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In freshwater ecosystems, dynamic hydraulic events (floods or dam maintenance) lead to sediment resuspension and mixing with waters of different composition. Microbial communities living in the sediments play a major role in these leaching events, contributing to organic matter degradation and the release of trace elements. However, the dynamics of community diversity are seldom studied in the context of ecological studies. Therefore, we carried out laboratory-induced leaching experiments, using sediments from the Villerest dam reservoir (Villerest, France). To assess whole microbial community diversity, we sequenced the archaeal and bacterial 16S rRNA genes using Illumina MiSeq. Our results suggest that the degree of dissolved oxygen found in the water during these resuspension episodes influenced community dynamics, with anoxic waters leading to drastic shifts in sedimentary communities compared to oxic waters. Furthermore, the release of microbial cells from sediments to the water column were more favorable to water colonization when events were caused by oxic waters. Most of the bacteria found in the sediments were chemoorganotrophs and most of the archaea were methanogens. Methylotrophic, as well as archaeal, and bacterial chemoorganotrophs were detected in the leachate samples. These results also show that organic matter degradation occurred, likely participating in carbonate dissolution and the release of trace elements during freshwater resuspension events.

Spatio-temporal distribution and leachability of some trace elements (TE) were investigated in sediments of the Xiangjiang River, tributary of the Yangtze River. Based on data collected during 2015–2017, a literature review and geoaccumulation indexes, the pollution level was the highest for Cd, Sb and Hg (Igeo > 3). Over the period reviewed, the TE contamination level displayed almost no temporal variation but an obvious spatial distribution. The most upstream contamination hotspot (Cd > Cr > As, Cu, Pb, Zn > Hg, Sb) was the Songbai section. This hotspot did not spread further downstream. The second hotspot identified was the Zhuzhou–Xiangtan section, impacted by Cd > Hg, Pb, Zn > Cu, with the Zhuzhou area being particularly highly impacted by Pb and Zn. A 30-day leaching experimental protocol under aerobic and anaerobic conditions was carried out to access TE mobility. Low percentages of TE released were calculated, showing that the TE fate mostly depends on the stability of bearing phases under specific physicochemical and microbial conditions. In this case, the studied sediments can be an important sink for these TE. However, some environmental issues have to be considered as some leachate concentrations of contaminants (As, Cr, Cu and U) released into water exceed freshwater aquatic life criteria. [ABSTRACT FROM AUTHOR]

Abstract: Potential contamination of rivers by trace elements can be controlled, among others, by the precipitation of oxyhydroxides. The streambed of the studied area, located in “La Châtaigneraie” district (Lot River Basin, France), is characterised by iron-rich ochreous deposits, acidic pH (2.7–4.8) and SO4–Mg waters. Beyond the acid mine drainage, the presence of As both in the dissolved fraction and in the deposits is also a problem. Upstream, at the gallery outlet, As concentrations are high (Asmax=2.6 μmol/l and up to 5 wt% locally, respectively, in the dissolved and in the solid fractions). Downstream, As concentrations decrease below 0.1 μmol/l in the dissolved fraction and to 1327 mg/kg in the solid fraction. This natural attenuation is related to the As retention within ochreous precipitates (amorphous to poorly crystalline Fe oxyhydroxides, schwertmannite and goethite), which have great affinities for this metalloid. Upstream, schwertmannite is dominant while downstream, goethite becomes the main mineral. The transformation of schwertmannite into goethite is observed in the upstream deposits as schwertmannite is unstable relative to goethite. Furthermore, thermodynamic calculations indicate that the downstream goethite is not able to precipitate in situ according to the water chemistry. Goethite mainly results from the transformation of schwertmannite and its solid transport downstream. Moreover, as highlighted by leaching experiments carried out on the ochreous precipitates, this transformation does not seem to affect the As-retention in solids as no release of As was observed in the solution. Arsenic may either be strongly trapped by co-precipitation in the present minerals or it may be quickly released and re-adsorbed on the precipitate surface. [Copyright &y& Elsevier]

An intensive surface sediment survey was carried out over 24 locations from the upstream to downstream sections of two large rivers (Adyar and Cooum) in Chennai (India) during the February dry season of 2015. Trace element concentrations were assessed on a <63 µm fraction using the Geoaccumulation Index (Igeo) and the newly proposed Geochemical Urban Footprint Index (GUFI), which can be performed to determine the pollution status of any megacity river influenced by urban development. The sediment quality of Chennai's rivers was also compared to worldwide megacity pollution using sediment quality guidelines (SQGs), and a new megacity pollution ranking was determined. The Igeo results indicate that the Chennai rivers studied are strongly to extremely polluted regarding trace element content of sediment. Silver (Ag), Cadmium (Cd) and Mercury (Hg) are the most significant tracers of urban contamination. Chromium (Cr) concentrations show an industrial contamination gradient in relation to levels of other trace elements (As, Cu, Ni, Pb, and Zn) at the Chennai megacity scale. The GUFI ranges from moderate to extreme contamination, particularly in the downstream stretches of the two rivers. This spatial trend is related to various point sources and identified at specific sampling stations, with a lack of identifiable buffer zones. According to the worldwide comparison of megacity pollution, Chennai is ranked in fifth position. The present position can be attributed to a number of explanations: a population explosion associated with the unplanned growth of the city and non-controlled point sources of pollution in Chennai's waterways. [ABSTRACT FROM AUTHOR]