Diagnostic et caractérisation d'un site pollué : comparaison de techniques d'échantillonnage des sols

The choice of a suitable soil sampling strategy plays an important role in the characterization of contaminated land. In order to achieve representative sampling it is necessary to minimize errors due to field investigation, i.e. soil sampling, tool sampling, soil types and land uses.The main objective of this work was a comparison of sampling strategies for mapping trace element concentrations at a given site. The studied area is a former arsenical pesticide manufactory site. Sampling area is approximately 5000 m2. Soil samples were collected during 1999 and 2001, on a regular grid of 10 x 10 m and 20 x 20 m. Additional sampling was also carried out on the basis of a regular grid of 5 x 5 m on two particular zones. At each sampling point two different tools were used (power shovel and hand auger). Random sampling was also used and compared to regular soil sampling.A total of 121 soil samples were collected. Arsenic concentration was analysed on each sample. Arsenic concentrations ranged from 100 mg·kg-1 to 14 000 mg·kg-1. The topsoil (0 to -20 cm) has a higher arsenic content than the lower layers (-20 to -40 cm and –40 to –60 cm). The regular sampling appears to be the better strategy to achieve representative soil sampling. On the other hand, an increase of the number of sampling points does not necessarily improve soil sampling representativity. The use of the two tools provides similar arsenic concentrations. This paper argues for more research on sampling strategies and the need for a multidisciplinary approach.Une connaissance précise de l’état de pollution des sols est une des conditions indispensables pour la mise en place d’une technique de dépollution efficace. Une telle connaissance passe par la mise en œuvre de campagnes d’échantillonnage permettant d’obtenir des données environnementales représentatives, c’est-à-dire minimisant les risques d’erreurs inhérents aux investigations de terrain. L’objectif du travail réalisé est de tester sur site la représentativité de l’échantillonnage des sols au travers de l’influence de la stratégie d’échantillonnage et de l’outil de prélèvement utilisé.Malgré les caractéristiques distinctes des outils utilisés (tarière et pelle mécanique) et des quantités différentes de sol prélevées, il ne semble pas y avoir d’influence de l’outil de prélèvement sur le résultat d’analyse.L’échantillonnage aléatoire prête à caution, car il n’a pas toujours permis de restituer les zones polluées détectées avec un échantillonnage systématique. En outre la représentativité de l’état de pollution n’est pas nécessairement améliorée par une échelle d’observation plus fine.La représentativité des mesures reste donc un objectif exigeant et avant de choisir une stratégie d’investigation, il est nécessaire de prendre en compte tous les éléments en notre connaissance en fonction de l’objectif poursuivi. De plus, pour la réussite d’un diagnostic une réflexion sur le programme d’échantillonnage doit être engagée dès le début car c’est sur sa base que seront estimés les volumes de terres à traiter

Urban groundwater age modeling under unconfined condition – Impact of underground structures on groundwater age: Evidence of a piston effect

International audienceIn this paper, underground structures are shown to have a major influence on the groundwater mean age distribution described as a dispersive piston effect. Urban underground development does not occur with- out impacts on subsoil resources. In particular, groundwater resources can be vulnerable and generate disturbances when this space is exploited. Groundwater age spatial distribution data are fundamental for resource management as it can provide operational sustainability indicators. However, the application of groundwater age modeling is neglected regarding the potential effect of underground structures in urban areas. A three dimensional modeling approach was conducted to quantify the impact of two under- ground structures: (1) an impervious structure and (2) a draining structure. Both structures are shown to cause significant mixing processes occurring between shallow and deeper aquifers. The design technique used for draining structures is shown to have the greatest impact, generating a decrease in mean age of more than 80% under the structure. Groundwater age modeling is shown to be relevant for highlighting the role played by underground structures in advective–dispersive flows in urban areas

Underground structures increasing the intrinsic vulnerability of urban groundwater: Sensitivity analysis and development of an empirical law based on a groundwater age modelling approach

International audienceIn a previous paper published in Journal of Hydrology, it was shown that underground structures are responsible for a mixing process between shallow and deep groundwater that can favour the spreading of urban contamination. In this paper, the impact of underground structures on the intrinsic vulnerability of urban aquifers was investigated. A sensitivity analysis was performed using a 2D deterministic modelling approach based on the reservoir theory generalized to hydrodispersive systems to better understand this mixing phenomenon and the mixing affected zone (MAZ) caused by underground structures. It was shown that the maximal extent of the MAZ caused by an underground structure is reached approximately 20 years after construction. Consequently, underground structures represent a long-term threat for deep aquifer reservoirs. Regarding the construction process, draining operations have a major impact and favour large-scale mixing between shallow and deep groundwater. Consequently, dewatering should be reduced and enclosed as much as possible. The role played by underground structures' dimensions was assessed. The obstruction of the first aquifer layer caused by construction has the greatest influence on the MAZ. The cumulative impact of several underground structures was assessed. It was shown that the total MAZ area increases linearly with underground structures' density. The role played by materials' properties and hydraulic gradient were assessed. Hydraulic conductivity, anisotropy and porosity have the strongest influence on the development of MAZ. Finally, an empirical law was derived to estimate the MAZ caused by an underground structure in a bi-layered aquifer under unconfined conditions. This empirical law, based on the results of the sensitivity analysis developed in this paper, allows for the estimation of MAZ dimensions under known material properties and underground structure dimensions. This empirical law can help urban planners assess the area of influence of underground structures and protect urban strategic reservoirs

Réutilisation en génie civil des résidus solides de l’assainissement pluvial routier : de la caractérisation aux techniques de traitement

Les résidus solides de l’assainissement pluvial routier représentent des tonnages importants de matériaux considérés comme des déchets à la charge des gestionnaires d’infrastructures routières. Afin de faciliter la gestion de ces résidus, une hiérarchisation de ces résidus en fonction de leurs caractéristiques environnementales et géotechniques a été établie. Près de 100 échantillons ont été prélevés dans des contextes routiers divers sur lesquels ont été réalisées des analyses des métaux lourds (Zn, Pb, Cu, Cd, Ni et Cr), des hydrocarbures totaux ainsi que des essais géotechniques. Pour faciliter la valorisation, des moyens de traitement simples et aisés à mettre en œuvre par les gestionnaires ont été envisagés. La réduction des teneurs en eau a été étudiée à l’aide d’essais de séchage à l’air libre. La réduction des teneurs en hydrocarbures par biodégradation aérobie a également été étudiée. La typologie, basée sur les critères environnementaux et géotechniques, a permis de subdiviser les résidus solides de curage de fossés et bassins routiers selon les critères suivants : origine des sédiments (bas-côtés routiers ou bassins), trafic routier moyen journalier annuel (TMJA), type d’ouvrage (sec ou en eau). Il ressort des essais et tests effectués que le stockage temporaire sur une épaisseur de 10 à 50 cm permet de passer d’un état saturé (w = 80 %) à une teneur en eau de 20 %, qui correspond à l’optimum Proctor. Ce changement d’état a été obtenu en une quinzaine de jours de stockage à l’abri des précipitations. Les tests de respiration des sédiments ont montré une activité bactérienne aérobie pouvant être assimilée à de la biodégradation des hydrocarbures. Selon la texture des résidus, une première estimation de la consommation en oxygène présente une variabilité entre 200 et 1000 mg O2/kg de MS pour 5 jours. La réutilisation de ces matériaux semble envisageable, après un séchage d’une quinzaine de jours sur faible épaisseur. Les teneurs en MO, assez variables, restreignent toutefois la réutilisation pour certains ouvrages. Enfin, une première version de fiche produits a été proposée pour résumer les étapes et traitements nécessaires à la valorisation en techniques routières et assimilées des terres de curages de fossés et les sédiments de bassins routiers

Review : Impact of underground structures on the flow of urban groundwater

International audienceProperty economics favours the vertical develop- ment of cities but flow of groundwater can be affected by the use of underground space in them. This review article presents the state of the art regarding the impact of distur- bances caused by underground structures (tunnels, basements of buildings, deep foundations, etc.) on the groundwater flow in urban aquifers. The structures built in the underground levels of urban areas are presented and organised in terms of their impact on flow: obstacle to the flow or disturbance of the groundwater budget of the flow system. These two types of disturbance are described in relation to the structure area and the urban area. The work reviewed shows, on one hand, the individual impacts of different urban underground structures, and on the other, their cumulative impacts on flow, using real case studies. Lastly, the works are placed in perspective re- garding the integration of underground structures with the aim of operational management of an urban aquifer. The literature presents deterministic numerical modelling as a tool capable of contributing to this aim, in that it helps to quantify the effect of an underground infrastructure project on groundwater flow, which is crucial for decision-making processes. It can also be an operational decision-aid tool for choosing construction techniques or for formulating strategies to manage the water resource

Deterministic modeling of the impact of underground structures on urban groundwater temperature

International audienceUnderground structures have a major in␣uence on groundwater temperature and have a major contribution on the anthropogenic heat ␣uxes into urban aquifers. Groundwater temperature is crucial for resource management as it can provide operational sustainability indicators for groundwater quality and geothermal energy. Here, a three dimensional heat transport modeling approach was conducted to quantify the thermally affected zone (TAZ, i.e. increase in temperature of more than +0.5 °C) caused by two common underground structures: (1) an impervious structure and (2) a draining structure. These design techniques consist in (1) ballasting the un- derground structure in order to resist hydrostatic pressure, or (2) draining the groundwater under the structure in order to remove the hydrostatic pressure. The volume of the TAZ caused by these underground structures was shown to range from 14 to 20 times the volume of the underground structure. Additionally, the cumulative im- pact of underground structures was assessed under average thermal conditions at the scale of the greater Lyon area (France). The heat island effect caused by underground structures was highlighted in the business center of the city. Increase in temperature of more than +4.5 °C were locally put in evidence. The annual heat ␣ow from underground structures to the urban aquifer was computed deterministically and represents 4.5 GW·h. Considering these impacts, the TAZ of deep underground structures should be taken into account in the geother- mal potential mapping. Finally, the amount of heat energy provided should be used as an indicator of heating po- tential in these areas