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Cascade dam has important effects on the magnitude and dynamics of sediment particles, heavy metals, and biogenic elements in reservoirs. However, systematic studies on the interception effect of cascade dam on the various elements that occur in rivers at the watershed scale are lacking. The aims of this study were to (1) assess the interception effect of a cascade dam on heavy metals and biogenic elements and (2) investigate the key factors of these effects of the cascade dam. Surface sediments were collected from 29 sites distributed in the Wujiang River Basin (WRB, a watershed scale in Southwest China), including from tributaries (7 sites), the main stream (13 sites), and cascade reservoirs (9 sites). In addition, the particle sizes, heavy metals (Fe, Mn, Zn, Cr, Cu, As, Pb, and Cd), and biogenic elements (TOC, TN, and TP) of sediments were analyzed. Compared with the tributaries, D50 (median particle size) was significantly reduced by 56.8% of cascade reservoirs. The proportion of 63–2,000 μm decreased from 13.78 to 1.34%, indicating that more coarse particles were intercepted in the cascade reservoirs. The contents of heavy metals (Fe, Zn, Cu, As, and Cd) declined significantly along the way. On the whole, the contents of TOC, TN, and TP were highest in the midstream and lower in the upstream and downstream. The hydrological condition (reservoir age, HRT, and flow) and the basin area and internal and external inputs of cascade reservoirs are important factors. The findings deepen the current understanding of the mechanisms by which cascade dam affects the river transport of heavy metals and biogenic elements at the watershed scale and provide an important reference for establishing hydropower developments along rivers and developing aquatic environment management strategies.

Accurate measurement of the oxygen isotopic composition of dissolved phosphate (δ18OP) of different phosphorus (P) fractions in lacustrine sediments is very difficult because of the influence of large amounts of impurities. In this study, we developed a five-step method for obtaining high purity Ag3PO4 sample for the analysis of δ18OP of different P fractions in freshwater sediments. Sedimentary P was divided into NaHCO3-P, NaOH-P and HCl-P by chemical sequential extraction. Pretreatment procedures for different sedimentary P fractions were improved in the following respects: 1) abandonment of the magnesium-induced coprecipitation method to avoid the introduction of impurity ions, such as Mg2+ and Cl-, and reduce the loss of P; 2) use of a small amount of non-phosphate activated carbon powder to efficiently remove organic matter in extracts of NaHCO3-P and NaOH-P, and reduce the loss of P; 3) adjustment of the HCl-P extract pH to 4 in order to form Fe(OH)3-PO43- coprecipitate, thereby removing most of metals and Cl-. This method reduces the pretreatment steps, simplifies the operation and increases the recovery of phosphate (90.98%-96.69%). The high purity Ag3PO4 sample can be obtained and the repeatability and accuracy of measured δ18OP is better than 0.3‰, demonstrating high reliability and accuracy. This new method was used to analyze the δ18OP of different P fractions in sediments of a eutrophic lake in southwestern China. The preliminary results indicated that the δ18OP in the sediments can be used to identify different P sources, and provide new insights into sedimentary P cycling. The method established in this study provides a powerful tool for investigating the sources and biogeochemical cycle of P in freshwater sediments.

Reservoirs in southwestern China are encountering the challenge of eutrophication, in which internal phosphorus (P) release from sediments plays an important role. Studies on the high-resolution profile variations and release mechanisms of P at the sediment-water interface (SWI) are rare in these reservoirs until now. In this study, monthly monitoring (Nov 2017 to Oct 2018) using a composite diffusive gradient in thin films (DGT) technique was taken to determine the temporal and vertical profile variations of the DGT-labile P (Fe) at the SWI in Hongfeng Reservoir. The results showed that the average concentrations of the DGT-labile P (Fe) in surface sediments were 0.63 ± 0.24 mg·L−1 and 4.61 ± 1.12 mg·L−1, respectively, with significantly higher concentrations during the summer anoxic period than that during the winter aerobic period. The DGT-labile P (Fe) concentrations in sediments presented a significant positive correlation (r2 > 0.70, p

Extensive algal bloom in the surface water is a pressing issue in Lake Dianchi that causes lake restoration to be difficult owing to complex and variable phosphorus (P) sources in the water column. P released from algae, suspended particles (SS), and sediment can provide sustainable P sources for algal blooms. However, little is known regarding the dynamic of P speciation in these substances from different sources. In this study, solution

Cascade dam has important effects on the magnitude and dynamics of sediment particles, heavy metals, and biogenic elements in reservoirs. However, systematic studies on the interception effect of cascade dam on the various elements that occur in rivers at the watershed scale are lacking. The aims of this study were to (1) assess the interception effect of a cascade dam on heavy metals and biogenic elements and (2) investigate the key factors of these effects of the cascade dam. Surface sediments were collected from 29 sites distributed in the Wujiang River Basin (WRB, a watershed scale in Southwest China), including from tributaries (7 sites), the main stream (13 sites), and cascade reservoirs (9 sites). In addition, the particle sizes, heavy metals (Fe, Mn, Zn, Cr, Cu, As, Pb, and Cd), and biogenic elements (TOC, TN, and TP) of sediments were analyzed. Compared with the tributaries, D50 (median particle size) was significantly reduced by 56.8% of cascade reservoirs. The proportion of 63-2,000 μm decreased from 13.78 to 1.34%, indicating that more coarse particles were intercepted in the cascade reservoirs. The contents of heavy metals (Fe, Zn, Cu, As, and Cd) declined significantly along the way. On the whole, the contents of TOC, TN, and TP were highest in the midstream and lower in the upstream and downstream. The hydrological condition (reservoir age, HRT, and flow) and the basin area and internal and external inputs of cascade reservoirs are important factors. The findings deepen the current understanding of the mechanisms by which cascade dam affects the river transport of heavy metals and biogenic elements at the watershed scale and provide an important reference for establishing hydropower developments along rivers and developing aquatic environment management strategies.

Aluminum-salt inactivating agents are extensively applied to the restoration of lakes polluted by internal phosphorus (hereinafter referred to as "P"). However, there is a lack of micromechanism information regarding the sediment P cycle and its interactions with aluminum salts, which has restricted the engineering applications of aluminum salts. In this study, a sediment core incubation system was used to simulate the influence of aerobic and anaerobic conditions on the effectiveness and stability of aluminum-modified clay (AMC). This study also investigated the millimeter-scale dynamics of P across the sediment-water interface (SWI) using the HR-Peeper and DGT techniques. According to the results, sediment P release mainly occurred under anaerobic conditions. When the incubation system was in an anaerobic state, AMC effectively reduced the internal-P loading. In pore water, there was a positive correlation between soluble Fe and SRP, suggesting that the reductive dissolution of Fe-P constituted the main mechanism of sediment P release. After with dosing AMC, the concentrations of SRP and labile P in the capping layer both dropped abruptly to low levels and the content of Al-P in surface sediments rose, suggesting that AMC had strongly adsorbed phosphates, formed inert Al-P and blocked the phosphate exchange between pore water and overlying water. This study elaborated on the micromechanism of the control of sediment internal P input by AMC and revealed that Al-P precipitation constituted the main mechanism of the inhibition of sediment P release by aluminum-salt inactivating agents. The research findings have a great significance for guiding field applications of aluminum-salt inactivating agents.

Accurate measurement of the oxygen isotopic composition of dissolved phosphate (δ

The oxygen isotope ratio in phosphate (δ18OP) is an effective tool for tracing the sources of phosphorus (P) and their biogeochemical cycles. The HCl-extractable inorganic phosphate (HCl-PTI) is the primary component of P and plays a key role in the dynamic transformation of P in sediment and soil. However, it has always been difficult to purity sample for analyzing the δ18OP composition of HCl-PTI due to the significantly interference of large amounts of Fe2+/3+, Ca2+, Cl− and other impurities in the HCl-PTI extracts. In this study, we developed a simple and promising purification method for δ18OP analysis of HCl-PTI in sediments and soils based on the superior selective enrichment and elution abilities of phosphate (PO4) and efficient synchronous removal of impurities by Zr-Oxide gel in highly acidic HCl-PTI extract. The enrichment rate and elution rate of PO4 in the HCl-PTI extract by the Zr-Oxide gel could achieve 100% respectively, and the synchronous removal rates of most anions, dissolved organic carbon (DOC), metals, and trace elements could be above 96% respectively, reducing the tedious operation of conventional methods and the difficulties in removal of impurities, such as Cl− in HCl-PTI extract. It's indicated that the purity of the Ag3PO4 solid for δ18OP composition analysis of HCl-PTI could reach 99.9%. The above results show that the new method is very simple, efficient and reliable for the δ18OP analysis of HCl-PTI in sediments and soils. This study provides new methodological support for identifying the sources of P and tracing their biogeochemical cycles in sediment or soil environment.

The phosphate oxygen isotope (δ18OP) ratio has been proven to be an effective tool to trace the sources and biogeochemical cycles of phosphorus (P) in aquatic ecosystems. However, the enrichment of phosphate (PO4) and the removal of impurities are quite complex and easy to cause PO4 loss in current δ18OP analytical methods. Moreover, the δ18OP value obtained by the commonly-used instantaneous sampling method is more of the instantaneous information of P, which is accidental or uncertain for accurate identification of the P source. In this study, a new method of in situ enrichment, elution, and purification of PO4 (ISEEP) was developed for δ18OP analysis in waters. This method utilized a PO4 binding phase (Zr-Oxide gel) to selectively in situ adsorb PO4 in water and exhibited an adsorption capacity per unit area of up to 789.3 μg P/cm2. The PO4 on the gel was eluted easily with a 1 M NaOH solution. More than 99.7% of the common anions, cations, and dissolved organic matter (DOM), as well as more than 90% of the trace elements were removed synchronously after adsorption and elution of PO4. The recovery rate of PO4 in the whole procedure was as high as 92.8%. The XRD and SEM examinations showed that the ISEEP can obtain high-purity Ag3PO4 solid for the δ18OP measurement. The reliability of the ISEEP method is confirmed by the measured δ18OP value and standard deviation of parallel samples from different types of natural waters obtained by both the ISEEP and the current popular McLaughlin (2004) method. It provides a good prospect of this new method for tracing the P sources and their biogeochemical cycling in aquatic ecosystems.

China has one of the largest areas of agriculture and one of the highest application rates of chemical fertilizer in the world. Agricultural non-point source pollution has became important source of nitrogen (N) pollution of surface water. In this study, the δ15N and δ18O isotopes of nitrate as a tool were used to reveal the sources of nitrate in water bodies in the Nala watershed, at a sugarcane planted area in Guangxi, China. According to the analyses based on a stable isotope analysis in R (SIAR), the mean contributions of different nitrate sources in the Nala watershed could be ranked as follows: manure (39.90 %) > soil N (SN) (34.94 %) > chemical fertilizer (CF) (17.77 %) > rainfall (7.39 %) in July; manure (42.30 %) > SN (33.54 %) > CF (24.16 %) in December. The CF and SN collectively mean contributing > 50 % of nitrate both in July and in December in the watershed, therefore, reducing the use of CF is the key to control nitrate pollution in the Nala watershed. Manure also composed a substantial contribution (mean 39.68 %, in July and December) to nitrate in the small watershed, it is necessary to provide recycle treatment for manure for the purpose of alleviating the pollution. The δ15N and δ18O isotopes of nitrate can accurately identify various sources of nitrate and SIAR offers a useful method to quantify the contribution rates of different N sources to the N pollution of watersheds.

The loss of phosphorus(P) and nitrogen from agricultural non-point source(ANPS) is one of the main causes of eutrophication in rivers, lakes and reservoirs in the Karst region, southern China. The lakeside buffer zone has proven to be effective in reducing the input of ANPS pollutants. However, the interception efficiency of natural soil for P and ammonia nitrogen is insufficient to control ANPS pollution. In this study, natural zeolite(NZ), aluminium modified clay(AMC) and lanthanum modified bentonite(LaMB) were used to amend the natural soil collected in the lakeside zone of Kelan Reservoir, Guangxi, China. An experimental study was conducted to examine the dissolved inorganic P(DIP) and ammonia nitrogen removal efficiency and fixation mechanism of three amended soils under simulation conditions. The results showed that natural soil had a high fraction of clay and silt (particle size 95 % for the DIP at the beginning of the experiment. With the intermittent input of farmland drainage, the P removal rate of natural soil from runoff decreased gradually. Compared with the control group, P removal rates of AMC- and LaMB amended soils remained at a high level within 46 days, with means of 93.6 % and 93.9 %. However, NZ amended soil has a weak interception capacity of runoff P pollutant. The chemical sequential extraction showed that AMC increased soil P capacity by forming NaOH-P and NaHCO3-P, while LaMB by forming NaOH-P, NaHCO3-P and Residual-P. In addition, LaMB amended soil can simultaneously remove ammonia from farmland runoff, with the removal rate of 89.3 %. AMC- and LaMB amended soils are proved to be potential and effective technologies for ANPS pollution control. This study provide an important basis for the treatment of ANPS pollutants and the construction of lakeside buffer zone in eutrophic watershed.

Lake eutrophication is a main water environmental problem. When the extraneous nutrients are effectively controlled, nutrients in water mainly source from endogenous release from the sediment. In situ passivation is an important pollution control technique for endogenous pollution in lakes or reservoirs. This study focused on Qianling Lake, a cascade-channel lake in Southwest China polluted by phosphorus (P), which was selected for an in situ passivation project using a novel Al-based passivator on the polluted water area. Accordingly, the release of endogenous nutrients from the sediment can be controlled to remediate the polluted water, and the remediation effect and ecological risk of the passivation were evaluated. The results showed that after 12 months of passivation, the release of P from the sediments can be effectively inhibited. Concentrations of total P (TP) and Chl-a within water of the passivation area were reduced by approximately 80% and 70%. Meanwhile, water transparency and the content of dissolved oxygen were remarkably enhanced. The application of the passivator remarkably improved the water quality. P in water and at the sediment-water interface was fixed on the surface of sediment in the form of Al-combined state. This passivator exhibits favorable P-controlling performance in the restoration of lakes and reservoirs polluted by endogenous P. This aluminum-modified clay is an effective passivator for remediation of internal P pollution in potentially similar lakes and reservoirs.

TSP and PM2.5 were sampled in Daihai, an Asian monsoon-sensitive region in Inner Mongolia, in summer. The aims were to evaluate the characteristics of trace elements and to investigate the influence of the summer monsoon on elemental composition there. Eighteen trace elements, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Br, Sr, Zr and Pb, were determined and their concentrations were evaluated. The TSP/PM2.5 ratio of element concentrations showed that crustal elements like K, Ca, Ti and Fe, as well as Pb, Ni and Sr were more abundant in TSP, while other elements predominated in PM2.5. Enrichment factor (EF) calculations revealed that non-crustal trace elements were more enriched in PM2.5 than in TSP. Back trajectory analysis indicated that crustal elements such as K, Ca, Ti and Fe had distinctly high concentrations associated with northerly flows, while non-crustal elements showed no clear pattern. However, non-crustal elements showed high EF values related to southeasterly flows. Principal component analysis (PCA) was also applied to the PM2.5 data to discuss the source appointment and five factors were determined.

We studied the influence of acid pretreatment on the effective distinction between elemental carbon (EC) and organic carbon (OC), and between char-EC and soot-EC. Though widely employed in the pretreatment of soils and sediments for EC quantification, the use of HCl, HF, and HNO(3) could decrease soot thermal stability as acid remains, leading to an underestimation of soot-EC by thermal methods. We compared thermal optical reflectance (TOR) measurements of EC concentrations in char reference materials and in lacustrine and marine sediments following pretreatment with various acids. The results showed that pretreatment with 2M HCl, concentrated HNO(3), 7 M HNO(3), and 1 M HNO(3) did not result in EC oxidation. However, hot concentrated HNO(3) oxidized EC significantly, leading to lower concentrations of EC, char-EC and soot-EC. By comparing the removal of potentially interfering materials, which contain little fire-derived carbon, with different acid pretreatments, we recommend the HCl-HF-HCl and concentrated (not hot) HNO(3)-HF-HCl pretreatments for the determination of EC, char-EC, and soot-EC in soils and sediments using the TOR method.