Changes In Sulfate Concentrations Research Articles

Deterioration of fire resistance of ultra-high-performance polypropylene-fibre-reinforced concrete (UHPPFRC) under long-term acid rain corrosion action

The application of ultra-high-performance polypropylene-fibre-reinforced concrete (UHPPFRC) has become widespread due to its superior mechanical properties and non-flammable characteristic. Compared to UHPC, the fire resistance of UHPPFRC, which is crucial in ensuring structural integrity and safety during fire incident, has been improved by the addition of PP fibres. In recent years, the escalation of industrial emission has resulted in a more pronounced pollution of acid rain, a major type of sulphate attack on outdoor concrete structures, leading to the aging of UHPPFRC. Given the long service life of civil engineering structures, the fire performance of aging UHPPFRC should be given significant consideration, yet few studies have taken it into account. This study, for the first time, analyses the deterioration of fire resistance of acid-rain-induced aging UHPPFRCs and compares their probability of failure under fire condition across different corrosion durations, providing guidance for the maintenance of UHPPFRC in acid-rain-prone areas. A novel coupled chemo-thermal-mechanical model is developed, which is employed to investigate the sulphate penetration process and internal stress development within UHPPFRC during the corrosion stage, and to predict temperature and internal stress distribution during the subsequent heating stage. The effects of sulphate concentration, heating rate and fibre dosage on the probability of failure of aging UHPPFRC are then studied. The results obtained from the numerical investigation indicate that the aging effect can significantly compromise the fire resistance of UHPPFRC and augment the probability of failure at elevated temperature. Also, the probability of failure of aging UHPPFRC is more sensitive to changes in sulphate concentration than changes in heating rate.

Evaluation of compatibility between formation and Injection water into the Reservoir Rock

Scale deposition represents a critical challenge encountered in water injection systems. The objective of this study is to assess the compatibility between the candidate smart water and the formation water in an oil reservoir rock sample. To achieve this, water samples were collected and analyzed to evaluate the extent of deposit formation under varying temperature and pressure conditions. Additionally, the study also examined the combined influence of temperature and pressure on deposit formation. The results indicated that as the temperature increased, there was a decrease in the concentration of sulfate in the seawater. Taking into account the low concentration of barium (zero) in seawater and the unchanged strontium concentration, the decrease in sulfate concentration can be attributed to the deposition of calcium sulfate. Contrarily, in various ratios of smart water, the sulfate concentration has shown an increase with rising temperatures. This can be attributed to the transformation of sulfite present in the formation water into sulfate due to the influence of temperature. The presence of a low concentration of barium in the formation water resulted in insignificant changes in sulfate concentration due to the precipitation of barium sulfate. Additionally, despite the high concentration of strontium, strontium sulfate deposition did not occur, possibly due to the solubility product constant. The analysis of different smart water ratios under various pressures at ambient temperature revealed that pressure had a minimal effect on the solubility of sulfate and calcium salts. During the sediment analysis, no evidence of strontium sulfate sediment formation was observed. However, the analysis indicated a high possibility of iron sulfide and iron oxide deposits forming. Ultimately, under the experimental conditions, significant sediment formation was not observed.

Effect of sulfate on arsenic migration and transformation in micro-cosmic experiments simulating mangrove sediment environment

Fragile mangrove wetlands are threatened by sea level rise, which results in the introduction of increased sulfate concentrations into mangrove sediment. Mangroves are also susceptible to heavy metal contamination, such as arsenic (As) contamination. However, little is known about whether sulfate affects the bioavailability of As in mangrove sediment. This study focuses on the effects of sulfate on arsenic mobilization and transformation in As-contaminated mangrove sediment, and modeled changes in sulfate concentration, As contamination levels, and pH conditions. The As release from sediment treated with sulfate (30 mM) significantly decreased (P < 0.05). In the moderate arsenic-contaminated sediment, the mean arsenic release was 181.95 ± 33.05 μg kg−1 without sulfate and 157.55 ± 40.98 μg kg−1 with sulfate. In addition, the release of As was affected by both pH and sulfate when pH conditions were changed. Arsenic release was promoted at pH 11 (P < 0.05), but sulfate still mitigating As release at this pH value (P < 0.001). Furthermore, the release of As after sulfate input was always accompanied by the release of iron (Fe) and aluminum (Al). Sulfate altered the distribution of forms of As in the sediment, with a significant increase in the proportion of strongly adsorbed As and silicate-bound states of As. Overall, sulfate intrusion mitigates As release, but it also increases the bioavailability of As in the sediment. This fact makes the toxic element most susceptible to long-term mobilization via biological processes. Therefore, the effect of persistent sulfate intrusion caused by sea level rise on toxic elements in mangrove sediment deserves long-term attention. The significance of this study is that it contributes to understanding the behavior of heavy metals in coastal wetland ecosystems affected by sea level rise.

Occurrence of ammonium in the acidic-circumneutral coastal groundwater of Beihai, Southern China: δ15N, δ13C, and hydrochemical constraints

High levels of ammonium (NH4+) in groundwater can threaten the health of surface ecosystems and the safety of water supplies. To ascertain the occurrence of NH4+ in the coastal acidic groundwater of Beihai, Southern China, a combined hydrochemical and isotopic (δ15N in nitrate (NO3−) and NH4+; δ13C in dissolved inorganic carbon (DIC)) study was performed. In addition to a chloride concentration-based fresh water-seawater mixing model, δ15NNO3 vs δ15NNH4, Δδ13CDIC,react (changes in δ13CDIC values caused by reaction), and ΔCSO4,react (changes in sulfate concentrations caused by reaction) were applied to interpret the genesis of NH4+. The results show groundwater NH4+ loadings range from < 0.04 to 6.2 mg/L, with typically higher loadings along the coastline than in the plains. Elevated NH4+ loadings (>0.2 mg/L) are commonly accompanied by enriched δ15NNO3 and depleted δ15NNH4 values in relation to the anthropogenic endmember, depleted or enriched Δδ13CDIC,react values, and ΔCSO4,react > 0.00 or < 0.00 mmol/L. Such isotopic fingerprints and hydrochemical features indicate dissimilatory nitrate reduction to ammonium (DNRA), glucose fermentation, methanogenesis (acetate fermentation), and sulfide oxidation or sulfate reduction. The findings suggest acidic rainwater, sulfide oxidation, and NO3− reduction by ferrous iron (Fe2+) could jointly account for the formation of acidic groundwater (pH as low as 4.0) that is of meteoric origin. Fermentative and sulfide-driven DNRA with anthropogenic NO3− is notably responsible for the enrichment of groundwater NH4+ concentrations, with a minor contribution from anthropogenic NH4+ infiltration and algae decomposition. As a result of silicate and carbonate weathering, seawater inputs, and fermentation, the addition of bicarbonate (HCO3−) results in circumneutral pH values. These results indicate DNRA plays a critical role in the enrichment of NH4+ and, hence, improve understanding of its genesis in the groundwater of Beihai and other similar areas.

Assessing effective radiative forcing from aerosol–cloud interactions over the global ocean

How clouds respond to anthropogenic sulfate aerosols is one of the largest sources of uncertainty in the radiative forcing of climate over the industrial era. This uncertainty limits our ability to predict equilibrium climate sensitivity (ECS)-the equilibrium global warming following a doubling of atmospheric CO2. Here, we use satellite observations to quantify relationships between sulfate aerosols and low-level clouds while carefully controlling for meteorology. We then combine the relationships with estimates of the change in sulfate concentration since about 1850 to constrain the associated radiative forcing. We estimate that the cloud-mediated radiative forcing from anthropogenic sulfate aerosols is [Formula: see text] W m-2 over the global ocean (95% confidence). This constraint implies that ECS is likely between 2.9 and 4.5 K (66% confidence). Our results indicate that aerosol forcing is less uncertain and ECS is probably larger than the ranges proposed by recent climate assessments.

Sulfate concentrations affect sulfate reduction pathways and methane consumption in coastal wetlands

Coastal wetlands are an important source of methane emissions, and understanding the mechanisms that control methane emissions from coastal wetlands is of great significance to global warming. Anaerobic oxidation of methane driven by sulfate is an important process to prevent methane emissions from coastal wetlands. The effects of environmental changes on this process and the function of the sulfate-methane transition zone (SMTZ) are poorly understood. In this study, spatiotemporal variations in pore-water geochemistry (concentrations of SO42-, CH4 and DIC as well as δ13C-DIC and δ13C-CH4) in the Beidagang wetland, Tianjin, China, were investigated to unravel factors controlling the role of anaerobic oxidation of methane in coastal wetlands. Results show that the geochemical profile of pore-water is characterized by significant spatial and temporal variability, which may be related to changes in sulfate concentration, temperature and dissolved oxygen. The carbon isotope fractionation factors (εC) during methane oxidation range from 8.9‰ to 12.5‰, indicating that the sulfate-driven anaerobic oxidation of methane (S-AOM) dominates the methane oxidation in the Beidagang coastal wetland in both winter and summer, in both high and low salinity wetlands, and in both open water and littoral areas. However, sulfate concentration has a strong influence on the sulfate reduction pathways and methane consumption. The consumption of methane and sulfate by S-AOM is more significant in coastal wetlands with high sulfate concentrations, with S-AOM consuming nearly all of the upward-diffusing methane (96%) and downward-diffusing sulfate (96%). In addition, the dissolved inorganic carbon (DIC) produced in the pore-water mainly comes from methanogenesis, accounting for more than 80% of the total DIC pool, but in the areas with high sulfate concentrations in water column, the contribution of S-AOM to the DIC pool is greater, although only a small fraction of the total DIC pool (9%). The depth and width of the SMTZ show a clear spatial and temporal pattern, with active methanogenesis activity and upward high methane flux shoaling the SMTZ and increasing the risk of high methane emissions from coastal wetlands with low sulfate concentrations. Our findings highlight the importance of sulfate-driven anaerobic oxidation of methane in coastal wetlands and the effect of sulfate concentration on it. It contributes to our understanding of the mechanism of methane production and emissions from the coastal wetland system, particularly in light of the increased demand for coastal wetland restoration under global warming.

The Carbon-Sulfur Link in the Remineralization of Organic Carbon in Surface Sediments

Here we present the carbon isotopic composition of dissolved inorganic carbon (DIC) and the sulfur isotopic composition of sulfate, along with changes in sulfate concentrations, of the pore fluid collected from a series of sediment cores located along a depth transect on the Iberian Margin. We use these data to explore the coupling of microbial sulfate reduction (MSR) to organic carbon oxidation in the uppermost (up to nine meters) sediment. We argue that the combined use of the carbon and sulfur isotopic composition, of DIC and sulfate respectively, in sedimentary pore fluids, viewed through a δ13CDIC vs. δ34SSO4 cross plot, reveals significant insight into the nature of carbon-sulfur coupling in marine sedimentary pore fluids on continental margins. Our data show systemic changes in the carbon and sulfur isotopic composition of DIC and sulfate (respectively) where, at all sites, the carbon isotopic composition of the DIC decreases before the sulfur isotopic composition of sulfate increases. We compare our results to global data and show that this behavior persists over a range of sediment types, locations and water depths. We use a reactive-transport model to show how changes in the amount of DIC in seawater, the carbon isotopic composition of organic matter, the amount of organic carbon oxidation by early diagenetic reactions, and the presence and source of methane influence the carbon and sulfur isotopic composition of sedimentary pore fluids and the shape of the δ13CDIC vs. δ34SSO4 cross plot. The δ13C of the DIC released during sulfate reduction and sulfate-driven anaerobic oxidation of methane is a major control on the minimum δ13CDIC value in the δ13CDIC vs. δ34SSO4 cross plot, with the δ13C of the organic carbon being important during both MSR and combined sulfate reduction, sulfate-driven AOM and methanogenesis.

An increase in the slope of the concentration-discharge relation for total organic carbon in major rivers in New England, 1973 to 2019.

The mobilization and transport of organic carbon (OC) in rivers and delivery to the near-coastal ocean are important processes in the carbon cycle that are affected by both climate and anthropogenic activities. Riverine OC transport can affect carbon sequestration, contaminant transport, ocean acidification, the formation of toxic disinfection by-products, ocean temperature and phytoplankton productivity. There have been many studies reporting temporal trends in OC concentrations in comparatively small streams with minimal anthropogenic influences but there have been fewer studies on larger rivers and fewer still that have investigated changes in OC concentration-discharge (C-Q) relations. This study examined changes in C-Q relations for total organic carbon (TOC) from 1973 to 2019 in 8 rivers in New England, USA. TOC concentrations declined in all rivers, and in most rivers, and in most seasons, the slope of the C-Q relation increased between 1973 to 1995 and 1996 to 2019. The increase in C-Q slope between periods may be related to changes in the magnitude of TOC sources. The most likely sources to have changed are wastewater inputs, urban runoff, production through photosynthesis in aquatic systems, and runoff from agricultural and forestry practices. Changes in wetland abundance and changes in sulfate concentrations can be ruled out as drivers of the observed changes in C-Q.

Bacterial and Archaeal Diversity and Abundance in Shallow Subsurface Clay Sediments at Jianghan Plain, China.

Clay layers are common in subsurface where microbial activities play an important role in impacting the biogeochemical properties of adjacent aquifers. In this study, we analyzed the community structure and abundance of bacteria and archaea in response to geochemical properties of six clay sediments at different depths in a borehole (112°34′0″E, 30°36′21″N) of Jianghan Plain (JHP), China. Our results suggested that the top two clay layers were oxic, while the remaining bottom four clay layers were anoxic. Both high-throughput sequencing and qPCR of 16S rRNA gene showed relatively high abundance of archaea (up to 60%) in three of the anoxic clay layers. Furthermore, microbial communities in these clay sediments showed distinct vertical stratification, which may be impacted by changes in concentrations of sulfate, HCl-extractable Fe2+ and total organic carbon (TOC) in the sediments. In the upper two oxic clay layers, identification of phyla Thaumarchaeota (11.2%) and Nitrosporales (1.2%) implied nitrification in these layers. In the two anoxic clay layers beneath the oxic zone, high abundances of Anaeromyxobacter, Chloroflexi bacterium RBG 16_58_14 and Deltaproteobacteria, suggested the reductions of nitrate, iron and sulfate. Remarkably, a significant portion of Bathyarchaeota (∼25%) inhabited in the bottom two anoxic clay layers, which may indicate archaeal anaerobic degradation of TOC by these organisms. The results of this study provide the first systematic understandings of microbial activities in subsurface clay layers at JHP, which may help develop microorganism-based solutions for mitigating subsurface contaminations.

Quantifying the Impacts of Cold Airmass on Aerosol Concentrations Over North China Using Isentropic Analysis

AbstractWinter air quality can vary markedly due to cold air activities. However, the impacts of the polar cold airmass on aerosol variations has not been clarified in a quantitative manner. Using an isentropic analysis method, we investigated changes in the cold airmass and sulfate aerosol distributions over North China during the winter of 2014/2015. The cold airmass and sulfate concentrations exhibited pronounced out‐of‐phase variations, with comparable amplitudes at subseasonal (30–60 days) and synoptic (4–6 days) scales. Subseasonal sulfate variations were closely associated with distribution of the polar cold airmass, as it shifted between zonal and meridional patterns, regulating cold airmass fluxes over North China. Typically, subseasonal surges in the cold airmass consisted of several synoptic disturbances, including one cold air outbreak event and 3–6 renewal/decay oscillations. These synoptic inflows of clean cold airmass repeatedly pushed the warm polluted airmass away from North China. Thus, spatiotemporal variations in sulfate better reflect cold airmass distributions and fluxes than northerly wind regimes. Using diagnostic equations derived herein, we estimated the contribution of various physical processes to aerosol variations during cold air anomalies. We found that the local changes in sulfate concentrations were mainly attributed to advection of warm airmasses, as well as advection and vertical displacement by horizontal convergence of the polar cold airmass. The latter two processes contributed most to sulfate decreases, leading to a relatively strong aerosol reductions during cold air outbreaks, while first two processes led to weaker aerosol reductions during cold air oscillations.

Influence of emission from copper–nickel smelters on the chemical composition of lake water: Acidification forecast

The influence of the emission of sulfur dioxide and solid substances from Pechenganickel and Severonickel copper–nickel combines (Murmansk oblast) on the chemical compositions of lake water and the development of acidification is analyzed. The temporal dynamics of ∼100 lakes, studied in 1990, 1995, 2000, 2005, 2009, and the response of the chemical composition of the lake water to the impact of acid-forming substances depending on the load level (the distance of combines), geologically controlled sensitivity of catchment areas of the lakes studied to acid deposition, and the lake areas is discussed. The likely further changes in sulfate concentration and pH value in the lake water under the scenarios of increase/decrease of sulfur dioxide emissions from smelter are estimated.

Volatile earliest Triassic sulfur cycle: A consequence of persistent low seawater sulfate concentrations and a high sulfur cycle turnover rate?

Marine biodiversity decreases and ecosystem destruction during the end-Permian mass extinction (EPME) have been linked to widespread marine euxinic conditions. Changes in the biogeochemical sulfur cycle, microbial sulfate reduction (MSR), and marine dissolved sulfate concentrations during the Permian-Triassic transition can provide insights into the role of ocean chemistry change in the largest mass extinction in Earth history. In this study, we constrain marine dissolved sulfate concentrations using the MSR-trend method of Algeo et al. [Algeo, T.J., Luo, G.M., Song, H.Y., Lyons, T.W., Canfield, D.E., 2015. Reconstruction of secular variation in seawater sulfate concentrations. Biogeosciences 12, 2131–2151] on sulfur (S) isotope records from Iran (the Kuh-e-Ali Bashi and Zal sections) and Hungary (the Bálvány North and Bálvány East sections). This empirically derived transfer function is based on the S isotope fractionation between sulfate and sulfide associated with MSR in natural aquatic environments. This fractionation is proxied by the difference in S isotope compositions between chromium-reducible sulfur (CRS) and carbonate-associated sulfate (CAS), i.e., Δ34SCAS-CRS. We show that, despite region-specific redox conditions, Δ34SCAS-CRS exhibits a nearly invariant value of 15–16‰ in both study sections. By comparing our record with a Δ34Ssulfate-sulfide density distribution for modern marine sediments, we deduce that porewater Rayleigh distillation, carbonate diagenesis, and other effects are unlikely to have appreciably altered the S isotope offset between CRS and CAS in the study sections. In addition, differences in sedimentary regimes and organic carbon (OC) fluxes between the Iranian and Hungarian sections exclude major influence of the electron donor on MSR-S isotope fractionation and point to a more universal control, i.e., contemporaneous seawater sulfate concentration.The MSR-trend transfer function yielded estimates of seawater sulfate of 0.6–2.8mM for the latest Permian to earliest Triassic, suggesting a balanced oceanic S-cycle with equal S inputs and outputs and no major changes in sulfate concentrations during this interval. However, a secular trend toward heavier δ34SCAS (by >5‰) in the earliest Triassic can be explained only by increasing the turnover rate of the S-cycle (by ca. one order of magnitude) and a concomitant change in terrestrial S sources in a box model experiment. Exposure of evaporite deposits having a high δ34S may account for the source change, with a possible role for the Siberian Traps volcanism by magmatic remobilization of Cambrian rock salt. A high sulfur cycle turnover rate would have left the ocean system vulnerable to development of widespread euxinic conditions, posing a sustained threat to marine life during the Early Triassic.

Character of spatiotemporal variations in the chemical composition of lake water under the influence of emission from copper–nickel plants: Prediction of acidification

In this paper, we analyze the influence of variations in the emission of sulfur dioxide and solid substances by the Pechenganikel and Severonikel copper–nickel plants in Murmansk oblast on the chemical composition of lake water and development of acidification. The dynamics of ~100 lakes examined in 1990, 1995, 2000, 2005, and 2009 and response of the chemical composition of the lake waters on the impact of acidifying substances was explored depending on the magnitude of load (distance from the plants), geologically controlled vulnerability of the lake catchments to acid precipitation, and the size of the lakes. Possible further changes in the sulfate concentration and pH values of lake waters were estimated for scenarios assuming an increase or a decrease in sulfur dioxide emission from the plants. It was shown that, in the zone of maximum and high load, a 20% change in sulfur dioxide emission will result in a mean change in sulfate concentration of ±8 μeq/L (which is comparable with the regional background) and a change in pH value of ±0.1 in acid-sensitive lakes and will have almost no effect on these parameters in lakes insensitive to acid precipitation.

Offsetting effects of aerosols on Arctic and global climate in the late 20th century

Abstract. We examine the impacts of atmospheric aerosols on Arctic and global climate using a series of 20th century transient simulations from Community Climate System Model version 4 (CCSM4). We focus on the response of surface air temperature to the direct radiative forcing driven by changes in sulfate and black carbon (BC) concentrations from 1975 to 2005 and we also examine the response to changes in sulfate, BC, and organic carbon (OC) aerosols collectively. The direct forcing from sulfate dominates the aerosol climate effect. Globally averaged, simultaneous changes in all three aerosols produce a cooling trend of 0.015 K decade−1 during the period 1975–2005. In the Arctic, surface air temperature has large spatial variations in response to changes in aerosol concentrations. Over the European Arctic, aerosols induce about 0.6 K decade−1 warming, which is about 1.8 K warming over the 30-year period. This warming is triggered mainly by the reduction in sulfate and BC emissions over Europe since the 1970s and is reinforced by sea ice loss and a strengthening in atmospheric northward heat transport. Changes in sulfate concentrations account for about two thirds of the warming and BC for the remaining one third. Over the Siberian and North American Arctic, surface air temperature is likely influenced by changes in aerosol concentrations over Asia. An increase in sulfate optical depth over Asia induces a large cooling while an increase in BC over Asia causes a significant warming.

Long-term Changes in Sulfate Concentrations and Soil Acidification of Forested Umbrisols and Andosols of Japan

Long-term changes in sulfate concentrations and soil acidification in two adjacent forest soils (Andosols and Umbrisols) were investigated using soil samples archived from 1967 to 2009. The amounts of phosphate-extracted sulfate in Andosols (≤51.0 mmol kg) were generally higher than that in Umbrisols (≤13.7 mmol kg). Both soils contained extremely high amounts of sulfate compared with the soils of polluted regions of Europe and North America in the 1980s and 1990s. A strong positive correlation was obtained between iron oxide minerals (acid oxalate–extractable Fe minus pyrophosphate-extractable Fe [Feo - Fep]) and phosphate-extracted sulfate, whereas soil organic matter was suggested to inhibit sulfate adsorption especially by allophane (acid oxalate–extractable Al minus pyrophosphate-extractable Al [Alo - Alp]). The amounts of phosphate-extracted sulfate in Umbrisols have gradually decreased by about one third between 1967 and 2009, resulting from the decline of sulfur emissions. However, those in Andosols have remained at high levels, indicating the very slow or irreversible equilibration of adsorbed sulfate in these soils. In addition, surface soils of Andosols from 1994 and 2009 showed soil acidification indicated by the decrease in soil pH, the dissolution of aluminum, and the formation of the precipitates, such as aluminum hydroxysulfate and basic iron sulfate. It is considered that irreversible adsorption of sulfate is an important factor contributing to the decrease in acid-neutralizing capacity. Although acidification in Andosols is slow to appear because of their high acid-neutralizing and sulfate adsorption capacities, acidification persists for a long time once the critical loads of acid deposition have occurred.

Dissipation of Pentachlorophenol in the Aerobic–Anaerobic Interfaces Established by the Rhizosphere of Rice (Oryza sativa L.) Root

Phytoremediation is an emerging technology for the detoxification and remediation of organic pollutants such as pentachlorophenol (PCP). To investigate the dissipation behavior of PCP in the aerobic-anaerobic interfaces established by the rhizosphere of rice ( L.) root, a glasshouse experiment was conducted using a specially designed rhizobox. The possible biogeochemical mechanisms were also studied through illustration of the dynamic behavior of important electron acceptors and donors that are potentially involved in the reductive dechlorination and aerobic catabolism processes of PCP. The soil was spiked with 20 ± 0.25 and 45 ± 0.25 mg of PCP kg soil. Soil in the rhizobox was divided into five different compartments at various distances from the root surface. Maximum dissipation of PCP in planted soil was observed at 3-mm distance from the root zone as well as rapid changes in concentrations of sulfate, chloride, nitrate, and ammonium at the same distance from the root. In contrast, in the unplanted soil, no difference was observed in the PCP concentration with increasing distance. After 45 d, a significantly higher concentration of PCP was degraded in planted soil compared with unplanted soil. In the unplanted microcosms, about 45% of the initial PCP was lost at both low and high added rates, respectively. This was, proportionately, a significantly smaller percentage compared with the planted rhizosphere (an average of 66 and 64.5%, respectively). Moreover, the correlations of PCP dissipation with SO, NO, and Fe were significantly negative, while the correlations of PCP dissipation with NH, Fe, and Cl were significantly positive. This suggested the oxidization of soil constituents can inhibit aerobic catabolism of PCP by consuming O, and the reduction of soil constituents can inhibit anaerobic reductive dechlorination of PCP. Therefore, the significance of the rhizosphere in phytoremediation of chlorinated compounds such as PCP differs significantly between wetland and rainfed systems.

Increasingly ice-free winters and their effects on water quality in Sweden’s largest lakes

Mean global air temperatures have steadily increased during recent decades, resulting in an earlier timing of lake ice breakup. In Sweden’s largest lakes, Vanern and Vattern, the breakup of ice has occurred considerably earlier since 1979 and ice-free winters have become more frequent. Comparison between the years when the lakes were ice covered with those when they remained ice-free in terms of 37 lake variables revealed significant differences in water temperatures, sulphate concentrations and the biomass of diatoms in May after ice breakup (P < 0.01). In particular, the biomass of the genus Aulacoseira increased significantly, which may explain increasing complaints about algae that clog fishing-nets, filter-beds and micro-strainers in waterworks in Vanern and Vattern. We assume that Aulacoseira is mainly affected by changes in climate-driven water circulation patterns. In contrast, other observed water quality changes such as changes in sulphate concentration might rather be attributed to changes in atmospheric deposition. To explain water quality changes in Sweden’s largest lakes it is important to consider changes in both climate and atmospheric deposition as well as catchment measures.

Evaluation of Wetland Methyl Mercury Export as a Function of Experimental Manipulations

Mercury associated with natural enrichment, historic mining, and ore processing is a contaminant of concern in watersheds of the western USA. In this region, water is a highly managed resource and wetlands, known to be important sites of methyl mercury production, are often an integral component of watersheds. This study applied controlled manipulations of four replicated experimental wetland designs with different water and soil mercury concentrations to determine the potential impacts on methyl mercury export. Wetlands were manipulated by drying and wetting, changing hydraulic retention time, and adding sulfate and nitrate to influent waters. In a summer drying and wetting manipulation, an immediate increase in total methyl mercury release was observed with rewetting, however, concentrations decreased quickly. Drying all wetlands over the winter and rewetting in the spring resulted in high net methyl mercury output relative to that observed before drying. Net methyl mercury output was not influenced by changes in hydraulic retention time from 4 to 8 h or to 30 min, or by increasing the nitrate concentration from 0.1 to 10 mg L(-1). The addition of sulfate to the inlet waters of two mesocosms to increase concentrations from approximately 100 to 250 mg L(-1) did not result in a clear effect on methyl mercury output, most likely due to sulfate concentrations being higher than optimal for methyl mercury production. Despite the lack of response to sulfate amendments, the change in sulfate concentration between the inlet and outlet of the mesocosms and temperature were the parameters best correlated with methyl mercury outputs.

Nitrogen Cycling and Mass Balance for a Forested Catchment in the Italian Alps. Assessment of Nitrogen Status

During 1999–2001 the chemical composition and fluxes were measured in rainfall, throughfall, soil solution and stream water in a remote forested site in the Italian Alps. The analysis of temporal patterns revealed the differential behaviour of nitrogen and sulphur and suggested that different mechanisms controlled their flux. No important changes in sulphate concentration and fluxes emerged as the solution passed through the various components of the forest ecosystem, and temporal variations of SO4 in the soil solution and stream were likely driven by the physical process of dilution. The availability of nitrate and ammonia, by contrast, was drastically reduced as throughfall water entered the soil and passed through the mineral layers, irrespective of season. The calculated hydrochemical budget based on throughfall and soil solution N fluxes revealed that ~80% N retention in the forest soil, corresponding to 12 kg ha−1 yr−1, despite a relatively high N deposition loading (15 kg ha−1 yr−1). Most of the leached nitrogen (90%) was in the organic form. Indicators of the N status of this ecosystem, such as C/N ratio in solid and solution phase of the soil and N foliage content as well as land use history were examined. Despite the strong N retention in the forested part of the catchment, the stream water N–NO3 levels were consistently above 10 μg l−1 suggesting that the Val Masino catchment as a whole was less efficient in processing atmospheric N inputs. This contrasting N behaviour illustrates the role of landscape features, such as the soil cover and vegetation type, that is characteristic of an alpine catchment.

Long‐term (18‐year) changes in sulphate concentrations in two Ontario headwater lakes and their inflows in response to decreasing deposition and climate variations

AbstractSulphate concentrations in two headwater lakes and their major inflows were evaluated over an 18 year period (1980–81 to 1997–98) during which time sulphate bulk deposition declined by approximately 40%. The two lake catchments represent either end of the spectrum of acid sensitivity in the Muskoka–Haliburton region of Ontario. Between 1980 and 1998, sulphate concentrations in Harp and Plastic Lakes decreased, but the decrease was much less than expected (28% and 21% respectively) given the magnitude of change in deposition. Sulphate export in streams draining into the lakes greatly exceeded sulphate input to catchments in most years, which quantitatively explains the response of lake‐sulphate concentration. Furthermore, temporal patterns in mean annual sulphate concentrations in streams were similar, and appeared to be related to climate factors. Specifically, catchment export of sulphate was greater and stream‐sulphate concentrations were higher in years that had warm, dry summers, i.e. when streamflow in many catchments ceased for up to several weeks. Increased sulphate export from catchments resulted in higher sulphate concentrations in lakes, but the response of lake sulphate was not as immediate or dramatic as the response of stream sulphate to changes in catchment dryness. Factors that affect sulphate retention or export in catchments exert a strong influence on sulphate concentrations in lakes and streams and need to be considered when evaluating the response of surface water chemistry to changes in sulphate deposition. Copyright © 2004 John Wiley &amp; Sons, Ltd.