The development of anoxic conditions in riparian wetland (RW) soils is widely known to release dissolved phosphorus (DP), but the respective roles of reductive dissolution of Fe-oxyhydroxide, pH ...changes and sediment inputs in this release remain debated. This study aimed to identify and quantify these respective roles via laboratory anaerobic/aerobic incubation of RW soils with and without the addition of sediment. The investigated soils came from two RWs with contrasting P status and organic matter (OM) content in their soils, while the added sediment came from an adjacent cultivated field. Results showed that the amount and speciation of the DP released during anaerobic/aerobic incubations were controlled by soil P status and soil OM content. During anaerobic incubation, DP release in the soil with high extractable P and low OM contents was controlled by reductive dissolution of Fe-oxyhydroxides (83%), whereas that released in the soil with low extractable P and high OM contents was controlled by an increase in pH (88%). Anaerobic incubation of a mixture of eroded sediments and RW soils increased the release of DP, dissolved organic carbon and Fe(II) (by 16%, 4% and 18%, respectively) compared to the simple addition of the amounts released during their separate incubations. Management practices should decrease soil erosion from upland fields to avoid deposition of P-rich sediments on RW soils. Management efforts should focus preferentially on RWs whose Fe:P molar ratios in the soil solution during reduction are the lowest, since they indicate a high risk that the DP released will be transferred to watercourses.
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•Dissolved P release increased in wetland soils under anoxic conditions.•Soil properties controlled the amount and speciation of the DP released.•FeO(OH) reduction controlled DP release in high extractable-P but low OM soils.•pH increase controlled DP release in low extractable-P but high OM soils.•Sediment input into RW soils increased its DP release under anoxic conditions.
An essential aspect of eutrophication studies is to trace the ultimate origin of phosphate ions (P-PO4) associated with the solid phase of river sediments, as certain processes can make these ions ...available for algae. However, this is not a straightforward task because of the diversity of allochthonous and autochthonous sources that can supply P-PO4 to river sediments as well as the existence of in-stream processes that can change the speciation of these inputs and obscure the original sources. Here, we present the results of a study designed to explore the potentials, limitations and conditions for the use of the oxygen isotope composition of phosphate (δ18Op) extracted from river sediments for this type of tracing. We first tested if the method commonly applied to soils to purify P-PO4 and to measure their δ18Op concentrations could be adapted to sediments. We then applied this method to a set of sediments collected in a river along a gradient of anthropogenic pressure and compared their isotopic signatures with those from samples that are representative of the potential P-PO4 inputs to the river system (soils and riverbank material).
The results showed that following some adaptations, the purification method could be successfully transposed to river sediments with a high level of P-PO4 purification (>97%) and high δ18Op measurement repeatability and accuracy (<0.4‰). The values for the potential allochthonous sources varied from 11.8 to 18.3‰, while the δ18Op value for the river sediments ranged from 12.2 to 15.8‰. Moreover, a sharp increase (>3‰) in the sediment δ18Op value immediately downstream from the discharge point revealed the strong impact of municipal wastewater. The calculation of the theoretical equilibrium δ18Op values using the river water temperature and δ18Ow showed that the downstream sediments were in equilibrium, which was not the case for the upstream sediments. This difference could be related to the contrast between the short residence time of the transfer system in the catchment head, which can preserve the isotopic variability of the source materials, and the longer residence times and higher P bioavailability in the lower catchment, possibly fostering the recycling of P-PO4 by the biota and the equilibration of the oxygen isotope signature in P-PO4. These results demonstrate the potential of the isotopic approach to assess the sources and in-stream turnover of sedimentary P in river systems.
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•We provided the first assessment of the oxygen isotopic composition of HCl-extracted phosphate in river sediments.•We observe a contrast in the oxygen isotope composition between upstream and downstream sediments.•This contrast can be related to anthropogenic impacts on the river system and to the sediment residence time.•We have shown that in some situations it is possible to trace particulate-P sources of sediments with this isotopic tool.
Accurate identification of phosphorus (P) forms is crucially important for understanding the geochemical cycle of P; however, until now the role of ferrous iron P (Fe(II)-P) buried in sediments has ...been completely ignored in nearly all sequential extraction procedures developed. Using sediment cores sampled from Donghu Lake in Wuhan, China, this study explored a modified version of widely used sequential P extraction method (SEDEX; Ruttenberg, 1992) in which Fe(II)-P was identified as an independent fraction. Based on the high selectivity of the extractant (0.2% 2,2′–bipyridine+0.1 M KCl) and the dissolution equilibrium of P, procedures for extracting Fe(II)-P were optimized using a 1:100 solid:liquid ratio and extraction at 50 ± 1 °C for 24 h. The sedimentary P extracted was divided into five fractions: loosely-bound P, Fe(II)-P, CDB-P, Ca-P and O-P. Fe(II)-P was the predominant fraction in fresh sediments in Donghu Lake, accounting for 15.7–49.9% of TP, with a mean of 31.6%. The mean values of Ca-P, O-P, CDB-P and loosely-bound P were 28.4%, 22.7%, 17.1% and 4.3%, respectively. Combined with component analysis of extracts and recovery experiments of standard reference minerals (vivianite, Fe3(PO4)2·8H2O) in natural sediments, extraction of Fe(II)-P with 0.2% 2,2-bipridine and 0.1 M KCl was robust, with a good recovery rate (88.7–100.6%) and little of the Ca-P dissolved. It is possible to use this innovative SEDEX not only to distinguish the contribution of different P matrices in fresh sediments, but also to investigate the transformation of sedimentary P under different redox conditions. Therefore, greater focus on Fe(II)-P is necessary, because it is a major sink for the geochemical process of sedimentary P.
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•An innovative SEDEX for anaerobic sediments was developed.•Fe(II)-P was identified as an independent fraction in this innovative approach.•The sedimentary P extracted was divided into five fractions: loosely-bound P, Fe(II)-P, CDB-P, Ca-P and O-P.•This innovative SEDEX can also be utilized to investigate the transformation of sedimentary P during sediment oxidation.
The complexation of Fe(II) with organic matter (OM) and especially with humic acids (HAs) remains poorly characterized in the literature. In this study, batch experiments were conducted on a pH range ...varying from 1.95 to 9.90 to study HA-mediated Fe(II) binding. The results showed that high amounts of Fe(II) are complexed with HA depending on the pH. Experimental data were used to determine a new set of binding parameters by coupling PHREEPLOT and PHREEQC-Model VI. The new binding parameters (log KMA=2.19±0.16, log KMB=4.46±0.47 and ΔLK2=3.90±1.30) were validated using the LFER (linear free energy relationship) method and published adsorption data between Fe(II) and Suwannee River fulvic acid (SRFA) (Rose and Waite, 2003). They were then put in PHREEQC-Model VI to determine the distribution of Fe(II) onto HA functional groups. It was shown that Fe(II) forms mainly bidentate complexes, some tridentate complexes and only a few monodentate complexes with HA. Moreover, Fe(II) is mainly adsorbed onto carboxylic groups at acidic and neutral pH, whereas carboxy-phenolic and phenolic groups play a major role at basic pH. The major species adsorbed onto HA functional groups is Fe2+; Fe(OH)+ appears at basic pH (from pH 8.13 to 9.9). The occurrence of OM and the resulting HA-mediated binding of Fe(II) can therefore influence Fe(II) speciation and bioavailability in peatlands and wetlands, where seasonal anaerobic conditions prevail. Furthermore, the formation of a cationic bridge and/or the dissolution of Fe(III)-(oxy)hydroxides by the formation of Fe(II)-OM complexes can influence the speciation of other trace metals and contaminants such as As.
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•We measured Fe(II)-humic acid adsorption isotherm and pH sorption edge.•Results were modeled using a coupling of PHREEPLOT–PHREEQC-Model VI.•We determined the binding parameters for Fe(II)-humic acid complex.•We validated the binding parameters using LFERs and published datasets.•We determined the speciation of Fe(II)-humic acid complexes using PHREEQC-Model VI.
The binding of rare earth elements (REE) to humic acid (HA) was studied by combining ultrafiltration and Inductively Coupled Plasma Mass Spectrometry techniques. REE–HA complexation experiments were ...performed at various pH conditions (ranging from 2 to 10.5) using a standard batch equilibration method. Results show that the amount of REE bound to HA strongly increases with increasing pH. Moreover, a Middle-REE (MREE) downward concavity is evidenced by REE distribution patterns at acidic pH. Modelling of the experimental data using Humic Ion Binding Model VI provided a set of log
K
MA values (i.e., the REE–HA complexation constants specific to Model VI) for the entire REE series. The log
K
MA pattern obtained displays a MREE downward concavity. Log
K
MA values range from 2.42 to 2.79. These binding constants are in good agreement with the few existing datasets quantifying the binding of REE with humic substances but quite different from a recently published study which evidence a lanthanide contraction effect (i.e., continuous increase of the constant from La to Lu). The MREE downward concavity displayed by REE–HA complexation pattern determined in this study compares well with results from REE–fulvic acid (FA) and REE–acetic acid complexation studies. This similarity in the REE complexation pattern suggests that carboxylic groups are the main binding sites of REE in HA. This conclusion is further illustrated by a detailed review of published studies for natural, organic-rich, river- and ground-waters which show no evidence of a lanthanide contraction effect in REE pattern. Finally, application of Model VI using the new, experimentally determined log
K
MA values to World Average River Water confirms earlier suggestions that REE occur predominantly as organic complexes (≥
60%) in the pH range between 5–5.5 and 7–8.5 (i.e., in circumneutral pH waters). The only significant difference as compared to earlier model predictions made using estimated log
K
MA values is that the experimentally determined log
K
MA values predict a significantly higher amount of Light-REE bound to organic matter under alkaline pH conditions.
The CPES (Channel Payments for Ecosystem Services) project developed PES schemes remunerating farmers for their activities in improving water quality by reducing the emissions of nutrients (both ...nitrogen and phosphorus) or erosion from agricultural activities. Catchment-wide approaches were tested in six case studies, and ecology, hydrogeology, agroeconomy, law, and farming expertise was combined. Collaborations were established with farmers, their associations, chambers of agriculture, and local and regional stakeholders. One case study concerned Lac au Duc (Brittany), a reservoir suffering from recurrent cyanobacterial blooms. Curative actions to control cyanobacteria had limited success. The main sources of excess P entering the lake were of farming origin but varied between the sub-catchments according to hydrogeological characteristics and agricultural practices. Long-term prevention possibilities to ameliorate agricultural practices and their costs were developed with the farmers and included permanent cover or anti-erosive hedges. During the project, PESs were successfully applied for ground water protection by the water supply companies as drinking water protection has a strong business case to preserve this ecosystem service. For recreational or multi-purpose lakes and reservoirs, it remains difficult to find financing to meet the monetary requirements to address farmer’s transition to ameliorate land management.
In this paper, we outline several recent insights for the priorities and challenges for future research for reducing phosphorus (P) based water eutrophication in the agricultural landscapes of ...Northwest Europe. We highlight that new research efforts best be focused on headwater catchments as they are a key influence on the initial chemistry of the larger river catchments, and here many management interventions are most effectively made. We emphasize the lack of understanding on how climate change will impact on P losses from agricultural landscapes. Particularly, the capability to disentangle current and future trends in P fluxes, due to climate change itself, from climate driven changes in agricultural management practices and P inputs. Knowing that, future climatic change trajectories for Western Europe will accelerate the release of the most bioavailable soil P. We stress the ambiguities created by the large varieties of sources and storage/transfer processes involved in P emissions in landscapes and the need to develop specific data treatment methods or tracers able to circumvent them, thereby helping catchment managers to identify the ultimate P sources that most contribute to diffuse P emissions. We point out that soil and aqueous P exist not only in various chemical forms, but also in range of less considered physical forms e.g. dissolved, nanoparticulate, colloidal and other particulates, all affected differently by climate as well as other environmental factors, and require bespoke mitigation measures. We support increased high resolution monitoring of headwater catchments, to help verify not only the effectiveness of catchments mitigation strategies, but add research data to develop new water quality models (e.g. those include Fe-P interactions) and can deal with climate and land use change effects within an uncertainty framework. We finally conclude that there is a crucial need for more integrative research efforts to deal with our incomplete understanding of the mechanisms and processes associated with the identification of critical source areas, P mobilization, delivery and biogeochemical processing, as otherwise even high-intensity and high-resolution research efforts will only reveal an incomplete picture of the full global impact of the terrestrial derived P on downstream aquatic and marine ecosystems.
Recently developed measurement technologies can monitor surface water quality almost continuously, creating high-frequency multiparameter time series and raising the question of how best to extract ...insights from such rich data sets. Here we use spectral analysis to characterize the variability of water quality at the AgrHys observatory (Western France) over time scales ranging from 20 min to 12 years. Three years of daily sampling at the intensively farmed Kervidy-Naizin watershed reveal universal 1/f scaling for all 36 solutes, yielding spectral slopes of 1.05 ± 0.11 (mean ± standard deviation). These 36 solute concentrations show varying degrees of annual cycling, suggesting different controls on watershed export processes. Twelve years of daily samples of SO4, NO3, and dissolved organic carbon (DOC) show that 1/f scaling does not continue at frequencies below 1/year in those constituents, whereas a 12-year daily record of Cl shows a general 1/f trend down to the lowest measurable frequencies. Conversely, approximately 12 months of 20 min NO3 and DOC measurements show that at frequencies higher than 1/day, the spectra of these solutes steepen to slopes of roughly 3, and at time scales shorter than 2–3 h, the spectra flatten to slopes near zero, reflecting analytical noise. These results confirm and extend the recent discovery of universal fractal 1/f scaling in water quality at the relatively pristine Plynlimon watershed in Wales, further demonstrating the importance of advective-dispersive transport mixing in catchments. However, the steeper scaling at subdaily time scales suggests additional short-term damping of solute concentrations, potentially due to in-stream or riparian processes.
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Up until now, only a small number of studies have been dedicated to the binding processes of As(III) with organic matter (OM) via ionic Fe(III) bridges; none was interested in Fe ...(II). Complexation isotherms were carried out with As(III), Fe(II) or Fe(III) and Leonardite humic acid (HA). Although PHREEQC/Model VI, implemented with OM thiol groups, reproduced the experimental datasets with Fe(III), the poor fit between the experimental and modeled Fe(II) data suggested another binding mechanism for As(III) to OM. PHREEQC/Model VI was modified to take various possible As(III)–Fe(II)–OM ternary complex conformations into account. The complexation of As(III) as a mononuclear bidentate complex to a bidentate Fe(II)–HA complex was evidenced. However, the model needed to be improved since the distribution of the bidentate sites appeared to be unrealistic with regards to the published XAS data. In the presence of Fe(III), As(III) was bound to thiol groups which are more competitive with regards to the low density of formed Fe(III)–HA complexes. Based on the new data and previously published results, we propose a general scheme describing the various As(III)–Fe–MO complexes that are able to form in Fe and OM-rich waters.
The objective of this work is to identify hydrological processes controlling nitrate export and base flow concentration at the year scale in agricultural headwater catchment streams fed by an ...unconfined aquifer. The study is based on the hydrological and hydrochemical monitoring of the stream and shallow groundwater of three headwater catchments (0.1–5
km
2) in Western France over three to five water years. Results show that at the year scale nitrate export from the three catchments is a transport-limited process. The stream nitrate flux depends on how much water flows in the stream and not on the distribution of the flow over the year. Seasonal variations of concentration over the water year were complex. Variations were different for a given year from one catchment to another, and also different for a given catchment from one year to another. We show that the seasonal variations are controlled by water table depth dynamics along hillslope associated with spatially distributed nitrate concentration in the groundwater. The groundwater displays high nitrate concentrations, from 6 to 22
mg
N–NO
3
-
/
L
, in upland and in the deeper zones of bottom lands. Persistence of such high concentrations results from the geochemical and mineralogical properties of the aquifers that consist of old, very oxidised and strongly weathered material, such that any primary electron donors have been leached. In riparian zones, concentrations are close to zero due to denitrification with oxidation of organic matter. In winter, stream nitrate concentration is controlled by the nitrate rich groundwater flow from upland. In summer, groundwater flow from upland decreases and stream concentration is controlled by bottom land hydrological and biogeochemical processes. The shift between winter and summer control depends on the water table dynamics along hillslopes. As long as the water table remains deep in upland, summer controls prevail. As soon as water table rises in upland, hydraulic gradient and groundwater flow from this zone increase, leading to an increase in the stream nitrate concentration. The shift from summer to winter control can be considered as the result of a connection between the upland nitrate rich groundwater and the stream, connection triggered by upland water table rise.
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