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Oxygen depletion (temporal or permanent) in freshwater ecosystems is a widespread and globally important environmental problem. However, the factors behind increased hypolimnetic anoxia in lakes and reservoirs are often diverse and may involve processes at different spatial and temporal scales. Here, we evaluate the combined effects of different anthropogenic pressures on the oxygen dynamics and water chemistry of Lake Enol, an emblematic mountain lake in Picos de Europa National Park (NW Spain). A multidisciplinary study conducted over a period of four years (2013–2016) indicates that the extent and duration of hypolimnetic anoxia has increased dramatically in recent years. The extent and duration of hypolimnetic anoxia is typical of meso-eutrophic systems, in contrast with the internal productivity of the lake, which remains oligo-mesotrophic and phosphorus-limited. This apparent contradiction is ascribed to the combination of different external pressures in the catchment, which have increased the input of allochthonous organic matter in recent times through enhanced erosion and sediment transport. The most important among these pressures appears to be cattle grazing, which affects not only the import of carbon and nutrients, but also the lake microbiology. The contribution of clear-cutting, runoff channelling, and tourism is comparatively less significant. The cumulative effects of these local human impacts are not only affecting the lake metabolism, but also the import of sulfate, nitrate- and ammonium-nitrogen, and metals (Zn). However, these local factors alone cannot explain entirely the observed oxygen deficit. Climatic factors (e.g., warmer and drier spring and autumn seasons) are also reducing oxygen levels in deep waters through a longer and increasingly steep thermal stratification. Global warming may indirectly increase anoxia in many other mountain lakes in the near future. [ABSTRACT FROM AUTHOR]

Summary Identifying groundwater retention times near managed aquifer recharge (MAR) facilities is a high priority for managing water quality, especially for operations that incorporate recycled wastewater. To protect public health, California guidelines for Groundwater Replenishment Reuse Projects require a minimum 2–6 month subsurface retention time for recycled water depending on the level of disinfection, which highlights the importance of quantifying groundwater travel times on short time scales. This study developed and evaluated a new intrinsic tracer method using the naturally occurring radioisotope sulfur-35 ( 35 S). The 87.5 day half-life of 35 S is ideal for investigating groundwater travel times on the <1 year timescale of interest to MAR managers. Natural concentrations of 35 S found in water as dissolved sulfate ( 35 SO 4 ) were measured in source waters and groundwater at the Rio Hondo Spreading Grounds in Los Angeles County, CA, and Orange County Groundwater Recharge Facilities in Orange County, CA. 35 SO 4 travel times are comparable to travel times determined by well-established deliberate tracer studies. The study also revealed that 35 SO 4 in MAR source water can vary seasonally and therefore careful characterization of 35 SO 4 is needed to accurately quantify groundwater travel time. More data is needed to fully assess whether or not this tracer could become a valuable tool for managers. [ABSTRACT FROM AUTHOR]

• Stable isotopes of sulfate in wastewater, fertilizers and manure in the Upper Silesia are generally more homogeneous compared to the literature data. • Isotopic composition of sulfate from landfill leachates is enriched with heavy isotopes of S and O. • Isotopic studies of local sources of contamination reduce the uncertainty in the interpretation of sulfate sources in groundwater. Isotopic signatures of anthropogenic sulfate sources within the Upper Silesian urban area (southern Poland) were investigated to increase the reliability of the origin assessment of groundwater contaminants. To analyze δ34S and δ18O in sulfate, 64 samples were collected, representing several sulfate sources: synthetic and organic fertilizers, raw and treated wastewater, mine water, leachate from landfills, mine heaps, and industrial waste dumps. Additionally, 10 surface water samples were taken from contaminated rivers regarded as potential groundwater pollution sources. Results of δ34S and δ18O in sulfate derived from anthropogenic sources were compared with literature data, which indicated significant discrepancies for some contamination sources. The results were applied to assess SO 4 2- origin in groundwater samples from the Gliwice aquifer. Apart from the δ34S and δ18O data, the study included measurements of major ions, inorganic nitrogen forms, trichloroethene and tetrachloroethene in groundwater. Isotopic analyses of river water samples confirmed the impact of urbanized and industrialized areas on surface water pollution and possible influence on groundwater contamination. The values of δ34S and δ18O similar to wastewater were reported for most groundwater samples from the Gliwice carbonate aquifer. Groundwater contamination was also manifested by higher values of electrical conductivity and concentrations of chloride, sodium, trichloroethene and tetrachloroethene. The comparison of recent isotopic data on SO 4 2- sources with literature values clearly shows that it is very important to investigate local potential contamination sources to increase their reliability in determining the source of groundwater or river water pollution. [ABSTRACT FROM AUTHOR]

• Stable isotopes of sulfate in wastewater, fertilizers and manure in the Upper Silesia are generally more homogeneous compared to the literature data. • Isotopic composition of sulfate from landfill leachates is enriched with heavy isotopes of S and O. • Isotopic studies of local sources of contamination reduce the uncertainty in the interpretation of sulfate sources in groundwater. Isotopic signatures of anthropogenic sulfate sources within the Upper Silesian urban area (southern Poland) were investigated to increase the reliability of the origin assessment of groundwater contaminants. To analyze δ34S and δ18O in sulfate, 64 samples were collected, representing several sulfate sources: synthetic and organic fertilizers, raw and treated wastewater, mine water, leachate from landfills, mine heaps, and industrial waste dumps. Additionally, 10 surface water samples were taken from contaminated rivers regarded as potential groundwater pollution sources. Results of δ34S and δ18O in sulfate derived from anthropogenic sources were compared with literature data, which indicated significant discrepancies for some contamination sources. The results were applied to assess SO 4 2- origin in groundwater samples from the Gliwice aquifer. Apart from the δ34S and δ18O data, the study included measurements of major ions, inorganic nitrogen forms, trichloroethene and tetrachloroethene in groundwater. Isotopic analyses of river water samples confirmed the impact of urbanized and industrialized areas on surface water pollution and possible influence on groundwater contamination. The values of δ34S and δ18O similar to wastewater were reported for most groundwater samples from the Gliwice carbonate aquifer. Groundwater contamination was also manifested by higher values of electrical conductivity and concentrations of chloride, sodium, trichloroethene and tetrachloroethene. The comparison of recent isotopic data on SO 4 2- sources with literature values clearly shows that it is very important to investigate local potential contamination sources to increase their reliability in determining the source of groundwater or river water pollution. [ABSTRACT FROM AUTHOR]

Summary The reclamation of shale overburden dumps from oil sands mining requires the placement of reclamation covers comprised of salvaged organic and mineral soils. The primary issues associated with the long-term performance of these covers are their ability to store sufficient water to meet transpiration demands and the potential threat of salt ingress into the cover from the underlying shale. The first issue has been addressed in previous studies, so the objective of this study was to evaluate controls on salt ingress through observations and modelling of the transport of the stable isotopes of water and salt within reclaimed profiles at the South Bison Hills overburden dump located north of Fort McMurray, Alberta. The water flow model was based on a dual porosity soil–vegetation–atmosphere model calibrated to observed field data. This model was then used to simulate deuterium and sulphate transport within the soil profiles. The optimized transport model for deuterium was used to estimate net percolation rates through the cover soil into the underlying shale. This model was then used to assess the controls on sulphate migration, including the rate of sulphate generation as a result of ongoing oxidation of the pyritic shale. The model results indicate that the average net percolation rate is a function of topographic location, ranging from 2.2 × 10 −5 m/d at slope locations to 20.8 × 10 −5 m/d at plateau locations during the unfrozen days. These rates of net percolation should have produced observable patterns of salt flushing from the cover and upper shale. However, the observed sulphate levels could only be simulated by including a production term related to pyrite oxidation of the shale. The simulated oxidation rates ranged from 0.4 to 5.65 mg/L/d, similar to those estimated from previous laboratory and field investigations. [ABSTRACT FROM AUTHOR]

Summary Seasonal hydrochemical monitoring has been conducted at a well-known arsenic (As) contaminated site to understand the critical controlling processes on As mobilization and enrichment in groundwater. The hydrochemical results indicate that the water chemistry at the studied site is mainly controlled by evaporates dissolution, and redox reactions during irrigation and non-irrigation periods, respectively. The measured redox-sensitive components indicate that the groundwater experienced periodic redox changes from irrigation to non-irrigation periods and that an enhanced reducing environment prevailed during the non-irrigation period. The observed high As concentration and the strong positive correlation between As and Fe support the conclusion that the Fe oxides/hydroxides reduction and Fe sulfide oxidation releases As during the irrigation period. However, the relatively low As concentration and the close inverse correlation between As, Fe and SO 4 2− show that Fe(III) and SO 4 2− reduction accompanied by the formation of Fe sulfide precipitates, retains As during the non-irrigation periods. The geochemical inverse modeling performed for the selected wells confirms that Fe oxide/hydroxide and Fe sulfide dissolution as well as siderite precipitation are the dominant processes related to the mobilization of As during the water evolution from the pre-irrigation to irrigation period, while Fe sulfide precipitation immobilize As during the groundwater evolution from the irrigation to the post-irrigation period. In general, the combined results of the hydrochemical monitoring and the geochemical inverse modeling support the conclusion that the Fe(III)–SO 4 2− redox cycling induced by periodic irrigation practices controls the mobilization of As in groundwater at this site. [ABSTRACT FROM AUTHOR]

Highlights: [•] Prediction of mining impact on groundwater and surface water quality. [•] Mining impact controlled by mixing and sulfate reduction. [•] Predicted diffuse mine drainage into a river corresponds to measurements. [•] Mining impact on river water is expected to quintuple within the next 40years. [ABSTRACT FROM AUTHOR]

Highlights: [•] Qualitative sulfate source ID using dual isotopes (34S and 18O) at a mining site. [•] Quantification of the sources (sulfide oxidation vs. sulfuric acid) by two methods. [•] Sulfate from sulfuric acid application served as the major source for 40years. [•] Proposed accumulation of sulfate salts formed from sulfuric acid in arid climate. [ABSTRACT FROM AUTHOR]

Highlights: [•] We provide a thorough literature review on prairie hydrology and salt dynamics in the subsurface. [•] Areas of salt accumulation are identified. [•] A conceptual model is proposed that explains salt dynamics and accumulation. [•] The effect of land-use and climatic changes on the salt dynamics and distribution is discussed. [ABSTRACT FROM AUTHOR]

Highlights: [•] Distribution and mobilization of As in the Guandu wetland were investigated. [•] As mobility occurred in the boundary between the Fe-reducing and sulfate-reducing conditions. [•] High aqueous As was caused by the reductive dissolution of solid Fe compounds. [•] Enriched and values implies significant dissimilatory sulfate reduction. [•] Arsenic mobility in wetland ecosystem is controlled by redox cycling of Fe, S and C. [ABSTRACT FROM AUTHOR]

Summary: Groundwater within the unconfined or semi-confined parts of Triassic carbonate aquifers in Upper Silesia (Poland) contains high concentrations of sulphate (up to 290mg/L), sometimes in excess of drinking water limits (>250mg/L). To assess the influence of different possible sulphate sources, isotopic analyses of S and O were performed on groundwater sulphate and potential sulphate sources and combined with literature data. Three dominant sources of sulphate were delineated, based on the geological and literature study and supported by the mixing relations between inverse concentration of sulphate and its isotopic compositions. These sources are: (i) sulphate from rainfall; (ii) weathering of sulphide minerals in ore deposits in the aquifer-forming carbonate rocks; (iii) dissolution of sulphate evaporites in the Triassic sequence. Fortunately these three sources have distinctive S and O isotope compositions and thus their contributions to the total dissolved sulphate could be estimated. The application of linear mixing models for three sources in the dual isotope system allowed the impact of the three different sulphate sources on particular parts of the aquifers to be calculated. The average isotopic composition of sulphate in abstracted groundwater indicates that the most important source of sulphate is sulphide weathering, contributing about 50% of total sulphate. The second most significant source of sulphate input is rainfall and it is characterised by a mean contribution of 30%. Application of Monte Carlo analysis that incorporates the full variability in distributions of isotopic compositions for the three sources and all mixing fractions between them gave the most probable ranges of the dissolved in groundwater sulphate. This analysis indicated that the proportion of sulphate derived by sulphide oxidation is comparable with the estimations based on linear models. This study has shown that the water quality of these important groundwater resources is under threat from both natural sources, i.e. metal sulphide oxidation and gypsum dissolution. Analysis of the mathematical models analysis shows that the first process is the predominant source of sulphate in groundwater. However, the highest concentrations of dissolved sulphate are positively correlated with the increasing proportion of sulphate derived from gypsum dissolution. Moreover, one should keep in mind that natural processes might be anthropogenically accelerated due to variable water demands and groundwater abstraction. Eventually, the statistically second-order source of sulphate – rainfall might contain surface-derived contaminants, and its contribution to the total load of sulphate might indirectly indicate the vulnerability of aquifers for the pollution. [Copyright &y& Elsevier]

Summary: The northern prairie region of North America has numerous closed-basin wetlands. Salinity (i.e. sulphate concentration) of these wetland ponds has a profound effect on their ecological function. Despite the apparent lack of surface and subsurface outflow from the closed basins, and strong evaporative enrichment due to the dry climate, many of the closed-basin wetlands maintain moderate salinity (1–10gL−1 as total dissolved solids) instead of evolving into hyper-saline wetlands. The objective of this study is to characterize the surface and groundwater conditions of a typical saline wetland in Saskatchewan, Canada, quantify the surface and subsurface distribution of sulphate, and understand the processes controlling the wetland salinity. The analysis of sediment core samples and the electrical resistivity imaging of the wetland basin indicated a large mass (ca. 107 kg) of subsurface sulphate, most of which is in the form of gypsum. In comparison, the sulphate dissolved in pond water was only 0.13×106 kg at its maximum, indicating that only a small fraction of sulphate is in the pond water column at any given time. The analysis of a 19-year mass balance of the wetland pond showed that complex water–sediment exchange processes transfer sulphate from surface water to the underlying sediments during the drying phase of wetland, and transfer part of it back to surface water during the wetting phase. We hypothesize that this wet–dry cycle, repeated many times since the deglaciation of the region has allowed the subsurface sulphate to accumulate and maintained the moderate salinity of the wetland. [Copyright &y& Elsevier]

Summary: The North China Plain (NCP) is a typical Cenozoic sedimentary basin and one of the largest alluvial plains in eastern Asia. It consists of piedmont alluvial plain, central alluvial-lacustrine plain, and littoral alluvial plain from west to east, where Quaternary aquifer systems developed in different depths. Groundwater from these aquifers is the most important water resource and supports a population of 200 million in the NCP. Currently the Quaternary groundwater is depleting fast due to both human and natural factors. Characterizing biogeochemical processes in groundwater is central to understanding groundwater chemistry. To identify the sources of dissolved sulfate () and address subsequent biogeochemical processes in groundwater, we conducted a study of spatial distributions of δ34SSO4 and δ18OSO4 in the Quaternary aquifers in the northern part of the NCP. The [], δ34SSO4, and δ18OSO4 along the west–east groundwater flowpaths in the three alluvial plains are analyzed and examined. The Quaternary groundwater (∼20°C) has a [] of 37.25–177.41mg/L with []/[Cl−] ratios ranging from 0.12 to 2.79. The δ34SSO4 and δ18OSO4 range from 4.9‰ to 30.9‰ and from 4.7‰ to 14.0‰, respectively. Supported by other geochemical and isotopic parameters such as total dissolved solids (TDS) concentrations, δD and δ18O of groundwater (δD and δ18OH2O), and 14C contents in groundwater, our study concludes that: (1) The shallow groundwater in the piedmont area is characterized by higher concentration of 153.22mg/L with δ34SSO4 of 10.0–14.5‰ and δ18OSO4 of 4.7–9.0‰, whereas the groundwater in the piedmont area is characterized by lower concentration of 37.25mg/L with δ34SSO4 of 4.9–7.3‰ and δ18OSO4 of 5.8–7.5‰. Therefore, the in the shallow groundwater is dominated by weathered sulfate minerals from the Taihang mountains and sulfate derived from oxidation of sulfides in infiltration paths, whereas the in the deep groundwater is dominated by atmospheric sulfate from precipitation and sulfate derived from decomposition of organic matter during its recharge processes; (2) Dissimilatory microbial sulfate reduction (DMSR) occurred in the deep confined aquifers in both central and littoral plains, where δ34SSO4 and δ18OSO4 increase along the groundwater flowpaths and present specific patterns for each aquifer which imply variable contributions from DMSR processes; (3) The deep confined aquifer in the littoral plain has a lower DMSR rate than the other aquifers, probably due to the effect of its lower permeability on physical exclusion of bacteria from the aquifer matrix. This study can provide a biogeochemical background for assessing groundwater evolution and its quality in the NCP. [Copyright &y& Elsevier]

Summary: 3H, δ 18O and δ 2H measurements on groundwater and chemical and isotope analyses on groundwater sulfate were used to determine the origin and mean transit times of groundwater and the sources affecting groundwater sulfate in a high-alpine karstic catchment area in southern Germany. Modelling results using a lumped parameter approach yielded mean transit times of water between 4 and 12 years for different karst springs. However, results obtained from δ 18O and δ 2H measurements on groundwater showed that a calculated mean transit time of 12 years for groundwater flow in one karst system can only be explained by mixing of young and old tritium-free ice and snowmelt water. Groundwater sulfate characterized by δ 34S values of around 20‰ and δ 18O values of ca. 12‰ in concert with sulfate concentrations of approximately 11mg/L are probably affected by evaporites. In contrast, δ 34S values of around 6‰ and δ 18O values up to 9.4‰ in concert with sulfate concentrations between 1 and 7mg/L mainly derive from atmospheric deposition. However, a δ 34S value of −7.3‰ identified for one of the karst springs demonstrates that the oxidation of sulfide-containing minerals must be considered as an additional source, affecting sulfate concentrations in groundwater. The estimated transit time distribution of groundwater shows relatively high mean transit times between 2 and 5 years with a low contribution of very short transit times of less than 1 year. The high contribution of long mean transit times of groundwater in the alpine karst system, which represents an important drinking water resource, is an important function for drinking water supplies. [Copyright &y& Elsevier]

• TF , SF and Ei had no significant differences between UP and LP. • TF NO 3 −-N and SO 4 2−-S fluxes were slightly/significantly lower in UP compared to LP. • SF NO 3 −-N and SO 4 2−-S fluxes were quite similar between the two slope plots. • NTF NO 3 −-N and SO 4 2−-S fluxes were negative at both UP and LP. • Canopy exchange may be dominant factor on these hydrochemical fluxes in watershed. Forest canopies create a high spatial variability of both canopy interception (Ei) and acid deposition. However, spatial characteristics of these hydrochemical fluxes along a slope gradient in forest watersheds remain unclear. Here we monitored bulk precipitation (BP), throughfall (TF), stemflow (SF), and associated nitrate (NO 3 −) and sulfate (SO 4 2−) in an upper slope plot (UP) and a lower slope plot (LP) in a subtropical broadleaved forest, Southeast China. Results showed that annual rates of TF , SF and Ei to BP in UP were 85.2%, 2.6%, and 12.2%, respectively. Corresponding values in LP were 88.6%, 2.2%, and 9.2%. There were no significant differences among these rainfall partitioning components between UP and LP. Additionally, annual volume weighted mean concentrations of NO 3 −-N in TF and SF were significantly lower than in BP , whereas no significant differences were found for SO 4 2−-S among BP , TF and SF. Unlike annual TF NO 3 −-N fluxes were similar in UP and LP, annual TF SO 4 2−-S fluxes significantly increased along the UP to LP. Net TF + SF (NTF) fluxes were negative for NO 3 −-N and SO 4 2−-S at the two slope plots, illustrating retained/uptake in the canopy. Dormant season NTF S fluxes were close to zero, indicating that dry deposition does not appear to play a major role, whereas exchange with foliage surfaces should be the dominant factor controlling the spatial patterns of TF N and S fluxes in the watershed. This study can contribute to the evaluation of watershed-scale water and acidifying input into the soil during canopy passage, and further consequent effects on forest ecosystems. [ABSTRACT FROM AUTHOR]