Impact of land-use on PAH transfer in sub-surface water as recorded by CaCO3 concretions in urban underground structures (Paris, France).

In densely populated urban areas, the pressure on water resources is considerable and will tend to intensify over the next decades. Preserving water resources therefore seems fundamental, but many questions remain as to the transfer of contaminants to subsurface waters in these largely sealed areas. Because of their toxicity and persistence in the environment, this work focused on the study of polycyclic aromatic hydrocarbons (PAHs), ubiquitous pollutants mainly produced by human activities. To better understand the main factors leading to the retention or transport of these pollutants in urban environments, vertical transects, from the surface to several meters down, were established on three study sites in or near Paris (France), selected according to an urbanization gradient. Soil samples collected at the surface and urban secondary carbonate deposits (USCD), similar to cave speleothems, sampled underground in quarries and aqueducts were analyzed. As the hydrophobic properties of PAHs favor their sorption onto organic matter, the latter was also studied using organic carbon analysis and UV fluorescence spectroscopy. The USCD located closest to the urbanized surface contained high concentrations of PAHs (76.8 ± 5.3 ng g-1), while the USCD located at greater depth with organic soil on the surface contained the lowest amount of PAHs (2.9 ± 0.4 ng g-1), and no PAHs with log KOC > 5. The results highlight the predominant role played by the presence of organic topsoil at the surface in retaining and storing large amounts of PAHs (1914-2595 ng. gsoil-1), particularly the most hydrophobic ones (i.e. 60% of the 15 PAHs are characterized by a Log KOC >5), which are also the most toxic. The lithology and thickness of the bedrock (between the surface and the USCD) also play an important role in the retention of PAHs, particularly those adsorbed on the particulate phase.

Polycyclic aromatic hydrocarbon record in an urban secondary carbonate deposit over the last three centuries (Paris, France).

Preserving water resources and limiting pollution are central environmental issues in the current context of intense anthropization. Among organic pollutants, polycyclic aromatic hydrocarbons (PAHs) are commonly analysed as part of water quality assessments. After being emitted into the atmosphere, these persistent organic pollutants are deposited on the continental surface, where they are transported to the aquatic environment by run-off and infiltration waters. Mainly due to anthropogenic emissions, PAHs can therefore be considered as a proxy for human activities. Urban secondary carbonate deposits (USCDs), similar to cave speleothems, have recently been studied for their potential as natural archives of water quality. However, USCDs have never been used to trace water organic pollution and only a few studies on PAHs in speleothems are available. This study focuses on a well-dated USCD covering the last 300 years from the Great Aqueduct of Belleville (north-east of Paris, France). The aim is to determine the nature and variation of trapped organic compounds over time and to discuss their origin, transport, and link with changes in soil occupation due to human activities. To do so, high-resolution solid-phase UV fluorescence imaging analyses were combined with chemical analyses of PAHs and organic carbon carried out on low-weight samples. The results show that PAHs have been present in urban surface water for 300 years. Over the last few decades, a 7-fold increase is observed, accompanied by a change in the pollution source, enriched in high-molecular-weight PAHs, probably linked to urban dust. This study also reveals modes of transport directly influenced by changes in the soil occupation that are very different from those usually encountered in natural environments. This work thus paves the way for a better long-term understanding of the impact of human activity on the transfer of pollutants to sub-surface waters.

Exploring carbonate rock wettability across scales: Role of (bio)minerals.

HYPOTHESIS: The wettability of carbonate rocks is expected to be affected by the organic components of biominerals which are complex, nanostructured organo-mineral assemblages. Elucidating the nanoscale mechanisms driving the wettability of solid surfaces will enable a better understanding of the role of biominerals in the wetting properties of carbonate rocks to control various geological, environmental and industrial processes. EXPERIMENTS: Using Atomic Force Microscopy and Spectroscopy (AFM/AFS) we probed the wettability properties of carbonate rocks with different amounts of organic material. The adhesion properties of two types of limestones were determined in liquid environments at different length scales (nm to mm) using functionalized tips with different chemical groups to determine the extent of surface hydrophobic and hydrophilic organo-mineral interactions. FINDINGS: We observed homogeneous hydrophobic areas at length scales below < 5 µm. The origin of this hydrophobicity is linked to the presence of organics, whose amount and spatial distribution depend on the rock composition. Specifically, our results reveal that the biogenic vs non-biogenic origin of the mineral grains is the main rock property controlling the wettability of the solid surface. Overall, our methodology offers a multi-scale approach to unravel the role that organic moieties and biominerals play in controlling the wettability of rock-water interfaces.

Sedimentary structure discrimination with hyperspectral imaging in sediment cores.

Hyperspectral imaging (HSI) is a non-destructive, high-resolution imaging technique that is currently under significant development for analyzing geological areas with remote devices or natural samples in a laboratory. In both cases, the hyperspectral image provides several sedimentary structures that must be separated to temporally and spatially describe the sample. Sediment sequences are composed of successive deposits (strata, homogenite, flood) that are visible depending on sample properties. The classical methods to identify them are time-consuming, have a low spatial resolution (millimeters) and are generally based on naked-eye counting. In this study, we compare several supervised classification algorithms to discriminate sedimentological structures in lake sediments. Instantaneous events in lake sediments are generally linked to extreme geodynamical events (e.g., floods, earthquakes), so their identification and counting are essential to understand long-term fluctuations and improve hazard assessments. Identification and counting are done by reconstructing a chronicle of event layer occurrence, including estimation of deposit thicknesses. Here, we applied two hyperspectral imaging sensors (Visible Near-Infrared, VNIR, 60 µm, 400-1000 nm; Short Wave Infrared, SWIR, 200 µm, 1000-2500 nm) on three sediment cores from different lake systems. We highlight that the SWIR sensor is the optimal one for creating robust classification models with discriminant analyses (prediction accuracies of 0.87-0.98). Indeed, the VNIR sensor is impacted by the surface reliefs and structures that are not in the learning set, which causes mis-classification. These observations are also valid for the combined sensor (VNIR-SWIR) and the RGB images. Several spatial and spectral pre-processing were also compared and enabled one to highlight discriminant information specific to a sample and a sensor. These works show that the combined use of hyperspectral imaging and machine learning improves the characterization of sedimentary structures compared to conventional methods.

Microplastic pollution of worldwide lakes.

Studies on microplastic (MP) pollution in lakes are recent, although the problem of MP particles in the oceans was first discovered in the 1970s. The first study on lakes was published in 2011. Since then, to our knowledge, 98 lakes have been investigated worldwide. In recent years, studies on this topic have increased worldwide, particularly those focusing on urbanised lakes. Most of the plastic waste in the seas and oceans originates from the terrestrial environment and inland waters. Moreover, lakes are potential temporary or long-term MP accumulators, according to the residence time of water. They are also of high interest for biodiversity, ecology, and the economy. Lacustrine ecosystems may suffer the same fate as marine ecosystems, or even worse, owing to their greater exposure. With the significant focus on ocean and sea contamination, contamination of freshwater ecosystems and lakes is a new and rising topic. However, as a new field of research, several methodological issues have been raised. The team diversity worldwide has led to contrasting sampling techniques and materials, sample treatments, analyses, and presentation of results. Consequently, it is necessary to determine several consensuses between scientific teams in order to work together with accuracy, produce comparable results, speed up knowledge sharing and reduce the reproducibility crisis. This review focuses on (1) MP contamination in 98 worldwide lakes. We identify (2) the theoretical sources of MPs and provide (3) an estimate of MP pollution in different compartments of the lakes based on current state-of-the-art methods. In addition, we also report (4) the predominant MP size classes and polymer types. Finally, we suggest (5) several recommendations to build a consensus between all the working teams to facilitate decision-making by public authorities.

High-resolution prediction of organic matter concentration with hyperspectral imaging on a sediment core.

In the case of environmental samples, the use of a chemometrics-based prediction model is highly challenging because of the difficulty in experimentally creating a well-ranged reference sample set. In this study, we present a methodology using short wave infrared hyperspectral imaging to create a partial least squares regression model on a cored sediment sample. It was applied to a sediment core of the well-known Lake Bourget (Western Alps, France) to develop and validate a model for downcore high resolution LOI550 measurements used as a proxy of the organic matter. In lake and marine sediment, the organic matter content is widely used, for example, to reconstruct carbon flux variations through time. Organic matter analysis through routine analysis methods is time- and material-consuming, as well as not spatially resolved. A new instrument based on hyperspectral imaging allows high spatial and spectral resolutions to be acquired all along a sediment core. In this study, we obtain a model characterized by a 0.95 r prediction, with 0.77 wt% of model uncertainty based on 27 relevant wavelengths. The concentration map shows the variation inside each laminae and flood deposit. LOI550 reference values obtained with the loss on ignition are highly correlated to the inc/coh ratio used as a proxy of the organic matter in X-ray fluorescence with a correlation coefficient of 0.81. This ratio is also correlated with the averaged subsampled hyperspectral prediction with a r of 0.65.

Investigation of organic matter entrapped in synthetic carbonates--a multimethod approach.

Organic matter (OM) entrapped in calcite is regularly used for environmental studies; however, insertion mechanisms and types of interaction remain poorly understood. The present study used a new methodology to investigate interactions between OM and the calcite matrix during crystallization processes with humic acid (HA) entrapment. A multimethod approach confirmed that HA is both adsorbed onto the calcite surface and incorporated into the calcite lattice during crystallization. Our results also confirm the log-linear correlation between fluorescence intensity and calcite matrix HA concentration. Fourier transform infrared spectroscopy showed that HA in colloidal conformation is adsorbed onto the calcite surface as a result of the structure of the OH stretching band. We also developed a new method based on synchrotron analysis that uses sulfur as a tracer element for entrapped HA and that localizes the OM electrostatically adsorbed onto the calcite surface. Changes in the sulfur environment, determined using X-ray absorption near-edge structure spectroscopy, indicated more complex insertion mechanisms than simple adsorption of HA during calcite crystallization. Desorption experiments revealed the stability of the OM atomic structure and its layered nature. These results allowed us to draw up a general model of OM insertion in calcite.