Benoit Laignel

Assessment of direct transfer and resuspension of particles during turbid floods at a karstic spring

Turbid water can be the source of important sanitary problems in karstic regions. It is the case of the Pays de Caux, in Haute Normandie, where the main resource in drinking water is provided by the chalk aquifer. In the case of the typical binary karst of the Pays de Caux, turbidity results from the input in sinkholes of turbid surface water induced by erosion on the plateaus. At some spring tappings, water may be very turbid in period of intense rainfall. The turbidity observed at a karstic spring is a complex signal which contains a part of direct transfer and a part of resuspension of the particles being transported. The aim of this study is turbidigraph separation, which would permit to distinguish the direct transfer and resuspension components of the turbidigraph. These two components are separated by comparing the elementary surface storm-derived water fluxes and elementary turbidity signals at the spring. The procedure takes place in three phases: (i) spring hydrograph separation by means of a two components mixing model (surface water and karstic groundwater) using specific electrical conductivity, (ii) decomposition of storm-derived water flux and turbidity thanks to the second-derivative method, (iii) comparison of the transfer times (< modal times) of the elementary turbidity and surface water flux signals, respectively. The mass corresponding to direct transfer, computed after signal decomposition, is then used to re-calculate a particle recovery rate, which passes so from 51 ^ 4 to 37 ^ 3%. Relations between particle flux and hydrodynamics show that resuspension can be either the fact of the dynamics of the introduction system, or that of the chalk karstic aquifer in general (case of resuspension not associated to surface water flux). In this sense, evolution of particle flux (and consequently of turbidity) can be also a marker of the karst structure. q 2003 Elsevier Science B.V. All rights reserved.

Polycyclic aromatic hydrocarbon transport processes on the scale of a flood event in the rural watershed of Le Bebec, France

Transport processes of polycyclic aromatic hydrocarbons (PAHs) were investigated in a rural watershed in France, which led to new quantitative results on the scale of a flood event. For that purpose, principal components analysis (PCA) combined with signal decomposition of PAH Breakthrough curves were used. The results obtained by PCA enabled us to distinguish (i) the transport of low molecular weight PAHs in dissolved form and (ii) the transport of high molecular weight PAHs in particulate form. The intermediate molecular weight PAHs are distributed in these two clusters. The application of a signal decomposition method confirms the results of PCA and makes it possible to quantify the mass PAH resulting, respectively, from resuspension (13.3mg, i.e. 77% of the total mass) and from direct transfer (2.8mg=16% of the total mass). For the resuspension phase, desorption of PAH (38% of the total exported mass) could be also distinguished from sediment resuspension (39%).

Filling of a wetland (Seine estuary, France): natural eutrophication or anthropogenic process? A sedimentological and geochemical study of wetland organic sediments

For over a century the Seine estuary has been highly affected by human activities, resulting in a reduction of the surface of wetland habitat. Several ponds of the Vernier Marsh, one of the main wetlands of the estuary, are being filled in by sediment. The St. Aubin canal, the only inlet from the Seine to the wetland, has regularly been opened in summer to maintain a sufficiently high water level in the system. The purpose of this study was to identify the origin of filling: natural evolution of eutrophication or human intervention by the opening of the St. Aubin canal? Sediment cores were sampled at 19 stations randomly distributed in the ponds of the wetland. Granulo-metric, micro-granulometric and geochemical analyses (ignition loss, C/N/P, chemical fractionation of particulate phosphate) were performed on the top 10 cm of sediment. Moreover nutrient concentrations (ammonium and phosphate) were measured in the interstitial waters using diffusion samplers. The results revealed that sediments sampled from the north are weakly organic (ignition loss: 19%, 15% C, 0.9% N, 40% organic P of total P) whereas those sampled from the southern stations are highly organic (ignition loss: 50–93%, 35% C, 1% N: 59–81% organic P of total P). In summary, sediments accumulating in the north of the ecosystem mainly consist of minerogenic particles from the Seine transported by the St. Aubin canal and thus are a result of the human interference. Interestingly the accumulation of sediments in the southern part is the result of a natural evolution of eutrophication: a peat bog. These sediments are highly organic, enriched with organic particulate phosphate, specifically a humic-acid-bound-phosphorus, a refractory form of phosphorus.

Hydrological variability of major French rivers over recent decades, assessed from gauging station and GRACE observations

Abstract This study was carried out in the framework of the Surface Water and Ocean Topography (SWOT) programme of the French National Centre of Space Studies (CNES). Based on discharge measurements and Gravity Recovery and Climate Experiment (GRACE) determination of total water storage (TWS), we have investigated the hydrological variability of the main French drainage basins (Seine, Loire, Garonne and Rhône) using a wavelet approach (continuous wavelet analyses and wavelet coherence analyses). The results of this analysis have shown a coherence ranging between 82% and 90% for TWS and discharge, thus demonstrating the potential use of TWS for characterization of the hydrological variability of French rivers. Strong coherence between the four basin discharges (between 73% and 92%) and between their associated TWS data (from 82% to 98%) suggested a common external influence on hydrological variability. To determine this influence, we investigated the relationship between hydrological variability and the North Atlantic Oscillation (NAO), considered as an index of prevailing climate in Europe. Basin discharges show strong coherence with NAO, ranging between 64% and 72% over the period 1959–2010. The coherence between NAO and TWS was 62% to 67% for 2003–2009. This is similar to the coherence between NAO and basin discharges detected for the same period. According to these results, strong influence of the NAO was clearly observed on the TWS and discharges of the major French river basins. Editor Z.W. Kundzewicz

Polyphase karst system in Cretaceous chalk and calcarenite of the Belgian-Dutch border

Along the Belgian-Dutch border, underground and surface quarries dug in Cretaceous calcarenite and chalk intersect many karst features as well as deep large nodes of weathered rock. Their observation allows the reconstruction of the genesis of an original karst system resulting from the merging of initially independent endokarsts and exokarsts. Deep weathering has developed within the Cretaceous formations, creating nodes of weathered chalk and closed cavities. These phenomena are expanded over time and can form interconnected voids. Near the surface, solution pipes are generated under the coarsest deposits of a fluvial terrace capping the Cretaceous formations. These pipes develop vertically and may be related to the progressive lowering of the water table in connection with the incision of the Meuse valley. Some of these phenomena cut up the older endokarsts and organize complex systems of out-flow within the chalk.

Variabilité du cortège argileux des formations résiduelles à silex de l'Ouest du bassin de Paris

Abstract The study of the clay fraction mineralogy of the clay with flints of the Western Paris Basin have brought to light a geographic differentiation of the latter and the existence of a mineral never described within this type of formation, i.e. pyrophyllite. This study shows the important role of the Cenozoic deposits in the genesis and the mineralogical composition of the clay with flints. Cenozoic deposits condition the age of the clay with flints, and they are the sources of allochtonous input contaminates which supplied the day with the flints matrix.

Spatial and temporal variability of groundwater quality of an Algerian aquifer: the case of Soummam Wadi

ABSTRACT Five-year monitoring of physicochemical parameters was performed with two campaigns in low and high water periods of the Lower Soummam catchment. Data from 18 wells were processed by multivariate statistical tools in order to identify the principal factors influencing groundwater chemistry. Two matrices of 14 and 8 physicochemical parameters with 18 groundwater samples collected in wells were obtained. The correlation matrix showed strong associations between nine variables: K+, Ca2+, Na+, SO42−, Cl−, Mg2+, NO2−, Zn2+ and Sr2+. Principal component analysis and factor analysis showed that the cumulated variance of high and low water periods was of 83.19% and 78.55%, respectively. The variables assigned to the mineralization effect or to pollution indicators were presented by the factor analysis. The bivariate plots confirmed a mineralization model, ascribed to dissolution of geological materials, and to high levels of saline contamination attributed to leakages from sanitary systems. They also showed an increase “upstream to downstream” of the mineralization, visualization of temporal variations, and a dilution process identification of the natural mineralization during the recharge of the aquifer. EDITOR D. Koutsoyiannis; ASSOCIATE EDITOR X. Chen

On the Investigation of the Sea-Level Variability in Coastal Zones Using SWOT Satellite Mission: Example of the Eastern English Channel (Western France)

The future mission of surface water and ocean topography (SWOT), launched in 2020 over a period of 3-5 years, will be designated to address the issue of combining surface water hydrology with physical oceanography aiming to present new perspectives of applications for coastal areas. The extent to which the synthetic SWOT measurements can reproduce the temporal variability of the sea level was investigated. The eastern English Channel (NW France) was considered as a case of application. The hourly sea-level records were filtered from the aliased harmonic tides by classic harmonic analyses to obtain the nontidal residual. This residual was used to simulate synthetically the satellite samples based on the number of overpasses per repeat cycle at each geographical station. Both real and synthetic SWOT measurements were compared by the use of different approaches of inference statistics and wavelets. The statistical behavior, deduced from the functions of probability density (pdf) and cumulative distribution (cdf), shows correlations between 65% and 75% for hourly measurements, which increase to 85% for monthly average ones. The frequency of positive and negative extreme values is under-estimated with an order less than 25%. The potential use of SWOT depends on the number of measurements and the sampling interval between SWOT overpasses per repeat orbit. In the time- frequency domain, the wavelet multiresolution analysis of the nontidal sea level displays four components: 1) 1 year; 2) ~4-7 month; 3) ~2-3 month; and 4) <;2 month bands. Such modes seem to be well manifested by SWOT samples with a mean explained variance more than 75%. The aliasing frequency of the altimeter generates a dispersion and an overexpression of the energy spectrum, which increases with the number of overpasses per repeat orbit and the high frequency (~2-3 and <;2 month bands). The reconstructed wavelet components evidence the capacity of SWOT to estimate the annual and the seasonal variability of the nontidal sea level. In particular, SWOT is able to reproduce the most of extreme storm surges in the English Channel. The main finding of this research clearly shows the utility of SWOT satellite altimetry in observing and understanding the sea-level variability and storm surges, complementing tide-gauge observations for the validation and improvement of coastal models.

Holocene evolution of a wetland in the Lower Seine Valley, Marais Vernier, France

Estuaries like that of the Seine River in NW Europe developed in incised fluvial valleys after the last glacial maximum. Since the 1940s, several authors have studied the largest wetland of the Seine estuary, the Marais Vernier, to understand depositional environments during Holocene infilling. We reinterpret previous research based on new and published data (for example fill thickness and material source) to (1) describe facies and depositional environments; (2) reconstruct palaeoenvironmental evolution; (3) show the influence of local and global forcing on depositional environments. Before 7000—6000 cal. BC, terrestrial material was deposited because of catchment erosion related to changes in climate. Just before 7000—6000 cal. BC, estuarine material began to be deposited in low-lying areas in response to sea-level rise, while terrestrial material still settled at higher elevations. After this, but before 5850—5710 cal. BC, estuarine material areas began to accumulate at both high and low elevations. This marked a general flooding of the Marais Vernier, synchronous with that at the Seine estuary mouth. Soon after, peat accumulated over a wide area as a response to a local change in accommodation and a worldwide drop in sea level. A tidal channel developed to the west of the Marais Vernier, providing minerogenic material. After 1130—900 cal. BC, human influence becomes increasingly clear in the record. This record of regional change during the Holocene can serve as a reference for further studies in the area.

Monitoring Sea Level and Topography of Coastal Lagoons Using Satellite Radar Altimetry: The Example of the Arcachon Bay in the Bay of Biscay

Radar altimetry was initially designed to measure the marine geoid. Thanks to the improvement in the orbit determination from the meter to the centimeter level, this technique has been providing accurate measurements of the sea surface topography over the open ocean since the launch of Topex/Poseidon in 1992. In spite of a decrease in the performance over land and coastal areas, it is now commonly used over these surfaces. This study presents a semi-automatic method that allows us to discriminate between acquisitions performed at high tides and low tides. The performances of four radar altimetry missions (ERS-2, ENVISAT, SARAL, and CryoSat-2) were analyzed for the retrieval of sea surface height and, for the very first time, of the intertidal zone topography in a coastal lagoon. The study area is the Arcachon Bay located in the Bay of Biscay. The sea level variability of the Arcachon Bay is characterized by a standard deviation of 1.05 m for the records used in this study (2001–2017). Sea surface heights are very well retrieved for SARAL (R~0.99 and RMSE 0.93 and RMSE 0.82 but with a higher RMSE >0.92 m). For the topography of the intertidal zone, very good estimates were also obtained using SARAL (R~0.71) and CryoSat-2 (R~0.79) with RMSE lower than 0.44 m for both missions.