Damien Tedoldi
email: damien.tedoldi@enpc.fr

Kelsey Flanagan
email: kelsey.flanagan@enpc.fr

Ghassan Chebbo
email: ghassan.chebbo@enpc.fr

Philippe Branchu
email: philippe.branchu@cerema.fr

Daniel Pierlot

Marie-Christine Gromaire
email: marie-christine.gromaire@enpc.fr

Zirconium deficit as a tracer of urban sediment accumulation in Sustainable Urban Drainage Systems -Application to the calibration of a filtration model

Keywords: Runoff infiltration, Soil, Urban environment, Urban sediment, X-ray fluorescence, Zirconium

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INTRODUCTION

Stormwater management close to the source, referred to variously as Low Impact Development (LID), Best Management Practices (BMPs), Water Sensitive Urban Design (WSUD), or Sustainable Urban Drainage Systems (SUDS), has gained popularity across the world [START_REF] Fletcher | WSUD and more -The evolution and application of terminology surrounding urban drainage[END_REF], in order to mitigate the adverse effects of urban development and rising levels of impervious cover on the water cyclenamely, increased peak flows and annual volumes of runoff, faster hydrological response of the catchments, reduced infiltration and groundwater recharge [START_REF] Fletcher | Understanding, management and modelling of urban hydrology and its consequences for receiving waters; a state of the art review[END_REF][START_REF] Miller | Assessing the impact of urbanization on storm runoff in a peri-urban catchment using historical change in impervious cover[END_REF], along with several qualitative impacts on the receiving water bodies [START_REF] Hatt | The influence of urban density and drainage infrastructure on the concentrations and loads of pollutants in small streams[END_REF][START_REF] Mcgrane | Impacts of urbanisation on hydrological and water quality dynamics, and urban water management: a review[END_REF]. In addition to their widely recognized hydrologic and hydraulic benefits, these systems, and especially soil-or media-based devices, offer interesting perspectives towards the interception of diffuse pollutant fluxes in urban environments [START_REF] Dierkes | General technical approvals for decentralised Sustainable Urban Drainage Systems (SUDS) -The current situation in Germany[END_REF][START_REF] Napier | Evidence of traffic-related pollutant control in soil-based Sustainable Urban Drainage Systems (SUDS)[END_REF][START_REF] Paus | Assessment of the hydraulic and toxic metal removal capacies of bioretention cells after 2 to 8 years of service[END_REF]. On one hand, particle-bound contaminants in runoff are likely to be trapped with suspended solids via deposition and filtration, sometimes leading to the progressive formation of a sediment layer at the surface of infiltration systems [START_REF] El-Mufleh | Review on physical and chemical characterizations of contaminated sediments from urban stormwater infiltration basins within the framework of the French observatory for urban hydrology (SOERE URBIS)[END_REF][START_REF] Erickson | Optimizing stormwater treatment practices: a handbook of assessment and maintenance[END_REF]. On the other hand, dissolved species may undergo sorption onto various reactive constituents of the solid matrix [START_REF] Sposito | The chemistry of soils[END_REF][START_REF] Tedoldi | Impact of runoff infiltration on contaminant accumulation and transport in the soil/filter media of Sustainable Urban Drainage Systems: A literature review[END_REF]. Since several contaminants such as trace metals are present in both dissolved and particulate forms in urban runoff, their accumulation in soil (or filter media) results from a combination of these physical and physico-chemical mechanisms.

Conventional experimental assessments of soil contamination in SUDS, which consist of collecting and analyzing soil samples after a known operation time, provide an overall and "time-integrated" vision of these retention processes [START_REF] Kluge | Heavy metal pattern and solute concentration in soils along the oldest highway of the worldthe AVUS Autobahn[END_REF]Tedoldi et al., 2017a). However, they do not enable to differentiate between the relative contributions of settling/filtration and sorption, especially because runoff-generated solids -hereafter referred to as "sediment"are not quantified in the solid phase.

As discussed by [START_REF] Clark | Targeting treatment technologies to address specific stormwater pollutants and numeric discharge limits[END_REF], while infiltration practices are often designed according to empirical recommendations, and their treatment performance extrapolated from a limited number of field studies, a better understanding of the retention mechanisms taking place could valuably improve their choice, design and maintenance. Additionally, soil clogging by runoff sediment has often been reported as a major concern in infiltration systems, but it remains difficult to characterize and quantify the state of clogging in a given device, or even to anticipate the occurrence of hydraulic malfunction [START_REF] Cannavo | Water balance prediction in stormwater infiltration basins using 2-D modeling: An application to evaluate the clogging process[END_REF]. Finally, if the long-term fate of contaminants in SUDS soil is to be appraised with a modelling approach, it seems necessary to consider the behavior of particle-bound species in addition to solute transport. Although several filtration models are available for this purpose [START_REF] Logan | Transport Modeling in Hydrogeochemical Systems[END_REF], few authors attempted to do so (Li and Davis, 2008a), possibly because of the difficulty to calibrate such models with usual field data [START_REF] Tedoldi | Impact of runoff infiltration on contaminant accumulation and transport in the soil/filter media of Sustainable Urban Drainage Systems: A literature review[END_REF]. To this end, the vertical distribution of filtered particlesprovided it may be accurately determined may constitute useful calibration data. All these points highlight the interest of identifying a tracer of urban sediment accumulation in SUDS.

The latter should (i) be conservative in a pedological acceptation, i.e. its content in soil should not be affected by runoff infiltration (neither via sorption nor via lixiviation), and (ii) display an anthropogenic signal different from the local background level in soils. The first point typically prevents the use of heavy metals such as copper or zinc, which are also present in the dissolved form in runoff (their average dissolved fractions reported by [START_REF] Huber | Critical review of heavy metal pollution of traffic area runoff: Occurrence, influencing factors, and partitioning[END_REF] in road runoff were 38 and 31%, respectively) and have a strong affinity for the soil constituents [START_REF] Tedoldi | Impact of runoff infiltration on contaminant accumulation and transport in the soil/filter media of Sustainable Urban Drainage Systems: A literature review[END_REF].

Due to their very low mobility in soils under almost all environmental conditions, resistance to weathering, and very low availability to plants, elements such as titanium (Ti) and zirconium (Zr) are generally considered as geochemical invariants [START_REF] Kabata-Pendias | Trace Elements in Soils and Plants[END_REF][START_REF] Schulz | Soil chemistry of radionuclides[END_REF][START_REF] Shahid | Behavior and impact of zirconium in the Soil-Plant System: Plant uptake and phytotoxicity[END_REF]; hence, they are commonly used as references to calculate enrichment or depletion factors of other elements [START_REF] Egli | Formulation of pedologic mass balance based on immobile elements: a revision[END_REF][START_REF] Stockmann | Utilizing portable X-ray fluorescence spectrometry for in-field investigation of pedogenesis[END_REF]. However, Ti has been quantified at relatively high levels (> 1 g/kg) in urban road dust [START_REF] Apeagyei | Distribution of heavy metals in road dust along an urban-rural gradient in Massachusetts[END_REF], and has numerous anthropogenic sources including paint pigments, car manufacturing, and metallic alloys (Salminen, 2005). Conversely, the anthropogenic sources of Zr are quite limitednuclear fallout and ceramic dust being the most frequently cited ones (Salminen, 2005;[START_REF] Shahid | Behavior and impact of zirconium in the Soil-Plant System: Plant uptake and phytotoxicity[END_REF] -, as a result of which the contents in road dust were found to be much lower (~200 mg/kg) [START_REF] Apeagyei | Distribution of heavy metals in road dust along an urban-rural gradient in Massachusetts[END_REF]. It was observed that the presence of a dense urban area did not significantly impact Zr concentrations in both water and sediment from an urban stream [START_REF] Mohiuddin | Geochemical distribution of trace metal pollutants in water and sediments of downstream of an urban river[END_REF]. Furthermore, the lithogenic sources of Zr and its consequent abundance in most soils enable a systematic quantification with analytical methods such as X-ray fluorescence (Kabata-Pendias, 2011), thus providing the opportunity to extend the number of analyses and to achieve a finer description of its spatial variability.

Therefore, the objective of the present work is to assess the potential use of Zr as an unequivocal tracer of sediment accumulation in SUDS, and to illustrate a possible application with the calibration of a simple filtration model. The approach relies on high-resolution horizontal and vertical soil sampling from 11 infiltration systems, and subsequent analysis of both Zr and three typical urban-and traffic-derived trace metals (Cu, Pb, and Zn). The latter, which are ubiquitous in stormwater runoff [START_REF] Göbel | Storm water runoff concentration matrix for urban areas[END_REF][START_REF] Huber | Critical review of heavy metal pollution of traffic area runoff: Occurrence, influencing factors, and partitioning[END_REF], will be considered as a signature of runoff-induced contamination, and used as a comparison basis.

MATERIALS & METHODS

Description of the study sites

Four small sized infiltration basins, five swales, and two grassed filter strips (Table 1), located in the Paris region (Figure 1), were selected for their contrasting hydraulic configurations, soil types, watersheds, and runoff contamination potentials (Tedoldi et al., 2017a). Inflow of water into the infiltration systems consists of either an inlet pipe (Dourdan, Greffiere, Alfortville, Vaucresson), or surface runoff directly flowing from the pavement, with a localized (Sausset2) or diffuse inflow (Sausset1,Chanteraines,Vitry,Compans1,Compans2,Compans3). In some devices, superficial outflow is possible in addition to infiltration: in the case of Dourdan, Chanteraines, and Vitry, this occurs when water ponding exceeds a given level, whereas in Compans1 and 2, water storage is supposedly achieved in a downstream longitudinal ditch; however, previous studies have shown that most water infiltrates in the roadside filter strips [START_REF] Flanagan | Caractérisation du fonctionnement d'un ouvrage de gestion à la source du ruissellement de voirie : approche par modélisation hydrologique et par cartographie de la contamination du sol[END_REF].

Compans3 is equipped with a V notch weir.

In Chanteraines, Vitry, Compans1 and 2, the surface soil displays a 5 to 15% slope perpendicular to the pavement; Alfortville and Compans3 have a V-shaped transversal section; the rest of the facilities were constructed with flat bottoms and sharp embankments. Chanteraines, Vitry, and Compans2 were excavated and backfilled with topsoil from another site during construction, as were Compans1 and 3 with an engineered filter media (mixture of calcareous sand and loamy soil). The other systems were implemented upon the preexisting soil.

Grass Herbaceous plants

Spontaneous vegetation

Grass Grass Grass ‡ The given value corresponds to the area of the sampled section during the initial field investigations (cf. Section 2.2).  Amended topsoil (different from the preexisting soil).

Figure 1 -Location and photographs of the study sites

: A) Dourdan, B) Greffiere, C) Alfortville, D1) Sausset1, D2) Sausset2, E) Chanteraines, F) Vitry, G) Vaucresson, H) Compans.

Sampling procedure

The field investigations were undertaken in two phases between April 2015 and May 2016, so as to consecutively obtain the distribution of the chemical species of interest at the surface and along vertical profiles (Figure 2). Firstly, a systematic sampling of the surface horizon was carried out using a rectangular grid, with a mesh size adapted to each site so as to meet the strictest of these two criteria: (i) collect ≥ 20 samples per device, and (ii) collect ≥ 35 samples/100 m 2 . At each node, the vegetation was removed if present, then ~50 g of surface soil (upper 2-3 cm) was composited from ≥ 4 subsamples surrounding the sampling location, using a stainless steel trowel which was subsequently cleaned and rinsed twice with ultrapure water. Altogether, nearly 550 soil samples were collected during this phase, all of which were conserved in individual high-density polyethylene bottles prior to analysis (Section 2.3).

This first characterization enabled the identification of the most and the least contaminated areas of each facility, defined as the domains where metal contents were respectively (i) higher than the 9 th decile, and

(ii) lower than the 1 st decile of the measurements in the site; the latter was considered as a "reference zone"

(Figure 2). In each area, 4 core samples were collected with a stainless steel gouge auger, then the upper 30 cm were subdivided into 6 depth sections, with a higher resolution near the surface (2.5-cm-thick segments). For each depth, a composite sample (~250 g) was formed from the 4 corresponding subsamples.

This step could not be completed in Vaucresson because of maintenance work undertaken by the municipality, which destructured the soil of the swale after the initial sampling campaign. In Compans1 and 2, due to the presence of the road sub-base, the soil thickness in the most contaminated zone was insufficient to perform corings. A different methodology was thus applied, which consisted of collecting soil cores at several distances from the road (30, 70, 120, and 180 cm), the thickness of which depended on the depth of the sub-base (7-30 cm); each core was then subdivided into 2 or 3 depth sections, and the samples were analyzed separately.

Whenever present, additional samples of raw sediment were collected on the nearby road pavement (Figure S1); such deposits could not be found in the immediate vicinity of Alfortville and Sausset.

Sample preparation and analysis

All samples (soil and sediment) were pre-treated according to the international standard on the preservation and pre-treatment of solid samples for the analysis of non-volatile species (ISO 11464, 2006), i.e. ovendried at 40°C for 7 days, ground with a pestle, then passed through a 2-mm nylon sieve. Elemental analysis was performed via X-ray fluorescence (XRF) spectrometry (Thermo Scientific, Portable Niton™ analyzer XL3t). 5 to 6 homogenized subsamples were poured as loose powder into polyethylene cells with a thin transparent film at the bottom, and analyzed independently. The elements of interest were copper, lead, zinc, and zirconium. The analysis time was set to 60 s per beam in a standard soil mode. The repeatability of the measurements was checked by calculating the relative standard deviation (RSD) of the 5-6 values for each element, which was always < 10%, with an average of 4-5% for metals and 3% for Zr; similar RSD were found for soil and sediment samples. Each sample was then assigned its average content of Cu, Pb, Zn, and Zr, calculated from the 5-6 measurements.

In the following developments, unless otherwise specified, soil concentrations will be given in milligrams per kilogram of dry matter (DM). The limits of quantification are sample-dependent, as they vary according to the signal received by the analyzer, but they were always ≤ 20, 10, and 30 mg/kg DM for Cu, Pb, and Zn, respectively, and ≤ 10 mg/kg DM for Zr.

Since XRF is a non-conventional practice for elemental analysis, a comparison with two reference approaches was carried outi.e. ICP-based analyses on acid-digested samples (for Cu, Pb, Zn) and NIST soil standards (for all elements)in order to assess the reliability of the XRF-determined contents. The first approach, which has been published in a previous work (Tedoldi et al., 2017a), evidenced a satisfactory precision for Cu (mean deviation of 7% from the reference contents) and Zn (14%), and showed that Pb contents were consistently underestimated by 16 mg/kg (which in proportion is higher than for the other two metalsup to 80%because Pb contamination is lower) when considering XRF measurements. The second approach displayed relative errors ranging from -12 to 5% for Cu, -7 to 16% for Zn, -35 to -13% for Pb, and 4 to 18% for Zr (negative values indicate an underestimation of the certified contents). Besides, it should be added that this analytical technique has been widely used for the quantification of Zr levels, e.g. for the construction of the Geochemical Atlas of Europe (Salminen, 2005), and the analysis of road dust [START_REF] Apeagyei | Distribution of heavy metals in road dust along an urban-rural gradient in Massachusetts[END_REF].

Additionally, a fraction (~10 g) of each sample was calcined at 550°C for 6 hours, so as to determine its volatile matter content from mass difference.

Data handling

Surface contents were interpolated so as to generate cartographies of trace metals and zirconium, using a 10 cm × 10 cm interpolation grid covering the whole sampled area of each device. In the subsequent results and discussion, the mention of the SUDS "surface soil" will refer to the layer sampled during this first step, i.e. the upper 2-3 cm. The data acquired during the second phase were represented as vertical profiles, i.e.

the evolution of the metal and Zr contents with increasing depth, in the two sampled zones (Figure 2). For

Compans1 and 2, instead of concentration profiles, the values were interpolated along the sampled transversal sections; the z-coordinate which was associated to each segment corresponded to its average depth, assuming that concentrations varied linearly within vertical samplesthus leading to a correspondence between the measured contents and the midpoint of the segments.

As some variables could not be considered as normally distributed, statistical correlations were assessed via

RESULTS

Spatial distribution of metals and Zr in the surface soil

Whatever the study site, Cu, Pb, and Zn exhibited similar distributions in the surface soil, which were characterized by a significant buildup near the inflow area, followed by a marked decrease in concentrations with increasing distance. Figures 3 and4 present the results obtained for Zn, while the distributions of the other two metals are presented as supplementary data (Figure S2). The difference in metal contents between the most and the least contaminated areas of each device is illustrated on Figure S3. Zn and Cu were strongly correlated with 𝜌 ≥ 0.86 (Table S1), which was statistically significant (𝑝-value < 10 -10 ). This observation was also applicable for the Zn-Pb correlations in most sites (𝜌 ≥ 0.84); loweralbeit significantcorrelations were found in Sausset1, Sausset2 and Chanteraines, where Pb contamination was inferior (because of limited loads from the watersheds) and the contents rapidly fell within the natural variability of the geochemical background. As a result, zinc could be considered representative of the metal's behavior in the topsoil of the devices, and will be focused on in the subsequent developments.

This spatial distribution, the origins and implications of which have been thoroughly discussed elsewhere (Tedoldi et al., 2017a), constituted a precise, "time-integrated" signature of both non-uniform infiltration fluxes and settling processes at the surface of each device. In particular, it provided evidence that the area affected by runoff-derived contamination was horizontally localizedwith the exception of the Alfortville basin (Figure 3), whose atypical behavior was due to the presence of a clay horizon at 25 cm depth, thus preventing the fast infiltration of water near the inlet pipe: hence, the lowest part of the V-shaped basin (located at the coordinate y = 0) was almost evenly polluted by trace metals.

Remarkably, Zr commonly showed "mirror" distributions with respect to metals: in other words, a deficit of this element could be observed in the most contaminated zone, with an opposite increasing trend toward the reference zone (Figures 3,4, and S3). The markedalbeit reversesimilarity between Zr and metal distributions was generally characterized by negative correlations with various levels of significance (Table S1), leading to the following classification:

i.

In Alfortville, Sausset1, Vitry, Compans1, and Compans2, the correlations were significant with 𝜌 ≤ -0.78 (excluding Pb in Sausset1 for the above-mentioned reasons).

ii. Dourdan, Sausset2, and Vaucresson displayed correlation coefficients between -0.79 and -0.53 (𝑝 < 10 -3 ).

iii.

Zr was heterogeneously distributed at the surface of the Greffiere, Chanteraines, and Compans3 devicesalthough in the second site a deficit could still be observed near the road pavement -, as a result of which no particular tendency could be noticed in relation to metals.

Zr levels in soil and raw sediment

Zr content was fairly consistent between sites in samples of road-deposited sediment, ranging from 175 to 214 mg/kg DM (Table 2), in accordance with the previous observations of [START_REF] Apeagyei | Distribution of heavy metals in road dust along an urban-rural gradient in Massachusetts[END_REF].

Conversely, the contents found in the reference zone were site-specific, but always greater than in sediment.

These values were coherent with the reported levels in soils from the Paris area (Salminen, 2005) albeit slightly higher on average. The Compans3 swale was the only site where the difference between soil and sediment was < 50 mg/kg DM. In all sites except Greffiere, Chanteraines, and Compans3, the values measured in raw sediment and in non-contaminated soil constituted the two bounds of Zr contents at the surface of the infiltration facilities. American soils [START_REF] Coughtrey | Radionuclide distribution and transport in terrestrial and aquatic ecosystems : a critical review of data[END_REF]) 70-890 ‡ Sediment deposits could not be found in the immediate vicinity of the sites Alfortville and Sausset.

Vertical distribution of Zr

In Compans1 and 2, the cross-sections showed that Zr deficit was globally restricted to the upper 5 centimeters alongside the road; the rest of the filter strips displayed almost uniform contents (Figure 4). In the most contaminated zone of the other facilities, Zr content generally tended to increase with depthwhich, as for the cartographies, was the opposite trend compared to metals (Tedoldi et al., 2017b). In comparison with the reference profiles, which showed little variability, Zr deficit in the contaminated zone was visible up to a depth of 5 to 15 cm (Figure 5). In Alfortville, the transition between the surface soil and the above-mentioned clay horizon caused a sharp decrease in the measured values (Figure 5a). The observations were somewhat different in Chanteraines, where the contents measured in the contaminated zone increased with increasing depth, but were significantly higher than in the reference zone (Figure 5b). In Greffiere and Compans3, similar to what was found at the surface, Zr contents did not show any particular trend with depth and fell in a relatively narrow range of values (225-330 and 185-310 mg/kg DM, respectively).

DISCUSSION

Quantification of the sediment fraction

Both horizontally and vertically, a depletion of zirconium (with respect to the site-specific reference values) has been noticed near the inflow zone. Yet, because of its very low mobility in soil, Zr is not likely to have been lixiviated from the upper horizons by the infiltrating water. Hence, the present observations may be explained by the fact that the collected samples were actually a mixture of original soil and other material(s)

with different elemental compositions and lower contents of zirconium, resulting in a "dilution" effect in the solid phase. These materials may consist of (i) accumulated sediment and/or (ii) humus, both of which are likely to be preferentially found near the inflow zone of the devicesvegetation growth was observed to be fostered in the most frequently flooded area, resulting in higher volatile contents [START_REF] El-Mufleh | Review on physical and chemical characterizations of contaminated sediments from urban stormwater infiltration basins within the framework of the French observatory for urban hydrology (SOERE URBIS)[END_REF]Tedoldi et al., 2017a). The typically increasing Zr profiles (i.e. decreasing "dilution") with increasing depth over 5-15 cm are also consistent with this interpretation, because of (i) filtration processes in the soil matrix, and (ii) formation of soil organic matter from the surface litter.

So as to isolate the contribution of sediment accumulation versus organic matter production, Zr contents in every collected sample were expressed per kilogram of mineral matter (MM) rather than per kilogram of dry matter, knowing the volatile matter content of each sample. This approach relies on the assumption that Zr content in the organic fraction is negligible with respect to the mineral fraction, which seems accurate given its lithogenic sources and low availability to plants [START_REF] Shahid | Behavior and impact of zirconium in the Soil-Plant System: Plant uptake and phytotoxicity[END_REF]; for instance, Kabata-Pendias (2011) reported very low Zr contents in histosols, which essentially consist of organic materials (~30 mg/kg DM). Therefore, the "corrected" contents 𝐶 * , which are assumed to reflect only the mixture of soil and sediment, may be written as: words, the importance of sediment accumulation could be spatialized (Figure 6). Logically, 𝑓 was higher in the inflow zone of each investigated system (up to 80% in Vitry and Vaucresson, and almost 100% in Compans1 and 2), demonstrating that particle settling predominantly occurs in a relatively narrow area. As regards the "exception sites" with dissimilar behaviors, it should be underlined that the Greffiere facility was implemented in a residential area with a priori low suspended solid loads, while the heterogeneity observed in the Chanteraines and Compans3 swales may be attributed to an uneven mixture of the soil material during their construction. In the former case, this would also explain the dissimilarity in Zr profiles between the contaminated and the reference zones (Figure 5b), which was also visible with the distribution of other metals or pedological parameters such as cation exchange capacity or carbonates (Tedoldi et al., 2017b). In the latter case, not only was the differential between the sediment and soil relatively small, but also the operating time of the system (1 year) was probably insufficient to observe a significant sediment buildup in the swale.

𝐶 * = 𝑓𝐶 sediment * + (1 -𝑓)𝐶 reference * (
By applying equation (1) to Zr profiles, the fraction 𝑓 could also be determined and plotted as a function of depth. 𝐶 reference * was taken as the mean value of the corrected contents over 0-30 cm depth in the reference zone, excluding the deepest sample in Alfortville, as it corresponded to a different horizon of soil. However, the retained value for 𝐶 reference *

in Chanteraines was the content in the deepest sample from the most contaminated zone (700 mg/kg MM) because of the differences in elemental composition between the two parts of the swale. Although the results from Compans1 and 2 suggested a "cake filtration" effect near the pavement (Figure 4), the profiles obtained in the other sites (except Greffiere and Compans3) depicted the accumulation of suspended solids within the soil matrix and thus proved the occurrence of bed filtration over 5 to 15 cm, in addition to sediment accumulation at the surface (Figure 7). The calculations showed that there were almost 20% of sediments in the 5-10 cm segment of Alfortville and Vitry, indicating a state of advanced clogging, contrary to the other sites. The penetration of such quantities of sediments at this depth is plausible, considering the age of the devices, the significant suspended solid production from their catchments (industrial areas with high truck traffic), and the renewal of the soil porosity near the surface due to vegetation roots and biological activity, as described by [START_REF] Paus | Assessment of the hydraulic and toxic metal removal capacies of bioretention cells after 2 to 8 years of service[END_REF] and [START_REF] Cannavo | Water balance prediction in stormwater infiltration basins using 2-D modeling: An application to evaluate the clogging process[END_REF].

Calibration of a filtration model

Theoretical background. A series of models with contrasting complexity are available to describe the attachment and transfer of suspended solids within a porous material, using either a microscopic or a macroscopic approach [START_REF] Logan | Transport Modeling in Hydrogeochemical Systems[END_REF]Li and Davis, 2008b). Among the second category, the simplest one is usually referred to as Iwasaki's model [START_REF] Iwasaki | Some notes on sand filtration[END_REF], which consists of a set of two equations:

𝑞 𝜕𝐶 𝑝 𝜕𝑧 + 𝜕𝜎 𝑝 𝜕𝑡 = 0 (2) 𝜕𝐶 𝑝 𝜕𝑧 = -𝜆𝐶 𝑝 (3)
where 𝐶 𝑝 is the mass concentration of particles in the percolating water [M.L -3 ], 𝑞 is the Darcy velocity

[L.T -1 ], 𝜎 𝑝 is the mass of deposited solids per unit bed volume [M.L -3 ], 𝑧 is the vertical coordinate [L]
(increasing downwards), and 𝜆 is called the filter coefficient [L -1 ]. Equation ( 2) is a simplified form of the continuity equation, which expresses the mass balance between the solids removed from suspension and those deposited in the pores of the media. In equation (3), a high value of 𝜆 indicates a good efficiency of the media for particle retention, resulting in an important accumulation near the surface. Some studies introduced an empirical relationship 𝜆 = 𝑓(𝜎 𝑝 ) to represent the effects of progressive soil clogging [START_REF] Logan | Transport Modeling in Hydrogeochemical Systems[END_REF]. In case 𝜆 is assumed constant, this parameter is often estimated experimentally from the ratio between influent and effluent concentrations, considering either soil columns, or in-situ bioretention systems (Li and Davis, 2008a,b). Yet, it is onerous and not always possible to collect the effluent water, especially in infiltration devices which are not equipped with an underdrain. Although several authors proposed analytical expressions for the parameter 𝜆, e.g. as a function of the bed porosity and size of the "collectors", they still introduced empirical parameters such as the "attachment efficiency" or the "collision efficiency" of the particles [START_REF] Yao | Water and waste water filtration: Concepts and applications[END_REF], which are not immediately available.

As the present approach directly provides the vertical distribution of filtered solids in the soil, it may be valuably used to calibrate such models. The mass content of retained particles 𝜎 𝑚 (i.e. expressed per unit bed mass) is linked to the variable 𝜎 𝑝 via the relationship 𝜎 𝑝 = 𝜌𝜎 𝑚 , where 𝜌 is the bulk density of the porous media [M.L -3 ]so combining equations 2 and 3 yields:

𝜕𝜎 𝑚 𝜕𝑡 = 𝜆 𝜌 𝑞𝐶 𝑝 (4)
Time integration of equation ( 4) finally leads to:

𝜎 𝑚 (𝑧, 𝑡) = 𝜎 0 (𝑧) + 𝜆 𝜌 ∫ 𝑞(𝑧, 𝜏) 𝐶 𝑝 (𝑧, 𝜏) 𝑑𝜏 𝑡 0

(5)

If the soil is assumed to be initially clean (𝜎 0 = 0), and 𝑞 is assumed to be uniform on average over the considered soil depth (30 cm)which seems acceptable for the most contaminated zone, as it corresponds to the most frequently flooded area where the infiltration fluxes are concentratedthen the variable 𝜎 𝑚 theoretically follows the same spatial variations as 𝐶 𝑝 , i.e. the exponential distribution imposed by equation ( 3).

Application to the data. A linear regression was carried out on the log-transformed fraction 𝑓 as a function of 𝑧, assigning to each vertical segment its average depth. The satisfactory adjustment provided by the linear model in Alfortville, Sausset1, Sausset2, Chanteraines, and Vitry (R 2 > 0.85) confirmed the validity of these theoretical developments. The filter coefficient 𝜆 was estimated as the slope of the regression and ranged from 0.12 cm -1 (in Vitry) to 0.20 cm -1 (in Alfortville). These values are consistent with the data reported by [START_REF] Leclerc | Filtration en profondeur -Aspects théoriques[END_REF], which varied between 0.08 and 0.50 cm -1 . They are slightly lower than the experimental measurements carried out by Li and Davis (2008b) in soil columns, which fell between 0.34 and 0.55 cm -1 , but higher than the same authors' in-situ estimations on a bioretention cell (0.01-0.06 cm -1 ) (Li and Davis, 2008a). These differences can be explained by the fact that (i) for column experiments, the soil material is generally homogenized and uniformly packed, so the filtration performance may be expected to be higher than for a real system, (ii) the particle size and pore size distributions are likely to be different between the presently assessed devices and the above-mentioned bioretention cell (made of 50% sand, 30% topsoil, and 20% mulch), and (iii) when studying a real system, soil particles may be lixiviated during water infiltration, thus leading to an underestimation of the filter coefficient.

Uncertainties, applicability and limits of the method

The objective of this section is to assess the robustness of the results with respect to the uncertainties on the Zr contents, which can be classified into four components. (i) Analytical uncertainties are related to the repeatability of the XRF measurements, which was characterized by an average coefficient of variation of 3% (calculated from the 5 to 6 analyses carried out on different subsamples) and modelled with a normal distribution centered about the mean value for each sample. (ii) Zr contents in the reference zone were never exactly uniform, as a result of which the retained values for 𝐶 reference * may be somewhat biased. The "local", site-specific variability of reference contents was represented with a normal distribution for each device.

(iii) In spite of a relative inter-site stability of the Zr content in raw sediment, some uncertainty is associated with the estimation of 𝐶 sediment * when sediment samples could not be collected on the nearby pavement.

Between sites, values ranged from 195 to 238 mg/kg MM, and this interval extended to 187-247 mg/kg MM when accounting for the analytical uncertainties. Accordingly, the uncertainty about 𝐶 sediment * in Alfortville

and Sausset was represented with a uniform distribution. (iv) Finally, it should be mentioned that the particles which are most likely to settle on the road are the coarser ones, and thus may be non-representative of the entire distribution of suspended solids in runoff (e.g. with lower organic matter contents), but this point was not accounted for in the subsequent calculations.

These uncertainties imply the existence of a minimal degree of dilution below which sediment accumulation cannot be considered as significant (i.e. the hypothesis that 𝑓 = 0 cannot be rejected). Evidently, the greater the difference between 𝐶 reference * and 𝐶 sediment * , the lower is the "limit of quantification" of the sediment fraction, which was found to lie between 7 and 11% (w/w) for Dourdan, Alfortville, Sausset1 and 2, Chanteraines, Vitry, Vaucresson, and Compans2. Conversely, this value was higher in Compans1 (∼25%)

and Greffiere (∼75%), while it was almost 100% in Compans3 due to notably low Zr contents in the filter media.

Finally, a stochastic approach was carried out in order to assess the impact of these uncertainties on the calculated filter coefficient. The estimation procedure was repeated 10 000 times for each site, with 𝐶 sediment * and 𝐶 reference * randomly sampled from their respective distributions, leading to statistical data for the parameter 𝜆. In the three examples displayed in Figure 7, the associated 95% confidence intervals were 0.11-0.30 cm -1 for Alfortville, 0.08-0.34 cm -1 for Chanteraines, and 0.08-0.20 cm -1 for Vitry. Therefore, although the proposed method appears to be somewhat sensitive to the estimated zirconium content in both raw sediment and non-contaminated soil, the resulting uncertainty on the parameter 𝜆 is quite comparable to the other experimental methods, and one of its major improvements lies in the fact that the procedure does not require sampling soil water.

The study sites Greffiere and Compans3, along with Chanteraines for the cartography stage, provided three examples where the proposed methodology is not applicable:

i. the geochemical background of Zr is close to the content in raw sediment;

ii. sediment accumulation is not sufficient to discriminate between Zr deficit and the natural variations of this elementeither due to a short operating time or to low suspended solid loads from the watershed;

iii. the mixing of the soil material before backfilling the infiltration devices is uneven.

Regarding the first point, the variability of Zr levels in worldwide soils (as reported in Table 2) suggests that this method may be successfully implemented in many contexts beyond the Paris region, excluding the zones with Zr contents close to the "critical" value of 200 mg/kg DMor perhaps biofiltration systems composed of an engineered filter media, the composition of which may be different from natural soils. In such cases, the present approach is certainly transposable with another tracer among the usual geochemical invariants (e.g. titanium or aluminum), which should be further investigated and may enable the validation of the results obtained with zirconium.

CONCLUSIONS AND PERSPECTIVES

The present work validated the use of zirconium as a tracer of sediment accumulation in soil-based Sustainable Urban Drainage Systems. Zr and metal levels were analyzed in road-deposited sediment as well as more than 500 surface soil samples and 175 samples along vertical profiles from 11 study sites. Zr content in "raw" sediment was found to be significantly lower than in non-contaminated soil, with a noteworthy inter-site stability. In most devices, Zr showed "mirror" distributions with respect to metals, i.e. a deficit of Zr could be noticed horizontally and vertically in the most contaminated zone. Since Zr is unlikely to be leached or taken up by plants because of its physicochemical properties, this observation was interpreted as the result of a "dilution" in the solid phase due to sediment accumulation. The "dilution ratio" enabled the calculation of the sediment fraction, the spatial variability of which provided evidence for the joint occurrence of particle settling over a limited part of the surface and bed filtration within the soil matrix.

Several sources of uncertainty, including the repeatability of the analyses and the natural variability of Zr in soil, resulted in a site-specific "limit of quantification" for the sediment fraction, which was generally found to be around 10% (w/w), except when the geochemical background was close to the Zr level in sediment.

Hence the proposed approach appeared to be reasonably precise and appropriate for many geological contexts beyond the present experimental area. These findings were used to calibrate a simple, onedimensional filtration model. In most study sites, the measured profiles were consistent with the exponential distribution derived from Iwasaki's equation, and the filter coefficient could be calculated with a controlled confidence interval. The accuracy of the measurement was comparable with other methods available in the literature, and the approach presented the considerable advantage that it did not require sampling soil water.

Another perspective of this work may be to further investigate the occurrence of soil clogging in infiltration systems, so as to better understand the antagonistic effects of particle accumulation versus biological renewal of the soil porosity.

  Spearman's rank correlation coefficient 𝜌, and the associated non-parametric test of significance. 𝜌𝑋𝑌 corresponds to the Pearson's correlation coefficient between the rank values of the two variables 𝑋 and 𝑌.

Figure 2 -

 2 Figure 2 -Schematic representation of the two-step methodology for soil sampling and analysis.

Figure 3 -

 3 Figure 3 -Spatial distribution of zinc and zirconium [mg/kg DM] in the surface soil of the study sites Sausset2, Vitry, and Alfortville (plan view). The x-and y-coordinates are given in meters. The arrows indicate the water inflow.

Figure 4 -

 4 Figure 4 -Spatial distribution of zinc and zirconium [mg/kg DM] in the surface soil and along four crosssections of the filter strip Compans2.

Figure 5 -

 5 Figure 5 -Vertical distribution of the Zr contents [mg/kg DM], in the most and the least contaminated zones of the study sites (a) Alfortville, (b) Chanteraines, and (c) Vitry.

  1) where 𝐶 sediment * and 𝐶 reference * are the Zr contents in the mineral fraction of raw sediment, and reference soil, respectively [mg/kg MM], and 𝑓 is the mass fraction of sediment in the sample [-]. In case raw sediment could not be sampled in the immediate vicinity of the infiltration systems (Alfortville, Sausset), the retained value for 𝐶 sediment * was the average of the measurements in sediment samples across all sites (210 mg/kg MM). Eventually, in all facilities except Greffiere, Chanteraines, and Compans3, the spatial distribution of 𝐶 * (𝑥, 𝑦) in the surface soil enabled the determination of the coefficient 𝑓(𝑥, 𝑦); in other

Figure 6 -

 6 Figure 6 -Spatial distribution of the sediment fraction 𝑓 [%] at the surface of the study sites (a) Sausset1, (b) Vitry, and (c) Vaucresson (plan view). The arrows indicate the water inflow.

Figure 7 -

 7 Figure 7 -Measured (solid lines) and calibrated (dotes lines) sediment fraction 𝑓 [%] as a function of depth, in the study sites Alfortville, Chanteraines, and Vitry.

Table 1 -

 1 Main characteristics of the investigated infiltration facilities.

	Site name	Dourdan	Greffiere	Alfortville	Sausset1	Sausset2
	Type of device	Infiltration-	Infiltration	Infiltration	Infiltration	Small swale
		retention basin	basin	basin	basin
	Catchment	Two-lane	Residential	Industrial	Car parking lot	Car parking lot
	characteristics	departmental	catchment:	activities +	(210 veh/day)	(210 veh/day)
		road (4900	houses with tile	service roads +
		veh/day) + car	roofs and	logistics area
		parking lot	metallic	(high truck
			elements	traffic)
	Active area of	7000 m 2	3000 m 2	20000 m 2	400 m 2	160 m 2
	the catchment
	Device area ‡	120 m 2	65 m 2	130 m 2	68 m 2	10 m 2
	Inflow of water	Pipe	Pipe	Pipe	Surface runoff,	Surface runoff,
		(Ø600 mm)	(Ø300 mm)	(Ø800 mm)	large opening	small opening
		followed by a			(90 cm)	(15 cm)
		concrete apron
	Superficial outlet Elevated pipe	None	None	None	None
		(30 cm above
		the ground)
	Operating time	> 20 years	15 years	16 years	14 years	14 years
	Soil texture	Sandy loam	Sandy clay	Clay loam	Silt loam	Silt loam
			loam
	(0-10 cm)
	Vegetation	Spontaneous	Spontaneous	Spontaneous	Shrubs and	Herbaceous
		vegetation	vegetation	vegetation	grass	plants

Table 1 (continued) -

 1 Main characteristics of the investigated infiltration facilities.

	Chanteraines	Vitry	Vaucresson	Compans1	Compans2	Compans3
	Swale	Swale	Swale	Filter strip	Filter strip	Swale
	Service road	T-junction	Two-lane	Highway	Highway	Highway
	(< 1500 veh/day)	within an	departmental	(11000 veh/day	(11000 veh/day	(11000 veh/day
	+ small car	industrial	road (4000	in each	in each	in each
	parking lot	catchment (high	veh/day)	direction) in the	direction) in the	direction) in the
		truck traffic)		vicinity of an	vicinity of an	vicinity of an
				industrial area	industrial area	industrial area
	470 m 2	350 m 2	400 m 2	250 m 2	250 m 2	330 m 2
	54 m 2	19 m 2	12 m 2	33 m 2	33 m 2	23 m 2
	Surface runoff,	Surface runoff,	Pipe	Surface runoff,	Surface runoff,	Surface runoff,
	diffuse inflow	multiple lateral	(Ø200 mm)	diffuse inflow	diffuse inflow	diffuse inflow
		openings
	Elevated pipe	Elevated pipe	None	Longitudinal	Longitudinal	V notch weir
	(25 cm above the	(25 cm above		ditch	ditch
	ground)	the ground)
	10 years	10 years	> 25 years	3 years	3 years	1 year
	Sandy loam 	Loam 	Clay loam	Sandy loam 	Silt loam 	Sandy loam 

Table 2 -

 2 Zr contents in the collected sediment samples and in the reference zone of the study sites.Comparison with literature data for road dust and worldwide soils.

	Site	Zrsediment [mg/kg DM]	Zrreference [mg/kg DM]
	Dourdan	175	420
	Greffiere	214	280
	Alfortville	Not sampled ‡	418
	Sausset1	Not sampled	626
	Sausset2	Not sampled	501
	Chanteraines	178	340
	Vitry	209	650
	Vaucresson	195	720
	Compans1	201	370
	Compans2	190	695
	Compans3	203	241
	Urban road dust (Apeagyei et al., 2011)	180 ± 52 (N = 41)
	Soils from the Paris area (Salminen, 2005)		406 ± 137 (N = 21)
	Worldwide soils (Kabata-Pendias, 2011)		200-850
	Worldwide soils (Pais and Jones, 1983)		60-2000

ACKNOWLEDGEMENTS

This research was carried out under the OPUR and ROULEPUR research programs. The authors gratefully acknowledge OPUR partners (AESN, SIAAP, CD92, CD93, CD94, Ville de Paris), French Agency for Biodiversity (AFB) and Seine-Normandie Water Agency for their financial support, as well as the French territorial collectivities which allowed the authors to perform soil samplings in their devices.


Rémy Crassard

Céline Thiébaut

Levallois points production from eastern Yemen and some comparisons with assemblages from East-Africa, Europe and the Levant

Introduction

The recent discovery of evidence for the production of Levallois points in Hadramawt, in the east of Yemen, marks a milestone in the development of definitions of the Palaeolithic in the Arabian peninsula. These industries, still undated, but very likely from the Middle Palaeolithic, are currently almost exclusively documented by cores found on the surface of several sites at the top of the Hadramawt limestone plateaus. The scope of this paper is to structure, in a preliminary approach, the Levallois production schemes observed in Yemen. This work is based on a still limited corpus of materials and might be somewhat arbitrary. Further investigations will show which of the production schemes present in fact reflect the prevailing southern Arabian mainstream. This study of the last phases of production on the cores allowed six production patterns to be identified, which show the making of two types of Levallois points; the so-called "classical" points and "constructed" points. These patterns reflect a variability of production within the limited geographical area in which this study has been conducted. Through a comparative approach with other sites of production of Levallois points in neighbouring and more distant regions (East Africa, the Levant and Europe), we attempt to determine to what extent the production of Levallois points displays technical, and therefore cultural, similarities in the Hadramawt and around the world.

Palaeolithic of Arabia

In-depth studies on the Palaeolithic in the Arabian Peninsula are relatively recent, compared with those in Europe, Africa or the Levant. In recent years, the multiplication of excavations and survey operations on surface sites in southern Arabia [START_REF] Amirkhanov | Stone Age of South Arabia[END_REF]Crassard 2009a; [START_REF] Delagnes | Middle Paleolithic settlement in Arabia: First evidence from a stratified archaeological site in Western Yemen[END_REF][START_REF] Rose | Among Arabian Sands: Defining the Palaeolithic of Southern Arabia[END_REF]) implies many discussions and reflections on the role that this region could have played during prehistory [START_REF] Amirkhanov | Cave al-Guza the multilayer site of Oldowan in South Arabia[END_REF]Crassard 2008aCrassard , 2008bCrassard , 2009aCrassard , 2009b;;[START_REF] Marks | Into Arabia, perhaps, but if so, from where[END_REF][START_REF] Rose | Defining the Palaeolithic of Arabia? Notes on the Roundtable Discussion[END_REF][START_REF] Petraglia | The Evolution of Human Populations in Arabia[END_REF]. As regards more particularly the Middle Palaeolithic, the first studies on lithic materials indicate a relatively abundant presence of remains from this period in Arabia. These vestiges are mainly evidenced by lithic industries of the Levallois tradition. They come mainly from surface sites and, in this case, their high degree of patina confirms Pleistocene dating without much doubt, although this criterion should be considered with caution (Crassard 2009a). The problem remains to date this material radiometrically and to be able to find archaeological contexts that combine Levallois production with human and faunal remains. This would associate the industries with a chrono-cultural frame and would allow us to learn more about the Middle Palaeolithic knappers' environment, as well as the nature of the population and its dispersal: what origins and what species? While awaiting more details, which will be provided by the excavation of stratified sites, it is essential to provide a first comparative element, at intercontinental and micro-regional scales, from our sole source of information: lithic industries, and more particularly the Levallois points industries, which are relatively convenient to identify and therefore to compare.

The production of Levallois points: definition and geographical distribution

The Levallois concept

The Levallois concept consists in producing in a predetermined manner flakes, blades or points, thanks to the implementation of different methods of flaking (débitage) involving technical traditions that can be understood from the study of reduction patterns (Boëda 994). This concept of debitage was used for nearly 500,000 years, from the African Acheulean until the end of the Middle Palaeolithic, and even in an isolated way during the Upper Palaeolithic and the Holocene. Levallois flake production appears with the Acheulean, at isotopic stages 0 and 9, but is generalised to the Middle Palaeolithic from stage 8 [START_REF] Delagnes | Les technocomplexes du Paléolithique moyen en Europe occidentale dans leur cadre diachronique et géographique[END_REF]). The Levallois concept has been widely described and illustrated through the study of various assemblages (e.g. Bordes 96; Boëda 99, 994; Delagnes 992; Van Peer 992). Levallois production schemes are evidenced on different continents; in Europe, the Middle East and northeast Africa (e.g. [START_REF] Crew | An evaluation of the relationship between the Mousterian complexes of the Eastern mediterranean: a technology perspective[END_REF]Meignen & Bar-Yosef 988, 99, 992, 2004;[START_REF] Van Peer | The Levallois Reduction Strategy[END_REF]Dibble & Bar-Yosef 995;[START_REF] Meignen | Levallois lithic production systems in the Middle Paleolithic of the Near East: the case of the undirectional method[END_REF][START_REF] Delagnes | Diversity of lithic production systems during the Middle paleolithic in France. Are there any chronological trends?[END_REF][START_REF] Delagnes | Les technocomplexes du Paléolithique moyen en Europe occidentale dans leur cadre diachronique et géographique[END_REF]). In the Arabian Peninsula, the presence of Levallois debitage has been relatively recently identified in Yemen, first by [START_REF] Caton-Thompson | Geology and archaeology of the Hadramaut, Southern Arabia[END_REF][START_REF] Caton-Thompson | Some Palaeoliths from South Arabia[END_REF] and then by Van Beek [START_REF] Van Beek | An archaeological reconnaissance in Hadramaut: a preliminary report[END_REF]), Inizan (Inizan & Ortlieb 987) and Amirkhanov (99,994). Since then, archaeological studies, including surveys of surface sites, have reported the presence of Levallois debitage in Saudi Arabia in the Jubbah basin (Petraglia & Alsharekh 2003: 675, 677), in the United Arab Emirates in the region of Fili close to Sharjah [START_REF] Scott-Jackson | Investigating Upper Pleistocene stone tools from Sharjah, UAE: Interim report[END_REF][START_REF] Wahida | Barakah: a Middle Palaeolithic site in Abu Dhabi Emirate[END_REF], in the centre of the Sultanate of Oman with the Sibakhan facies and its rare unipolar convergent Levallois cores [START_REF] Rose | Among Arabian Sands: Defining the Palaeolithic of Southern Arabia[END_REF], and in Yemen in Wadi Wa'shah, Wadi Sana and the region of Hadramawt in general (eastern Yemen : Crassard 2008a: Crassard , 2009a)), as well as in the foothills of the Western Highlands at the interface of the Tihamah coastal plain with the sites of Shibat Dihya, including SD site in Wadi Surdud [START_REF] Delagnes | Middle Paleolithic settlement in Arabia: First evidence from a stratified archaeological site in Western Yemen[END_REF]. This last site apart, which is dated by OSL method to around 60 ka BP, the Levallois presence in Arabia is not precisely dated.

Definition of the Levallois points production

Within the Levallois concept, a relatively important variability exists in the implementation of knapping operations. Production objectives can also be varied and getting oriented to the obtainment of points. In this case we have to speak of the production of Levallois points (débitage Levallois à pointes), recurrent or not, which aims at the making of triangular flakes, sometimes standardised. It was in 1961 that Bordes described for the first time the flaking of a Levallois point, from the cores and points encountered in different industries from northern France (Seine Maritime and Somme) and Jordan (the site of Abu Sif). Later, Bordes (980) described the production of Levallois points according to two modalities of preparation; preparation by unipolar convergent removals, flaked from the striking platform of the future point, or a unipolar divergent preparation by removals made from a striking platform opposite to that of the future point. Bordes also resumés schemes defined on the "Nubian" cores (Guichard & Guichard 965), of which two types have been distinguished. The first type corresponds to "a Levallois point core characterised by a special technique", which Bordes brings closer to the Levallois point cores with a preparation by two unipolar divergent removals from an opposite striking platform to that of the point, and a second type with an elaborated centripetal preparation on a block of triangular morphology from which will be produced a Levallois point, but not in a "classical" way . For Bordes, the objective of this second scheme is not the production of a Levallois point, but a triangular flake. A few years later, a third production scheme was proposed for obtaining a Levallois point, while pointing out the existence of many variants (Inizan et al. 1995: 69). This scheme is the production of a Levallois point resulting from a strict bidirectional preparation. More recently, from the material found on the site of Umm el-Tlel (Syria), Boëda illustrated the diversity of the procedures implemented for the production of points (Boëda et al. 998). After analysing the points and subproducts, Boëda defined two main groups: the so-called "three hits" (trois coups) points (that we qualify here as "classical" points), which are distinguished from the "constructed" points in which different schemes coexist depending on the direction of the preparation removals. Furthermore, Boëda had previously proposed around 30 theoretical patterns of Levallois "three hits" points production, from an experimental corpus (Boëda,982), an approach previously developed by Crew (975). It is important to emphasise the heuristic value of such a study, allowing us to consider the variability of the Levallois concept despite the existence of a single objective, that is, the production of "classical" points.

Geographical distribution of Levallois points production

The production of Levallois points seems less geographically widespread than the production of Levallois flakes (Fig. ). It is especially attested in Eastern and Western Europe (OIS 7 and 6). At the Koulichivika site in Ukraine, and in the Bohunician in general (Meignen et al. 2004) the Levallois points show great morphological variation and are produced by the exploitation of the surface and then the thickness of the block, after a bidirectional or bipolar preparation. In the north of France, a few assemblages from open-air deposits have shown a production of Levallois points [START_REF] Bordes | Les limons quaternaires du bassin de la Seine. Stratigraphie et Archéologie paléolithique[END_REF]Vallin 988, 992;Delagnes & Roppars 996;[START_REF] Watté | Un gisement moustérien typique, à fort débitage Levallois et faciès Laminaire en Haute-Normandie: La mare d'Oissel, à Oissel (Seine-Maritime)[END_REF][START_REF] Locht | Le Mont de Bouguillemont[END_REF]Locht et al. , 2001[START_REF] Locht | Bettencourt-Saint-Ouen (Somme): Cinq occupations paléolithiques au début de la dernière glaciation[END_REF][START_REF] Locht | Le gisement paléolithique moyen et les séquences pléistocènes de Villiers-Adam (Val-d'Oise)[END_REF]. Like the majority of the Levantine assemblages, the classical production scheme (unipolar convergent) is the more common; for instance, the lithic material from the site of Houppeville (Vallin 988,992), the B assemblage from Le Pucheuil (Delagnes & Roppars 996), the N2b layer at Bettencourt-Saint-Ouen (Locht et al. 2001[START_REF] Locht | Bettencourt-Saint-Ouen (Somme): Cinq occupations paléolithiques au début de la dernière glaciation[END_REF] or the sector 1 at Le Petit-Saule [START_REF] Locht | Le gisement paléolithique moyen et les séquences pléistocènes de Villiers-Adam (Val-d'Oise)[END_REF]. Only the collection from Therdone site (89-67 kaBP, [START_REF] Locht | Le Mont de Bouguillemont[END_REF] differs from this set of Levallois points from the north of France by the presence of a greater diversity of patterns of preparation of the convexities: preparation by unipolar convergent removals, sometimes reworked by distal removals; preparation by unipolar opposed and bidirectional removals; or preparation of the convexities by centripetal removals [START_REF] Gadebois | Étude technologique et technique de la production de pointe Levallois et d'éclats Levallois triangulaires: un aspect comportemental des modalités de gestion lithique des Néandertaliens[END_REF]). In the Rhone Valley, if some industries have points that are morphologically close to the Levallois point (at Mandrin, at Néron layer III: 43 ka BP and at Abri du Maras), their realisation seems to be far from the Levallois concept, according to [START_REF] Slimak | Les dernières expressions du Moustérien entre Loire et Rhône[END_REF]. The production of Levallois points is relatively abundant and characteristic of some assemblages from the Levant (OIS 4 and 3), from the Lebanese sites of Ksar Akil (Meignen & Bar-Yosef 1998[START_REF] Slimak | Les dernières expressions du Moustérien entre Loire et Rhône[END_REF] and Bezez Cave (Copeland 1983), from the Israeli sites of Rosh Ein Mor (Marks & Crew 972), Abu Sif (Neuville 95,Copeland 975), Tabun (Copeland 975, Jelinek 1982[START_REF] Meignen | Variabilité technologique au Proche Orient: l'exemple de Kebara[END_REF], Kebara (layers IX and X : 64-48 ka BP, Meignen 1995[START_REF] Meignen | Variabilité technologique au Proche Orient: l'exemple de Kebara[END_REF][START_REF] Meignen | Les outillages lithiques moustériens de Kebara (fouilles 1982-1985): premiers résultats[END_REF][START_REF] Slimak | Les dernières expressions du Moustérien entre Loire et Rhône[END_REF], Meignen et al. 2006), Qafzeh XV (Hovers 1997) and Amud Cave (layer B1 : 58-53 ka BP, Watanabe 968, Hovers 998, Meignen 995), or from Jordan at Tor Faraj/Tor Sahiba (69-44 ka BP, [START_REF] Henry | Prehistoric Cultural Ecology and Evolution. Insights from Southern Jordan[END_REF][START_REF] Henry | The Prehistoric Archaeology of Jordan[END_REF][START_REF] Henry | Neanderthals in the Levant. Behavioral Organization and the Beginnings of Human Modernity[END_REF]Meignen 1995) and in Syria at Umm al-Tlel (65-50 ka BP for layer VI3b', Boëda et al. 998). In most cases, concerning the production of elongated points or shorter wide-based points, the convexity is created by a unipolar dominant removal, or sometimes by two proximal unipolar convergent removals, even if some of the Negev series testify to the existence of a preparation from the distal part . The site of Umm el-Tlel seems characterised by more varied preparation schemes (mainly unipolar convergent, sometimes recurrent, centripetal, bipolar, or-thogonal) (Boëda et al. 998). The production of Levallois points is also attested in Nubia and Egypt during the Middle Stone Age (with very little chronometric data, these are dated between 300 and 50 ka BP), but the evidence is much less abundant than in the Levant. Preparation types are very different from those encountered in Levantine deposits: either centripetal (in reference to Nubian debitage type 2) or unipolar from the distal part of the core (Guichard & Guichard 1965[START_REF] Hours | Les industries paléolithiques du Proche-Orient[END_REF], Van Peer 1992).

In the Horn of Africa, several sites have delivered as-semblages featuring Levallois points. This is particularly the case in Ethiopia at the Gorgora rockshelter (no dating, [START_REF] Moysey | Excavation of a rock shelter at Gorgora, Lake Tana, Ethiopia[END_REF][START_REF] Leakey | The industries of the Gorgora Rockshelter, Lake Tana[END_REF]), or at Pork-Epic Cave (70-60 ka BP, Clark et al. 984,Pleurdeau 200), where they are uncommon and come from a unipolar convergent management, more rarely bipolar (Pleurdeau 2003), or of a Nubian type as at Kone (no dating, [START_REF] Kurashina | An examination of prehistoric lithic technology in East-Central Ethiopia[END_REF]). Industries that have shown a significant number of Levallois points are known in northern Somalia at Midhishi 2 (no dating, Brand & Gresham 1989). Some points have been found at Omo Kibish (site AHS 95±5 ky, [START_REF] Shea | The Middle Stone Age archaeology of the Lower Omo Valley Kibish Formation: Excavations, lithic assemblages, and inferred patterns of early Homo sapiens behavior[END_REF]. The lack of technological descriptions of these finds means it is not always possible to determine which method was used to obtain these points. In the East African MSA tradition, tools are characterised by points with unifacial and bifacial retouch on blanks likely Levallois, as is the case at Gademotta (ETH-72-8B before 276±4 ka BP, Wendorf & Schild 1974; Nubian at ETH-72-6 after 183±10 ka BP, Morgan & Renne 2008) and at Kulkuletti (200-300 ka BP, Wendorf & Schild 974) and Tiya (surface, Joussaume 995), Aduma (00-80 ky, [START_REF] Brooks | Projectile technologies of the African MSA: Implications for modern human origins[END_REF], Melka Kunture (Garba III, Hours 1976), Gorgora [START_REF] Leakey | The industries of the Gorgora Rockshelter, Lake Tana[END_REF] and in Somalia at Gogoshiis Qabe (no dating, Brand & Gresham 989, Clark 988). The great difference between these and the Levantine Middle Paleolithic is a much less systematic production of Levallois points during the East African MSA. The production of Levallois points in Hadramawt, in the east of Yemen, presents a relative diversity of reduction patterns. Careful analysis of the material gathered during surveys allows us to complete the production models proposed by our predecessors and emphasises a greater diversity of the already known production modalities. Here, the proposed study details and refines the previously proposed nomenclature (Crassard 2009a), thanks to the contribution of new sites which were discovered during surveys in January 2008. In the future it will be interesting to confront the different reduction patterns in the production of Levallois points that are known in Hadramawt with a broader geographical context, in order to identify any technical similarities between these industries and those from East Africa, the Near East and Europe.

Production of Levallois points in Hadramawt region: context of dscovery and presentation of the studied assemblage

Hadramawt, covering part of the centre and the east of Yemen, is a region of limestone plateaus formed during the Palaeocene and Eocene which can reach altitudes of more than 000 m. Erosive activity over the millennia has formed an impressive network of canyons and steep valleys. Two main areas have been selected in this study; Wâdî Wa'shah to the north and Wâdî Sanâ to the south, two wadis located on either side of Wâdî Hadramawt (or Wâdî Masîlah), whose orientation follows a west-east axis. The sites that have delivered cores for Levallois points are located at the top of the limestone plateaus. They were discovered during archaeological operations in two distinct projects; The Roots of Agriculture in Southern Arabia Project (RASA) in Wâdî Sanâ and the French Archaeological Mission in Jawf-Hadramawt (HDOR) in Wâdî Wa'shah. A total of 27 surface sites with artefacts reflecting the production of Levallois points have been studied (18 by HDOR and 9 by RASA Fig. 2). They were mostly characterised by the discrete presence of lithic industries directly found on the surface. These Levallois debitage collections very rarely included typical Holocene pieces (arrowheads, less patinated lithic material). A few sites, however, delivered abundant material bringing together several lithic production phases (reduction flakes, Levallois flakes and points, etc.), but unfortunately in a context too uncertain to make an accurate study of all the vestiges. It has thus been decided to focus this study on some cores and points, and therefore on the very last visible phases of the Levallois production, visible through the removal scars on the abandoned cores. A total of 50 cores used for the production of Levallois points has been analysed, with the four Levallois points that have been collected. Well aware of the limits inherent in the almost exclusive analysis of cores in the general understanding of schemes of production, nevertheless it seemed interesting to deliver here our observations which, to our mind, participate in the recognition of a greater diversity of the schemes of production of Levallois points realised by prehistoric human groups.

Analysis of the cores for Levallois points from Hadramawt

In previous studies [START_REF] Crassard | Apport de la technologie lithique à la définition de la préhistoire du Hadramawt, dans le contexte du Yémen et de l'Arabie du Sud[END_REF](Crassard , 2008a, in press), in press), the different procedures attested by the Levallois debitage in Hadramawt have been defined through three broad categories: Group A for the Levallois debitage with one (or two) preferential flakes, Group B for the Levallois debitage of points, and Group C for the centripetal recurrent Levallois debitage. Groups A and B include several modalities. We resume here Group B, which brings together the procedures for obtaining Levallois points. Thus, to the four previously identified schemes (B1, B2, B3 and B4, Crassard 2007), a fifth one has been added (B5), while group B2 has been associated with group B. The categories of points production have been established based on the direction of the preparation removals seen on the debitage surface of the cores. The categories are divided into subgroups based on the absence or presence of scars which accentuate the distal or lateral convexities by removals of more centripetal directions (Fig. 3). Thus, we find patterns corresponding to the "classical" points and to the so-called "constructed" points from Boëda's work. However, we preferred a first-level categorisation based on the direction of preparation removals because, regarding the material collected in Hadramawt, some production schemes of the so-called "constructed" points are more an improvement of the production of convexities prior to a truly independent conceptualisation of the production schemes of the "classical" points.

Scheme B1

This is the production scheme of the "classical" points and the one most commonly encountered. It is characterised by the prior production of two convergent unipolar removals from the proximal part of the core. These scars will prepare the lateral and distal convexities (HDOR 2000 No. 1 and2003 No. 1). This is scheme B1 for "classical" points (Fig. 4). In a few rare cases, the two convergent unipolar removals may be accompanied by a few removals that accentuate the distal convexity (RASA 2004-66-, former scheme B2). They correspond to scheme B for "constructed" points (Fig. 5).

Scheme B3

Two sub-schemes have been distinguished: B3 opposed unipolar and B3 bipolar.

Scheme B3 opposed unipolar

This first sub-scheme includes unipolar preparation removals from the distal part of the core. They are therefore opposed to the striking platform that will be used for extraction of the point. These two removals contribute to the creation of the distal and lateral convexities, and no other preparation is present. This is thus a production of "classical" points. However, the plunging negative of the point still present on one of the cores shows that the distal convexity is sometimes insufficient (HDOR 2000 No. 2). The cores can then benefit from a new preparation of convexities by some distal and/or lateral removals (HDOR 2005 No. 5). There is then a production of "constructed" points (Fig. 6). Around the core HDOR 2005 No. 5, the two unipolar removals from the distal part are still visible, but the right lateral part has undergone a reorganisation of its convexity by the production of shorter flakes of a centripetal direction, which have here hinged. Two removals in the left proximo-lateral part probably allow accentuation of the convexity obtained by the first removal. In this case, the presence of secondary removals seems thus more related to a lack of convexity than to an independent scheme.

Scheme B3 bipolar

This second sub-scheme differs from the first by the presence of negatives of bipolar removals. It is somehow a mixture of schemes B1 and B3 unipolar described above. The convexity may thus be made by a series of multiple bipolar removals from the distal and proximal parts of the core (HDOR 2003 No. 8 andHDOR 2004 No. ), thereby producing "classical" points (Fig. 7). As with previous schemes, when lateral or distal convexities are not quite pronounced, a new phase of preparation is implemented and lateral or distal removals of a centripetal direction can thus overlap the first negatives of removals, thereby causing the knapper to consider the production of "constructed" points. In two of the cores belonging to this category, the lateral centripetal removals overlap bipolar scars, and two others feature lateral centripetal removals overlapped by unipolar or bipolar removals (HDOR 566 No. 1, Fig. 8).

Scheme B4

By its characteristics, scheme B4 exclusively includes the modalities of production of "constructed" points. It is subdivided into two sub-schemes: B4 proximal and B4 distal (Fig. 9).

Scheme B4 proximal

This scheme includes the preparation of a lateral convexity by a major invasive removal from the proximal part (HDOR 2003 No. 5) while the convexity of the opposite side is prepared by shorter removals of centripetal direction. The strict independence of this method from previous schemes is not obvious. The centripetal negatives may hide previous, more invasive, unipolar or bipolar removals.

Scheme B4 distal

These show the same preparation of convexities, but this time from the distal part of the core (HDOR 2004 No. 4).

Scheme B5

Its originality from previous schemes is in the preparation of a striking surface by two lateral bidirectional removals (production of "classical" points). One of the cores classified in this scheme could also testify to a recurrent production of bipolar points (HDOR 2003 No. 4). This schema is fairly widespread (Fig. 0) and may be supplemented by lateral removals, emphasising lateral convexities ("constructed" points production). Three cores pertaining to this scheme feature one or two more centripetal lateral removals (HDOR 2016 No. 2 and HDOR 2004 No. 5).

Analysis of Levallois points

The points collected are very rare, just four. Their small number is due to the near absence of these pieces from the surveyed sites. They feature scars of unipolar convergent removals, linking them to the B1 group. One of them contains negatives of removals on the distal part which suggest a more sustained preparation of the distal convexity (potentially linked to the former scheme B2, i.e., scheme B for "constructed" points) (Fig. ). Since the reference corpus of the Levallois points is extremely limited for Hadramawt, it seemed relevant to investigate the morphological and dimensional charac- teristics of the negatives of points, from the cores themselves. With regard to the morphological characteristics, the negatives of points observed on the cores are rather heterogeneous (Fig. 2). A relatively large variation exists in the final shape of the resulting point, being long and thin, wide and short, wide and long, or short and thin. Analysis of the dimensional data (lengths and widths) for each method of production group does not particularly distinguish particular morphometric groups which could indicate a type of product for a particular method of production (Fig. 13 ). Thus, diversity of preparation schemes seems not to relate to any particular type of point. Accordingly, it is questionable if this diversity is due rather to the shape of the blocks of raw material, to some special technical knowledge, or to the final state of the debitage which does not allow us to identify the possible existence of the different stages of the schemes on a block due to the possibilities or the accidents of knapping that occurred.

Conclusions

Despite a limited number of cores, it was possible finally to identify many procedures for obtaining Levallois points. Such variation in the methods implemented for the production of Levallois points could, however, be typical to the Hadramawt region. At a regional scale, the presence of Levallois debitage in general, for making points or not, can be explained by a diffusionist approach. Its presence in the plateaus in the east of Yemen could match the dispersal of the Levallois concept from the African coasts and/or from Levantine regions. Furthermore, the possibility of an adaptive local development of the knapping modalities is quite likely. From an exogenous population base, future generations could very well have developed their own conceptual systems of preferential productions influenced by types of raw materials and technical or cultural traditions specific to those regions, which would explain the presence of a greater variation and even a greater diversity of knapping schemes in Hadramawt.

If the analysis of the scarce lithic material here cannot answer these questions, it does however offer a few elements of comparison with the assemblages from Africa and the Levant. The first dated archaeological data from Yemen and the first detailed comparisons thus tend to favour the hypothesis of the existence of an area of endemic development in southern Arabia, and this at different times of prehistory (Crassard 2008a(Crassard , 2009a(Crassard , 2009b)). Nevertheless it is fair to nuance the scope of this study on the material from Hadramawt region. Indeed, as the results of this analysis are based on a relatively small number of cores and just four points, it seems difficult to rule on the strictly independent character or not of the schemes described here. Do they attest to a real diversity of procedures in the production of Levallois points, or of a mere variation reflecting the adaptation by the knappers to the morphology of the blocks, to the stages of exhaustion of the exploited cores, and to the accidents of knapping? A common reflection with all researchers working on these issues may allow us to apprehend better the archaeological reality, at a micro and macro-regional scale, but also at a purely theoretical scale of the anthropology of techniques.

Fig

  Fig. : sites mentioned in text, worldwide distribution of the Levallois points productions.

Fig. 2 :

 2 Fig. 2: Sites from Hadramawt, Yemen, where Levallois points production has been documented.

Fig. 3 :

 3 Fig. 3: Group B schemes.

Fig. 5 :

 5 Fig. 5: Scheme B.

Fig. 4 :

 4 Fig. 4: Scheme B.

Fig. 6 :

 6 Fig. 6: Scheme B3 opposed unipolar. Fig. 7: Scheme B3 opposed unipolar, "constructed" points.

Fig. 0 :

 0 Fig. 0: Scheme B5.

Fig. :

 : Fig. : Levallois points from Hadramawt, Yemen.

Fig. 2 :

 2 Fig. 2: Reconstructed shapes of Levallois points, from the analysis of the cores.

Fig. 13 :

 13 Fig. 13: Dimensions of the points by types (classical / constructed).

Acknowledgements

We gratefully acknowledge the assistance provided by Dr Abdullah Bawazir, President of the General Organization for Antiquities and Museums of the Republic of Yemen, and his able staff, particularly Abdul-Rahman Assaqaf and Abdulaziz bin Aqil from Hadramawt region. Rémy Crassard thanks the directors of the RASA Project: Joy McCorriston, Erich Oches and Abdulaziz bin Aqil, and of the French Archaeological Mission in Jawf-Hadramawt: Michel Mouton and Anne Benoist. For their comments on previous drafts and general discussion about South Arabian, European and East African prehistory, we gratefully acknowledge the help of Pierre Bodu, Anne Delagnes, Katja Douze, Marie-Louise Inizan, Reto Jagher, Liliane Meignen, Michael Pet-raglia and Jeffrey Rose. Rémy Crassard gratefully acknowledges the Fondation Fyssen and Céline Thiébaut the Fondation des Treilles, both for providing financial support.