Bogdan Muresan

PAH dynamics in roadside environments: Influence on the consistency of diagnostic ratio values and ecosystem contamination assessments.

Roadside contamination (of air, soils and organisms) by polycyclic aromatic hydrocarbons (PAHs) was examined in an arable field and a mature forest (central France). Measured contents accounted for minute fractions of the cumulative vehicular exhaust emissions. The fate of vehicular PAHs was affected by many factors, including: atmospheric load dispersal, deposition on soils and vegetation, incorporation into water and organic matter cycles, and accumulation in species. Given these empirical results, we evaluated the consistency of a set of well-known diagnostic ratios. This effort has revealed that: i) most diagnostic ratio values vary considerably across roadside samples, including exhaust emissions; and ii) the first few meters from the carriageway or the road verge/forest interface or remote areas where surface water accumulates actually define turning or inflection points in the ratio profiles. These variations constitute a major obstacle to delimitating the extent of roadside contamination due to PAHs, in addition to raising questions over the applicability of ratios routinely used to designate sources. New ratios, namely (Flt+BkF)/(Pyr+BbF) and (Flt+BkF+BghiP)/Σ10PAH, have been specifically developed to address this challenge. The higher consistencies exhibited among environmental compartments as well as between surface soil and exhaust emissions still yield differentiated values relative to several industrial sources.

A multiscale study of mercury transformations and dynamics at the chemocline of the Petit-Saut tropical reservoir (French Guiana)

This study investigated, on both metric and centimetric scales, mercury (Hg) transformations and dynamics within a water column chemocline of a tropical reservoir. Data collected included conventional measurement of Hg in water samples, diffusive gradients in thin-films (DGT) assessments, and thermodynamic speciation modeling in order to portray the biogeochemical processes that control elemental Hg (EM) and dissolved monomethylated Hg (MeHgD) production. The primary contribution of this study is demonstration that the DGT technique can be successfully implemented to examine labile Hg compound mobilization, and estimation of how local substratum facilitates Hg reduction and methylation reactions. DGT profiles with a resolution of 1cm revealed a fine sequence of prominent Hg reduction/oxidation reactions at the chemocline level. This is interpreted as a manifestation of both: i) kinetic effects capable of arising inside the diffusive layer of DGT devices, and ii) extremely localized production or consumption of reducible and methylable Hg. Another key result obtained at the metric scale is that EM and MeHgD production at a water column chemocline are intricately linked, as both are fueled by nutrients episodically released during the decomposition of falling epilimnetic organic particles or inhibited by dissolved organic matter and inorganic compounds continuously transported from the deeper monimolimnion. Finally, it is worth noting that the chemocline acts as an accumulation and recycling domain for falling MeHg-loaded organic particles, whereas the high primary productivity layer in the epilimnion represents the principal reactor with respect to Hg methylation and reduction.