300 years of organic pollution recorded in an urban speleothem (Paris, France)

The preservation of water resources and the limitation of pollution are an environmental central issue in the current intense anthropization context. Considered as sensitive recorders of past changes, speleothems offer an under investigated natural archive for the reconstruction of water quality. Urban speleothems have recently been used to show the impact of urbanization over the water quality using inorganic trace elements. Speleothems thus represent a promising archive of water quality on short and long-time scales. However, they have never been used to trace organic pollution. Within the organic and anthropogenic proxies, polycyclic aromatic hydrocarbons (PAHs) are commonly used in water quality analysis. These persistent organic pollutants (POPs) are mainly due to anthropogenic emissions. The use of speleothem to trace the variations in quantity and quality of organic matter, including organic pollutant as PAHs, over the last centuries, is unprecedented.

For this purpose, high resolution (10 µm) solid phase UV fluorescence imaging analyses were crossed with chemical analyses (PAHs, Non Purgeable Organic Carbon (NPOC)) carried out on low weight samples (a few mg to g) from a Parisian aqueduct flowstone. Solid-phase fluorescence imaging, although poorly applied yet to speleothems, is a non-destructive technique. To obtain quantitative information, solid phase spectroscopy is coupled with liquid phase compound analysis and NPOC analysis. Due to their low concentration, the analysis of PAHs required a long development phase. The protocol consists of an extraction and analysis process using high performance liquid chromatography coupled with a fluorescence detector. The first results reveal the presence of PAHs for 300 years in runoff water with an increase, in particular in heavy molecular weight PAHs, over the last two decades. These data will be crossed with modelled imaging of quantitative variations in organic matter. This work opens the way to a better long term understanding of the impact of anthropization on transfer of pollutants in subsurface waters.