Internal loading of phosphate in rivers reduces at higher flow velocity and is reduced by iron rich sand application: an experimental study in flumes

Emerging investigator series: impacts of land use on dissolved organic matter quality in agricultural watersheds: a molecular perspective

In aquatic systems, dissolved organic matter (DOM) has important ecological and biogeochemical functions, where the molecular composition of DOM has larger-scale implications for climate change and global carbon cycles. However, there is limited information about the relationships between landscape characteristics and human disturbance that influence the molecular composition of DOM changes in watersheds. In this study, we collected water samples from 22 sites across a gradient of topographically characterized agricultural land coverage and community infrastructure development in the Kawartha region in Ontario, Canada. We employed a combination of Fourier Transform Ion-Cyclotron Resonance Mass Spectrometry (FT-ICR-MS) and absorbance spectroscopy to investigate changes in the molecular composition of DOM with increasing agricultural and community development disturbance on the optical and molecular characteristics of DOM. We found that dissolved organic carbon (DOC) concentrations in disturbed (>75%) watersheds ranged from 3.67-32.8 mg L-1 and were significantly higher than in watersheds with more abundant forest coverage (3.78-9.13 mg L-1). In addition, watersheds with higher phosphorus concentrations had more negative nominal oxygenation state of carbon (NOSC) values, suggesting biologically processed DOM correlating with increased phosphorus levels in aquatic systems. To relate the molecular properties of DOM to landscape metrics, we used Spearman's correlation analysis to reveal that agriculturally impacted and community developments enhanced the molecular signature of unsaturated hydrocarbon. In addition, we identified 65 dissolved organic phosphorus (DOP) molecules that significantly increased in abundance with disturbance, likely due to microbial mineralization of existing DOM with the addition of phosphorus to form larger, biologically inaccessible molecules. The overall recalcitrance of the identified molecules can serve as molecular signatures when evaluating the level of disturbance of a watershed.

Internal loading of phosphorus in streams described by a Sediment-Water Exchange Model for Phosphorus (SWEMP): From lab to field scale

The reaction of phosphorus (P) between sediments and water in streams strongly affects the surface water P concentrations. A new reactive transport model (SWEMP: Sediment-Water Exchange Model for Phosphorus) was developed to describe redox dependent P sorption in the sediment and vertical diffusive transport of solutes to the overlying stream. The model parameters were independently obtained to first predict P release in ten different sediment-water batch systems and in two flumes. Input parameters are the degree of P saturation of the sediment, its organic matter content, dissolved oxygen (DO) concentration and temperature. The dissolved P concentrations in the overlying waters ranged from 0.02 to 1.2 mg P L-1 in these systems and were correctly predicted by the model within, on average, a factor 1.3 (batch) or 1.1 (flume). The P flux from the sediment towards the overlying water increased with increasing sediment P:Fe ratio and respiration rates, and with decreasing DO and water pH. After validation of the model with experimental data, it was used to predict monthly P concentrations in Flemish rivers using the total P emission data, total discharge, average sediment properties and the monthly averaged water temperatures, DO concentrations and electric conductivity. The monthly average P concentrations oscillate annually between 0.24 and 0.73 mg P L-1 and predictions matched the long-term monitoring data within 10 % using only one adjustable parameter for the entire water system (N > 250,000). The model predicts that summer peaks in P are related to internal loading from the sediment under anoxic conditions rather than to emission-dilution effects, i.e. external input of P and/or its concentration at lower flow rates. This suggests that, surface water P concentrations can be lowered by enhanced DO in the water, the addition of Fe and Al rich binding agents to the sediments and by reducing P emissions.

Effect of external and internal loading on source-sink phosphorus dynamics of river sediment amended with iron-rich glauconite sand

Glauconite sands (GS) are abundantly available iron (Fe)-rich minerals that are efficient in lowering the release of phosphorus (P) from sediments to the overlying water. Many river sediments are, however, net sinks for P rather than sources and it is unclear if these GS minerals also enhance the P uptake from water. This is because the concentration of Fe(III) minerals at the sediment-water interface (SWI) depends on the redox potential that is affected by physicochemical processes. This study was set-up to investigate if a sediment amendment with GS can both lower P release from the sediment and enhance P uptake from the overlying water. The P fluxes across the SWI were compared between GS-amended (added at 10% weight fraction) and non-amended river sediment in static (incubation) and dynamic (flume) systems. The net P uptake was measured in response to a pulse external P loading (0.5-5 mg P L^-1). Sodium glutamate was added to all treatments to simulate water with a high oxygen demand. Before the P pulse, the GS-amended sediments released significantly less P to the overlying water than the non-amended sediments in both static as dynamic systems. Spiking the water reverted the net P flux over the SWI only in the dynamic system, and the net P uptake in the sediment was factor two larger in GS-amended sediment compared to the non-amended sediment. This study showed that GS addition not only reduced internal P release, but also enhanced P uptake from the overlying water. However, the long-term efficiency in streams likely decreases over time due to saturation processes.

Sediment phosphorus immobilization with the addition of calcium/aluminum and lanthanum/calcium/aluminum composite materials under wide ranges of pH and redox conditions

Aquatic environment factors often influence and regulate the direction of phosphorus (P) flow at the sediment-water interface (SWI). High pH and low DO, common in eutrophic lakes, would induce large releases of P from sediment, and thus cause the negative effect on the efficiency of some P-passivators. Hence, the development of P passivators that could function over a wide range of pH condition and redox state in the overlaying water with reduced undesirable side effects is critical for the eutrophic lake remediation. In the present study, a calcium (Ca)/aluminum (Al) composite (CA) and a lanthanum (La)/Ca/Al composite (LCA) were prepared for P immobilization in lake sediments, using calcium and lanthanum coprecipitated with aluminum. CA and LCA were shown to have good P sorption performance at pH 4-11, particularly at pH 8-11. Furthermore, CA and LCA have an ability to correct the pH of water that deviates from neutral. The maximum P adsorption (Q_max) of sediment amended by 4 % CA and 4 % LCA increased by 83 % and 103 %, and their equilibrium P concentration (EPC₀) decreased by 76 % and 88 %, respectively. Under various pH and DO conditions, the P concentration in overlying water was significantly decreased by CA and LCA amendment, and their addition could effectively counteract the P release from sediments induced by high pH and low DO. The mechanisms of P immobilization in amended sediments under various pH and DO levels are primarily the conversion of reactive P to stable P. The P immobilization performance of CA and LCA could cope with a wide range of pH and redox conditions in eutrophic lakes, and they would help to correct extreme pH values, thus they are expected to be a new generation of commercial P-passivators.

Nano-scale analysis of uranium release behavior from river sediment in the Ili basin

Due to the limitations of the conventional water sample pretreatment methods, some of the colloidal uranium (U) has long been misidentified as "dissolved" phase. In this work, the U species in river water in the Ili Basin was classified into submicron-colloidal (0.1-1 μm), nano-colloidal (0.1 μm-3 kDa) and dissolved phases (< 3 kDa) by using high-speed centrifugation and ultrafiltration. The U concentration in the river water was 5.39-8.75 μg/L, which was dominated by nano-colloidal phase (55-70%). The nano-colloidal particles were mainly composed of particulate organic matter (POM) and had a very high adsorption capacity for U (accounting for 70 ± 23% of colloidal U). Sediment disturbance, low temperature, and high inorganic carbon greatly improved the release of nano-colloidal U, but high levels of Ca²⁺ inhibited it. The simulated river experiments indicated that the flow regime determined the release of nano-colloidal U, and large amounts of nano-colloidal U might be released during spring floods in the Ili basin. Moreover, global warming increases river flow and inorganic carbon content, which may greatly promote the release and migration of nano-colloidal U.

Iron rich glauconite sand as an efficient phosphate immobilising agent in river sediments

The reductive dissolution of iron (Fe) (oxy)hydroxides in sediments releases phosphorus (P) to the overlying water and may lead to eutrophication. Glauconite sands (GS) are rich in Fe and may be used as readily available P sorbents. This study was set up to test effects of dose and type of GS on the P immobilisation in sediments under hypoxic conditions. Three different GS were amended to a P-rich river sediment at doses of 0% (control), 5% and 10% (weight fractions) and incubated with overlying water in batch laboratory conditions. Glutamate was added to the solution after 15 days to deplete any residual dissolved oxygen from the sediment-water interface. In the first 15 days, the P concentration in the overlying water peaked to 1.5 mg P L^-1 at day 9 in the control and decreased to 0.9 mg P L^-1 at lowest Fe-dose and to 0.03 mg P L^-1 at the highest Fe-dose, the effects of GS type and dose were explained by the Fe dose. After 15 days, the added glutamate induced a second, and larger peak of P in the overlying water in sediment, that peak was lower in amended sediments but no GS dose or type related effects were found. This suggests that freshly precipitated P species at the sediment-water interface can be remobilised. This study highlights the potential for using this natural mineral as a cheap and easily available sediment remediation material, but its longevity under rare extreme conditions needs to be further investigated.