Seasonal geochemical trends and pollution assessment of bottom sediments in the São Francisco hydrographic basin, Brazil: the Três Marias Reservoir.

The Três Marias Reservoir is the ninth largest reservoir in Brazil, becoming crucial for national strategic development. However, many anthropic activities may affect the sediment quality, promoting the need for a proper environmental assessment. This research appraised the seasonal influences on the Três Marias Reservoir's sediment geochemistry, elucidating possible anthropogenic impacts. The concentrations of Mg, Al, Ca, Cr, Fe, Co, Cu, Cd, Ti, Mn, Ni, Zn, Ba, and Pb were measured in 78 samples of bottom sediments regarding the two seasons of the area, a dry winter and rainy summer. The median ± 2 median absolute deviation (MAD) settled the geochemical background and environmental thresholds for the two seasons. The sediment quality guidelines CONAMA 344/12 highlight the possible adverse ecological effects of pollutants. The hierarchical clustering analysis, the geoaccumulation index, and the pollution load index delineated the polluted zones. The pollution load index ranges from 0.25 to 2.28 in the dry season and 0.56 to 2.11 in the rainy season, defining three affected zones in the reservoir. Forestry and agriculture are the probable pollution sources, reaching warning levels that should be considered in further environmental strategies.

Geochemical signature and environmental background of bottom sediments in a tropical aquatic system: the Três Marias Reservoir, Brazil.

Understanding geochemical provenances and their respective geochemical background values is crucial to sustain a proper comprehension of a tropical ecosystem and provides a compatible evaluation for further environmental policy and monitoring. In this research, the concentrations of Mg, Al, Ca, Cr, Fe, Co, Cu, Cd, Ti, Mn, Ni, Zn, Ba, Pb, and organic matter were measured in 78 samples of bottom sediments in order to evaluate the influence of geogenic and anthropogenic chemical sources in the Três Marias reservoir. Data exploration analysis, multivariate techniques using compositional data, multielemental mapping, geochemical background based on land-use management and geological domains, and enrichment factors were applied to understand the environmental dynamics. Geochemical signatures highlight three pronounced geological structures that control the sediment supply in the Três Marias reservoir: ultramafic rocks from the Mata da Corda Formation, carbonates from the Três Marias Formation, and pedogenic cover formed by tropical Latossoils. The median values of Cu, Ti, and Pb are 28.83 mg/kg, 20.36 mg/kg, and 24.92 mg/kg, respectively, all of which exceed the average upper continental crust concentrations. Pb and Cu ratios reach the threshold effect level, whereas Cr reaches the probable effect level proposed in the CONAMA 454/2012 (Brazilian environmental guidelines). This research conjugates multielemental mapping, background evaluation, and enrichment factors to elucidate the geochemistry dynamics of the Três Marias reservoir in the sense of understanding the behavior of the bottom sediments and to define geochemical reference values for the upper São Francisco basin.

Site partitioning of Cr3+ in the trichroic alexandrite BeAl2O4:Cr3+ crystal: contribution from x-ray absorption spectroscopy.

X-ray absorption spectroscopy measurements at the Cr K-edge of a trichroic crystal of alexandrite BeAl(2)O(4):Cr(3+) for different orientations of the crystal with respect to the polarization and direction of the x-ray incident beam have been performed. Analysis of the experimental spectra with the help of first-principles calculations of x-ray absorption spectra allowed us to estimate the proportion of chromium Cr(3+) cations among the two different octahedral sites of the alexandrite structure (70% in the C(s) site-30% in the C(i) site). The methodology presented in this work opens up new possibilities in the field of mineralogy for the study of complex minerals containing several sites potentially occupied by several transition elements or for solid solutions.