Pierre Pichet

The Partitioning of Mercury in the Solid Components of Dry and Flooded Forest Soils and Sediments from a Hydroelectric Reservoir, Quebec (Canada)

Upon inundation, the soils in a hydroelectric reservoir are subjected to several years of physical, biological, and chemical changes as the transition from a terrestrial to an aquatic ecosystem is achieved. It is suspected that changes in soil Eh and pH alter the metal binding capacity of organic matter, reactive iron (Fe) oxides/oxyhydroxides, and clay minerals, and may cause the mercury associated with these phases to be remobilized. Four cores were collected along a transect from an unflooded forest soil to a pre-impoundment lake bottom sediment. They were subjected to a customized sequential extraction procedure to determine the distribution of Hg between three operationally-defined solid compartments: organic carbon, reactive Fe oxides/hydroxides, and the solid (clay and sulfide) residue. Results indicate that up to 80% of the Hg in the O-horizon of forest soils and flooded soils and up to 85% of the Hg in lake sediments is bound to the NaOH-extractable organic carbon fraction. Furthermore, it was observed that the highest Hg concentrations are associated with degraded organic matter. In the B-horizon of a podzol, 40–60% of the total Hg was found associated with reactive Fe minerals. In contrast, the flooded podzol contains almost no reactive Fe at any depth and associated Hg concentrations are low. We propose that upon inundation, Fe oxides are reduced and Hg released to the pore waters where it is rapidly bound to other available substrates. Analyses of the extractions residues suggest that there is an enrichment of Hg in this fraction immediately above the B-horizon in a flooded soil.

Presence of organophosphate insecticides in fish of the Amazon River

Trace levels of three organophosphate insecticides (OPI) were detected in eight fish species from the region of Santarem, State of Para, Brazil. Individual concentrations of OPI in fish ranged from less than the detection limit to 2,1 ppb. Mean concentrations of chlorpyrifos, malathion, and methyl-parathion were 0,3 ± 0,3, 0,1 ± 0,1, and 0,3 ± 0,3 ppb, respectively. Pellona flavipinnis, the largest and fattest piscivorous species analyzed, was the most contaminated. Since an inhabitant of this Amazonian region consumes 220 g of fish per day on average, ingested doses of chlorpyrifos, malathion, and methyl-parathion may reach up to 308, 220, and 462 ng·d-1, respectively. Compared to acceptable daily intakes (ADI), quantities of OPI absorbed via fish consumption on a daily basis are far below deleterious levels. We estimated that even considering the highest OPI contents detected, the average daily fish consumption of anadult of 60 kg would have to increase by ca. 1 950, 5 450, and 2 600 times to reach ADI of chlorpyrifos, malathion, and methyl-parathion, respectively. Neither fish diet nor fish lipid content enabled us to completely explain the interspecific differences observed.

A petrological and geochemical study of the calcareous sandstone of West African marine Pleistocene (Cap des Biches, Senegal)

Abstract In Senegal, the marine Pleistocene is represented by several outcrops of calcareous sandstone slabs which underly the recent sandy shoreline south of Dakar. These outcrops show dissolution and recrystallization phenomena increasingly evident as one moves south. They disappear at Toubab Dialao, i.e. approximately 40 km from Dakar. The outcrop at Cap des Biches has been studied mainly because of the exceptional preservation of some Ostrea calcitic tests which allowed isotopic U Th measurements. This outcrop of metric thickness was subjected to petrographic and mineralogical examination and to several geochemical analyses (trace elements, stable isotopes, amino acids) which lead to the conclusion that the base of the deposit has been preserved. Two U Th apparent ages obtained (178 ka and > 240 ka) are older than the only age of the isotopic stage 5c obtained in the area. A post-depositional mobility of U cannot be rejected, and ages may not be reliable. However, a minimal age, considerably older than finite 14C ages previously published, may be proposed (Eemian Interglacial, i.e. isotopic stage 5). In these environments, geochemical analyses show high Pleistocene sea levels of either warmer and/or more diluted waters than the present levels, i.e. a probable Northern extension of Guinean waters.