# Influence of early pedogenesis on sediments deposited on land with a focus on sulfur dynamics

C. Tiberg, Samuel Coussy, L. Haouche, M. Lefebvre, Gabriel Billon, Lesven

Ludovic, Philippe Bataillard

► To cite this version:

C. Tiberg, Samuel Coussy, L. Haouche, M. Lefebvre, Gabriel Billon, et al. Influence of early pedogenesis on sediments deposited on land with a focus on sulfur dynamics. Journal of Soils and Sediments, 2025, 10.1007/s11368-025-03976-1. hal-04928472

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1 **Influence of early pedogenesis on sediments deposited on land with focus on**
2 **sulfur dynamics**

3 Tiberg C.<sup>a,b</sup>, Coussy S.<sup>b,*</sup>, Haouche L.<sup>c</sup>, Lefebvre M.<sup>c</sup>, Billon G.<sup>d</sup>, Lesven L.<sup>d</sup>, Bataillard P.<sup>b</sup>

4 <sup>a</sup> Swedish Geotechnical Institute, SGI, Dep. for management of contaminated sites, SE-581 93

5 Linköping, Sweden, OrcidID: 0000-0003-2809-2261 (Tiberg C.)

6 <sup>b</sup> Bureau de recherches géologiques et minières, BRGM, 3, av. Claude Guillemin 45100 Orléans,

7 France, OrcidID: 0000-0003-4696-1088 (Coussy S.) 0000-0003-1866-7076 (Bataillard P.)

8 <sup>c</sup> Institut scientifique de service public, ISSeP, Rue de la Platinerie B-7340 Colfontaine, Belgium

9 <sup>d</sup> UMR8516 Laboratoire de Spectroscopie pour les Interactions, la Réactivité et l'Environnement,

10 LASIRe, Avenue Paul Langevin 59655 Villeneuve d'Ascq, France. OrcidID: 0000-0002-4693-5106

11 (Billon G.) 0000-0002-4575-5891 (Lesven L.)

12 *Corresponding author: s.coussy@brgm.fr

13 **Abstract**

14 **Purpose:** When sediments are deposited on land, large themodynamic disequilibria are induced. We
15 studied early pedogenesis of sediments deposited 1971 to 2016 in a depot with well-known history to
16 characterize transformation processes and estimate their rate of transformation into Technosols.

17 **Methods:** Total and extractable concentrations of elements were determined in samples from several
18 depths of four sampling pits. Surface samples were subjected to mineralogical investigations and
19 weathering cell test. Investigations focused on sulfur dynamics, but iron and manganese
20 transformation and trace metals were also studied.

21 **Results:** Concentrations of soluble salts were low in leachates from older sediments. Oxidation
22 progressed downwards through the profiles converting sulfides to SO₄²⁻ at a rate of about 6-10 cm per
23 year and faster in sediments with high sand content. There were significant amounts of S left in the
sediments also after depletion of leachable SO₄²⁻ but weathering cell tests indicate that this pool is not reactive. Leaching of trace metals was generally low and did not increase with sulfide oxidation, indicating that released metals redistributed quickly to scavengers, probably newly formed Fe and Mn (hydr)oxides and organic matter.

**Conclusion:** The first processes occurring after sediment deposition are the release of soluble salts, including SO₄²⁻ kinetically controlled by the oxidation of sulfides. The evolution during this critical stage may not be fully captured by the total trace metal content and simple leaching tests. For sediment managers, we recommend complementing with analysis of total S and in case of high S content with the weathering cell test.

Keywords: Sediment, sulfide, pedogenesis, Technosol, iron and manganese (hydr)oxides, trace metal

35

## 1. Introduction

Every year, dredging activities in Europe moves around 200 million m³ of sediments (SedNet 2004). In France, between 950,000 and 2,500,000 m³ of sediments are dredged annually from rivers belonging to the public river domain (Cerema 2021) and between 28 and 52 million m³ from ports and estuaries (Cerema 2020). Around 40% of the river sediments are managed on land. In the case of marine sediments, only a few percent of the total volume is extracted from the aquatic environment and deposited on land, but this represents several million of m³, due to the huge volumes handled. In France, sediments removed from the aquatic environment as part of the maintenance or creation of river and port infrastructures have the status of waste (Ministère de la Transition Ecologique 2022). They are usually landfilled in dedicated deposit sites located in the perimeter of the watercourses or ports where they were dredged (e.g. Piou et al. 2009; Dorleon 2024). Some of these sediments are contaminated, and many studies dealt with the environmental risks associated with contaminants (Tack et al. 1996; Stephens et al. 2001; Caille et al. 2003; Lors et al. 2004; Isaure et al. 2005; Piou et al. 2009; Crocetti et al. 2022). When sediments are deposited on land and stored under aerobic conditions large thermodynamic disequilibria occur which drives geochemical changes in the
sediments. This can lead to the release of metals or metalloids to the environment. The geochemical evolution will affect the whole sediment which will evolve into a soil (Betancur-Corredor et al. 2020).

Time is a crucial parameter for this evolution. The sediments in depots created after dredging are the parent material of soils which can be classified as Technosols because they are the product of a human activity and their properties and pedogenesis are dominated by their technical origin (Schad 2018; IUSS Working Group WRB 2022).

Pedogenesis is usually very slow and includes various mechanical, geochemical and biological processes through which our soils evolve on a timescale of thousands of years (Duchaufour 1976; Costantini et al. 2006; Salvador-Blanes et al. 2007; Minasny et al. 2008). However, due to the drastic change in environment when depositing sediments on land, the transformation processes are accelerated compared to natural soils (Séré et al. 2010; Huot et al. 2015). Indicators related to redox status evolution and weathering can be used to monitor those transformations in dredged sediments. In anoxic sediments, sulfur is reduced by bacteria and buried in the sediment as sulfide minerals (Berner 1984). Sulfur in organic rich anoxic sediments is in general initially transformed to iron monosulfides such as mackinawite, pyrrhotite and greigite (Bush et al. 2004; Rickard and Luther 2007). These phases are metastable and very reactive and can be further converted to pyrite under anoxic conditions (Hurtgen et al. 1999). After deposition on land, the water content of sediments decreases and oxygen concentration increases. Redox sensitive elements, compounds and minerals oxidize, e.g. sulfides will be converted to metal ions and SO₄²⁻ and may release trace metals (Piou et al. 2009; Choppala et al. 2017; Johnson et al. 2022). In this context, sulfur dynamics is a reliable indicator of sediments evolution.

Another indicator of pedogenesis in dredged sediments is the dynamics of iron oxides, since Fe minerals are reported to be reliable and well-accepted markers of soil evolution in soil sciences (Buol et al. 1973; Uzarowicz and Skiba 2011). The presence of secondary Fe minerals such as ferrihydrite in the shallow horizons of Technosols are indicative of soil evolution processes occurring in those systems (Coussy et al. 2017).
The transition period observed at the start of pedogenesis in these dredged sediments is therefore critical. The release of metal(oid)s and high $SO_4^{2-}$ concentrations over time may be problematic for the environment. The impact of these deposits on groundwater is generally assessed in the light of the criteria for landfills for inert waste (OJEC 2003). However, the method used to assess these criteria (batch leaching test) does not take into account the evolution of sediments over time, in particular sulfide oxidation. As sulfides oxidize, it may well be that $SO_4^{2-}$ or metal(oid) concentrations exceed the criteria with time even if criteria are met at the time of deposition. The time-dependent release needs to be understood for a safe and sustainable management of sediments.

In this study, an exhaustive characterization of dredged sediments deposited several decades ago was carried out, with a focus on sulfur dynamics and iron status evolution as indicators of pedogenesis, along with the concomitant release of $SO_4^{2-}$ and trace metals. The well-known history of the sediment depot, and especially the record of the deposit episodes, made it possible to assess the early pedogenetic processes occurring in sediment-based Technosols.

The aim of the present study was to try to 1) characterize the initial transformation processes and their influence on leaching, based on sulfur dynamics and iron status evolution 2) estimate the rate at which the sediments are transformed into soil, linked to their physio-chemical characteristics.

# 2. Material and methods

## 2.1 Study site

The study site was a depot (TD26) of VNF (Voies navigables de France) where material from dredging of a canal had been deposited. The depot was located on agricultural land along the right bank of the Aa canal in the Hauts-de-France region in France. Samples were taken in two parts of the depot where material from canal Aa had been deposited. In the first part (cell 2 of TD26; Fig. 1), the sediments have been successively deposited between 1971 (approximately) and 2004. In the second part of the depot (cell 3 of TD26, Fig. 1), the sediments have been deposited in 2016.

The history of cell 2 of TD26 was investigated based on VNF records and on aerial photos from 1966 to 2012 (Fig. S1). The photos indicate that the depot was created between 1967 and 1971, along with
the widening of channel Aa. It is therefore likely that the oldest deposited material at the bottom of the
depot to a large extent consists of excavated earth from widening of the waterway. Around 1988 to
1990 more sediments were deposited, probably originating from dredging of the canal. A third
depositing event took place in 1998. After that the depots gradually dried and revegetated. Then, in
2004, there was a final large influx of dredged material. Since then, the area has been revegetated and
maintained regularly by mowing to keep an open habitat for animals and plants. The history of cell 3
is less well known than the one of cell 2, but the last recorded deposition of sediments in cell 3 was in
2016, 12 years after the last deposition of sediments in cell 2. VNF lists at least 6 more depositing
events between 1979 and 2004 in deposit TD26 but it is not clear if, and how, they affected cells 2 or
3. According to VNF, approximately 40,000 m³ material was deposited at each event, except in 2004,
with around 60,000 m³. In parts of TD26, the depot now reaches and probably exceeds 4.50 m in
thickness. The sediments were pumped into the depots through pipes. At that time, it was fluid which
led to preferential sedimentation of coarser particles near the pipe outlet and a horizontal gradient
regarding particle size within cell 2. The site is currently used as a natural area (renaturation).

## 2.2 Sampling

Samples have been collected on two occasions. First, three samples of surface material (0-10 cm) were
collected in March 2018 (at later locations of Pit 1, 3 and 4). Then, in May 2019, four pits were dug
and sampled (Fig. 1). Three of them were located in cell 2, the first (Pit 1) close to the former location
of pipe outlets, and the other two (Pits 2 and 3 respectively) 15 and 25 m from Pit 1 to give a gradient
of grain size. The fourth pit (Pit 4) was dug in the younger sediments in cell 3. The pits were about 1.5
m deep, and samples were collected at several depths from the walls (Fig. 1 and 2). In addition,
samples were taken from greater depths with a hand-held auger through the bottom of the sampling
pits. The samples were put in plastic bags and stored at 4 °C, except for the Acid Volatile Sulfide
(AVS) analysis. Samples for AVS were taken in triplicates using 2 cm diameter plastic syringes (the
end of which was cut to allow the soil to be transferred inside the syringe), which were then sealed
with a cap. These syringes were stored in small plastic bags in an inert atmosphere (filled with
nitrogen gas) in order to preserve the AVS-rich anoxic soil particles. The plastic bags were frozen at
130 -18°C until analysis. AVS was analyzed on one sample from each depth, combined from all three
131 syringes. All other analyses of soil profiles except the CaCl₂ extraction were performed on three
132 samples from each depth.

133

134 **2.3 Analysis of surface samples**

135 **2.3.1 Mineralogical investigations by X-ray diffraction**

136 The XRD patterns of each material in powder form were collected at BRGM (Orléans, France) using a
137 BRUKER D8 ADVANCE diffractometer at the CuKα wavelength (λKα=0.1541 nm), equipped with
138 the high-speed energy-dispersive LYNXEYE XE-T detector with unprecedented filtering of
139 fluorescence and Kβ radiation. The XRD patterns were collected between 4 and 90°2θ with a step size
140 of 0.03°2θ and a counting time per step of 480 seconds. The diffractograms were interpreted with
141 JCPDS international references and processed using the DIFFRAC suite software. To quantify the
142 phases, the overall profile of the diagrams was then refined according to the Rietveld method (Rietveld
143 1969) using the SiroQuant V.4 software. For the clay fraction, oriented patterns were realized onto
144 glass slides, and X-Rayed after being air-dried, glycolated and thermally treated at 490°C to determine
145 the clay mineralogy.

146 **2.3.2 Total content of elements**

147 Chemical analysis was carried out by X-ray fluorescence spectrometry using a Zetium spectrometer
148 (Malvern PANalytical). Sample preparation was performed according to the fused bead technique,
149 dissolving the material with a lithium tetraborate flux before analysis. The particle size was reduced to
150 a powder and mixed with the glass product (“flux”) and then heated to approximately 1025°C (ISO
151 12677 2011). The proportion are about 1/5 of material for 4/5 of flux.

152 **2.3.3 Weathering cell tests**

153 The accelerated weathering in humidity cells aimed at mimicking the conditions at the surface of the
154 deposit but with shorter drying-wetting cycles to increase the weathering. The protocol used was
adapted from the ASTM “Standard Test Method for Accelerated Weathering of Solid Materials Using
a Modified Humidity Cell” (ASTM 1996).

Experiments were performed in triplicate with solid samples (800 g) in 20 cm high glass cylinders
with diameter 14.5 cm in an air-conditioned room at 25°C. First, dry air was circulated in the cells for
3 days. Then, for the next 3 days, the air was water-saturated by bubbling trough 30°C water before
injection into the cell. This cycle was repeated 22 times. The air flow rate was 5 L/min for both dry
and wet periods. To flush out the weathering products at the end of the wet step, a peristatic pump
injected deionized water into the cell from below (flow rate of 30 mL/h) until the sample was saturated
and covered by a 1 mm water layer (injection volume approximately 140 mL). The samples were
saturated for 24h before draining and collecting the leachate. Conductivity, pH and redox potential
were measured on all leachates and leachates from weathering cycles 1, 2, 4, 6, 9, 12, 15, 19 and 22
were analyzed for cations and anions by ion chromatography and inductively coupled plasma with
atomic emission spectrometry (ICP-AES).

## 2.4 Analysis of samples from sampling pits

### 2.4.1 Identification of soil colors

The soil colors of the soil profiles were described using the Munsell Color Chart (FAO 2006). For
illustrations they were converted to RGB colors.

### 2.4.2 Particle size distribution

The particle size distribution was determined in four samples from each pit, for the 1-2 mm fraction,
by sieving and for the 0.04 µm to 1 mm fraction by a laser diffraction Beckman CoulterTM LS 13 320
with an aqueous liquid module. Sediments smaller than 1 mm were transferred to a 20 mL plastic
container and 10 mL of MiliQ® water were added to liquefy the sediment. After at least one week,
sediments were analyzed. Several portions of liquefied sediment were transferred with a plastic pipette
to the aqueous liquid module until reaching an obscuration between 7 and 10%. The measure was
repeated three times and averaged. Acid (to destroy the carbonates) or hydrogen peroxide (to destroy
organic matter) was not added. The clay fraction may therefore be underestimated.
181 **2.4.3 Total content of elements**

182 Chemical analysis was carried out by X-ray fluorescence spectrometry, in the same way as the surface
183 samples (2.3.2).

184 **2.4.4 AVS – Acid volatile sulfides**

185 Briefly, approximately 1 g of wet soil was taken from each of the three syringes that were used for
186 sampling, weighed and transferred to a reactor under an inert atmosphere. Then, 40 ml of 6M HCl
187 was added to the soil to extract the AVS in the form of H₂S gas, which was transported via a nitrogen
188 stream into a basic trap. An aliquot of this trap was then titrated with a cadmium solution (1000 ppm)
189 by potentiometry using a Ag₂S electrode coupled to a calomel reference electrode.

190 **2.4.5 Extractions**

191 The release of trace elements was assessed by extraction with 10⁻³M CaCl₂. Batch tests were
192 performed with approximately 50 g sample at L/S 10 according to ISO 21268-2 2009. The eluates
193 were analyzed as follows: pH at 20°C ± 2°C, Cl⁻, SO₄²⁻, Na⁺, K⁺, Ca²⁺ and Mg²⁺ by liquid phase ion
194 chromatography, As, Be, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Sn, Zn by mass spectrometry with
195 inductively coupled plasma ICP-MS and dissolved organic carbon (DOC) was determined by thermal
196 and catalytic oxidation with infrared detection. A blanc value based on 16 measurements on the
197 leaching liquid was subtracted from measurements of Ca²⁺, Cl⁻, Na⁺, NO₃⁻ and SO₄²⁻.

198 The presence of poorly crystalline Fe and Mn oxides and hydroxides was assessed by ascorbic acid
199 extraction according to ISO 12782-1 2012. Briefly, 10 g of dry matter was extracted with 200 mL
200 solution containing 50 g.L⁻¹ of sodium hydrogen carbonate, 50 g.L⁻¹ of trisodium citrate dihydrate and
201 20 g.L⁻¹ of l(+) ascorbic acid. After 4h of agitation, extracts were centrifuged and filtrated through 0.2
202 µm cellulose acetate filters. Fe and Mn analyses were carried out by ICP-AES. Statistical evaluation
203 was performed by one-way Analysis of Variance (ANOVA) followed by a post-hoc Tukey (HSD) test
204 to determine if there were significant differences along soil profiles.
## 2.4.6 Saturation indexes

Saturation indexes (SI) for gypsum, anhydrite and Fe (hydr)oxides, e.g. ferrihydrite, were calculated with the free software Visual Minteq 4.0 (Gustafsson 2022) according to equation 1,

$$SI = \log (IAP/K_s) \qquad \text{Eq 1}$$

where IAP is the ion activity product and Ks the solubility constant. Calculations were based on results from the 10⁻³M CaCl₂ extraction (ISO 21268-2 2009). Values -1 < SI < 1 were regarded to be close to saturation.

# 3. Results

## 3.1 Soil profiles and properties

The soil profiles of Pits 1 to 3 were bedded, except for the top layers, while the whole profile of Pit 4 was massive silt with vertical cracks down to about 1 m (Fig. 2). The observed grain size decreased from Pit 1 to 3, consistent with deposition of heavier particles closer to the outlet of the pipes transporting the sediment. This was supported by particle size distribution analysis (Table 1) which also revealed a rather similar size fractionation down to 140 cm depth within each pit. At the time of sampling (in 2019), the sediments in the upper parts of Pit 1 to 3 had developed on land for about 15 years and in Pit 4 for about 3 years. Soil moisture increases with depth in the profiles, but the ground water table was below the depths of the profiles. Soil moisture was between 4 and 37% in Pits 1-3 and increased with decreasing grain size (i.e. lowest in Pit 1). Soil moisture in Pit 4 was more consistent, starting at 25% close to the surface and increasing to 37% at 135 cm (Fig. S2). Munsell colors followed the same trend indicating oxic conditions (brown) in the upper parts of the profiles and increasing anoxic conditions (grey or gleyey) further down (Fig. 2).

Main minerals in surface samples were quartz, calcite and clay minerals (Table 2). The content of clay minerals increased from Pit 1 to Pit 3, in consistency with size fractionation (Table 1) and Al content (Fig. S2).

The TOC content in the sediments was about 1-3.5%, the pH between 7.5 and 8.5, and the CaO content about 10% (Fig. S2). There was no clear pattern related to depth or grain size for these
parameters. The consistently high pH indicates that the carbonate content is high enough to buffer acid produced by oxidation of the sediment sulfides.

The leachable content of the very mobile elements Cl⁻ and Na⁺ seem to be depleted in whole profiles of the older pits 1, 2 and 3 (Fig. 3). Very little Cl⁻ was leached down to about 70 cm from younger sediment in Pit 4 while Na⁺ was low in leachates from the first 30 cm. Below these levels there were “leaching fronts”. However, below about 200 cm depth, the concentrations in leachates decreased sharply. The general pattern for K⁺ (Fig. S3) was similar to Na⁺.

The leachable concentration of Ca²⁺ was very low throughout Pit 1, the oldest pit with the largest grain size (Fig. 3). Concentrations in leachates from Pit 2 and 3 were low except at around 150 cm depth, where a leaching front seems to be moving down the profile. Below 150 cm Ca²⁺ concentrations were low again. Here the material is a mix of sediment and soil. In Pit 4 only the topsoil was depleted. Although present in much lower concentrations the overall pattern of leachable Mg²⁺ was very similar to Ca²⁺. Mg²⁺ and Ca²⁺ have similar properties in soil but Ca²⁺ is smaller and adsorbs stronger to ion exchange sites (e.g. Jalali et al. 2020). The Ca concentration in the sediment is much higher than the Mg concentration and Ca²⁺ therefore replaces Mg²⁺ on the surface of soil particles when it moves downwards in the soil profile.

## 3.2 Sulfur behavior in the soil profiles

The total S concentration in the profiles varied from a few 100 to 4000 mg kg⁻¹ dw with the lowest concentrations near the surface of Pit 1, 2 and 3 and at around 200 cm depth in all pits. The highest S concentrations were found around 150 cm depth in Pits 2 and 3 and from surface to about 200 cm in Pit 4 (Fig. 4). The XRD measurements performed on surface samples did not identify any sulfur containing minerals except gypsum in Pit 4 which is in line with a lower in S content in the oxidized surface samples from Pit 1, 2 and 3.

The results from total S, AVS and SO₄²⁻ extraction with CaCl₂ (Fig. 4) are overall consistent showing that sulfides in the surface layers have been oxidized and S has leached. Both AVS and leachable SO₄²⁻ were depleted down to at least 100 cm depth in Pits 1-3 but only in the surface layer of Pit 4.
The high sulfide content (higher than total sulfur) in the sample from 160-170 cm in Pit 1 coincides with observation of blackish noodles, delineated to this depth (Fig. 2). Total S and SO₄²⁻ leaching decreased in the material below 150-200 cm depth. If the material on this depth comes from the widening of canal Aa in the late 60's (as indicated for at least Pits 1-3 in the historical investigation) it is a mixture of soil and sediment and should contain less S than the pure sediments.

The leached SO₄²⁻ concentrations had the same pattern as and were very well correlated with leached Ca (Fig. 3). When sulfide is oxidized in presence of Ca²⁺, secondary minerals such as anhydrite (CaSO₄) and gypsum (CaSO₄·2H₂O) precipitate (Blowes et al. 2005). According to the calculated SI:s both gypsum and anhydrite were close to saturation in Pit 4 but undersaturated in Pits 1-3 except at around 150 cm depth in 2 and 3 (Fig. S4).

## 3.3 Iron and manganese in the soil profiles

Total content of Fe and Mn as well as the percentage of Fe and Mn extractable with ascorbic acid displayed very similar patterns throughout the soil profiles (Fig. 5). The total Fe and Mn content increased with smaller grainsize (i.e. from Pit 1 to 3). More Fe and Mn was extracted with ascorbic acid in the first 150 cm of Pits 1-3 (except the surface sample in Pit 1). This was verified by statistical tests indicating significant differences between the results above 150 cm and below (p<0.05). These results can be interpreted as the presence of higher proportion of poorly crystalline metal (hydr)oxides at the surface than in depth. On the contrary, in the younger profile P4, Fe and Mn ascorbic acid extractable concentrations were lower and not significantly different along the soil profile (p>0.05), except for one point at 205 cm depth. The results are in line with calculated SI for Fe (hydr)oxides. Ferrihydrite was close to or above saturation in pits 1-3 (whole profile) but undersaturated in Pit 4 except at the surface (Fig. S4). Higher concentrations of extractable Fe and Mn near the surface can be explained by a depth-dependence of oxidation of primary Fe- and Mn-bearing species. The neoformation of secondary Fe- and Mn- (hydr)oxides induced by weathering processes in surface horizons is a common process in natural soils (Schwertmann and Fitzpatrick 1993) and in Technosols (Coussy et al., 2017). These (hydr)oxides are formed by oxidation of Fe- and Mn- primary minerals, and especially Fe-bearing sulfides such as pyrite (Schwertmann and Fitzpatrick 1993; Blowes et al.
284 2005). For some points above 150 cm, the amount of extractable Mn exceeds 100% of total Mn. This
285 is probably due to inhomogeneities between samples, which have low total concentrations. Greater
286 mobility of Mn compared with Fe is a phenomenon often observed in the literature (e.g. Coussy et al.
287 2017).

288 **3.4 Trace metal content and leaching**

289 Among the trace metal investigated (As, Cd, Co, Cr, Cu, Hg, Ni, Pb, and Zn) only Cu, Hg, Pb and Zn
290 were present above background concentrations in the region in all pits (Sterckeman et al. 2006). In
291 addition, As was slightly above the background in the upper part of Pit 4. Cu, Pb and Zn had no
292 obvious relation with grain size or depth (Cu in Fig 6, Zn and Pb in Fig. S5). Hg was above
293 background in all pits and the concentrations increased below 150-200 cm depth. Concentrations of
294 the redox sensitive anion As were low but increased somewhat from pits 1 to 3 and were highest in Pit
295 4. Concentrations of Cu, Pb, Zn and As were all at the same level as median concentrations in
296 sediments in the north of France (Table 3) and well below the French quality guidelines for dredged
297 sediment (100 mg kg⁻¹ dw for Cu and Pb, 300 mg kg⁻¹ dw for Zn and 30 mg kg⁻¹ dw for As (JORF
298 2006)).

299 The only metal exceeding the quality guideline value was Hg (1 mg kg⁻¹ dw) below about 2 m. In the
300 first 2 m of the profile Hg concentrations were similar to the median values in Table 3. The higher Hg
301 levels in the deeper material can probably be explained by the age and origin of this material. At least
302 in Pits 1-3 it was deposited before 1988 (Fig S1) and likely consists of sediments and soil from
303 widening of channel Aa in the early 1970s. The Hg concentration in sediments has in general
304 decreased during the latest decades due to regulations and conventions limiting use of Hg. For
305 example, the estimated background value of Hg in suspended particulate matter in the Seine river has
306 decreased from 0.5-1 mg kg⁻¹ dw in 1979 to below 0.015-0.5 mg kg⁻¹ dw in 2002 (Meybeck et al.
307 2007). The high Hg concentrations below 250 cm depth probably origin from some Hg using activity,
308 for example hat-making that used mercuric nitric acid to felt furs, which was quite common in France
309 until the early 20th century.
Results from batch tests indicate that the release of these trace elements is randomly dispersed throughout the profiles (Fig. S6) and does not exceed the inert-waste criteria of the Council decision 2003/33 (OJEC 2003). Their behavior was not directly associated with the SO₄²⁻ release, since no trace metal “front” was observed in the profiles. Correlations of total content of Cu, Pb and Zn with pH, TOC and Feₐscorbic (Table S1) indicate that Feₐscorbic was the most important factor for Pb and Zn retention (R²=0.57 and 0.71 respectively). The correlation with pH was rather weak, especially for Pb (R²=0.19) which can be explained by a narrow pH interval. Correlations with TOC did not show clear trends (R² between 0.35 and 0.41).

An association of Pb and Zn with Fe and Mn (hydr)oxides was supported by Pb and Zn extracted by ascorbic acid, especially above 150 cm where ~50% of Pb and up to 80% of Zn was extracted (Fig 7). In depth, the lower extraction of metals with ascorbic acid can be explained by the probable greater crystallinity of the (hydr)oxides, as well as the possible presence of metal sulfides. That Cu seems less associated with Fe and Mn (hydr)oxides (Fig. 7 and Table S1) is reasonable given that Cu is generally closely associated with organic matter, and that copper sulfides can be relatively stable (Zhou et al. 1999; Weng et al. 2001).

## 3.5 Weathering cell tests

Weathering cell tests were carried out on surface samples from the locations of pits 1, 3 and 4. pH increased slightly at the beginning of the tests for all samples and then remained stable, between 7.6 and 8, until the end of the tests (Fig. 8). Conductivity was higher during the 8 first cycles for samples corresponding to Pit 4 than for samples corresponding to Pit 1 and 3 (Fig. 8). This was mainly due to a higher release of SO₄²⁻ from the Pit 4 sample during the first 8 cycles. After 8 cycles, the SO₄²⁻ release decreased to the level of samples corresponding to Pits 1 and 3. This behavior is consistent with a higher total S content in Pit 4 (2880 mg kg⁻¹) than in Pit 3 (1170 mg kg⁻¹) and Pit 1 (430 mg kg⁻¹) in the 0-20 cm horizons (Fig. 4).

The release of Ca²⁺ in the samples corresponding to Pit 4 during the weathering cell tests also decreased after 8 cycles, in consistence with a presence of gypsum in the surface horizon of Pit 4 (Table 2).
337

The release of trace metals such as Cu, Pb and Zn is essentially constant over time during the weathering cell test.

338

339

# 4. Discussion

340

## 4.1. Early pedogenetic mechanisms in sediments deposited on land

341

From the historical mapping of deposition events the first 90-100 cm of sediment (down to the buried old humus-containing soil surface, Fig 1) in pits 1-3 were deposited about 15 years before sampling.

342

343

The sediments between about 100 and 170 cm were probably deposited 8 years earlier and are thus 23 years old. They were vegetated for a few years between the deposition events. The exact "age" of the material at different depths further down in the profile is more uncertain but historical investigation (Fig. S1) show that the deepest material of Pits 1-3 was deposited at least 50 years before sampling.

344

345

346

347

The upper parts of Pit 4 were deposited about 3 years before sampling. Below about 200 cm depth the chemical characteristics of the material in Pit 4 changes to some extent (lower content of S, higher Hg and more leachable Na⁺ and Cl⁻) which indicates that this material is not the same, i.e. was deposited before 2016. Therefore, the discussion on pedogenic processes focus on sediments down to about 170-200 cm.

348

349

350

351

352

The main early "ageing" mechanisms in the sediments of the present study were leaching of soluble salts (i.e. Cl⁻, Na⁺) and a progressive oxidation downwards through the sediment profiles. This oxidation front lead to (i) sulfide dissolution into leachable SO₄²⁻ and (ii) Fe and Mn (hydr)oxide precipitation, due to the high pH. Calcium, coming from the abundant calcite content in sediments, combines with SO₄²⁻ to temporary precipitates of gypsum that are subsequently dissolved. The Ca²⁺ then pushes out Mg²⁺ from particle surfaces. Overall, the results indicate that S in the form of sulfide and leachable SO₄²⁻ has been partly depleted in pits 1-3 during the 15 years since the last deposition event and that the extent of depletion depends on particle size, with a faster depletion in the coarser sediments. Based on available data the depletion of sulfides is extended downwards with about 100 to 150 cm (Fig. 4) in 15 years leading to a rate of about 6-10 cm per year. However, there are also significant amounts of S left in all the profiles. This may be partly due to presence of more stable, well

353

354

355

356

357

358

359

360

361

362
363 crystallized, pyritic compounds remaining in the system. Another explanation is the well-known
364 phenomenon of sulfide passivation that may occur when sulfides oxidize in a high pH environment
365 (e.g. carbonated media). In this case, Fe-hydroxide coatings form on the sulfide surfaces, physically
366 protecting them from air (Holmström et al. 1999). Whatever the explanation, the weathering cell tests
367 of the surface samples confirmed that those remaining sulfides are not reactive in the older sediments
368 (Pit 1-3), and that the reactive sulfides will be depleted in the younger sediments (Pit 4) after a
369 depletion period. Sulfide oxidation in dredged sediments may lead to the release of associated trace
370 metals (Stephens et al. 2001). However, in our case these metals have been redistributed and bound to,
371 for example, metal (hydr)oxides and organic matter. It seems that amorphous Fe and Mn (hydr)oxides
372 formed after deposition of the sediments were important sorbents of Pb and Zn, which is in line with
373 previous studies about Technosol evolution (Piou et al. 2009; Coussy et al. 2017).

374 From a pedological point of view, the surface horizons can be easily distinguished from the deeper
375 horizons since they are not bedded and show a crumbly structure with a dense net of roots and signs of
376 biological activity. The surface horizons are deeper for Pits 1 to 3 (15 to 60 cm) than for Pit 4 (5 cm),
377 indicating that early pedogenesis is more advanced in the older pits. The structure acquired in those
378 surface horizon is due to biological processes and incorporation of organic matter (Chenu et al. 2000),
379 but can be also linked to the Ca and/or Fe dynamics observed in the profiles (Fig. 3 and 5), since Ca²⁺
380 cations and "free" Fe (hydr)oxides are known to play a role in the structural stability of soil
381 aggregates. They promote flocculation between soil organic matter and clays through a "bridging"
382 effect of Ca²⁺ and amorphous Fe (hydr)oxides between humic acids and clays (Muneer and Oades
383 1989; Barral et al. 1998).

384 **4.2. Classification of soil profiles**

385 According to the last edition of the World Reference Base for Soil Resources (IUSS Working Group
386 WRB 2022), profiles from Pit 1 to 3 can be classified as Spolic Technosols (Calcaric, Fluvic, Loamic),
387 due to the presence of artefacts (dredged sediments) with a calcite content > 2%, still showing fluvic
388 properties (stratification visible in all the profiles) and a texture class of sandy loam in one or more
389 layers with a combined thickness of ≥ 30 cm, occurring within 100 cm of the mineral soil surface.
Profile from Pit 4 only differs from a texture point of view because the horizons are composed of silt and silt loam, and is consequently classified as Spolic Technosol (Calcaric, Fluvic, Siltic). All these soil profiles are assigned to the “Technosols” key of the WRB since they are mainly composed of artefacts (>20% by volume of dredged sediments in the upper 100 cm) and the pedogenetic processes are not advanced enough to hide the human origin of the parent materials (e.g. Schad 2018). If we consider their evolution over a long-term period (>100 years, e.g. Séré et al. 2010; Coussy et al. 2017), they may become a Cambisol (i.e. moderate soil differentiation), due to the progressive disappearance of stratifications linked to the deposits, as it is visible in the first horizon of the Pit 3 soil profile, as well as the SO₄²⁻ depletion and the expected decarbonatization of the calcite-rich sediments.

## 5. Conclusions and implications

Our study confirms that the first critical processes occurring after sediment deposition on land are the release of soluble salts, including SO₄²⁻ which is kinetically controlled by the oxidation of sulfides, and that the release of metal(oid)s like Cu, Pb and As is linked to Fe- and Mn hydroxides in the sediments. However, due to the time dependence of these processes, the evolution during this critical early stage may not be fully captured by the total trace metal content and simple leaching tests generally used to characterize sediments. For a careful characterization of the environmental risks during this critical early stage we therefore recommend sediment managers and authorities to complement with analysis of total S and in case of high S content more specific tests such as the weathering cell test used in this study. The aim of these tests is to identify the risk of leaching of contaminants and their ligands, and to verify the stability of the remaining phases, under disturbance conditions more representative of ambient conditions. This methodology, already used in France for the management of excavated soils containing sulfides (Coussy and Quesnel 2021), could easily be implemented to improve predictions of the evolution of sediment depots. Depending on the results, this characterization step can limit the need for monitoring of the groundwaters downstream the sediment depots.

Today, there is no generic model to set limits beyond which the potential release of metals, Cl⁻ or SO₄²⁻ is too high to allow deposition. Our study gives a first contribution to the development of such a
416 decision-support tool, which would be particularly useful to any stakeholder confronted with sediment
417 deposition.

418 Acknowledgements

419 We acknowledge the Interreg V FWV1 program for funding this research through the VALSE project
420 and Voies Navigables de France (French Waterways) for giving us access to the site and its archives.

421 We thank BRGM members Pascal Auger and Safae El Farricha for their help during the field
campaign, and Virginie Derycke for her help with the statistical processing of the data.

423 Conflict of interest

424 The authors declare no competing interests.

425 References

426 ASTM (1996) D 5744, Standard Test Method for Accelerated Weathering of Solid Materials Using a
427 Modified Humidity Cell

428 Barral MT, Arias M, Guérif J (1998) Effects of iron and organic matter on the porosity and structural
429 stability of soil aggregates. Soil Tillage Res 46:261-272

430 Berner RA (1984) Sedimentary pyrite formation: An update. Geochim Cosmochim Acta 48:605-615

431 Betancur-Corredor B, Loaiza-Usuga JC, Denich M, Borgemeister C (2020) Changes of Technosol
432 properties and vegetation structure along a chronosequence of dredged sediment deposition in
433 areas with alluvial gold mining in Colombia. J Soil Sediment 20:2377-2394

434 Blowes DW, Ptacek CJ, Jambor JL, Weisener CG (2005) The geochemistry of acid mine drainage.
435 Treatise on Geochemistry, Vol 9: Environ Geochem, 149-204

436 Buol S, Hole F, McCracken R (1973) Soil genesis and classification. The Iowa State University Press,
437 Ames, pp. 360

438 Bush RT, Fyfe D, Sullivan LA (2004) Occurrence and abundance of monosulfidic black ooze in
439 coastal acid sulfate soil landscapes. Soil Res 42:609-616

440 Caille N, Tiffreau C, Leyval C, Morel JL (2003) Solubility of metals in an anoxic sediment during
441 prolonged aeration. Sci Total Environ 301:239-250

442 Cerema (2020) Enquête dragage 2017 - Enquête nationale sur les dragages des ports maritimes

443 Cerema (2021) Rétrospective des dragages fluviaux en France - Période 2011-2017

444 Chenu C, Le Bissonnais Y, Arrouays D (2000) Organic Matter Influence on Clay Wettability and Soil
445 Aggregate Stability. Soil Sci Soc Am J 64:1479-1486

446 Choppala G, Bush R, Moon E, Ward N, Wang ZH, Bolan N, Sullivan L (2017) Oxidative
447 transformation of iron monosulfides and pyrite in estuarine sediments: Implications for trace
448 metals mobilisation. J Environ Manage 186:158-166

449 Costantini EAC, Lessovaia S, Vodyanitskii Y (2006) Using the analysis of iron and iron oxides in
450 paleosols (TEM, geochemistry and iron forms) for the assessment of present and past
451 pedogenesis. Quatern Int 156-157:200-211

452 Coussy S, Grangeon S, Bataillard P, Khodja H, Maubec N, Faure P, Schwartz C, Dagois R (2017)
453 Evolution of iron minerals in a 100years-old Technosol. Consequences on Zn mobility.
454 Geoderma 290:19-32

455 Coussy S, Quesnel F (2021) Recommandations sur l'élaboration de valeurs limites sur le soufre pour
des déblais provenant du bassin de Paris (chantiers du Grand Paris Express) - Rapport final .
456 BRGM/RP-71252-FR.
457
458 Crocetti P, González-Camejo J, Li K, Foglia A, Eusebi AL, Fatone F (2022) An overview of
459 operations and processes for circular management of dredged sediments. Waste Manage
460 146:20-35 https://doi.org/10.1016/j.wasman.2022.04.040.

461 Dorleon G, Rigaud S, Techer I (2024) Management of dredged marine sediments in Southern France: main keys to large-scale beneficial re-use. Environ Sci Pollut R
462
463 https://doi.org/10.1007/s11356-024-33129-9

464 Duchaufour P (1976) Dynamics of organic matter in soils of temperate regions: Its action on pedogenesis. Geoderma 15:31-40

466 FAO (2006) Guidelines for Soil Description. Food and Agriculture Organization of the United Nations, Rome

468 Gustafsson (2022) Visual MINTEQ 4.0 (4.02). Visual MINTEQ – A freeware chemical equilibrium model for the calculation of metal speciation, solubility equilibria, sorption etc. for natural waters (vminteq.com)

471 Holmström H, Ljungberg J, Öhlander B (1999) Role of carbonates in mitigation of metal release from mining waste. Evidence from humidity cells tests. Environ Geol 37:267-280

473 Huot H, Simonnot M-O, Morel JL (2015) Pedogenetic trends in soils formed in technogenic parent materials. Soil Sci 180:182-192

475 Hurtgen MT, Lyons TW, Ingall ED, Cruse AM (1999) Anomalous enrichments of iron monosulfide in euxinic marine sediments and the role of H 2 S in iron sulfide transformations; examples from Effingham Inlet, Orca Basin, and the Black Sea. Am J Sci 299:556-588

478 Isaure M-P, Manceau A, Geoffroy N, Laboudigue A, Tamura N, Marcus MA (2005) Zinc mobility and speciation in soil covered by contaminated dredged sediment using micrometer-scale and bulk-averaging X-ray fluorescence, absorption and diffraction techniques. Geochim Cosmochim Acta 69:1173-1198

482 ISO 21268-2 (2009) Soil quality - Leaching procedures for subsequent chemical and ecotoxicological testing of soil and soil-like materials - Part 2: Batch test using a liquid to solid ratio of 10 l/kg dry matter

485 ISO 12677 (2011) Soil Quality - Chemical analysis of refractory products by X-ray fluorescence (XRF) - Fused cast-bead method

487 ISO 12782-1 (2012) Soil quality - Parameters for geochemical modelling of leaching and speciation of constituents in soils and soil materials - Part 1: Extraction of amorphous iron oxides and hydroxides with ascorbic acid

490 IUSS Working Group WRB (2022) World Reference Base for Soil Resources. International soil classification system for naming soils and creating legends for soil maps. 4th edition. International Union of Soil Sciences (IUSS), Vienna, Austria.

493 Jalali M, Arian TM, Ranjbar F (2020) Selectivity coefficients of K, Na, Ca, and Mg in binary exchange systems in some calcareous soils. Environ Monit Assess 192(80)

495 Johnson A, Högfors-Rönnholm E, Engblom S, Österholm P, Åström M, Dopson M (2022) Dredging and deposition of metal sulfide rich river sediments results in rapid conversion to acid sulfate soil materials. Sci Total Environ 813:151864 https://doi.org/10.1016/j.scitotenv.2021.151864.

498 JORF (2006) Arrêté du 9 août 2006 relatif aux niveaux à prendre en compte lors d'une analyse de rejets dans les eaux de surface ou de sédiments marins, estuariens ou extraits de cours d'eau ou canaux relevant respectivement des rubriques 2.2.3.0, 4.1.3.0 et 3.2.1.0 de la nomenclature annexée à l'article R. 214-1 du code de l'environnement. Journal Officiel de la République Française n°0222.

503 Lors C, Tiffreau C, Laboudigue A (2004) Effects of bacterial activities on the release of heavy metals from contaminated dredged sediments. Chemosphere 56:619-630

505 Mancioppi L, Padox J-M, Hennebert P (2010) Qualité chimique des sédiments fluviaux en France - Synthèse des bases de données disponibles

507 Meybeck M, Lestel L, Bonte P, Moilleron R, Colin JL, Rousselot O, Herve D, de Ponteves C, Grosbois C, Thevenot DR (2007) Historical perspective of heavy metals contamination (Cd, Cr, Cu, Hg, Pb, Zn) in the Seine River basin (France) following a DPSIR approach (1950-2005). Sci Total Environ 375:204-231

511 Minasny B, McBratney AB, Salvador-Blanes S (2008) Quantitative models for pedogenesis — A review. Geoderma 144:140-157
<table><tr><td>513</td><td>Ministère de la Transition Ecologique (2022) Note d'explication de la nomenclature ICPE des installations de gestion et de traitement de déchets</td></tr><tr><td>514</td><td>Muneer M, Oades JM (1989) The role of Ca-organic interactions in soil aggregate stability. III. Mechanisms and models. Soil Res 27:411-423</td></tr><tr><td>515</td><td>OJEC (2003) Council Decision 2003/33/EC of 19 December 2002 establishing criteria and procedures for the acceptance of waste at landfills pursuant to Article 16 of and Annex II to Directive 1999/31/EC. Official Journal of the European Communities</td></tr><tr><td>516</td><td>Piou S, Bataillard P, Laboudigue A, Férard J-F, Masfaraud J-F (2009) Changes in the geochemistry and ecotoxicity of a Zn and Cd contaminated dredged sediment over time after land disposal. Environ Res 109:712-720</td></tr><tr><td>517</td><td>Rickard D, Luther GW (2007) Chemistry of Iron Sulfides. Chem Rev 107:514-562</td></tr><tr><td>518</td><td>Rietveld H (1969) A profile refinement method for nuclear and magnetic structures. J Appl Crystallogr 2:65-71</td></tr><tr><td>519</td><td>Salvador-Blanes S, Minasny B, McBratney AB (2007) Modelling long-term in situ soil profile evolution: application to the genesis of soil profiles containing stone layers. Eur J Soil Sci 58:1535-1548</td></tr><tr><td>520</td><td>SedNet (2004) Contaminated Sediments in European River Basins, The Netherlands</td></tr><tr><td>521</td><td>Schad P (2018) Technosols in the World Reference Base for Soil Resources – history and definitions. Soil Sci Plant Nutr 64(2):138–144. https://doi.org/10.1080/00380768.2018.1432973</td></tr><tr><td>522</td><td>Schwertmann U, Fitzpatrick RW (1993) Iron minerals in surface environments. Catena Supp 21:7-30</td></tr><tr><td>523</td><td>Séré G, Schwartz C, Ouvrard S, Renat JC, Watteau F, Villemin G, Morel JL (2010) Early pedogenic evolution of constructed Technosols. J Soil Sediment 10(7):1246–1254.</td></tr><tr><td>524</td><td>Stephens SR, Alloway BJ, Parker A, Carter JE, Hodson ME (2001) Changes in the leachability of metals from dredged canal sediments during drying and oxidation. Environ Pollut 114:407-413</td></tr><tr><td>525</td><td>Sterckeman T, Douay F, Baize D, Fourrier H, Proix N, Schvartz C, Carignan J (2006) Trace element distributions in soils developed in loess deposits from northern France. Eur J Soil Sci 57:392-410</td></tr><tr><td>526</td><td>Tack FM, Callewaert OWJJ, Verloo MG (1996) Metal solubility as a function of pH in a contaminated, dredged sediment affected by oxidation. Environ Pollut 91:199-208</td></tr><tr><td>527</td><td>Uzarowicz Ł, Skiba S (2011) Technogenic soils developed on mine spoils containing iron sulphides: Mineral transformations as an indicator of pedogenesis. Geoderma 163:95-108</td></tr><tr><td>528</td><td>Weng L, Temminghoff EJ, Van Riemsdijk WH (2001) Contribution of individual sorbents to the control of heavy metal activity in sandy soil. Environ Sci Technol 35:4436-43</td></tr><tr><td>529</td><td>Zhou W, Hesterberg D, Hansen PD, Hutchison KJ, Sayers DE (1999) Stability of copper sulfide in a contaminated soil. Synchrotron Radiat 6:630-632</td></tr></table>

557 Tables

<table><caption>Table 1. Particle size distribution in the 4 pits as a function of depth</caption><thead><tr><th rowspan="2">Pit</th><th rowspan="2">Horizon (cm)</th><th colspan="3">Texture (%)</th><th rowspan="2">Textural class<sup>1</sup></th></tr><tr><th>Clay</th><th>Silt</th><th>Sand</th></tr></thead></table>
<table><tbody><tr><td rowspan="4">Pit 1</td><td>0-10</td><td>2.6</td><td>33.1</td><td>64.3</td><td>SL</td></tr><tr><td>20-30</td><td>2.2</td><td>26.1</td><td>71.7</td><td>SL</td></tr><tr><td>80-90</td><td>1.5</td><td>16.9</td><td>81.6</td><td>LS</td></tr><tr><td>130-140</td><td>0.6</td><td>4.4</td><td>95</td><td>S</td></tr><tr><td rowspan="4">Pit 2</td><td>0-10</td><td>3.8</td><td>43.4</td><td>52.8</td><td>SL</td></tr><tr><td>20-30</td><td>4.3</td><td>52.3</td><td>43.4</td><td>SiL</td></tr><tr><td>80-90</td><td>3.3</td><td>41.7</td><td>55</td><td>SL</td></tr><tr><td>130-140</td><td>3.9</td><td>47.4</td><td>48.7</td><td>SL</td></tr><tr><td rowspan="4">Pit 3</td><td>0-10</td><td>6.9</td><td>68.9</td><td>24.2</td><td>SiL</td></tr><tr><td>20-30</td><td>5</td><td>53.7</td><td>41.3</td><td>SiL</td></tr><tr><td>80-90</td><td>4.5</td><td>49.3</td><td>46.2</td><td>SL</td></tr><tr><td>130-140</td><td>3.5</td><td>41.5</td><td>55</td><td>SL</td></tr><tr><td rowspan="4">Pit 4</td><td>0-10</td><td>9.7</td><td>81</td><td>9.3</td><td>Si</td></tr><tr><td>20-30</td><td>10</td><td>78.7</td><td>11.3</td><td>SiL</td></tr><tr><td>80-90</td><td>9.4</td><td>77.6</td><td>13</td><td>SiL</td></tr><tr><td>130-140</td><td>9.3</td><td>75.6</td><td>15.1</td><td>SiL</td></tr></tbody></table>

559

560

561

<caption>Table 2. Relative proportion of minerals in surface samples from 0-10 cm depth. Results from XRD estimated by Rietveld analysis (n.d.: not detected).</caption>

562

563

<table><thead><tr><th></th><th>Quartz<br>(%)</th><th>Calcite<br>(%)</th><th>Plagioclase<br>(albite) (%)</th><th>Potassium<br>feldspar<br>(%)</th><th>Gypsum<br>(%)</th><th>Amphiboles<br>(hornblende)<br>(%)</th><th>Clay<br>minerals<fnref n="1" /><br>(%)</th></tr></thead><tbody><tr><th>Error<br>(%)</th><td>3</td><td>3</td><td>3</td><td>3</td><td>3</td><td>3</td><td>5</td></tr><tr><th>Pit 1</th><td>47</td><td>31</td><td>3</td><td>8</td><td>n.d</td><td>n.d</td><td>6</td></tr><tr><th>Pit 2</th><td>61</td><td>9</td><td>7</td><td>9</td><td>n.d</td><td>1</td><td>14</td></tr><tr><th>Pit 3</th><td>41</td><td>11</td><td>6</td><td>8</td><td>n.d</td><td>2</td><td>32</td></tr><tr><th>Pit 4</th><td>47</td><td>11</td><td>7</td><td>8</td><td>2</td><td>1</td><td>24</td></tr></tbody></table>

564

565

566

<caption>Table 3. Median concentrations in sediments for northwest of France (in mg kg⁻<fnref n="1" />). Data is from 1994-2008, but sampling was more frequent in the 2000s. Data from VNF are higher as they mainly come from channels while data from the water agencies are to a greater extent from rivers (Mancioppi et al. 2010)</caption>

567

568

569

<table><thead><tr><th></th><th>Cu (number of samples)</th><th>Hg (number of samples)</th><th>Pb (number of samples)</th><th>Zn (number of samples)</th><th>As (number of samples)</th></tr></thead><tbody><tr><th>Agence de l'Eau Artois Picardie</th><td>23.0 (2183)</td><td>0.2 (2497)</td><td>39 (2503)</td><td>170 (2159)</td><td>4.50 (2178)</td></tr><tr><th>VNF NPdC</th><td>41.7 (1758)</td><td>0.55 (1765)</td><td>104 (1766)</td><td>357 (1762)</td><td>7.41 (1731)</td></tr></tbody></table>

570

571

Figure captions

572

<footnote n="1">¹Textural class terminology follows the FAO guidelines for soil description (S: Sand; SL: Sandy loam; LS: Loamy sand; SiL: Silt loam; Si: Silt)</footnote>

<footnote n="1">¹ Mainly smectite and/or interstratified illite/smectite. Some illite and/or micas and small amounts of kaolinite, chlorite and talc.</footnote>
573 **Fig. 1** Localization of the TD 26 depot (only 2 depot cells shown in this photo). Pit 1, 2 and 3 were dug in cell 2 of
574 TD26 and pit 4 was dug in cell 3

575
576 **Fig. 2** Sampling pits description based on field observations and particle size analysis. Textural class terminology
577 (S: Sand; SL: Sandy loam; LS: Loamy sand; SiL: Silt loam; Si: Silt) and colors follow the FAO guidelines for soil
578 description (FAO 2006)

579
580 **Fig. 3** Results from leaching tests. Leachable Cl⁻, Na⁺, Ca²⁺ and Mg²⁺ in the soil profile

581
582 **Fig. 4** Sulfur, leached sulfate and acid volatile sulfides in the soil profiles. The two × on the y-axis indicate
temporary surfaces between deposition periods at about 90 cm and 170 cm depth

584
585 **Fig. 5** Total concentrations and percentage extractable with ascorbic acid of Fe and Mn. The two × on the y-axis
586 indicate temporary surfaces between deposition periods at about 90 cm and 170 cm depth

587
588 **Fig. 6** Concentrations of Cu, Hg and As in the soil profiles. The two × on the y-axis indicate temporary surfaces
589 between deposition periods at about 90 cm and 170 cm depth. Straight lines are background values for soils in
590 France (Sterckeman et al. 2006)

591
592 **Fig. 7** Percentage of Cu, Pb and Zn extractable with ascorbic acid in the soil profiles

593
594 **Fig. 8** Results from weathering test on surface samples from the locations of Pits 1, 3 and 4. Conductivity, pH,
595 SO₄²⁻ and Ca²⁺

596

597