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A coupled methodology of nitrogen isotopes, hydrogeochemical characterization, multivariate statistical analysis, and SIAR Bayesian modeling has been employed to identify the sources of NO3− and N transformation processes in three alluvial aquifers (Schinos, Thiva, and Central Evia) located in central Greece where geogenic Cr(VI) co-occurs with agricultural activity and rural development. Hexavalent chromium concentrations exceed 50 μg/L in many sampling stations of the studied groundwater bodies, while nitrate contamination is evident in all three study areas with concentrations well over 50 mg/L. The mean δ15N-NO3– and δ18Ο-NO3− values are 6.67 ± 1.77‰ and 2.68 ± 1.77‰ in C. Evia, 8.72 ± 4.74‰ and 3.96 ± 4.57‰ in Schinos and 4.44 ± 1.71‰ and 2.91 ± 1.02‰ in Thiva, respectively. Domestic sewage and N-bearing fertilizers are contributing in various degrees to the observed nitrification which is the dominant transformation process of N in the studied aquifers. Multivariate statistics indicated that the main processes identified in the study areas are salinization, silicate dissolution, and groundwater contamination due to fertilizer use. It is suggested that ultramafic rock–related alluvial aquifers must be closely monitored in terms of nutrient inputs as an effective measure for controlling Cr(VI) release in groundwater. [ABSTRACT FROM AUTHOR]

This study investigates the performance of support vector machine (SVM), multivariate adaptive regression spline (MARS), and random forest (RF) models for predicting the lead (Pb) adsorption by attapulgite clay. Models are constructed using batch stochastic data of heavy metal (HM) concentrations under different physicochemical conditions. Implementation of auto-hyper-parameter tuning using grid-search approach and comparative analysis is performed against the benchmark artificial intelligence (AI) models. Models are constructed based on Pb concentration (IC), the dosage of attapulgite clay (dose), contact time (CT), pH, and NaNO3 (SN). Principle component analysis (PCA) and correlation analysis (CA) methods are integrated to assess the importance of the applied predictors and their relationship with the target. Research findings approved the potential of the grid-RF model as a marginal superior predictive model against the grid-SVM in terms of MAE, i.e., 3.29 and 3.34, respectively; moreover, the md scored the same, i.e., 0.93, which reveals the potential predictability for both. Nonetheless, grid-MARS and standalone MARS models remained likewise in their predictability. IC parameter demonstrated the highest influential among all the predictors with the highest value of importance in the case of all three evaluators. The solution pH and dose stands together with marginal differences in case of PCA method; however, solution pH and CT appeared with similarity impact using the PCA method. [ABSTRACT FROM AUTHOR]

The holistic approach of Driver-Pressure-State-Impact-Response (DPSIR) methodology was applied to selected Cr(VI) impacted groundwater bodies of Central Greece. The main driving forces in the study areas are agricultural activities, urban and industrial development as well as tourism. The main pressures induced by the anthropogenic activities are fertilizer use, uncontrolled urban sewage disposal and industrial effluents discharges. Groundwater stress is caused by the qualitative degradation due to Cr(VI), NO3−, Cl− and SO42− contamination. Hexavalent chromium occurrence is attributed to both geogenic and anthropogenic sources. The maximum Cr(VI) concentration (11.7 mg/L) was measured in Oinofyta area. Important impacts are the deterioration of groundwater body chemical status as well as the decline of groundwater use efficiency. Based on the applied DPSIR, a management framework is proposed in order to address the complex environmental issue of Cr(VI) in the study areas. [ABSTRACT FROM AUTHOR]

This research aims to determine the hydrogeochemical characteristics, origin, and geochemical evolution of the Cr enriched alkaline waters in the ultramafic rock-related water bodies of Loutraki basin in Northeast Peloponnese, Greece. The significance of the particular research lies in the fact that Cr occurrence raises concerns with respect to health risks associated with the utilization of such aquifers for drinking water supply. Therefore, the investigation of water–rock interaction processes leading to Cr mobilization in such environments is of great importance. A total of 41 water samples including groundwater from the alluvial and fractured aquifers as well as surface water were collected and analyzed for major and trace elements during the wet and dry seasons of 2015. Solid samples including soils, sediments and rocks were also collected to provide evidence of elemental mobilization due to water–rock interaction, with special focus on Cr. Τhe main Cr-bearing minerals identified were Cr-spinel, ferrichromite, magnetite, clinopyroxene, chlorite and serpentine. The observed dissolution textures within the magnetite rim and across the magnetite–chromite boundary indicate that these latter can be also active sources of Cr(III). Both groundwater and surface waters are of Mg–HCO3 type due to CO2-driven dissolution of serpentine minerals and Mg-carbonates/hydroxides. The formation of hydromagnesite aggregates is attributed to precipitation from Mg2+-rich alkaline waters. The highest Cr(VI) concentrations were measured in the alluvial aquifer (6.7–74.3 μg L−1) and the lowest in the fractured ophiolitic aquifer (1.9–14.3 μg L−1); while in surface water, the maximum measured Cr(VI) concentration was up to 3.9 μg L−1. Finally, the identification of Mn-rich hematite indicates that the oxidation of Cr(III) to Cr(VI) is probably performed by manganese–iron oxides. [ABSTRACT FROM AUTHOR]