The freshwater lens of Benjamín Aceval,Chaco, Paraguay: a terrestrial analogue of an oceanic island lens

The occurrence of a freshwater lens in the Paraguayan Chaco, 900 km away from the ocean, is reported. It is located underneath sandstone hills, surrounded by lowlands with predominantly saline groundwater. Its geometry was delineated using geoelectrical and electromagnetic investigations. The unusual height of the fresh groundwater level can be attributed to the presence of a confining layer at depth. The lens receives its recharge exclusively from rainfall during the hot and humid summer months. It predominantly contains water predating the atmospheric atomic bomb tests, some of it probably up to a thousand or more years old. The water balance shows that extraction currently does not exceed recharge in normal years. However, the available volume of groundwater leaves little room for a further increase of extraction in the future. Recharge is augmented by return flow from thousands of latrines and cess pits, and this has lead to widespread contamination of the groundwater by faecal bacteria.     

Understanding Perturbation on the Simplex: A Simple Method to Better Visualize and Interpret Compositional Data in Ternary Diagrams

Perturbation is an operation defined on the simplex and can be used for centering compositional data in a ternary diagram, applying objective criteria. Because a straight line in the original diagram is still astraight line in the perturbed diagram, gridlines or compositional fields defined by straight lines can easily be included in the operation. Simultaneous perturbation of data, gridlines, and/or compositional fields is shown to improve both visualization and graphical interpretation of compositions in ternary diagrams. This is illustrated by some examples using simulated as well as real data.     

Efficacy of Coagulants and Ballast Compounds in Removal of Cyanobacteria (<Emphasis Type="Italic">Microcystis</Emphasis>) from Water of the Tropical Lagoon Jacarepaguá (Rio de Janeiro,Brazil)

Eutrophication is considered the most important water quality problem in freshwaters and coastal waters worldwide promoting frequent occurrence of blooms of potentially toxic cyanobacteria. Removal of cyanobacteria from the water column using a combination of coagulant and ballast is a promising technique for mitigation and an alternative to the use of algaecides. In laboratory, we tested experimentally the efficiency of two coagulants, polyaluminium chloride (PAC) and chitosan (made of shrimp shells), alone and combined with two ballasts: red soil (RS) and the own lagoon sediment, to remove natural populations of cyanobacteria, from an urban brackish coastal lagoon. PAC was a very effective coagulant when applied at low doses (≤8 mg Al L^?1) and settled the cyanobacteria, while at high doses (≥16 mg Al L^?1) large flocks aggregated in the top of test tubes. In contrast, chitosan was not able to form flocks, even in high doses (>16 mg L^?1) and did not efficiently settle down cyanobacteria when combined with ballast. The RS itself removed 33–47 % of the cyanobacteria. This removal was strongly enhanced when combined with PAC in a dose-dependent matter; 8 mg Al L^?1 was considered the best dose to be applied. The lagoon sediment alone did not promote any settling of cyanobacteria but removal was high when combined with PAC. Combined coagulant and ballast seems a very efficient, cheap, fast and safe curative measure to lessen the harmful cyanobacteria bloom nuisance in periods when particularly needed, such as around the 2016 Olympics in Jacarepaguá Lagoon.     

Zinc-rich and copper-bearing amphiboles from sulfide-free ore occurrences of the Pelagonian Massif,Republic of North Macedonia

Mineralogy and Petrology - This paper describes specific features of isomorphism of unusual amphiboles containing up to 23&nbsp;wt% ZnO and up to 1.3&nbsp;wt% CuO from sulfide-free...     

Kriging Regionalized Positive Variables Revisited: Sample Space and Scale Considerations

Frequently, regionalized positive variables are treated by preliminarily applying a logarithm, and kriging estimates are back-transformed using classical formulae for the expectation of a lognormal random variable. This practice has several problems (lack of robustness, non-optimal confidence intervals, etc.), particularly when estimating block averages. Therefore, many practitioners take exponentials of the kriging estimates, although the final estimations are deemed as non-optimal. Another approach arises when the nature of the sample space and the scale of the data are considered. Since these concepts can be suitably captured by an Euclidean space structure, we may define an optimal kriging estimator for positive variables, with all properties analogous to those of linear geostatistical techniques, even for the estimation of block averages. In this particular case, no assumption on preservation of lognormality is needed. From a practical point of view, the proposed method coincides with the median estimator and offers theoretical ground to this extended practice. Thus, existing software and routines remain fully applicable.     

Influence of a permeable sand layer on the mechanism of backward erosion piping using 3D pipe depth measurements

Backward erosion piping (BEP) poses a threat to the stability of water-retaining structures. This can lead to severe erosion and collapse of embankments. A novel economically appealing measure against BEP is the coarse sand barrier (CSB). The CSB is a trench filled with coarse sand that is placed below the blanket layer on the landward side of the embankment, which prevents the pipe from developing upstream when it encounters the CSB. Inclusion of a CSB creates a vertically layered sand, which is the situation that can also exist in practice but is different from traditional BEP tests with one homogeneous sand. This paper presents new observations and measurements in medium-scale laboratory tests. 3D measurements of the pipe depth and dimensions are presented and analysed. This analysis indicates how the pipe dimensions evolve during the piping process and shows the erosion mechanism for BEP in vertically layered sands. The findings demonstrate the significance of three-dimensional study of the pipe rather than two dimensions. The pipe depth, width and depth-to-width ratios at the pipe tip in critical erosion stages are measured and presented. In the presented tests, two different erosion behaviours (stepwise pipe progression until failure and straight failure) are found and analysed with respect to possible influential parameters. Higher head drops and flow rates are found in tests with straight failure at the stage before progression. A linear relationship between the hydraulic conductivity contrast (k_c) and the critical head drops (h_c) is found and observations are used to investigate deviations from the line.

     

Influence of the Madden Julian Oscillation on precipitation and surface air temperature in South America

The regional influence of the Madden–Julian oscillation (MJO) on South America is described. Maps of probability of weekly-averaged rainfall exceeding the upper tercile were computed for all seasons and related statistically with the phase of the MJO as characterized by the Wheeler–Hendon real-time multivariate MJO (RMM) index and with the OLR MJO Index. The accompanying surface air temperature and circulation anomalies were also calculated. The influence of the MJO on regional scales along with their marked seasonal variations was documented. During December–February when the South American monsoon system is active, chances of enhanced rainfall are observed in southeastern South America (SESA) region mainly during RMM phases 3 and 4, accompanied by cold anomalies in the extratropics, while enhanced rainfall in the South Atlantic Convergence Zone (SACZ) region is observed in phases 8 and 1. The SESA (SACZ) signal is characterized by upper-level convergence (divergence) over tropical South America and a cyclonic (anticyclonic) anomaly near the southern tip of the continent. Impacts during March–May are similar, but attenuated in the extratropics. Conversely, in June–November, reduced rainfall and cold anomalies are observed near the coast of the SACZ region during phases 4 and 5, favored by upper-level convergence over tropical South America and an anticyclonic anomaly over southern South America. In September–November, enhanced rainfall and upper-level divergence are observed in the SACZ region during phases 7 and 8. These signals are generated primarily through the propagation of Rossby wave energy generated in the region of anomalous heating associated with the MJO.