Regional-scale airborne electromagnetic surveying of the Yucatan karst aquifer (Mexico): geological and hydrogeological interpretation

Geometry and connectivity of high-permeability zones determine groundwater flow in karst aquifers. Efficient management of karst aquifers requires regional mapping of preferential flow paths. Remote-sensing technology provides tools to efficiently map the subsurface at such scales. Multi-spectral remote sensing imagery, shuttle radar topography data and frequency-domain airborne electromagnetic (AEM) survey data were used to map karst-aquifer structure on the Yucatan Peninsula, Mexico. Anomalous AEM responses correlated with topographic features and anomalous spectral reflectance of the terrain. One known preferential flow path, the Holbox fracture zone, showed lower bulk electrical resistivity than its surroundings in the AEM surveys. Anomalous structures delineated inland were sealed above by a low-resistivity layer (resistivity: 1–5 Ωm, thickness: 5–6?m). This layer was interpreted as ejecta from the Chicxulub impact (Cretaceous/Paleogene boundary), based on similar resistivity signatures found in borehole logs. Due to limited sensitivity of the AEM survey, the subsurface configuration beneath the low-resistivity layer could not be unambiguously determined. AEM measurements combined with remote-sensing data analysis provide a potentially powerful multi-scale methodology for structural mapping in karst aquifers on the Yucatan Peninsula and beyond.     

Environmental assessment of Dhaka City (Bangladesh) based on trace metal contents in road dusts

Geochemical analysis of street dusts was conducted to evaluate the environment of Dhaka City, Bangladesh. Dust samples were collected from different areas (industrial, commercial, and residential) of Dhaka City, and their major, trace and rare earth elements (REE) were determined. Samples from the commercial area had Pb concentrations double those of the industrial and residential areas. Contents of Zn, Cu, Ni, and Cr in the industrial areas were greater than those in the commercial and residential areas. The REE patterns of all dusts in Dhaka are similar and are comparable to the average upper continental crust. The condition of the Dhaka environment was compared to that in Japan and other baseline sediments using Zn–Fe₂O₃ and Pb–Fe₂O₃ diagrams. Zn–Fe₂O₃ trends for the dusts show steep inclination compared to the baseline sediment and the Japanese urban sediment trends. Dhaka lake data show enrichment of Zn over the dusts, suggestive of Zn pollution from poorly controlled industrial sources. In contrast, on the Pb–Fe₂O₃ diagram, Dhaka dusts have greater Pb contents than lake sediments, probably due to the higher traffic density in the commercial area compared to the residential area including the area around the lake. The results suggest that higher levels of Pb and Zn in street dusts in Dhaka can most likely be attributed to the anthropogenic sources like vehicle emissions to the atmosphere and a rapid development.     

Assessment of metal concentrations in lake sediments of southwest Japan based on sediment quality guidelines

Geochemical analysis of the bottom sediments of Lakes Banryoko, Onbe, Hamahara and Kijima in Shimane prefecture of southwest Japan was carried out to determine their metal concentrations, and to assess the potential for ecological harm by comparison with sediment quality guidelines. The work conducted includes water quality measurement, and analyses of trace and major elements and rare earth element (REE) of sediments. Results showed that water quality of the lakes contrasts slightly between their upper and lower parts. Average abundances of As, Pb, Zn, Cu, Ni, and Cr in Banryoko sediments were 27, 33, 90, 27, 25, and 46 ppm, respectively, compared to 31, 52, 175, 44, 44, and 75 ppm at Onbe, 11, 26, 96, 13, 13, and 35 ppm at Hamahara, and 24, 43, 193, 31, 12, and 30 ppm at Kijima. These concentrations exceeded the lowest effect level that has moderate impact on aquatic organisms as proposed by the New York State Department of Environmental conservation. Pb and Cu abundances are comparable to the Coastal Ocean Sediment Database threshold, while As and Zn exceed this value, indicating the concentrations of these metals are potentially toxic. Increases in the abundances of these metals in lake sediments are probably related to the reducing condition of the sediments, anthropogenic sources and surface soil erosion. The REE patterns of sediments in the study areas are broadly comparable to the average upper continental crust, but show some contrast between lakes due to differing source litho type. Significant positive correlations between Fe₂O₃ and As, Pb, Zn, and Cu were found in the sediments, suggesting these metals may be adsorbed on Fe oxides in the lake sediments.     

Preparation of magnetic microgels for catalytic reduction of 4-nitrophenol and removal of methylene blue from aqueous medium

This work describes the synthesis of poly(acrylic acid) microgels and fabrication of magnetic cobalt nanoparticles in the prepared microgels. Cobalt nanoparticles were fabricated by loading the cobalt (II) ions in microgels from aqueous solution and their subsequent reduction with sodium borohydride (NaBH₄). Bare and composite microgels were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy and transmission electron microscopy. The catalytic properties of the prepared microgel composites were investigated by using them as catalyst for the reduction of 4-nitrophenol and methylene blue. The effect of temperature and catalyst dose on the rate of reduction of these toxic pollutants was investigated. The reusability of prepared catalysts was also studied for the five consecutive cycles, and an increase in catalytic activity was observed after every cycle. The prepared bare and magnetic microgels were found as very effective adsorbent for the removal of methylene blue from aqueous medium. Very rapid adsorption rate was found for the removal of methylene as its 100 mg was adsorbed on per gram of dried hydrogels in about 25 min. The effects of different parameters like amount of adsorbate and concentration of adsorbent on the adsorption process were studied. Langmuir, Freundlich and Temkin adsorption isotherms were applied, and it was found that adsorption of MB follows Freundlich model better than others. Furthermore, pseudo-first-order and pseudo-second-order kinetic models were also applied and adsorption of MB was found to abide by pseudo-second-order kinetics.     

AGN effect on cooling flow dynamics

We analyzed the feedback of AGN jets on cooling flow clusters using three-dimensional AMR hydrodynamic simulations. We studied the interaction of the jet with the intracluster medium and creation of low X-ray emission cavities (Bubbles) in cluster plasma. The distribution of energy input by the jet into the system was quantified in its different forms, i.e. internal, kinetic and potential. We find that the energy associated with the bubbles, (pV+γ pV/(γ−1)), accounts for less than 10 percent of the jet energy.     

Comparison of ordinary kriging and artificial neural network for spatial mapping of arsenic contamination of groundwater

In this technical note, we investigate the hypothesis that ‘non-linearity matters in the spatial mapping of complex patterns of groundwater arsenic contamination’. The spatial mapping pertained to data-driven techniques of spatial interpolation based on sampling data at finite locations. Using the well known example of extensive groundwater contamination by arsenic in Bangladesh, we find that the use of a highly non-linear pattern learning technique in the form of an artificial neural network (ANN) can yield more accurate results under the same set of constraints when compared to the ordinary kriging method. One ANN and a variogram model were used to represent the spatial structure of arsenic contamination for the whole country. The probability for successful detection of a well as safe or unsafe was found to be atleast 15% larger than that by kriging under the country-wide scenario. The probability of false hopes, which is a serious issue in public health monitoring was found to be significantly lower (by more than 10%) than that by kriging.     

Akaganéite (β-FeOOH) precipitation in inland acid sulfate soils of south-western New South Wales (NSW), Australia

The prevalence of sulphidic sediments in inland wetlands has been only recently recognized in many parts of the world, including Australia. The exposure of sulphidic sediments in these wetlands due to natural and human induced drying events has resulted in the oxidation of iron sulfide minerals, the formation of secondary iron minerals characteristic of acid sulfate soils and the release of highly acidic solutions. The objective of this study was to determine the mineralogy and morphology of sediments collected from the oxidized surface horizon (0-5 cm) of an inland acid sulfate soil located in south-western New South Wales (NSW), Australia. Random powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning transmission electron microscopy combined with energy dispersive X-ray spectroscopy (STEM-EDS) techniques were used to characterize the minerals present in these sediments. Akaganéite was identified as the major mineral phase in the sediments; K-jarosite was also determined in small amounts in some sediments. The XRD patterns of sequentially washed (E-pure® water-0.01 M HCl-0.01 M EDTA) sediment samples showed all akaganéite peaks; the Rietveld refinement of these patterns also revealed a predominance of akaganéite. The chemical analyses of the original and washed sediments using STEM-EDS clearly showed the presence of akaganéite as a pure mineral phase with an average Fe/Cl mole ratio of 6.7 and a structural formula of Fe₈O₈(OH)_6.8(Cl)_1.2. These findings show that the extreme saline-acidic solutions (pH ∼ 2, EC = 216 dS/m) at the Bottle Bend lagoon provide ideal conditions for the crystallization of this rarely forming mineral.