1,720,961 research outputs found
Spatial distribution, geochemical processes of high-content fluoride and nitrate groundwater, and an associated probabilistic human health risk appraisal in the Republic of Djibouti
No full textIn the northern East African Rift System, the Republic of Djibouti relies exclusively on groundwater, with levels of fluoride (up to 14 mg/L) and nitrate (up to 256 mg/L) posing potential health risks. To address this, 362 samples were considered, including 133 shallow groundwater samples, along with new and previously published data dating back to 2012 on deep (88) and thermal (141) groundwater samples. To understand the enrichment mechanisms, dissolved anion and cation constituents, geochemical and thermodynamic tools, and stable isotope ratios, such as δ2H(H2O), δ18O(H2O), δ15N(NO3−), and δ18O(NO3−), were used. In particular, two activity diagrams (Mg2+ vs. Ca2+ and Na+ vs. Ca2+), focused on aqueous and solid fluoride species in an updated thermodynamic dataset of 15 fluoride-bearing minerals, are shown for the first time. The dataset offers new and valuable insights into fluoride geochemistry (classic thermodynamic datasets combined with geochemical codes rely solely on fluorapatite and fluorite F-bearing minerals). Activity diagrams and geochemical modeling indicate that mineral dissolution primarily drives groundwater fluoride enrichment in all water types, whereas the elevated nitrate levels may stem from organic fertilizers like animal manure, as indicated by nitrate isotopes and NO3−/Cl− vs Cl− diagrams. Despite the arid climate and 2Hsingle bond18O enrichment in shallow waters, evaporation seems to play a minor role. Monte Carlo simulations and sensitivity analysis were used to assess the health risks associated with elevated F− and NO3− concentrations. Mapping-related spatial distribution analysis identified regional contamination hotspots using a global Moran's I and GIS tools. One fluoride and three nitrate contamination hotspots were identified at a p-value of 0.05. Groundwater chemistry revealed that 88 % of groundwater being consumed exceeded the permissible levels for fluoride and nitrate, posing potential health risks, particularly for teenagers and children. This study pinpoints specific areas with excessive nitrate and fluoride contamination, highlighting a high non-carcinogenic risk
Origin of nitrate and sulfate sources in volcano-sedimentary aquifers of the East Africa Rift System: An example of the Ali-Sabieh groundwater (Republic of Djibouti)
Full text linkWithin the East African Rift System (EARS), the complex Ali-Sabieh aquifers system, located in the south of the Republic of Djibouti, was overexploited and subjected to anthropogenic and/or geogenic pollution with high concentrations of dissolved nitrate (up to 181 mg/l) and sulfates (up to 1540 mg/l). This study is the first undertaken on the hydrochemistry of this aquifer system, combining geochemical tools and multi-isotope - δ2H(H2O), δ18O(H2O), δ18O(SO4), δ34S(SO4), δ15N(NO3), δ18O(NO3), δ13C(DIC), and 14C- was used to decipher the origin and fate of different nitrate and sulfate sources to groundwater. The groundwater samples of the region show a chemical evolution from fresh Ca(Na)-bicarbonate to brackish Na-Cl , mainly due to water-rock interaction. The combined chloride and water isotope data show that evaporation and transpiration are present, with the latter occurring primarily in the shallow alluvial aquifer waters. Inspection of δ15N(NO3) vs. δ18O(NO3) and NO3/Cl vs. Cl diagrams show that dissolved nitrates are primarily of anthropogenic origin. In particular, higher nitrate concentrations may be related to animal manure used as organic fertilizers during agricultural activities. Sulfates are from a natural origin related to the interaction of water with gypsum of hydrothermal or sedimentary origin. SO4/Cl ratio and isotopic composition show that dissolved sulfates in saline and ancient groundwater of the Cretaceous sandstone aquifer (between 7.4 ± 2.2 and 5.8 ± 1.4 k-years before the present) are generated by interaction with gypsum from oxidation of pre-existing (Jurassic?) sulfides. This work highlight that isotopic ratios of the two molecules -δ18O(SO4), δ34S(SO4), δ15N(NO3), δ18O(NO3)- are not sufficient for tracing the origin of nitrate and sulfates in groundwater, but that a complete hydrogeochemical study is needed. In the absence of this, the relatively high concentration of chloride and sulfates could be wrongly linked to the anthropogenic source of nitrate (manure or sewage)
Atmospheric drivers of rainfall events in the Republic of Djibouti
No full textInternational audienceThe Republic of Djibouti is a small country in an arid context coupled with a high variability of rainfall that generates flash floods causing severe damage to the population and infrastructure. The mechanisms controlling extreme rainfall events in this part of the Horn of Africa remain poorly understood. In this study, we document the atmospheric circulation patterns associated with such events. To that end, we use rain‐gauge data (a network of 36 stations on the period 2013–2020), satellite‐based rainfall estimates (CHIRPS, IMERG, MSWEP and RFE) and atmospheric reanalyses (ERA5), all at the daily timescale, over their common period 2001–2020. A multivariate Agglomerative Hierarchical Clustering of rainy days in Djibouti (≥10% of grid‐points exceeding 1 mm·day −1 , according to all four satellite products) reveal four clusters, which differentiate from each other by the intensity and spatial extent of rainfall. These clusters show a nonhomogeneous seasonal distribution, occurring mainly in the March–April–May (MAM) and July–August–September (JAS) seasons, and more rarely in October–November–December (OND). The atmospheric circulation anomalies associated with the clusters are quite similar and highly season‐dependent. In MAM most clusters display an anomalous trough over the Red Sea, from 700 to 200 hPa. In JAS, an anomalous low over the southern Red Sea drives a thicker than normal monsoon flow at 700 hPa, while upper northerlies prevail at 200 hPa. In OND, most rainy events result from moisture advection from the Western Indian Ocean, favoured by positive phases of the Indian Ocean Dipole. Some highly unusual atmospheric circulation patterns (e.g., associated with tropical cyclones) can also result in intense rainfall events in Djibouti. These findings provide new insights on the physical causes of extreme events in the Horn of Africa, with applications to improved early warning and skill evaluation of climate models for climate change projections
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Geochemical and isotopic studies of the Douda-Damerjogue aquifer (Republic of Djibouti): Origin of high nitrate and fluoride, spatial distribution, associated health risk assessment and prediction of water quality using machine learning
No full text: Groundwater from the East African Rift System (EARS), for which there is limited data available, is often characterized by high levels of dissolved fluoride and nitrate, which pose inherent health risks. Within the EARS, the Douda-Damerjogue aquifer system was overexploited and subjected to anthropogenic and/or geogenic pollution with high NO3-concentrations (up to 375.4 mg L-1) and F-(up to 4.5 mg L-1). This study is the first to examine the origin and cumulative health risk assessment of groundwater with high F- and NO3- contents in rifting zones, as well as the spatial patterns and the water quality forecasting. This study use a combination of geochemical and thermodynamic tools, geospatial analysis, MixSIAR model, Machine Learning (ML) model, as well as stable isotope ratios, including δ18O(H2O), δ2H(H2O), δ15N(NO3-), and δ18O(NO3-). A ML framework was developed to forecast NO3-, Electrical Conductivity (EC), and Irrigation Water Quality Index (IWQI) in such data-scarce environments. The key geochemical processes controlling the groundwater composition in the study area were: (i) basalt weathering; (ii) ion exchange; (iii) mixing with fossil groundwater; and (v) seawater intrusion. Fluoride enrichment (> 1.5 mg L-1) in the groundwater was likely driven by the dissolution of fluoride-bearing minerals and desorption from sorbent surfaces. The combined application of the MixSIAR model, stable nitrate isotopes, and the NO3/Cl vs Cl diagram identified soil organic nitrogen, NH4-fertilizers, sewage and manure as the primary anthropogenic sources of NO3- in the groundwater. Groundwater chemistry showed that 76 % of samples exceeded the permissible limits for fluoride and nitrate, posing potential health risks, especially for teenagers and infants. The proposed ML-based framework provides a robust, scalable solution for forecasting water quality in Djibouti and other regions facing similar challenges
- …
