2.37</td><td>6.85</td><td>4.54</td><td>2.60</td><td>0.72</td><td>11.68</td></tr><tr><td>3 kDa</td><td></td><td></td><td></td><td>42.7</td><td>0.89</td><td>11.4</td><td>2.45</td><td>7.01</td><td>4.34</td><td>2.53</td><td>0.67</td><td>10.99</td></tr></tbody></table>
<table><tbody><tr><td rowspan="12">Tributaries</td><td>LIM</td><td>0.22 µm</td><td>43.315000, 2.388000</td><td>7.66</td><td>10.1</td><td>138</td><td>44.5</td><td>3.27</td><td>13.71</td><td>2.04</td><td>6.74</td><td>4.71</td><td>3.02</td><td>0.72</td><td>10.84</td></tr><tr><td rowspan="4">GDL</td><td>0.22 µm</td><td></td><td></td><td></td><td></td><td>45.8</td><td>3.29</td><td>22.24</td><td>2.16</td><td>7.01</td><td>7.42</td><td>3.46</td><td>0.87</td><td>13.18</td></tr><tr><td>0.025 µm</td><td>43.308785, 2.393746</td><td>7.75</td><td>10.3</td><td>154</td><td>46.4</td><td>7.33</td><td>22.39</td><td>2.14</td><td>7.06</td><td>6.88</td><td>3.25</td><td>0.80</td><td>12.43</td></tr><tr><td>100 kDa</td><td></td><td></td><td></td><td></td><td>45.8</td><td>2.57</td><td>22.3</td><td>2.17</td><td>7.04</td><td>6.82</td><td>3.22</td><td>0.81</td><td>12.18</td></tr><tr><td>3 kDa</td><td></td><td></td><td></td><td></td><td>45.8</td><td>9.61</td><td>21.49</td><td>2.19</td><td>7.17</td><td>7.09</td><td>3.35</td><td>0.85</td><td>12.61</td></tr><tr><td>CNQ</td><td>0.22 µm</td><td>43.268000, 2.398000</td><td>7.71</td><td>9.9</td><td>228</td><td>72.6</td><td>2.91</td><td>32.38</td><td>2.47</td><td>8.15</td><td>7.63</td><td>5.58</td><td>0.96</td><td>21.49</td></tr><tr><td>MIR.TO</td><td>0.22 µm</td><td>43.408117, 2.328650</td><td>6.80</td><td>13.4</td><td>40.4</td><td>4.9</td><td>2.55</td><td>2.73</td><td>1.63</td><td>5</td><td>3.22</td><td>0.64</td><td>0.27</td><td>1.31</td></tr><tr><td rowspan="4">RT</td><td>0.22 µm</td><td></td><td></td><td></td><td></td><td>21.4</td><td>3.71</td><td>6.28</td><td>2.87</td><td>6.06</td><td>3.90</td><td>1.14</td><td>0.87</td><td>6.08</td></tr><tr><td>0.025 µm</td><td>43.371215, 2.377534</td><td>8.32</td><td>9.0</td><td>79.0</td><td>20.7</td><td>4.01</td><td>6.29</td><td>2.91</td><td>6.06</td><td>3.88</td><td>1.12</td><td>0.86</td><td>5.94</td></tr><tr><td>100 kDa</td><td></td><td></td><td></td><td></td><td>21.4</td><td>4.08</td><td>6.32</td><td>2.82</td><td>6.07</td><td>4.71</td><td>1.39</td><td>1.03</td><td>6.91</td></tr><tr><td>3 kDa</td><td></td><td></td><td></td><td></td><td>21.4</td><td>6.59</td><td>5.87</td><td>2.81</td><td>6.11</td><td>4.67</td><td>1.34</td><td>1.01</td><td>6.50</td></tr><tr><td>PES</td><td>0.22 µm</td><td>43.338000, 2.380000</td><td>7.43</td><td>14.3</td><td>245.0</td><td>115.3</td><td>0.73</td><td>16.05</td><td>3.15</td><td>8.07</td><td>5.16</td><td>6.38</td><td>0.71</td><td>26.48</td></tr><tr><td>RUV</td><td>0.22 µm</td><td>43.335735, 2.364788</td><td>6.97</td><td></td><td>2470.0</td><td>86.6</td><td>3.18</td><td>4461</td><td>24.5</td><td>22.11</td><td>1.09</td><td>23.27</td><td>0.46</td><td>30.04</td></tr><tr><td>GRE</td><td>0.22 µm</td><td>43.333861, 2.376938</td><td>7.25</td><td>13.1</td><td>128.0</td><td>25.0</td><td>1.97</td><td>12.93</td><td>1.09</td><td>8.72</td><td>6.87</td><td>2.36</td><td>0.72</td><td>6.72</td></tr><tr><td>SLT</td><td>0.22 µm</td><td>43.282000, 2.392000</td><td>8.1</td><td>10.90</td><td>472.0</td><td>169.6</td><td>1.62</td><td>79.78</td><td>3.04</td><td>11</td><td>6.88</td><td>15.67</td><td>1.17</td><td>49.19</td></tr></tbody></table>
<table><thead><tr><th></th><th>Al</th><th>Fe</th><th>As</th><th>Pb</th><th>La</th><th>Ce</th><th>Pr</th><th>Nd</th><th>Sm</th><th>Gd</th><th>Tb</th><th>Dy</th><th>Ho</th><th>Er</th><th>Tm</th><th>Yb</th><th>Lu</th><th>Ce.anom</th></tr><tr><th></th><th colspan="18">µg/L</th></tr></thead><tbody><tr><td rowspan="4">TOURB</td><td>218.3</td><td>195.6</td><td>0.29</td><td>0.604</td><td>0.182</td><td>0.314</td><td>0.053</td><td>0.200</td><td>0.051</td><td>0.058</td><td>0.009</td><td>0.057</td><td>0.012</td><td>0.031</td><td>0.005</td><td>0.026</td><td>0.004</td><td>0.743</td></tr><tr><td>206.7</td><td>151.2</td><td>0.29</td><td>0.451</td><td>0.177</td><td>0.294</td><td>0.050</td><td>0.195</td><td>0.050</td><td>0.056</td><td>0.009</td><td>0.056</td><td>0.011</td><td>0.031</td><td>0.004</td><td>0.027</td><td>0.004</td><td>0.716</td></tr><tr><td>210.8</td><td>147.2</td><td>0.28</td><td>0.449</td><td>0.161</td><td>0.276</td><td>0.046</td><td>0.178</td><td>0.047</td><td>0.052</td><td>0.008</td><td>0.051</td><td>0.010</td><td>0.029</td><td>0.004</td><td>0.025</td><td>0.004</td><td>0.737</td></tr><tr><td>21.7</td><td>8.2</td><td>0.18</td><td>0.032</td><td>0.011</td><td>0.020</td><td>0.004</td><td>0.015</td><td>0.004</td><td>0.004</td><td>0.001</td><td>0.005</td><td>0.001</td><td>0.003</td><td>0.001</td><td>0.004</td><td>0.001</td><td>0.721</td></tr><tr><td rowspan="4">MIR.AM</td><td>124.4</td><td>100.1</td><td>0.28</td><td>0.184</td><td>0.244</td><td>0.296</td><td>0.056</td><td>0.204</td><td>0.050</td><td>0.057</td><td>0.008</td><td>0.050</td><td>0.010</td><td>0.028</td><td>0.004</td><td>0.024</td><td>0.003</td><td>0.575</td></tr><tr><td>108.0</td><td>73.6</td><td>0.26</td><td>0.131</td><td>0.215</td><td>0.261</td><td>0.050</td><td>0.184</td><td>0.044</td><td>0.050</td><td>0.007</td><td>0.044</td><td>0.009</td><td>0.024</td><td>0.003</td><td>0.022</td><td>0.003</td><td>0.572</td></tr><tr><td>108.5</td><td>63.7</td><td>0.27</td><td>0.123</td><td>0.201</td><td>0.257</td><td>0.047</td><td>0.174</td><td>0.041</td><td>0.048</td><td>0.007</td><td>0.043</td><td>0.009</td><td>0.024</td><td>0.003</td><td>0.021</td><td>0.003</td><td>0.600</td></tr><tr><td>11.7</td><td>3.4</td><td>0.20</td><td>0.006</td><td>0.011</td><td>0.015</td><td>0.003</td><td>0.012</td><td>0.003</td><td>0.003</td><td>0.001</td><td>0.003</td><td>0.001</td><td>0.003</td><td>0.000</td><td>0.003</td><td>0.000</td><td>0.598</td></tr><tr><td rowspan="4">MIR</td><td>92.8</td><td>61.4</td><td>1.05</td><td>0.090</td><td>0.224</td><td>0.247</td><td>0.052</td><td>0.189</td><td>0.045</td><td>0.050</td><td>0.007</td><td>0.043</td><td>0.008</td><td>0.023</td><td>0.003</td><td>0.020</td><td>0.003</td><td>0.521</td></tr><tr><td>89.7</td><td>64.3</td><td>1.09</td><td>0.094</td><td>0.212</td><td>0.230</td><td>0.048</td><td>0.183</td><td>0.041</td><td>0.050</td><td>0.007</td><td>0.043</td><td>0.008</td><td>0.023</td><td>0.003</td><td>0.020</td><td>0.003</td><td>0.509</td></tr><tr><td>90.0</td><td>62.1</td><td>1.06</td><td>0.116</td><td>0.231</td><td>0.244</td><td>0.050</td><td>0.184</td><td>0.043</td><td>0.050</td><td>0.007</td><td>0.042</td><td>0.009</td><td>0.023</td><td>0.003</td><td>0.020</td><td>0.003</td><td>0.508</td></tr><tr><td>8.7</td><td>3.0</td><td>0.89</td><td>0.025</td><td>0.009</td><td>0.010</td><td>0.002</td><td>0.009</td><td>0.002</td><td>0.003</td><td>0.000</td><td>0.003</td><td>0.001</td><td>0.002</td><td>0.000</td><td>0.002</td><td>0.000</td><td>0.503</td></tr><tr><td rowspan="4">MAS</td><td>61.6</td><td>58.9</td><td>1.83</td><td>0.099</td><td>0.194</td><td>0.181</td><td>0.042</td><td>0.158</td><td>0.036</td><td>0.042</td><td>0.006</td><td>0.034</td><td>0.007</td><td>0.018</td><td>0.002</td><td>0.015</td><td>0.002</td><td>0.446</td></tr><tr><td>54.1</td><td>44.1</td><td>1.80</td><td>0.071</td><td>0.168</td><td>0.153</td><td>0.036</td><td>0.135</td><td>0.032</td><td>0.036</td><td>0.005</td><td>0.031</td><td>0.006</td><td>0.016</td><td>0.002</td><td>0.014</td><td>0.002</td><td>0.437</td></tr><tr><td>52.1</td><td>37.9</td><td>1.76</td><td>0.070</td><td>0.159</td><td>0.143</td><td>0.034</td><td>0.130</td><td>0.030</td><td>0.034</td><td>0.005</td><td>0.030</td><td>0.006</td><td>0.015</td><td>0.002</td><td>0.014</td><td>0.002</td><td>0.430</td></tr><tr><td>12.5</td><td>1.7</td><td>1.64</td><td>0.024</td><td>0.008</td><td>0.007</td><td>0.002</td><td>0.007</td><td>0.002</td><td>0.002</td><td>0.000</td><td>0.002</td><td>0.000</td><td>0.001</td><td>0.000</td><td>0.002</td><td>0.000</td><td>0.392</td></tr><tr><td rowspan="4">LST</td><td>20.6</td><td>22.3</td><td>3.28</td><td>0.045</td><td>0.064</td><td>0.054</td><td>0.016</td><td>0.059</td><td>0.014</td><td>0.016</td><td>0.002</td><td>0.014</td><td>0.003</td><td>0.008</td><td>0.001</td><td>0.007</td><td>0.001</td><td>0.384</td></tr><tr><td>15.2</td><td>12.2</td><td>3.20</td><td>0.026</td><td>0.038</td><td>0.030</td><td>0.009</td><td>0.037</td><td>0.009</td><td>0.010</td><td>0.002</td><td>0.009</td><td>0.002</td><td>0.005</td><td>0.001</td><td>0.005</td><td>0.001</td><td>0.357</td></tr><tr><td>17.0</td><td>11.5</td><td>3.17</td><td>0.024</td><td>0.049</td><td>0.037</td><td>0.011</td><td>0.042</td><td>0.010</td><td>0.012</td><td>0.002</td><td>0.011</td><td>0.002</td><td>0.006</td><td>0.001</td><td>0.006</td><td>0.001</td><td>0.352</td></tr><tr><td>11.9</td><td>0.7</td><td>2.76</td><td>0.008</td><td>0.003</td><td>0.002</td><td>0.001</td><td>0.003</td><td>0.001</td><td>0.001</td><td>0.000</td><td>0.001</td><td>0.000</td><td>0.001</td><td>0.000</td><td>0.001</td><td>0.000</td><td>0.318</td></tr></tbody></table>
<table><tbody><tr><td></td><td>LIM</td><td>46.9</td><td>54.5</td><td>4.76</td><td>0.128</td><td>0.136</td><td>0.139</td><td>0.033</td><td>0.123</td><td>0.030</td><td>0.033</td><td>0.005</td><td>0.028</td><td>0.006</td><td>0.016</td><td>0.002</td><td>0.013</td><td>0.002</td><td>0.472</td></tr><tr><td></td><td></td><td>46.6</td><td>68.6</td><td>13.29</td><td>0.126</td><td>0.135</td><td>0.133</td><td>0.032</td><td>0.122</td><td>0.029</td><td>0.033</td><td>0.005</td><td>0.029</td><td>0.006</td><td>0.015</td><td>0.002</td><td>0.013</td><td>0.002</td><td>0.456</td></tr><tr><td rowspan="3">GDL</td><td></td><td>38.2</td><td>40.6</td><td>12.34</td><td>0.058</td><td>0.088</td><td>0.087</td><td>0.022</td><td>0.083</td><td>0.020</td><td>0.024</td><td>0.004</td><td>0.021</td><td>0.004</td><td>0.011</td><td>0.002</td><td>0.009</td><td>0.001</td><td>0.450</td></tr><tr><td></td><td>36.8</td><td>40.9</td><td>12.40</td><td>0.057</td><td>0.094</td><td>0.090</td><td>0.023</td><td>0.087</td><td>0.021</td><td>0.024</td><td>0.004</td><td>0.021</td><td>0.004</td><td>0.011</td><td>0.002</td><td>0.010</td><td>0.002</td><td>0.439</td></tr><tr><td></td><td>32.9</td><td>3.2</td><td>12.68</td><td>0.007</td><td>0.008</td><td>0.007</td><td>0.002</td><td>0.008</td><td>0.002</td><td>0.003</td><td>0.000</td><td>0.003</td><td>0.001</td><td>0.002</td><td>0.000</td><td>0.002</td><td>0.000</td><td>0.409</td></tr><tr><td>CNQ</td><td></td><td>25.9</td><td>48.5</td><td>20.07</td><td>0.091</td><td>0.076</td><td>0.074</td><td>0.018</td><td>0.070</td><td>0.017</td><td>0.019</td><td>0.003</td><td>0.017</td><td>0.003</td><td>0.009</td><td>0.001</td><td>0.008</td><td>0.001</td><td>0.446</td></tr><tr><td></td><td>MIR.TO</td><td>34.9</td><td>21.7</td><td>0.61</td><td>0.092</td><td>0.157</td><td>0.256</td><td>0.043</td><td>0.168</td><td>0.039</td><td>0.040</td><td>0.005</td><td>0.029</td><td>0.006</td><td>0.015</td><td>0.002</td><td>0.014</td><td>0.002</td><td>0.709</td></tr><tr><td rowspan="4">Tributaries</td><td></td><td>92.4</td><td>71.0</td><td>3.85</td><td>0.184</td><td>0.199</td><td>0.173</td><td>0.044</td><td>0.167</td><td>0.039</td><td>0.045</td><td>0.007</td><td>0.040</td><td>0.008</td><td>0.022</td><td>0.003</td><td>0.018</td><td>0.002</td><td>0.412</td></tr><tr><td></td><td>83.2</td><td>47.1</td><td>3.71</td><td>0.094</td><td>0.149</td><td>0.130</td><td>0.033</td><td>0.128</td><td>0.029</td><td>0.035</td><td>0.005</td><td>0.032</td><td>0.006</td><td>0.017</td><td>0.002</td><td>0.016</td><td>0.002</td><td>0.409</td></tr><tr><td></td><td>102.7</td><td>43.8</td><td>4.31</td><td>0.078</td><td>0.137</td><td>0.129</td><td>0.032</td><td>0.122</td><td>0.029</td><td>0.033</td><td>0.005</td><td>0.031</td><td>0.006</td><td>0.018</td><td>0.003</td><td>0.015</td><td>0.002</td><td>0.437</td></tr><tr><td></td><td>55.3</td><td>4.3</td><td>4.24</td><td>0.010</td><td>0.015</td><td>0.014</td><td>0.004</td><td>0.015</td><td>0.004</td><td>0.004</td><td>0.001</td><td>0.004</td><td>0.001</td><td>0.003</td><td>0.000</td><td>0.003</td><td>0.001</td><td>0.404</td></tr><tr><td>PES</td><td></td><td>1.7</td><td>0.2</td><td>2.18</td><td>0.003</td><td>0.005</td><td>0.001</td><td>0.001</td><td>0.004</td><td>0.001</td><td>0.002</td><td>0.000</td><td>0.001</td><td>0.000</td><td>0.002</td><td>0.000</td><td>0.002</td><td>0.000</td><td>0.112</td></tr><tr><td>RUV</td><td></td><td>0.6</td><td>1.1</td><td>3.93</td><td>0.009</td><td>0.129</td><td>0.003</td><td>0.011</td><td>0.041</td><td>0.008</td><td>0.014</td><td>0.002</td><td>0.008</td><td>0.002</td><td>0.004</td><td>0.000</td><td>0.002</td><td>0.000</td><td>0.015</td></tr><tr><td>GRE</td><td></td><td>33.6</td><td>6.2</td><td>83.48</td><td>0.017</td><td>0.051</td><td>0.048</td><td>0.015</td><td>0.064</td><td>0.017</td><td>0.020</td><td>0.003</td><td>0.017</td><td>0.003</td><td>0.009</td><td>0.001</td><td>0.008</td><td>0.001</td><td>0.381</td></tr><tr><td>SLT</td><td></td><td>4.0</td><td>3.7</td><td>57.42</td><td>0.014</td><td>0.007</td><td>0.004</td><td>0.001</td><td>0.005</td><td>0.001</td><td>0.003</td><td>0.000</td><td>0.002</td><td>0.000</td><td>0.001</td><td>0.000</td><td>0.001</td><td>0.000</td><td>0.304</td></tr></tbody></table>
<figure><img src="image_13.png" /><figcaption>Old mining site of
Salsigne, SW France</figcaption></figure>

Orbiel River

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

Tailing dams

Cascade ultrafiltration
upstream to downstream

Trace element migration ?

Colloidal fraction 100 - 3 kDa

<figure><img src="image_16.png" /></figure>

UPSTREAM

<figure><img src="image_17.png" /></figure>

DOWNSTREAM

colloidal fraction

truly dissolved fraction

As enrichment
Colloidal matter decrease

Truly dissolved fraction

<figure><img src="image_18.png" /></figure>

33

As

Arsenic

74.9216

As truly dissolved:
more bioavailable ?

Colloidal fraction 100 - 3 kDa

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

Fe