Sediment-water Interactions Research Articles

Interactions between Mercury and Biogeochemical Features of Aquatic Ecosystems: A Critical Review

Background: Mercury (Hg) is a pollutant known to affect the nervous system. The most threatening form of Hg is methylmercury (MeHg), which can biomagnify in aquatic biota. This critique summarizes our current understanding of the major relationships between mercury speciation in aquatic environments and various environmental factors to determine the most suitable indicators of MeHg pollution. Methods: Searches were performed using Web of Science. Ultimately, 33 studies were chosen and reviewed. Priority was given to recent studies as this review focused on the current state of our knowledge. Summary: The total amount of Hg (THg) alone cannot be used as an indicator of MeHg since the relationship between THg and MeHg is poorly correlated. Microbial DNA is often used in research to give insight into the mercury cycle and fate in aquatic systems, but further studies are needed to accurately assess MeHg concentration using DNA. Based on an early study, it was thought that water colour could indicate the amount of Hg in aquatic biota. However, subsequent work has shown this to not be the case since dissolved organic matter (DOM) can help or hinder Hg bioavailability and bioaccumulation. There is a nonlinear relationship between dissolved organic carbon (DOC) and MeHg bioaccumulation where there is a threshold concentration (~8.5 mg C L-1 DOC) above which mercury bioaccumulation is hindered. A recent study found that the methylation rate of an aquatic system was correlated to the type of organic matter present in the sediments, although it could not predict the amount of MeHg present in the water since runoff brings Hg from the catchment to the downstream aquatic system. Recent advancements have been made to understand components of the mercury cycle (e.g. land-water interactions, microbial methylation, water-sediment interactions), but a model encompassing all components has yet to be constructed.

Removal of the algal toxin microcystin‐LR in permeable coastal sediments: Physical and numerical models

AbstractMicrocystin is one of the most common toxins associated with freshwater harmful algal blooms, but little is known about microcystin fate in the aquatic environment. Laboratory wave tank experiments were performed to determine whether exchange of surface water and pore water (benthic exchange) removes and dilutes microcystin‐LR (MC‐LR) at environmentally relevant concentrations in coastal waters overlying permeable sediments. Over the 100 h experiment, 60% of MC‐LR mass was removed due to interaction with sediment (via adsorption and/or biodegradation), while only 20% was removed in an experiment without sediment. The observed fate and transport of MC‐LR in sediments was adequately described with a one‐dimensional reactive transport model that uses an enhanced diffusion coefficient to represent benthic exchange of solutes. Numerical sensitivity studies showed that MC‐LR removal increases with hydraulic conductivity of sediment and wave height and decreases with water depth. For MC‐LR concentration at the WHO recreational guideline (20 ppb), sandy sediments can remove the equivalent MC‐LR mass in 1 m of surface water under typical nearshore wave conditions within tens of hours. In open water at large depths above a silty bed, removal times are much longer (on the order of weeks). Wave‐driven benthic exchange is therefore an important control on MC‐LR fate in energetic coastal areas but not in deep or calm settings where sediment–water interactions are greatly reduced. The nearshore fate of algal toxins is important to human health and socioeconomic vitality, since recreational activities and direct human exposures are concentrated along coasts.

Decoupled water-sediment interactions restrict the phosphorus buffer mechanism in agricultural streams

Our study aimed to explore the effects of agriculture on the phosphorus buffer capacity of 11 headwater streams in Austria. We used phosphorus adsorption curves and re-suspension experiments to determine both, the potential of the sediments to act as phosphorus source or sink and the actual phosphorus exchange between water and sediments. Additionally, we determined the alkaline phosphatase activity (APA) in epilithic and epipsammic biofilms as indicator for the phosphorus demand of the benthic and hyporheic community. We hypothesized that highly polluted streams will show decreased phosphorus buffer capacities, which were either due to saturation or restricted water-sediment interactions.Our results support the second hypothesis. Fine sediment accumulations, organic matter content, and phosphorus concentrations in water and sediments increased with percent cropland in the catchment. Below SRP concentrations of 120μgL−1 in the stream water, sediments showed a high potential for phosphorus release, with zero equilibrium phosphorus concentrations (EPC0) being more than twice as high as SRP concentrations. Above 150μgL−1, EPC0 reached only 20–50% of SRP concentrations, indicating a high potential of the sediments to act as phosphorus sinks. These findings were confirmed by phosphorus uptake of these sediments during re-suspension. While APA in epilithic biofilms decreased with increasing SRP concentrations, APA in epipsammic biofilms showed the reverse pattern, indicating a restricted phosphorus supply of the hyporheic community despite phosphorus surplus in the water column.Our study shows that inputs of fine sediments from agricultural sources may reduce the phosphorus buffering mechanism of stream sediments through restrictions of water-sediment interactions. Consequently, water column and sediment processes are increasingly decoupled and phosphorus-rich stream water will not effectively reach the reactive sites in the sediments responsible for uptake. Therefore, phosphorus mitigation measures in stream ecosystems must comprise sediment management in the catchment as well as in-stream measures for the rehabilitation of the hyporheic zone.

Arsenic in Holocene aquifers of the Red River floodplain, Vietnam: Effects of sediment-water interactions, sediment burial age and groundwater residence time

Water-sediment interactions were investigated in arsenic contaminated Holocene aquifers of the Red River floodplain, Vietnam, in order to elucidate the origin of the spatial variability in the groundwater arsenic concentration. The investigated aquifers are spread over an 8 × 13 km field area with sediments that varied in burial age from <1 kyr to 11 kyr. The groundwater age ranged from less than 2 yr, up to a maximum near 90 yr. Groundwater As concentrations are between 0 and 6.5 µM and there are no simple correlations between the As concentration and groundwater age or aquifer sediment burial age. The aquifers are anoxic with up to 2 mM CH4 and up to 0.5 mM DOC. The downward advective DOC flux is too small to support both methanogenesis and the reduction of As-containing Fe-oxides and sedimentary carbon is therefore considered the main carbon source for the redox processes. The groundwater H2 concentration ranged between 0.1 and 4 nM. These values are intermediate between ranges characteristic for Fe-oxide reduction and methanogenesis and suggest that both processes take place simultaneously. The groundwater pe was calculated from the H2/H+ and CH4/CO2 redox couples, giving almost similar results that apparently reflects the pe of the bulk groundwater. The pe calculated for the As(III)/As(V) redox couple was found in disequilibrium with the other redox couples. Using the pe calculated from the CH4/CO2 redox couple we show that the groundwater has a reducing potential towards Fe-oxides ranging from ferrihydrite to poorly crystalline goethite, but not for well crystalline goethite or hematite. Hematite and poorly crystalline goethite were identified as the Fe-oxides present in the sediments. Reductive dissolution experiments identify two phases releasing Fe(II); one rapidly dissolving that also contains As and a second releasing Fe(II) more slowly but without As. The initial release of Fe and As occurs at a near constant As/Fe ratio that varied from site to site between 1.2 and 0.1 mmol As/mol Fe. Siderite (FeCO3) is the main sink for Fe(II), based on saturation calculations as well as the identification of siderite in the sediment. Most of the carbonate incorporated in siderite originates from the dissolution of sedimentary CaCO3. Over time the CaCO3 content of the sediments diminishes and FeCO3 appears instead. No specific secondary phases that incorporate arsenite could be identified. Alternatively, the amount of arsenic mobilized during the dissolution of reactive phases can be contained in the pool of adsorbed arsenite. Combining groundwater age with aquifer sediment age allows the calculation of the total number of pore volumes flushed through the aquifer. Comparison with groundwater chemistry shows the highest arsenic concentration to be present within the first 200 pore volumes flushed through the aquifer. These results agree with reactive transport modeling combining a kinetic description of reductive dissolution of As-containing Fe-oxide with adsorption and desorption of arsenite. Understanding variability in groundwater arsenic concentration requires appreciating the coupling of the chemical processes to both sedimentary and hydrogeological cycling.

Sediment-Water Methane Flux in a Eutrophic Pond and Primary Influential Factors at Different Time Scales

Water bodies are major areas for methane release. Eutrophic water bodies may promote methane flux. The sediment-water interface is the major location for methane release, and studies on sediment-water interactions are necessary to regulate methane release in water. However, relevant studies on methane flux at the sediment-water interface are limited due to methodological difficulties. Using an innovative gas trapping device, this study investigated the seasonal characteristics and diel variation in summer methane flux from eutrophic water bodies and analyzed the correlations between temperature, dissolved oxygen (DO), different forms of nitrogen and the methane flux at different time scales. The results showed that methane flux in the eutrophic pond was high and had distinct seasonal variations and diel variations: the average value was 2.81 ± 0.19 mmol m−2 h−1 in summer, which was significantly greater than that in spring (0.62 ± 0.14 mmol m−2 h−1), autumn (0.63 ± 0.10 mmol m−2 h−1) (p &lt; 0.01) and winter (approached zero). The diel characteristics of methane flux in summer exhibited a unimodal pattern of increase at night and decrease during the day. The correlation analysis showed that the sediment-water methane flux rate of the water body was significantly positively correlated with the temperature and NH4+ concentration and significantly negatively correlated with DO, NO3− and NO2− concentration. Meanwhile, among different time scales, the correlations between NO3−/NH4+ concentration and methane flux were the highest at the diel scale in summer (R2 = 0.68, 0.87 respectively) when the temperature and DO vibration was low and the relationship between temperature/DO and methane flux was poor (R2 = 0.45, 0.87 respectively). This study considered that higher NH4+-N and lower NO3−-N/NO2−-N content in eutrophic water could have an effect on the high methane flux in summer as well as the low dissolved oxygen content.

Trace elements: water-sediment interactions in tropical rivers.

This research aimed to determine the water-sediment interaction (partition coefficient Kd) of trace element (Cd, Cu, total Cr, Pb, and Zn) in tropical rivers of the Upper Paraná River basin, Central Brazil. Three trace elements (Cu, total Cr, and Zn) presented quantifiable concentration values in the water and sediment. Neither the element trace considered nor the rivers displayed a similar water-sediment interaction. The evaluation of Kd values indicates the tendency of total Cr to be adsorbed into the sediment (min Kd=6.244, max Kd=131.389), mainly in one sampling station (São Francisco River, Kd=131.389) and the availability of Zn in the water column in all sampling stations (min Kd=0.234, max Kd=1.289). The sediment concentrations of Cr in the São Francisco sampling station (0.118mgL-1) are above international reference values suggesting a risk of contamination for the biota, whereas in four rivers, Cr concentrations represent a risk. The water-sediment interaction of Cu is influenced by water temperature, whereas the pH influenced the Zn interaction.

Transport of Conservative and “Smart” Tracers in a First-Order Creek: Role of Transient Storage Type

Using “smart” tracers such as Resazurin (Raz) allows assessment of sediment-water interactions and associated biological activity in streams. We compared two approaches to simulate the effects of transient storage (TS) on the transport of conservative and reactive tracers. The first approach considered TS as composed of metabolically active and metabolically inactive compartments, while the second model approach accounted for the surface transient storage (STS) and hyporheic transient storage (HTS). Experimental data were collected at a perennial first-order creek in Maryland, MD, USA, by injecting the conservative tracer bromide (Br) and the reactive (Raz) tracer and sampling water at two weir stations. The STS–HTS approach led to a more accurate simulation of Br transport and tails of the Raz and its product Rezorufin (Rru) breakthrough curves. Sediments support large microbial communities, and the STS–HTS model in creeks provides additional parameters to characterize the habitats of microbial water-quality indicator organisms.

The origin of Late Devonian (Frasnian) stratiform and stratabound mudstone-hosted barite in the Selwyn Basin, Northwest Territories, Canada

Multiple occurrences of bedded barite (BaSO4) are preserved in mudstones of the upper-most Canol Formation (Frasnian) of the Selwyn Basin (Canada). Samples of barite were collected from eight sites (seven measured sections), and examined using petrographic and isotopic techniques (87Sr/86Sr, δ34S and δ18O values) to determine whether the barite has a diagenetic or hydrothermal origin.Barite at all locations occurs in two morphologies: (1) laminated barite composed of thin layers of microcrystalline barite with lesser amounts of pyrite and hyalophane (Ba-K-feldspar) interlaminated with mudstone, and (2) nodular barite consisting of barite, hyalophane, cymrite (hydrous Ba-silicate) and minor pyrite. At the Cowan occurrence, one sample has a mineral assemblage comprising barite and nodular barium carbonates (witherite, norsethite).The δ34S and δ18O values of the laminated barite co-vary, and range between +24.1 and +35.3‰ and +14.8 to +18.3‰ respectively, with the exception of one sample with δ34S and δ18O values of +32.9‰ and −0.5‰. The laminated samples have an average 87Sr/86Sr of 0.7085 (n = 8), which is consistent with Frasnian seawater (0.7081) that has undergone some minor water sediment interaction during early diagenesis.The nodular barite has highly variable δ34S (+4.8 to +56.5‰) and δ18O (+9.2 to +19.3‰) values, which developed from more evolved diagenetic pore fluids with limited sulphate re-supply (i.e., as a relatively closed system). In these conditions sulphate reduction likely occurred at a slower rate, and the sulphate in the barite records both kinetic (δ34SSO4) and equilibrium (δ18OSO4) isotope effects. One sample of nodular barite has a 87Sr/86Sr ratio of 0.7086. The single sample with a barite and barium-carbonate assemblage at Cowan has a 87Sr/86Sr ratio of 0.7133, a δ34S value of +23.9‰, and a δ18O value of +10.7‰ and may represent a local hydrothermal event at this site.Our data supports the interpretation that the barite horizon is a stratigraphic marker throughout the Canol Formation. Laminated and nodular barite formed as a result of diagenetic processes, which recycled Ba from organic matter in the sediment below, and reprecipitated it above, the sulphate methane transition zone in the sediments.The results of this study are similar to those obtained for other Late Devonian sediment-hosted barite occurrences in North America (Yukon, Nevada, New York State), meaning these occurrences likely formed by similar processes and diagenetic pore water conditions within oceanic sediments that covered the North American continent during the Late Devonian. We suggest that the combination of high biological productivity and low seawater sulphate at this time contributed to the efficient cycling of Ba from seawater into the sediments on the continental margin.

Isotope constraints on seasonal dynamics of dissolved and particulate N in the Pearl River Estuary, south China

Isotope measurements were performed on dissolved NO3−, NH4+ and suspended particulate total N along a salinity gradient in the Pearl River Estuary (PRE) to investigate seasonal changes in main N sources and its biogeochemical processing under the influence of monsoon climate. Our data revealed that municipal sewage and re-mineralized soil organic N were the major sources of DIN (NO3− and/or NH4+) in freshwater during winter and summer, respectively, whereas phytoplankton biomass was a major component of PN in both seasons. In low salinity waters (<2–3), nitrification was proved to be a significant NO3− source via NH4+ consumption, with N isotope effects of −15.3‰ in summer and −23.7‰ in winter for NH4+ oxidation. The contribution of nitrification to the total NO3− pool was smaller in summer than in winter, most likely due to freshwater dilution. At mid-salinities (3–20), δ15N values of PN were similar to those of NO3− and NH4+ in summer, reflecting a strong coupling between assimilation and remineralization. In winter, however, higher δ15NNH4 but lower δ15NNO3 than δ15NPN were observed, even though δ15NPN was similar between summer and winter. Intense sediment-water interaction and resuspension of sediments during winter appeared largely responsible for the decoupling. At high salinities, the greater enrichment in δ18ONO3 than in δ15NNO3 (up to 15.6‰) in winter suggests that atmospheric deposition may contribute to NO3− delivery during the dry season. Overall, these results show the importance of seasonal variability in physical forcing on biological N sources and its turnover processes in the highly dynamic river-dominated estuary. This article is protected by copyright. All rights reserved.

Backstepping stabilization of the linearized Saint-Venant–Exner model

Using backstepping design, exponential stabilization of the linearized Saint-Venant–Exner (SVE) model of water dynamics in a sediment-filled canal with arbitrary values of canal bottom slope, friction, porosity, and water–sediment interaction, is achieved. The linearized SVE model consists of two rightward convecting transport Partial Differential Equations (PDEs) and one leftward convecting transport PDE. A single boundary input control strategy with actuation located only at the downstream gate is employed. A full state feedback controller is designed which guarantees exponential stability of the desired setpoint of the resulting closed-loop system. Using the reconstruction of the distributed state through a backstepping observer, an output feedback controller is established, resulting in the exponential stability of the closed-loop system at the desired setpoint. The proposed state and output feedback controllers can deal with both subcritical and supercritical flow regimes without any restrictive conditions.

Investigating geochemical aspects of managed aquifer recharge by column experiments with alternating desalinated water and groundwater

Managed aquifer recharge (MAR) events are occasionally carried out with surplus desalinated seawater that has been post-treated with CaCO3 in infiltration ponds overlying the northern part of the Israeli Coastal Aquifer. This water's chemical characteristics differ from those of any other water recharged to the aquifer and of the natural groundwater. As the MAR events are short (hours to weeks), the sediment under the infiltration ponds will intermittently host desalinated and natural groundwater. As part of comprehensive research on the influence of those events, column experiments were designed to simulate the alternation of the two water types: post-treated desalinated seawater (PTDES) and natural groundwater (GW). Each experiment included three stages: (i) saturation with GW; (ii) inflow of PTDES; (iii) inflow of GW. Three runs were conducted, each with different sediments extracted from the field and representing a different layer below the infiltration pond: (i) sand (<1% CaCO3), (ii) sand containing 7% CaCO3, and (iii) crushed calcareous sandstone (35% CaCO3). The results from all columns showed enrichment of K+ and Mg2+ (up to 0.4meq/L for 20 pore volumes) when PTDES replaced GW, whereas an opposite trend of Ca2+ depletion (up to 0.5meq/L) was observed only in the columns that contained a high percentage of CaCO3. When GW replaced PTDES, depletion of Mg2+ and K+ was noted. The results indicated that adsorption/desorption of cations are the main processes causing the observed enrichment/depletion. It was concluded that the high concentration of Ca2+ (relative to the total concentration of cations) and the low concentration of Mg2+ in the PTDES relative to natural GW are the factors controlling the main sediment–water interaction. The enrichment of PTDES with Mg2+ may be viewed as an additional post-treatment.

Effects of water-sediment interaction and irrigation practices on iodine enrichment in shallow groundwater

High iodine concentrations in groundwater have caused serious health problems to the local residents in the Datong basin, northern China. To determine the impact of water-sediment interaction and irrigation practices on iodine mobilization in aquifers, isotope (2H, 18O and 87Sr/86Sr) and hydrogeochemical studies were conducted. The results show that groundwater iodine concentrations vary from 14.4 to 2180μg/L, and high iodine groundwater (>150μg/L) mainly occurs in the central area of the Datong basin. Sediment iodine content is between <0.01 and 1.81mg/kg, and the co-occurrence of high iodine and high DOC/TOC concentrations of groundwater and sediment samples in the deeper aquifer indicates that the sediment enriched in iodine and organic matter acts as the main source of groundwater iodine. The 87Sr/86Sr values and groundwater chemistry suggest that aluminosilicate hydrolysis is the dominant process controlling hydrochemical evolution along groundwater flowpath, and the degradation of TOC/iodine-rich sediment mediated by microbes potentially triggers the iodine release from the sediment into groundwater in the discharge area. The vertical stratification of groundwater 18O and 2H isotope reflects the occurrence of a vertical mixing process driven by periodic surface irrigation. The vertical mixing could change the redox potential of shallow groundwater from sub-reducing to oxidizing condition, thereby affecting the iodine mobilization in shallow groundwater. It is postulated that the extra introduction of organic matter and O2/NO3/SO4 could accelerate the microbial activity due to the supplement of high ranking electron acceptors and promote the iodine release from the sediment into shallow groundwater.

Origin and weathering of landslide material in a loess area: a geochemical study of the Kulcs landslide, Hungary

Geochemical characteristics of sediments are responses to physical and chemical alteration in landslides. However, consequences of in situ interactions associated with landslides are difficult to distinguish from those related to long-term weathering in young soft sediments such as loess. In this study, geochemical characteristics of the Kulcs landslide in Hungary are studied to identify the provenance of the loess–paleosol–red clay sequence and geochemical signatures that can potentially be attributed to the effects of landsliding. Results indicate that sliding is largely initiated by the lithological changes within the landslide body. Sediments above the sliding zone closely resemble the non-slipped Pleistocene old loess deposits from Hungary. It is also confirmed that the sliding zone develops in old paleosols in the loess sequence and red clays at its base which are all characterized by the enrichment of Al, K, Na, H2O and considerable depletion in Ca and Mg associated with carbonates. Altogether, these geochemical characteristics indicate that chemical weathering trend of unconsolidated landslide sediments is slightly modified by the redistribution of carbonates and decomposition of plagioclase. It is assumed that the distribution of Mn and Ba is modified by the water–sediment interaction in the landslide.

Evolution of chemical and isotopic composition of inorganic carbon in a complex semi-arid zone environment: Consequences for groundwater dating using radiocarbon

Estimating groundwater age is important for any groundwater resource assessment and radiocarbon (14C) dating of dissolved inorganic carbon (DIC) can provide this information. In semi-arid zone (i.e. water-limited environments), there are a multitude of reasons why 14C dating of groundwater and traditional correction models may not be directly transferable. Some include; (1) the complex hydrological responses of these systems that lead to a mixture of different ages in the aquifer(s), (2) the varied sources, origins and ages of organic matter in the unsaturated zone and (3) high evaporation rates. These all influence the evolution of DIC and are not easily accounted for in traditional correction models. In this study, we determined carbon isotope data for; DIC in water, carbonate minerals in the sediments, sediment organic matter, soil gas CO2 from the unsaturated zone, and vegetation samples. The samples were collected after an extended drought, and again after a flood event, to capture the evolution of DIC after varying hydrological regimes. A graphical method (Han et al., 2012) was applied for interpretation of the carbon geochemical and isotopic data. Simple forward mass-balance modelling was carried out on key geochemical processes involving carbon and agreed well with observed data. High values of DIC and δ13CDIC, and low 14CDIC could not be explained by a simple carbonate mineral–CO2 gas dissolution process. Instead it is suggested that during extended drought, water–sediment interaction leads to ion exchange processes within the top ∼10–20m of the aquifer which promotes greater calcite dissolution in saline groundwater. This process was found to contribute more than half of the DIC, which is from a mostly ‘dead’ carbon source. DIC is also influenced by carbon exchange between DIC in water and carbonate minerals found in the top 2m of the unsaturated zone. This process occurs because of repeated dissolution/precipitation of carbonate that is dependent on the water salinity driven by drought and periodic flooding conditions. This study shows that although 14C cannot be directly applied as a dating tool in some circumstances, carbon geochemical/isotopic data can be useful in hydrological investigations related to identifying groundwater sources, mixing relations, recharge processes, geochemical evolution, and interaction with surface water.

Hydrochemical and sedimentological dynamics in a subtropical plain river: assessment by multivariate statistical analysis

Understanding the transport dynamics of sediments and soluble components is critical when ecosystems could be adversely affected. This study examined some environmental factors influencing the spatial and temporal variations of water–sediment interactions in Negro River, northeast of Argentina, a subtropical plain fluvial system with anthropogenic influence. Samples of water and bottom sediments were collected from four sampling stations in four sampling campaigns spread over 2 years. Chemical parameters, including pH, conductivity, major ions, concentration of suspended sediments, extractable major cations and particle size of sediments, were determined. The experimental data were interpreted by classical hydrogeochemical methods and statistical methods. Multivariate statistical tools were applied to reduce the dimensionality of the data set and to evaluate the relative importance of combinations of environmental variables on sediment dynamics. The results showed that the variability of the hydrochemistry and the sediment chemistry is controlled by longitudinal and seasonal factors, whereas differences across the river section were nonsignificant. The concentration of suspended solids and the abundance of fine-grained materials in bottom sediments were inversely correlated with the concentration of some metals with agglutinant and coagulant properties in sediments. This corroborates the presence of fine particle aggregates, also evidenced by classical grain-size analysis. Thus, particle disaggregation by chemical methods is not recommended, as it could lead to misinterpretation when the objective is to evaluate the dynamics of sediments in natural environments. This study highlights the importance of analyzing multiple dimensions (spatial and temporal) to understand the dynamics of fluvial sediments, especially in plain rivers.

Sediment–water interaction in phosphorus cycling as affected by trophic states in a Chinese shallow lake (Lake Donghu)

Lake sediment substantially accumulates nutrients, while little is known regarding the conditions under which it tends to be a sink or source of phosphorus. It is postulated that the above functions were largely dependent on trophic state. To test this hypothesis, composition, abundance, and size-spectrum of phytoplankton were studied in a Chinese shallow lake (Lake Donghu), together with concentrations and sorption behaviors of phosphorus in water column and sediment. Relationships among these variables were also examined by structural equation model. In the basins with the lowest phosphorus concentration, sediment was a phosphorus source as judged by equilibrium phosphorus concentration, directly affecting the abundance of phytoplankton with smaller size. Contrastingly, in the basins with the highest phosphorus concentration, sediment tended to uptake phosphorus, companied by the lowest ratio of nitrogen to phosphorus, diversity, and evenness of phytoplankton with the smaller size one dominating. Oscillatoria sp. was directly affected by ammonia and phosphorus. Moreover, with a balanced exchange of phosphorus between sediment and water column, phytoplankton increased markedly with the highest diversity and evenness in the basins having intermediate phosphorus concentration. Overall, trophic state modulated sediment functions to supply nutrient and was in turn greatly affected by the sediment in shallow lakes.

Redox processes and occurrence of arsenic in a volcanic aquifer system of Kumamoto Area, Japan

Sources, distributions, and controlling factors for mobilization of arsenic (As) in Kumamoto basin were investigated relating to the determination of redox processes of the study area. Groundwater and sediment core samples were analyzed. Nitric acid digestion and sequential leaching experiment of sediment core samples revealed that the source of As in the groundwater is geogenic and leached primarily due to the sediment–water interactions. Unlikely, similar relations of total As with total Fe, Mn, and Al in acid extracts, leaching experiments showed a positive relation of Astotal with Fetotal and Altotal indicated that Fe and Al oxides/hydroxides are abundant and may be the major adsorbent of As in low pH condition. Arsenic concentrations in groundwater ranges from 0.1 to 60.6 μg/l. High As concentrations occurred in anaerobic stagnant groundwaters from Kumamoto plain area with high dissolved Fetotal, Mntotal, and Altotal, moderately dissolved HCO3 −, PO4 3−, SO4 2−, and low concentrations of NO3 − and DOC suggesting the reducing condition of subsurface aquifer. There is a range of As(III)/As(T) ratios from mostly arsenate to mostly arsenite. Groundwater pH was relatively high, and high As occurred at higher pH range. It is assumed that desorption of As from metal oxide surfaces was facilitated by the elevated pH, which is considered as an important process for As mobilization. In addition to this, a wide range of δ34SSO4 values (8.3–57.6 ‰) indicates that sulfur is undergoing redox cycling mediated by microbial activities. Following δ34SSO4 results, it is anticipated that dissolved As is coprecipitated as sulfides in the presence of dissolved Fe(II) in some places, though at this moment, there is no direct evidence of coprecipitation or sequester of As with Fe and sulfide ion. Finally, a combination of the following three variables are considered potentially important causes for the high dissolved As concentrations in groundwater of Kumamoto area—(1) high groundwater pH, (2) anoxic redox conditions, and (3) stagnant groundwater in younger age sediments, which have not been well flushed since burial.

Trend of Heavy Metal Release According to Forecasted Climate Change in the Po Delta

The low-lying coastal areas, and especially the delta areas are prone to saltwater intrusion due to sea level rise, which may lead to a loss of ecosystem services brought about by salinization of already limited freshwater reserves. Despite the acknowledged importance of climate change impacts and land use practices on water resources availability, there is still a lack of studies on the issue of climate change induced alteration of water quality. To understand the hydrogeochemical processes occurring within the shallow groundwater system of the Po River delta (Italy), the contribution of local water-sediment interaction and the anthropogenic influence on groundwater quality were quantified. High-resolution multi-level sampling was used to capture chemical gradients within the aquifer. Data were employed to calibrate a density-dependent multicomponent reactive transport model implemented with PHT3D code along a flow line. The reactive network accounted for redox driven organic matter oxidation and cation exchange. Finally, a series of numerical scenarios were run following the projection of IPCC 2014 on sea level rise and climate change. Results of the predictive model highlight an increase of arsenic and lead in the shallow portion of the aquifer, drained by the reclamation network. The increase of heavy metals export toward surface waters could be of serious concern, since the drainage network operates also for irrigation purposes. Thus, the surface water quality could be negatively affected by climate change.

Mineralogical control on arsenic release during sediment–water interaction in abandoned mine wastes from the Argentina Puna

The sulfide-rich residues of La Concordia mine, Argentina Puna, are accumulated in tailing dams that remained exposed to the weathering agents for almost 30years. In such period of time, a complex sequence of redox and dissolution/precipitation reactions occurred, leading to the gradual oxidation of the wastes and the formation of weathering profiles. The sources of arsenic in the wastes were analyzed by XRD and SEM/EDS analysis while a standardized sequential extraction procedure was followed to define solid As associations. In addition, the release of As during sediment–water interaction was analyzed in a period of 10months. The results indicate that primary As-bearing minerals are arsenian pyrite and polymetallic sulfides. As-jarosite and scarce arsenates are the only secondary As-bearing minerals identified by XRD and SEM/EDS. However, the rapid release (i.e., <1h) of arsenic from suspensions of the studied sediments in water, seems to be associated to the dissolution of highly soluble (hydrous)sulfates, as it was determined in samples of the efflorescences that cover the entire site. Contributions from the more abundant As-jarosite are also expected in longer periods of sediment–water interaction, due to its low rate of dissolution in acid and oxic conditions. Finally, near 30% of As remains adsorbed onto Fe (hydr)oxides thus representing a hazardous reservoir with the potential of mobilizing As into porewaters and streamwaters if the acidic and oxidizing conditions that predominate in the region are altered.

Recalibration of the Lake Okeechobee Water Quality Model (LOWQM) to extreme hydro-meteorological events

The Lake Okeechobee Water Quality Model (LOWQM) was developed using an enhanced version of WASP5 to improve understanding of nutrient cycling – specifically total phosphorus – and to assess lake-wide averaged nutrient and algal responses to management alternatives. Previously, the LOWQM was calibrated/validated to hydro-meteorological and monitoring data from 1983 to 2000. Subsequently, a series of extreme hydro-meteorological events, including three hurricanes and two major droughts, occurred from 2005 to 2008 that were outside the range of observations in this original LOWQM model calibration (original simulation). These events affected the light environment, nutrients and phytoplankton communities in the lake. The LOWQM was recalibrated and revalidated (new simulation) to the time period 1983–2012. When compared to the original simulation using the same set of forcing functions (e.g. flow, rain, temperature, nutrient loads, and resuspension of sediments), there were some improvements in model predicted nitrogen and phytoplankton, based on goodness-of-fit measures to observed data. Major differences between the two simulations were found in nutrient cycles. The differences in these cycles and the similarity of goodness-of-fit measures between the new and original simulations indicate the importance of sediment-water interactions and the effect that model structure (e.g. equations) have on the ability to improve calibration results. Further evaluation of these two simulations suggest that model accuracy can be improved through changes of model segmentation, additional lake sampling of biological processes simulated by the LOWQM, and revising model equations to allow for variation in algal nutrient to carbon ratios and uptake rates of nitrogen.