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Effective, post-accidental management needs an accurate understanding of the biogeochemical behavior of radionuclides in surface environments at a regional scale. Studies on stable isotopes (element homologs) can improve this knowledge. This work focuses on the biogeochemical behavior of stable cesium (Cs) along a major European fluvial-estuarine system, the Gironde Estuary (SW France). We present results obtained from (i) a long-term monitoring (2014–2017) of dissolved (Cs d) and particulate (Cs p) Cs concentrations at five sites along the freshwater continuum of the Garonne watershed, (ii) Cs d and Cs p concentrations during four oceanographic campaigns at contrasting hydrological conditions along longitudinal profiles of the estuarine system, (iii) a 24 h cycle of Cs p at the estuary mouth, and (iv) a historical trend of Cs bioconcentration in wild oysters at the estuary mouth (RNO/ROCCH, 1984–2017). In addition, we model the partitioning of Cs within the estuarine environment for clay mineral interactions via PhreeqC. At fluvial sites, we observe a geogenic dependence of the Cs p and a seasonal variability of Cs d , with a downstream increase of the solid-liquid partitioning (log 10 Kd values from 3.64 to 6.75 L kg−1) for suspended particulate matter (SPM) < 200 mg L−1. Along the estuarine salinity gradients, Cs shows a non-conservative behavior where fresh SPM (defined as Cs-depleted particles recently put in contact with Cs d) act as a Cs sink during both flood and low discharge (drought) conditions. This sorption behavior was explained by the geochemical model, highlighting the relevance of ionic strength, water and SPM residence times. However, at high salinities, the overall log 10 Kd value decreases from 6.02 to 5.20 for SPM ∼300–350 mg L−1 due to the Cs d oceanic endmember. Despite wild oysters showing low bioconcentration factors (∼1220 L kg−1) at the estuary mouth, they are sensitive organisms to Cs fluxes. [Display omitted] • Unique comprehensive work on stable Cs in continent-ocean transition system. • Watershed reactivity depends on geological source and seasonal processes. • Non-conservative estuarine behavior, driven by SPM age and salinity. • Modeled conditions show main sorption driver is cation-exchange with clay fraction. • Low BCFs in oysters but general natural origin of Cs related to water discharge. [ABSTRACT FROM AUTHOR]

Abstract The objective of this study is to develop a Fluoride Index for mitigation of geogenic contamination by Managed Aquifer Recharge (FIMAR). This index was tested by applying to the Pambar river basin in southern India. About 40% of the study area had fluoride >1.5 mg/L, contributed from the dissolution of fluorite, fluorapatite, biotite and hornblende. The relationship between groundwater level and fluoride concentration exhibited two types of relationship. In shallow wells, groundwater recharge during monsoon increases the groundwater level and dilutes the fluoride concentration. During summer, evaporation decreases the groundwater levels and increases the fluoride content. In deep wells, the fluoride-rich salts that is deposited in the unsaturated zone due to evaporation in the pre-monsoon season is flushed during groundwater recharge and thus, the fluoride concentration increases with increase in groundwater level. This relationship was used as a key layer in FIMAR. Overlay analysis including the well type derived from the groundwater level and fluoride fluctuation, average fluoride concentration, geology, geomorphology, soil, drainage density, land use, lineament density and thickness of the weathered zone indicated 30% of the area was suitable for MAR to mitigate fluoride contamination. Since the construction of new MAR structures is cost-intensive, it is proposed to rejuvenate the existing ponds in the areas identified by FIMAR to induce recharge and dilute fluoride concentration. The positive impact was confirmed using numerical and geochemical modelling. This proves that FIMAR can be used as a planning tool in fluoride endemic areas and that the future MAR methods will be beneficial to mitigate geogenic fluoride contamination. Graphical abstract Image 1 Highlights • About 40% of the groundwater samples have fluoride above 1.5 mg/L. • New Fluoride Index applied to identify sites for mitigating geogenic contamination by Managed Aquifer Recharge (FIMAR). • FIMAR indicates 30% of the study area is suitable for MAR. • Results from FIMAR is validated through numerical and geochemical models. • Rejuvenating the existing water bodies will increase recharge by 40% and reduce fluoride below 1.5 mg/L. [ABSTRACT FROM AUTHOR]

To understand occurrence, distribution and source of arsenic, 175 groundwater samples from coastal aquifers of the Puducherry region were collected and analyzed for major ions and trace metals. The concentration of As in groundwater of study area ranges from not detectable — 28.88 μg/L during the post-monsoon and not detectable — 36.88 μg/L in the pre-monsoon. The desirable limit for As in groundwater is 10 μg/L as per World Health Organization and Bureau of Indian standard. About 13.64 and 11.50% of groundwater samples shows arsenic concentration higher than recommended limit. Hydrochemical facies which dominate during pre and post monsoon are Na-K-Cl-SO 4 , Ca-Cl and Ca-Mg-Cl-SO 4 type and Na-K-Cl-SO 4 , mixedCa-Na-HCO 3 , Ca-HCO 3 and mixed Ca-Mg-Cl type respectively. The Gibbs diagram suggested that rock-water interaction is major process controlling hydrochemistry of groundwater. From the Pourbaix diagram, it is inferred that H3AsO3 is the principal As species in groundwater. The PHREEQC modelling indicates supersaturation of ferric oxides and hydroxide mineral phases in aquifer system which on reductive dissolution releases arsenic into groundwater. Statistical analysis (Spearman Correlation and Principal Component Analysis) showed that reductive dissolution of As-bearing minerals and Fe-oxyhydroxides in the presence of organic matter is the major process contributing arsenic into groundwater. The relationship between As, K + and HCO − 3 indicates agricultural and competitive exchange process which is an additional contributor of arsenic in groundwater. The sources which act as a sink and responsible for the release of As into the groundwater are marine sediments enriched in As and Fe-bearing minerals and organic matter. [ABSTRACT FROM AUTHOR]

AOD (argon oxygen decarburization) slag, which is the by-product of the stainless steel refining process, is a recyclable slag because of its high content of calcium and silicon. The leaching toxicity cannot be ignored in the recycling process because the slag contains a certain amount of Cr. In this study, the mineral analysis, batch leaching tests and thermodynamic and kinetic modeling by PHREEQC combined with FactSage software were performed to explore the influence of the dissolution of primary minerals and the precipitation of secondary minerals on the elution of Cr from AOD slag. The results indicated that the main minerals in the original AOD slag are larnite, merwinite, pyroxene and periclase. Cr was dispersed in the mineral phases mentioned above. The simulation of Cr leaching controlled by Cr(III)-hydroxide corresponded better to the batch leaching tests, while the Cr leaching controlled by chromite or double control was underestimated. Increasing the L/S ratio enhances the pH of the leachate and restrains the elution of Cr from the AOD slag. [ABSTRACT FROM AUTHOR]

AOD (Argon Oxygen Decarburization) slag is a by-product of the stainless steel refining process. The leaching toxicity of chromium from AOD slag cannot be ignored in the recycling process of the AOD slag. To assess the leaching toxicity of the AOD slag, batch leaching tests have been performed. PHREEQC simulations combined with FactSage were carried out based on the detailed mineralogical analysis and petrophysical data. Moreover, Pourbaix diagram of the Cr–H 2 O system was protracted by HSC 5.0 software to explore the chromium speciation in leachates. It was found that AOD slag leachate is an alkaline and reductive solution. The Pourbaix diagram of the Cr–H 2 O system indicated that trivalent chromium, such as Cr ( OH ) 4 − , is the major chromium species in the experimental Eh-pH region considered. However, toxic hexavalent chromium was released with maximum concentrations of 30 µg L −1 and 18 µg L −1 at L/S 10 and 100, respectively, during the earlier leaching stage. It concluded that the AOD slag possessed a certain leaching toxicity. After 10 d of leaching, trivalent chromium was the dominant species in the leachates, which corresponded to the results of PHREEQC simulation. Leaching toxicity of AOD slag is based on the chromium speciation and its transformation. Great attention should be focused on such factors as aging, crystal form of chromium-enriched minerals, and electrochemical characteristics of the leachates. [ABSTRACT FROM AUTHOR]

The systematic analysis of groundwater in the Greek island of Skiathos revealed a seasonal increase of total mercury concentrations after the extensive groundwater abstraction during the busy and heavily touristic summer months. This contamination was accompanied by a corresponding increase of the chloride content of groundwater, attributed to seawater intrusion into the freshwater-depleted aquifer within mercury-rich bedrock. The effects of elevated concentrations of chloride anions in the mobilization of mercury and its speciation were addressed by geochemical equilibrium modeling, considering cinnabar (HgS) as the mineral source of mercury. Adsorption onto hydrous ferric oxide (Fe 2 O 3 ·H 2 O) was a necessary ingredient of the geochemical model for bringing the calculated concentrations in agreement with field measurements, after optimization of the cinnabar/adsorbent mass ratio to a value of 4.9 × 10−8. The speciation of mercury was found to depend on the acidity and redox status as well as on the chloride content of groundwater. Mercury concentrations in the groundwater of Skiathos rise above the World Health Organization limit of 1 μg L−1 for a seawater intrusion higher than 3 %, with HgCl 2 being the dominant species followed by HgClOH, HgCl 3 − and HgCl 4 2−. The assumed concentration of dissolved organic matter in groundwater had a negligible impact on the mercury speciation and its mobilization by chloride. [Display omitted] • Long-term examination of groundwater Hg contamination by non-anthropogenic sources is conducted. • Seasonal fluctuation in Hg contamination is consistent with aquifer saltwater intrusion, due to over-exploitation. • Successful simulation of Hg concentrations in groundwater is achieved when adsorption on minerals is incorporated. • Domination of HgCl 2 , HgClOH and HgCl 3 − species in groundwater is predicted for typical cases of seawater intrusion. • Groundwater Hg speciation is insensitive to the mass ratio of Hg minerals to Hg adsorbing minerals. [ABSTRACT FROM AUTHOR]

Abstract: The chemical and physical processes involved in the retention of 10−2 M Zn, Pb and Cd in a calcareous medium were studied under saturated dynamic (column) and static (batch) conditions. Retention in columns decreased in order: Pb≫Cd≈Zn. In the batch experiments, the same order was observed for a contact time of less than 40h and over, Pb≫Cd>Zn. Stronger Pb retention is in accordance with the lower solubility of Pb carbonates. However, the equality of retained Zn and Cd does not fit the solubility constants of carbonated solids. SEM analysis revealed that heavy metals and calcareous particles are associated. Pb precipitated as individualized Zn–Cd–Ca– free carbonated crystallites. All the heavy metals were also found to be associated with calcareous particles, without any change in their porosity, pointing to a surface/lattice diffusion-controlled substitution process. Zn and Cd were always found in concomitancy, though Pb fixed separately at the particle circumferences. The Phreeqc 2.12 interactive code was used to model experimental data on the following basis: flow fractionation in the columns, precipitation of Pb as cerrusite linked to kinetically controlled calcite dissolution, and heavy metal sorption onto proton exchanging sites (presumably surface complexation onto a calcite surface). This model simulates exchanges of metals with surface protons, pH buffering and the prevention of early Zn and Cd precipitation. Both modeling and SEM analysis show a probable significant decrease of calcite dissolution along with its contamination with metals. [Copyright &y& Elsevier]

In many parts of the world, wastewater irrigation has become a common practice because of freshwater scarcity and to increase resource reuse efficiency. Wastewater irrigation has positive impacts on livelihoods and at the same time, it has adverse impacts related to environmental pollution. Hydrochemical processes and groundwater behaviour need to be analyzed for a thorough understanding of the geochemical evolution in the wastewater irrigated systems. The current study focuses on a micro-watershed in the peri-urban Hyderabad of India, where farmers practice intensive wastewater irrigation. To evaluate the major factors that control groundwater geochemical processes, we analyzed the chemical composition of the wastewater used for irrigation and groundwater samples on a monthly basis for one hydrological year. The groundwater samples were collected in three settings of the watershed: wastewater irrigated area, groundwater irrigated area and upstream peri-urban area. The collected groundwater and wastewater samples were analyzed for major anions, cations and nutrients. We systematically investigated the anthropogenic influences and hydrogeochemical processes such as cation exchange, precipitation and dissolution of minerals using saturated indices, and freshwater-wastewater mixtures at the aquifer interface. Saturation indices of halite, gypsum and fluorite are exhibiting mineral dissolution and calcite and dolomite display mineral precipitation. Overall, the results suggest that the groundwater geochemistry of the watershed is largely controlled by long-term wastewater irrigation, local rainfall patterns and water-rock interactions. The study results can provide the basis for local decision-makers to develop sustainable groundwater management strategies and to control the aquifer pollution influenced by wastewater irrigation., (Copyright © 2019 Elsevier Ltd. All rights reserved.)