Méthode de détection et de caractérisation de pollutions du sous-sol par les sites contaminés à l'aide des traces inorganiques

Contaminated sites form one of the major causes of environmental pollutions, in particular those of the soil and of the underground. One of the main problems presented by these sites is their location. Since water is the main contamination support in soil and underground, it is necessary to get good indicators to be able to detect pollution sources in areas where suspected sites location remains unknown. This is also necessary for the monitoring of contaminated sites in time and space. The aim of this project is to enhance the use of inorganic trace elements by offering specialists a new investigation tool adapted to a survey strategy of contaminated sites in various geological conditions like those found in Switzerland. This study has shown, through numerous site descriptions (41 sites) and the statistical approach of collected samples (222 samples), that the use of inorganic trace elements is helpful for the survey of contaminated sites. The most interesting result shows specific indicators for hazardous waste. Principal results Some inorganic trace elements represent almost exclusively hazardous wastes, since they are missing in the domestic and in the inert wastes. We also showed that these trace elements are typical indicators allowing to detect the presence of hazardous waste mixed with domestic wastes. These elements are : Se, As, W, Cs, Sc, Bi, Ga, Hg, Ag. Domestic and inert wastes do not show any characteristical trace elements. In general one can find the following elements in more or less important amounts in domestic wastes : Cu, U, La, B, Al, Ni, Co and Zn, Li in inert wastes. We also noticed that main inorganic trace elements in landfill leachates are : B, Ba and Br, of which the concentration is often close to or greater than mg/l. Tritium is also often associated to landfills. The comparison between uninfluenced groundwater collected during this study and polluted groundwater has allowed to value some inorganic trace elements. These elements differ significantly from the natural background when polluted sites are encountered. We distinguished specific indicators that are not present in uninfluenced groundwater : Cs, Sb, Sn, Ag, Bi, La, Se and tritium. We also put forward the presence of typical tracers showing anomalous concentrations in case of pollution by landfills, such as : Ti, Co, V, Ni, Cu, Ba, Br and B. Guide values could be established for these elements for their use in detecting contaminated sites. The same process was adopted for a limited amount of samples in surface waters. The significant trace elements are the same as for groundwater, except Sn and La. Hg, Cd and Pb which, on the other hand, could be detected at significant concentrations in cases of pollutions by contaminated sites.

Estimation of the total VOC reactivity with OH by pump-and-probe OH lidar to determine a new indicator for ozone production

Many of our most severe problems of environmental pollution involve the Earth's atmosphere. The adverse effects of atmospheric pollution are particularly evident in the low troposphere, especially in urban/suburban areas where many sources contribute to the atmospheric burden. Ozone (O3) is one of the major constituents of the air pollution over cities commonly called photochemical smog. It is a secondary pollutant, produced by the reaction of primary pollutants, nitrogen oxides (NOx) and hydrocarbons (VOC), in the presence of sunlight. Although hydrocarbons and NOx have long been identified as the two key precursors to photochemical O3, the development of an effective strategy for reducing ozone production in photochemical smog by controlling the anthropogenic emissions of these precursors has proved to be problematic. The objective of the present study is to present and demonstrate the robustness of a new observable "indicator" for determining the sensitivity of ozone formation to VOC and NOx (VOC- and NOx-limited regime). This indicator is based on the reactivity of VOC and NOx with respect to OH. Simulation tests of the ability of this new indicator Θ = τOHVOC/τOHNOx to distinguish between a NOx and a VOC-limited regime were performed with a box model and a 3D model applied to a specific episode of photochemical smog (Athens) and of a less polluted area (the Swiss plateau). Whereas the NOx reactivity 1/τOHNOx can easily be measured by standard methods, the total VOC reactivity 1/τOHVOC will be estimated by measuring the inverse lifetime of OH with a new Pump-and-Probe Lidar instrument. The method consists of producing high OH concentrations by the flash photolysis of ozone and the subsequent O(1D) reaction with H2O and by following the OH relaxation by laser-induced fluorescence (LIF). Numerical simulation of the experiment will show that the total VOC reactivity with OH can be obtained by measuring the inverse lifetime of the OH and its absolute initial concentration with the Pump-and-Probe experiment as well as the NOx, CO and O3 concentrations with standard trace-gas detectors. This total in-situ VOC reactivity will be used to determine the new indicator Θ but will also help to test the accuracy of the VOC-lumped chemical mechanism in the numerical atmospheric simulation. In order to demonstrate the feasibility of the Pump-and-Probe experiment, laboratory investigations have been performed in controlled atmosphere: OH absorption and fluorescence emission spectra allowed the OH transition bands and the non-resonant fluorescence wavelength to be identified. High-resolution OH absorption spectra allowed the OH effective absorption cross section to be determined for assessing the experimental sensitivity and retrieving the OH concentration. The experimental retrieval of the rate constant for the OH+CH4 and OH+NO2 reactions reveals the ability of the pump-and-probe experiment to monitor the chemical kinetics of OH at atmospheric pressure and room temperature. OH relaxation measured in different VOC/NOx/O3 gas mixtures confirm the relevance of the model predictions of the OH relaxation approximated by a first-order chemical decay and the validity of the total VOC reactivity with respect to OH estimated by such an approximation. A first in-situ OH relaxation measurement realized directly in the air of the laboratory showed that the pump-and-probe method could be performed under typical PBL conditions. Preliminary range-resolved measurements yielding vertical profiles of the OH concentrations produced by flash photolysis of ambient ozone suggest a promising future for the Lidar pump-and-probe technique. The perspectives for the Pump-and-Probe Lidar experiment are expressed in the concluding section.

EPFL2000

Substance flow analysis as a tool for urban water management

Nathalie Chèvre Rossi, Luca Rossi

Human activity results in the production of a wide range of pollutants that can enter the water cycle through stormwater or wastewater. Among others, heavy metals are still detected in high concentrations around urban areas and their impact on aquatic organisms is of major concern. In this study, we propose to use a substance flow analysis as a tool for heavy metals management in urban areas. We illustrate the approach with the case of copper in Lausanne, Switzerland. The results show that around 1,500 kg of copper enter the aquatic compartment yearly. This amount contributes to sediment enrichment, which may pose a long-term risk for benthic organisms. The major sources of copper in receiving surface water are roofs and catenaries of trolleybuses. They represent 75% of the total input of copper into the urban water system. Actions to reduce copper pollution should therefore focus on these sources. Substance flow analysis also highlights that copper enters surface water mainly during rain events, i.e., without passing through any treatment procedure. A reduction in pollution could also be achieved by improving stormwater management. In conclusion, the study showed that substance flow analysis is a very effective tool for sustainable urban water management.

IWA Publishing2011

Impact of climate change on runoff pollution in urban environments

David Andrew Barry, Sylvain Coutu, Sébastien Kramer

Runoff from urban environments is generally contaminated. These contaminants mostly originate from road traffic and building envelopes. Facade envelopes generate lead, zinc and even biocides, which are used for facade protection. Road traffic produces particles from tires and brakes. The transport of these pollutants to the environment is controlled by rainfall. The interval, duration and intensity of rainfall events are important as the dynamics of the pollutants are often modeled with non-linear buildup/washoff functions. Buildup occurs during dry weather when pollution accumulates, and is subsequently washed-off at the time of the following rainfall, contaminating surface runoff. Climate predictions include modified rainfall distributions, with changes in both number and intensity of events, even if the expected annual rainfall varies little. Consequently, pollutant concentrations in urban runoff driven by buildup/washoff processes will be affected by these changes in rainfall distributions. We investigated to what extent modifications in future rainfall distributions will impact the concentrations of pollutants present in urban surface runoff. The study used the example of Lausanne, Switzerland (temperate climate zone). Three emission scenarios (time horizon 2090), multiple combinations of RCM/GCM and modifications in rain event frequency were used to simulate future rainfall distributions with various characteristics. Simulated rainfall events were used as inputs for four pairs of buildup/washoff models, in order to compare future pollution concentrations in surface runoff. In this way, uncertainty in model structure was also investigated. Future concentrations were estimated to be between ±40% of today’s concentrations depending on the season and, importantly, on the choice of the RCM/GCM model. Overall, however, the dominant factor was the uncertainty inherent in buildup/washoff models, which dominated over the uncertainty in future rainfall distributions. Consequently, the choice of a proper buildup/washoff model, with calibrated site-specific coefficients, is a major factor in modeling future runoff concentrations from contaminated urban surfaces.

2012

Impact of climate change on runoff pollution in urban environments

David Andrew Barry, Sylvain Coutu, Sébastien Kramer

2012

Estimation of the total VOC reactivity with OH by pump-and-probe OH lidar to determine a new indicator for ozone production

EPFL2000