A Comparison of Fluvial Sediment Phosphorus (P) Chemistry in Relation to Location and Potential to Influence Stream P Concentrations

Fluvial sediments are subject to cyclic submersion during changes in stream flow, which can affect their phosphorus (P) sorptive capacity. As fluvial sediments play a major role in determining P concentrations in stream flow, we compared the P chemistry of exposed stream bank and submerged bed sediments from an agricultural catchment in central Pennsylvania, USA. Total P concentration was greater in bank (417 mg kg^-1) than bed sediments (281 mg kg^-1), but because bed sediments contained more sand-sized material, they could release more P and support a higher solution P concentration (0.043 mg l^-1) than bank sediments (0.020 mg l^-1). Phosphorus release was a function of Mehlich-3 soluble Fe in stream sediments (r > 0.65), reflecting redox processes in the fluvial system. In contrast, P sorption maxima of bank and bed sediments were related to Mehlich-3 soluble Al (r > 0.78) and organic matter concentration (r > 0.79). Overall, our research suggested that erosion of bank sediments should contribute less P and may be a sink for P in the stream system compared with resuspension of bed sediment. However, bank sediments may have the potential to be a large source of P in downstream reservoirs or lakes, where increased microbial activity and reducing conditions may solubilise sediment-bound P.     

Organic matter compositions and DOM release from the sediments of the shallow lakes in the middle and lower reaches of Yangtze River region, China

The contents of different organic matter components and dissolved organic matter (DOM) release kinetics of the sediments from the middle and lower reaches of the Yangtze River region were investigated, and their relationships discussed. The results show that organic C (OC) ranged from 8.14 to 43.65 g kg⁻¹, dissolved organic C (DOC) from 0.38 to 1.38 g kg⁻¹, active organic C (AOC) from 1.12 to 4.45 g kg⁻¹, heavy fraction organic C (HFOC) from 6.86 to 39.08 g kg⁻¹, accounting for 2.42-9.34%, 8.66-29.72% and 84.29-93.18% of OC, respectively. With increasing of OC content the ratios of DOC to OC and AOC to OC decreased. The contents of AOC, DOC, light fraction organic C (LFOC) and their contribution ratios to OC in studied sediments were higher than those reported in soils. The DOM release process of the studied sediments includes rapid and slow stages, and the rapid release occurred within 30 min, mainly in 5 min. The DOM release kinetic data in this investigation can be best fitted by the Power Function model. The correlations between total N (TN), total P (TP), OC, DOC, AOC, LFOC, HFOC and the DOM release kinetic parameters (k, c, a, b, rate₃₀) of the sediments were significant. There were also significant correlations between TN, TP, OC, DOC, LFOC and HFOC in sediments. So the DOM release from sediment was not only related to the OC content, but also related to the organic matter composition characteristics, especially the contents of DOC, AOC and LFOC.     

Solid-state C NMR analysis of size and density fractions of marine sediments: Insight into organic carbon sources and preservation mechanisms

Burial of organic carbon (OC) in ocean sediments acts as the ultimate long-term sink for both terrestrial and marine carbon, however, the mechanisms controlling the preservation of this carbon are poorly understood. To better understand these mechanisms, we applied solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy, along with elemental, stable carbon isotopic (δ¹³C) and lignin phenol analyses, to size and density fractions of sediments influenced by either mixed terrestrial and marine OC inputs (Washington Coast slope) or dominantly marine inputs (Mexican Margin). Elemental, isotopic and lignin analyses all reveal that within the Washington Coast sediment, the OC mixes linearly between nitrogen-poor and ¹³C-depleted, lignin-rich OC in the large and light fractions and nitrogen-rich and ¹³C-enriched, lignin-poor OC in the small and dense fractions, suggesting that this sediment contains a two-component mixture of terrestrial vascular plant- and marine-derived OC. The integral areas of each of seven NMR spectral regions in the different samples trend linearly when plotted versus δ¹³C signature, with most R² values of 0.78 or greater, demonstrating that the NMR spectra of the two sources of carbon also mix linearly between the two endmembers. The terrestrial endmember in this sediment appears to be dominated by lignin and black carbon whereas the source of the marine endmember is less clear from the NMR spectra. In contrast, all of the analyses indicate that OC in the Mexican Margin sediment fractions is homogenous and derives almost exclusively from marine sources. It appears that selective preservation of (bio)chemically recalcitrant lignin and black carbon is the primary mechanism of preservation of terrestrial OC, whereas mineral-protection is the dominant mechanism preserving marine OC in the Washington coast sediment. There is little evidence showing that either preservation mechanism functions in the Mexican Margin sediments.     

Arsenic remobilization from sediments contaminated with mine tailings near the Adak mine in Västerbotten district (northern Sweden)

Weathering of mine tailings have resulted in high As concentrations in water (up to 2900 μg l^− 1) and sediment (up to 900 mg kg^− 1) samples around the Adak mine. Notably, As occurs as As(III) species (15–85%) in the oxic surface and ground water samples, which is not common. Time-series based sediment incubations were set up in the laboratory with contaminated sediments to study the microbial processes involved in transformation and remobilization of As across the sediment–water interface. The microcosm experiments indicate that microorganisms are capable of surviving in As-rich sediments and reduce As(V) to As(III). A decrease in total As concentration in sediments is coupled to an increase in As(III) concentration in the aqueous media. In contrast, the controls (treated with HgCl₂ and formaldehyde) did not show growth, and As(V) concentrations increased steadily in the sediments and aqueous medium. The results imply that active metabolism is necessary for As(V) reduction. These microorganisms possess reduction mechanisms that are not necessarily coupled to respiration, but most likely impart resistance to As toxicity.     

Disentangling marine, soil and plant organic carbon contributions to continental margin sediments: A multi-proxy approach in a 20,000 year sediment record from the Congo deep-sea fan

A 20 kyr long sediment sequence from the Congo deep sea fan (core GeoB 6518-1), one of the world’s largest deep sea river fans, has been analysed for bulk and molecular proxies in order to reconstruct the marine, soil and plant organic carbon (OC) contributions to these sediments since the last glacial maximum. The bulk proxies applied, C/N ratio and δ¹³C_org, ranged from 10 to 12.5 and from −24.5 to −21‰ VPDB, respectively. As molecular proxies, concentrations of marine derived alkenones and terrestrial derived odd-numbered n-alkanes were used, which varied between 0.2 and 4 μg/g dry weight sediment. In addition, the branched vs. isoprenoid tetraether (BIT) index, a proxy for soil organic matter input, was used, which varied from 0.3 to 0.5 in this core.Application of binary mixing models, based on the different individual proxies, showed estimates for terrestrial OC input varying by up to 50% due to the heterogeneous nature of the OC. Application of a three end-member mixing model using the δ¹³C_org content, the C/N ratio and the BIT index, enabled the distinction of soil and plant organic matter as separate contributors to the sedimentary OC pool. The results show that marine OC accounts for 20% to 40% of the total OC present in the deep sea fan sediments over the last 20 kyr and that soil OC accounts for about half (∼45% on average) of the OC present. This suggests that soil OC represents the majority of the terrestrial OC delivered to the fan sediments.Accumulation rates of the plant and soil OC fractions over the last 20 kyr varied by a factor of up to 5, and are strongly related to sediment accumulation rates. They showed an increase starting at ca. 17 kyr BP, a decline during the Younger Dryas, peak values during the early Holocene and lower values in the late Holocene. This pattern matches with reconstructions of past central African humidity and Congo River discharge from the same core and revealed that central African precipitation patterns exert a dominant control on terrestrial OC deposition in the Congo deep sea fan. Marine OC accumulation rates are only weakly related to sediment accumulation rates and vary only little over time compared to the terrigenous fractions. These variations are likely a result of enhanced preservation during times of higher sedimentation rates and of relative small fluctuations in primary production due to wind-driven upwelling.     

Phosphorus adsorption characteristics at the sediment–water interface and relationship with sediment properties in FUSHI reservoir, China

This study was designed to survey the reservoir sediment properties, assess the phosphorus (P) sorption isotherm, and analyze the relationship between sediment properties and sorption parameters. Physicochemical analysis indicated that sediment from the FUSHI reservoir in Zhejiang Province, China, has similar physical and chemical properties and has been contaminated by P. Sorption isotherm experiments showed that the sorption process could be described by Langmuir and Freundlich models. The parameters of Q _max (Phosphorus sorption maximum) and K (Freundlich adsorption isotherm constant) ranged from 618.98 to 825.70?mg?kg^?1 and 114.18 to 170.74?l?kg^?1, respectively. EPC₀ (zero P equilibrium concentration) ranged from 0.14 to 0.24?mg?l^?1, more than the total P concentration in the water of the reservoir. Thus, the reservoir sediment releases P into the water and acts as a ??P resource??. The clay, Fe_o, Al_t, and Fe_t?+?Al_t content were the main active components in P sorption. Q _max had a highly significant positive relationship with some properties and could be estimated by a combination of these.     

50-year record and solid state speciation of mercury in natural and contaminated reservoir sediment

Contaminated fluvial sediments represent both temporary sinks for river-borne pollutants and potential sources in case of natural and/or anthropogenic resuspension. Reservoir lakes play a very important role in sediment dynamics of watersheds and may offer great opportunities to study historical records of river-borne particles and associated elements transported in the past. The fate and potential environmental impact of Hg depends on its abundance, its carrier phases and its chemical speciation. Historical Hg records and solid state Hg speciation were compared in sediments from two contrasting reservoirs of the Lot River (France) upstream and downstream from a major polymetallic pollution (e.g. Cd, Zn) source. Natural (geochemical background) and anthropogenic Hg concentrations and their relationships with predominant carrier phases were determined. The results reveal important historical Hg contamination (up to 35 mg kg⁻¹) of the downstream sediment, reflecting the historical evolution of industrial activity at the point source, i.e. former coal mining, Zn ore treatment and post-industrial remediation work. Single chemical extractions (ascorbate, H₂O₂, KOH) suggest that at both sites most (∼75%) of the Hg is bound to organic and/or reactive sulphide phases. Organo-chelated (KOH-extracted) Hg, representing an important fraction in the uncontaminated sediment, shows similar concentrations (∼0.02 mg kg⁻¹) at both sites and may be mainly attributed to natural inputs and/or processes. Although, total Hg concentrations in recent surface sediments at both sites are still very different, similar mono-methylmercury concentrations (up to 4 μg kg⁻¹) and vertical distributions were observed, suggesting comparable methylation-demethylation processes. High mono-methylmercury concentrations (4–15 μg kg⁻¹) in 10–40 a-old, sulphide-rich, contaminated sediment suggest long-term persistence of mono-methylmercury. Beyond historical records of total concentrations, the studied reservoir sediments provided new insights in solid state speciation and carrier phases of natural and anthropogenic Hg. In case of sediment resuspension, the major part of the Hg historically stored in the Lot River sediments will be accessible to biogeochemical recycling in the downstream fluvial-estuarine environment.     

Natural-abundance radiocarbon as a tracer of assimilation of petroleum carbon by bacteria in salt marsh sediments

The natural abundance of radiocarbon (¹⁴C) provides unique insight into the source and cycling of sedimentary organic matter. Radiocarbon analysis of bacterial phospholipid lipid fatty acids (PLFAs) in salt-marsh sediments of southeast Georgia (USA)—one heavily contaminated by petroleum residues—was used to assess the fate of petroleum-derived carbon in sediments and incorporation of fossil carbon into microbial biomass. PLFAs that are common components of eubacterial cell membranes (e.g., branched C₁₅ and C₁₇, 10-methyl-C₁₆) were depleted in ¹⁴C in the contaminated sediment (mean Δ¹⁴C value of +25 ± 19‰ for bacterial PLFAs) relative to PLFAs in uncontaminated “control” sediment (Δ¹⁴C = +101 ± 12‰). We suggest that the ¹⁴C-depletion in bacterial PLFAs at the contaminated site results from microbial metabolism of petroleum and subsequent incorporation of petroleum-derived carbon into bacterial membrane lipids. A mass balance calculation indicates that 6-10% of the carbon in bacterial PLFAs at the oiled site could derive from petroleum residues. These results demonstrate that even weathered petroleum may contain components of sufficient lability to be a carbon source for biomass production by marsh sediment microorganisms. Furthermore, a small but significant fraction of fossil carbon is assimilated even in the presence of a much larger pool of presumably more-labile and faster-cycling carbon substrates.     

Study on vertical distribution and activity factor of P forms in sediments of three urban shallow lakes in People’s Republic of China

The vertical variation of P forms in sediments of urban shallow lakes in China, Xuanwu Lake, Daming Lake and Mochou Lake, were sequentially extracted and measured with the method of SEDEX. The results indicated the TP content in the sediment profiles ranged from 371.94 to 777.25 mg kg⁻¹ for Xuanwu Lake, 1,308.14 to 4,632.63 mg kg⁻¹ for Daming Lake, and 995.49 to 1,860.71 mg kg⁻¹ for Mochou Lake. The results of sequential extraction showed that Ca-P and Fe-P were the main fractions. Meanwhile, the proportions of Bio-P to TP were 35.24% for Xuanwu Lake, 29.57% Daming lake, and 25.26%, for Mochou Lake, indicating a high potential of P releasing. The content of Bio-P was significantly and positively correlated with TP (r = 0.978, P < 0.01). Lake hydrations conditions played an important role in the distribution and contents of Bio-P and TP. In the region with macrophytes, the contents of TP and Bio-P were relatively low. Physicochemical properties of sediments were significantly related to the fraction distribution and P contents, and might play an important role in controlling P activity and mobility. Moreover, Fe showed an evident influence on P fraction and the ratio Fe/P might be good indicator to the contents and composition of active P in sediments.     

Mercury distribution and transport in a contaminated river system in Kazakhstan and associated impacts on aquatic biota

The River Nura in Central Kazakhstan has been heavily polluted by Hg originating from an acetaldehyde plant. A number of studies were undertaken to investigate the transport, fate and bioavailability of Hg in this river system. The sediments within a 20 km section of the river downstream of the effluent outfall canal are highly polluted and are acting as a strong source of surface water contamination. Mercury transport in the river is dominated by the remobilization of contaminated bed sediments and river bank erosion during the annual spring flood. Peak Hg concentrations in unfiltered surface water samples during a larger than usual flood event in 2004 were in the order of 1600–4300 ng L⁻¹. The majority of the particulate-bound Hg appears to be sedimented in the shallow Intumak reservoir 75 km downstream of the source of the pollution, leading to a drop in aqueous Hg concentrations by an order of magnitude. Nevertheless, background concentrations of Hg in surface water are not reached until at least 200 km downstream, and during the flood period Hg is also detected in the terminal wetlands of the river.Mercury concentrations in sediment cores taken from the river bed in the most contaminated section of the Nura ranged from 9.95 to 306 mg kg⁻¹. Methylmercury (MeHg) levels in shallow sediment cores were highest in surface sediments and ranged between 4.9 and 39 μg kg⁻¹, but were generally less than 0.1% of total Hg (THg). A significant inverse relationship was found between THg concentrations and the percentage of MeHg formed in the sediments, irrespective of the sampling depth. The observed relationship was confirmed by comparison with results from a different river system, indicating that it may be true also for other highly contaminated aquatic systems. It is hypothesized that at high THg levels in severely contaminated sediments, the accumulation of MeHg may be limited by increasingly efficient demethylation processes, and that this underlying trend in sediments is the reason why MeHg levels in surface water are often found to be higher at less contaminated sites compared to upstream sites.Mercury concentrations in biota in the most contaminated section of the river were 15–20 times higher than background levels. Fish were found to be impacted for more than 125 km downstream from the source, indicating significant transport of dissolved MeHg to downstream areas and/or in-situ MeHg production in less contaminated downstream reaches. There were also indications that impoundments may increase the bioavailability of Hg.     

Organochlorine contaminants in a coastal lagoon in Argentina: Analysis of sediment, crabs, and cordgrass from two different habitats

Burrowing crabs (Chasmagnathus granulata), sediment collected from inside and outside crab burrows as well as outside the crab bed, and cordgrass (Spartina densiflora) were collected from intertidal mudflat and cordgrass marsh habitats and analyzed for concentrations of polychlorinated biphenyl (PCB) congeners and organochlorine (OC) compounds to test the hypothesis that there are differences in the distribution and bioaccumulation of OC contaminants in coastal lagoon habitats in Argentina. PCB concentrations were relatively low, although the penta- and hexachlorobiphenyl detected in sediments and biota indicated that there had been direct inputs of PCBs into the ecosystem. Heptachlor epoxide, dieldrin, endosulfan sulphate, chlordane compounds, DDT and metabolites, and hexachlorocyclohexanes (HCHs) were the major pesticides detected in sediment and biota samples. When lipid-normalized concentration data for all OC pesticides in crabs were summed together (ΣOCC), there were higher concentrations in crabs from the cordgrass habitat in comparison to crabs from the mudflat. In sediment samples, there were no significant differences in percent organic matter of marsh and mudflat sediments, but the concentrations of ΣOCC normalized to organic carbon were higher in the sediments collected in the cordgrass marsh. Samples of rhizomes and roots from the cordgrass contained high concentrations of OC compounds and it was estimated that 2.4 kg of heptachlor epoxide, the most abundant OC pesticide, may be present in the total cordgrass root biomass in Mar Chiquita lagoon. These data indicated that the cordgrass in coastal lagoon environments is an important factor in determining the distribution of persistent contaminants, and that a significant portion of the total burden of these hydrophobic compounds may be deposited in cordgrass biomass.     

Nitrogen and phosphorus mineralization in sediments of Taihu Lake after the removal of light fraction organic matter

Mineralization of organic matter (OM) in sediment is crucial for biogeochemical cycle of nitrogen (N) and phosphorus (P) in lake ecosystem. Light fraction OM (LFOM) is a reactive pool in sediment and is considered as labile fraction contributing to N and P cycling. In our study, the effect of LFOM on the process and characteristics of N and P mineralization in sediments of Taihu Lake were investigated with 77-day waterlogged incubation plus intermittent leaching at 27°C. Sediments from Yuantouzhu (Y) and Gonghu (G) were used, which were removed the LF. Results indicated that the organic nitrogen mineralized ranged from 14.3 to 19.5% of total nitrogen (193.49–378.93 mg kg⁻¹ sediment) and the organic phosphorus mineralized ranged from 5.7 to 7.9% of total phosphorus (19.86–60.65 mg kg⁻¹ sediment). The heavily polluted sediment had a higher mineralization rate and net mineral-N and mineral-P than slightly polluted sediment. LF stimulated the initial amounts of inorganic N and P and also can be the potential source for mineralization. After the LFOM removal, the net mineral-N of Y and G decreased 116.47 mg kg⁻¹ sediment and 48.03 mg kg⁻¹sediment, respectively, and the net mineral-P decreased 2.67 mg kg⁻¹sediment for Y and 4.82 mg kg⁻¹sediment for G. Two models were used to fit the observed mineral-N data vs. incubation days using a non-linear regression procedure: one is the effective cumulated temperature model, a thermodynamic model which assumes that N mineralization is affected by temperature; the other is the single first-order exponential model, which is a dynamic model. Based on root mean square error values for the two models, the effective cumulated temperature model made a better prediction of N mineralization than the other model for all the four treatments. The single first-order exponential model underestimated N mineralization during the first 14 days and the last 21 days, and overestimated it in the other days during the 77-day incubation. This indicated that temperature was the primary factor influencing N mineralization and the amount of mineral-N were correlated significantly with the effective cumulated temperature (T ≥ 15°C) and incubation time when waterlogged incubation plus intermittent leaching was used.     

Phosphorus status and sorption characteristics of some calcareous soils of Hamadan,western Iran

Phosphorus (P) application in excess of plant requirement may result in contamination of drinking water and eutrophication of surface water bodies. The phosphorous buffer capacity (PBC) of soil is important in plant nutrition and is an important soil property in the determination of the P release potential of soils. Phosphorus sorption greatly affects both plant nutrition and environmental pollution. For better and accurate P fertilizer recommendations, it is necessary to quantify P sorption. This study was conducted to investigate available P and P sorption by calcareous soils in a semi-arid region of Hamadan, western Iran. The soil samples were mainly from cultivated land. Olsen’s biocarbonate extractable P (Olsen P) varied among soils and ranged from 10 to 80 mg kg⁻¹ with a mean of 36 mg kg⁻¹. Half of the soils had an Olsen P > 40 mg kg⁻¹ and >70% of them had a concentration >20 mg kg⁻¹, whereas the critical concentration for most crops is <15 mg P kg⁻¹. Greater average Olsen P in soils occurred under garlic (56 mg kg⁻¹) and potato (44 kg kg⁻¹) fields than in dry-land wheat farming (24 mg kg⁻¹), pasture (30 mg kg⁻¹), and wheat (24 mg P kg⁻¹) fields. A marked increase in fertilizer P rates applied to agricultural soils has caused P to be accumulated in the surface soil. Phosphate sorption curves were well fitted to the Freundlich equation. The standard P requirement (SPR) of soils, defined as the amount of P sorbed at an equilibrium concentration of 0.2 mg l⁻¹ ranged from 4 to 102 mg kg⁻¹. Phosphorus buffer capacity was relatively high and varied from 16 to 123 l kg⁻¹ with an average of 58 l kg⁻¹. In areas of intensive crop production, continual P applications as P fertilizer and farmyard manure have been used at levels exceeding crop requirements. Surface soil accumulations of P are high enough that loss of P in surface runoff and a high risk for P transfer into groundwater have become priority management concerns.     

Variability of dissolved organic carbon in sediments of a seagrass bed and an unvegetated area within an estuary in Southern Texas

Pore-water dissolved organic carbon (PWDOC) concentrations were examined in vegetated and bare sediments of aHalodule wrightii seagrass bed, and in a mud bottom sediment of a southern Texas estuary. Temporal variability was examined at diel (dawn and noon) and bimonthly time scales. Distribution patterns of PWDOC were compared with physical, chemical, and biological factors thought to exert control on PWDOC. Concentration of PWDOC, bacterial production, and resultant PWDOC turnover times displayed statistically significant spatial and temporal variability. Concentration of PWDOC ranged from 14 mg C 1^?1 to 107 mg C 1^?1 of pore water, or 9–71 μg C cm^?3 wet sediment. PWDOC was more variable and was approximately 5 times higher than DOC concentrations in the water column. Low PWDOC concentrations (mean = 14.6 μg C cm^?3) and high bacterial production rates (mean = 1.92 μg C cm^?3 h^?1) were observed at the mud station, whereas PWDOC concentrations were high (mean = 24.6 μg C cm^?3) and bacterial production rates were low (mean = 0.43 μg C cm^?3 h^?1) at the bare station. PWDOC turnover times (T_t), assuming 50% bacterial growth efficiency (1–840 h) were shortest at the mud station (mean=13 h) and longest at the bare station (mean=180 h). In the overlying water column, T_t values were longer, ranging from 1,000–10,000 h. PWDOC concentrations were 25% higher in vegetated sediments than in neighboring bare sediments. This difference was probably due to inputs of labile photosynthetic excretia, since bacterial production rates in vegetated sediments displayed significant diel variability and were 4 times greater than that of bare sediments. Based upon the entire data set, PWDOC was significantly related to macrofaunal biomass, sediment POC, sediment C:N ratios, and oxygen metabolism, but was significantly correlated only to the latter two variables in stepwise multiple regression. Our findings suggest that organism activities and detrital quality are the major determinants controlling variability in PWDOC.     

Understanding the controls on sediment-P interactions and dynamics along a non-tidal river system in a rural–urban catchment: The River Nene

The release of Phosphorus (P) from river sediments has been identified as a contributing factor to waters failing the criteria for ‘Good Ecological Status’ under the EU Water Framework Directive (WFD). To identify the contribution of sediment-P to river systems, an understanding of the factors that influence its distribution within the entire non-tidal system is required. Thus the aims of this work were to examine the (i) total (P_Total) and labile (P_Labile) concentrations in sediment, (ii) the sequestration processes and (iii) the interactions between sediment P and the river water in the six non-tidal water bodies of the River Nene, U.K. Collection of sediments followed a long period of flooding and high stream flow. In each water body, five cores were extracted and homogenised for analysis with an additional core being taken and sampled by depth increments. Comparing the distribution of sediment particle size and P_Total data with soil catchment geochemical survey data, large increases in P_Total were identified in sediments from water body 4–6, where median concentrations of P_Total in the sediment (3603 mg kg⁻¹) were up to double those of the catchment soils. A large proportion of this increase may be related to in-stream sorption of P, particularly from sewage treatment facilities where the catchment becomes more urbanised after water body 3. A linear correlation (r = 0.8) between soluble reactive phosphate (SRP) and Boron in the sampled river waters was found suggesting increased STW input in water bodies 4–6.P_Labile concentrations in homogenised cores were up to 100 mg kg⁻¹ PO₄–P (generally < 2% of P_Total) and showed a general increase with distance from the headwaters. A general increase in Equilibrium Phosphate Concentrations (EPC₀) from an average of 0.9–∼1.7 μm L⁻¹ was found between water bodies 1–3 and 4–6. Fixation within oxalate extractable phases (Al, Fe and Mn) accounted for ∼90% of P binding in water bodies 4–6, but only between 31 and 74% in water bodies 1–3. Statistical models predicting P_Total (R² = 0.78), oxalate extractable P (R² = 0.78) and Olsen P (R² = 0.73) concentrations in river sediments identified Mn oxy-hydroxides (MnO_x) as a strong predictive variable along with the location within the river system. It is suggested that MnO_x within model predictions is identifying a pool of mixed Fe–Mn oxy-hydroxides (MnO_x–FeOOH) or Fe oxy-hydroxide (FeOOH) from the wider Fe_Oxalate pool that are particularly effective at sorbing and fixing P. The findings demonstrate how sediment and P may accumulate along a 100 km non-tidal river system, the extent to which a range of processes can fix P within mineral phases and how natural flooding processes may flush sediment from the river channel. The processes identified in this study are likely to be applicable to similar river systems over their non-tidal water bodies in eastern England.     

Multi-element signatures of stream sediments and sources under moderate to low flow conditions

This study assesses a simple sediment source tracing method using major- (Al, Ca, Fe, K, Mg, Mn, Na, P, Si, Ti) and trace-element (Ba, Be, Ce, Co, Cr, Mo, Nd, Pb, Sr, Th, V, Y, Zn) signatures of stream suspended particulate matter (SPM), bed sediments and soils in a small agricultural catchment in NE Scotland. Whilst most erosion studies characterise the large amounts of material mobilised at the highest flows, this study aimed to assess properties of sediments during moderate to low flow periods. These occur more frequent than intense storms and are important in linking stream sediments, near-channel sources and aquatic ecosystem impacts. Data were transformed by multivariate statistical methods to compare elemental signatures of SPM (ranging from 3 to 53 mg L⁻¹ in the stream) and stream bed sediments with a limited number of near-channel source soils. Increased concentrations of Ce, Nd, Th and Y in subsoils contributed to the ability to discriminate between surface fieldslope and stream bank erosion sources. Stream bed sediments showed close matches with compositions of stream bank and arable surface soils, but signatures of SPM differed greatly from any of the sources. Large concentrations of Cr, Pb and Zn in SPM, particularly during summer (677, 177 and 661 mg kg⁻¹, respectively) exceeded water quality standards and were linked to an accumulation of trace elements associated with biological material. The potential for within-stream alteration of SPM in relation to erosion sources was confirmed by changes in the nature of the SPM organic matter observed by IR spectroscopy. Thus the potential is shown for multi-element signatures to give information on catchment sediment sources to aid land management decisions, given careful consideration of the effects of in-stream alteration of eroded material. However, this combined information may be beneficial to process understanding linking land use and stream ecosystems at critical ecological periods.     

Zinc mobility and speciation in soil covered by contaminated dredged sediment using micrometer-scale and bulk-averaging X-ray fluorescence, absorption and diffraction techniques

The mobility and solid-state speciation of zinc in a pseudogley soil (pH = 8.2-8.3) before and after contamination by land-disposition of a dredged sediment ([Zn] = 6600 mg kg⁻¹) affected by smelter operations were studied in a 50 m² pilot-scale test site and the laboratory using state-of-the-art synchrotron-based techniques. Sediment disposition on land caused the migration of micrometer-sized, smelter-related, sphalerite (ZnS) and franklinite (ZnFe₂O₄) grains and dissolved Zn from the sediment downwards to a soil depth of 20 cm over a period of 18 months. Gravitational movement of fine-grained metal contaminants probably occurred continuously, while peaks of Zn leaching were observed in the summer when the oxidative dissolution of ZnS was favored by non-flooding conditions. The Zn concentration in the <50 μm soil fraction increased from ∼61 ppm to ∼94 ppm in the first 12 months at 0-10 cm depth, and to ∼269 ppm in the first 15 months following the sediment deposition. Higher Zn concentrations and enrichments were observed in the fine (<2 μm) and very fine (<0.2 μm) fractions after 15 months (480 mg kg⁻¹ and 1000 mg kg⁻¹, respectively), compared to 200 mg kg⁻¹ in the <2 μm fraction of the initial soil. In total, 1.2% of the Zn initially present in the sediment was released to the environment after 15 months, representing an integrated quantity of ∼4 kg Zn over an area of 50 m². Microfocused X-ray fluorescence (XRF), diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy techniques were used to image chemical associations of Zn with Fe and Mn, and to identify mineral and Zn species in selected points-of-interest in the uncontaminated and contaminated soil. Bulk average powder EXAFS spectroscopy was used to quantify the proportion of each Zn species in the soil. In the uncontaminated soil, Zn is largely speciated as Zn-containing phyllosilicate, and to a minor extent as zincochromite (ZnCr₂O₄), ^IVZn-sorbed turbostratic birnessite (δ-MnO₂), and Zn-substituted goethite. In the upper 0-10 cm of the contaminated soil, ∼60 ± 10% of total Zn is present as ZnS inherited from the overlying sediment. Poorly-crystalline Zn-sorbed Fe (oxyhydr)oxides and zinciferous phyllosilicate amount to ∼20-30 ± 10% each and, therefore, make up most of the remaining Zn. Smaller amounts of franklinite (ZnFe₂O₄), Zn-birnessite and Zn-goethite were also detected. Further solubilization of the Zn inventory in the sediment, and also remobilization of Zn from the poorly-crystalline neoformed Fe (oxyhydr)oxide precipitates, are expected over time. This study shows that land deposition of contaminated dredged sediments is a source of Zn for the covered soil and, consequently, presents environmental hazards. Remediation technologies should be devised to either sequester Zn into sparingly soluble crystalline phases, or remove Zn by collecting leachates beneath the sediment.     

Phosphorus fractions and bioavailability in relation to particle size characteristics in sediments from Lake Hongfeng, Southwest China

Information on the chemical composition of phosphorus (P) fractions in sediments is fundamental to understanding P bioavailability and eutrophication in lake ecosystems. Phosphorus fractions and its bioavailability in sediments cores of Lake Hongfeng, southwest China, were investigated using a chemical sequential extraction scheme. Relationships between P fractions, P bioavailability and particle sizes were discussed. P fractions concentrations were ranked in the order: Residual-P > NaOH–rP > NaOH–NRP > HCl–P > BD–P > NH₄Cl–P, and all of them decreased with increasing sediment depth. Statistical analysis showed that concentrations of bioavailable P (BAP) which includes the NH₄Cl–P, BD–P, NaOH–rP and NaOH–NRP fractions ranged from 404.68 to 1,591.99 mg/kg and accounted for 26.8–71.8 % of the concentrations of total phosphorus (TP) in the top 5 cm sediments, whereas in the whole sediment cores, their concentrations ranged from 239.70 to 1,591.99 mg/kg and accounted for 26.8–76.0 % of TP. The results suggested that the sediments were a large potential source of P for algae blooms in Lake Hongfeng. Phosphorus fractions and their potential bioavailability were influenced by the sediment particle sizes, especially the bioavailability of the NH₄Cl–P fraction, which was strongly affected by the presence of fine particle sizes in the sediments.     

Sediment Organic Carbon in Todos Santos Bay, Baja California, Mexico

Sediment grain size and organic carbon (OC) data collected over the past 50 years, together with δ¹³C values of OC in recently collected samples, were analyzed to improve understanding of sediment OC distribution and abundance in Todos Santos Bay. Sediments in the submarine canyon at the mouth of the bay and in quiet-water locations along the shore are fine grained, high in OC, and have generally low δ¹³C values; sediments in high-energy environments are low in OC and have high δ¹³C values. A bivariate isotopic mixing model indicates that none of the sediments contain >50% terrigenous OC (average ~30%), and that the terrigenous OC content of the sediments is a small proportion of the OC content of local soils. Sediment OC composition is apparently controlled by energy-related sorting and deposition, oxidation of much of the original terrigenous OC, and replacement of some terrigenous OC by marine OC.     

Sediment-water oxygen and nutrient exchanges along the longitudinal axis of Chesapeake Bay: Seasonal patterns,controlling factors and ecological significance

Sediment-water oxygen and nutrient (NH₄ ⁺, NO₃ ^?+NO₂ ^?, DON, PO₄ ^3?, and DSi) fluxes were measured in three distinct regions of Chesapeake Bay at monthly intervals during 1 yr and for portions of several additional years. Examination of these data revealed strong spatial and temporal patterns. Most fluxes were greatest in the central bay (station MB), moderate in the high salinity lower bay (station SB) and reduced in the oligohaline upper bay (station NB). Sediment oxygen consumption (SOC) rates generally increased with increasing temperature until bottom water concentrations of dissolved oxygen (DO) fell below 2.5 mg l^?1, apparently limiting SOC rates. Fluxes of NH₄ ⁺ were elevated at temperatures >15°C and, when coupled with low bottom water DO concentrations (<5 mg l^?1), very large releases (>500 μmol N m^?2 h^?1) were observed. Nitrate + nitrite (NO₃ ^?+NO₂ ^?) exchanges were directed into sediments in areas where bottom water NO₃ ^?+NO₂ ^? concentrations were high (>18 μM N); sediment efflux of NO₃ ^?+NO₂ ^? occurred only in areas where bottom water NO₃ ^?+NO₂ ^? concentrations were relatively low (<11 μM N) and bottom waters well oxygenated. Phosphate fluxes were small except in areas of hypoxic and anoxic bottom waters; in those cases releases were high (50–150 μmol P m^?2 h^?1) but of short duration (2 mo). Dissolved silicate (DSi) fluxes were directed out of the sediments at all stations and appeared to be proportional to primary production in overlying waters. Dissolved organic nitrogen (DON) was released from the sediments at stations NB and SB and taken up by the sediments at station MB in summer months; DON fluxes were either small or noninterpretable during cooler months of the year. It appears that the amount and quality of organic matter reaching the sediments is of primary importance in determining the spatial variability and interannual differences in sediment nutrient fluxes along the axis of the bay. Surficial sediment chlorophyll-a, used as an indicator of labile sediment organic matter, was highly correlated with NH₄ ^?, PO₄ ^3?, and DSi fluxes but only after a temporal lag of about 1 mo was added between deposition events and sediment nutrient releases. Sediment O:N flux ratios indicated that substantial sediment nitrification-denitrification probably occurred at all sites during winter-spring but not summer-fall; N:P flux ratios were high in spring but much less than expected during summer, particularly at hypoxic and anoxic sites. Finally, a comparison of seasonal N and P demand by phytoplankton with sediment nutrient releases indicated that the sediments provide a substantial fraction of nutrients required by phytoplankton in summer, but not winter, especially in the mid bay region.