Metal distribution in sediments of a drinking water reservoir: influence of reservoir morphometry and hydrodynamics

Bioavailability and ecological risk assessment of metal pollutants in ambient PM2.5 in Beijing

Metal pollutants in fine particulate matter (PM2.5) are physiologically toxic, threatening ecosystems through atmospheric deposition. Biotoxicity and bioavailability are mainly determined by the active speciation of metal pollutants in PM2.5. As a megacity in China, Beijing has suffered severe particulate pollution over the past two decades, and the health effects of metal pollutants in PM2.5 have received significant attention. However, there is a limited understanding of the active forms of metals in PM2.5 and their ecological risks to plants, soil or water in Beijing. It is essential that the ecological risks of metal pollutants in PM2.5 are accurately evaluated based on their bioavailability, identifying the key pollutants and revealing historic trends to future risks control. A two-year project measured the chemical speciation of pollution elements (As, Cd, Cu, Cr, Ni, Mn, Pb, Sb, Sr, Ti, and Zn) in PM2.5 in Beijing, in particular their bioavailability, assessing ecological risks and identifying key pollutants. The mass concentrations of total and active species of pollution elements were 199.12 ng/m3 and 114.97 ng/m3, respectively. Active fractions accounted for 57.7 % of the total. Cd had the highest active proportion. Based on the risk assessment code (RAC), most pollution elements except Ti had moderate or high ecological risk, with RAC exceeding 30 %. Cd, with an RAC of 70 %, presented the strongest ecological risk. Comparing our data with previous research shows that concentrations of pollution elements in PM2.5 in Beijing have decreased over the past decade. However, although the total concentrations of Cd in PM2.5 have decreased by >50 % over the past decade, based on machine model simulation, its ecological risk has reduced by only 10 %. Our research shows that the ecological risks of pollution elements remain high despite their decreasing concentrations. Controlling the active species of metal pollutants in PM2.5 in Beijing in the future is vital.

Bubbles dominated the significant spatiotemporal variability and accumulation of methane concentrations in an ice-covered reservoir

Climate-sensitive ice-covered reservoirs are critical components of methane (CH4) release. However, the mechanisms that influence CH4 dynamics during ice-covered periods remain poorly studied. To investigate the effects of bubbles on CH4 dynamics, we conducted intensive field and incubation experiments in an ice-covered reservoir (ice growth, stability, and melt period) in Northeast China. We found that the mean dissolved CH4 concentrations in the ice (625.9 ± 2419.7 nmol L-1) and underlying water (1218.9 ± 2678.9 nmol L-1) were high, making them atmosphere CH4 sources. The visible bubble bands (bubble area) in the riverine zone and the vertical profile of the CH4 concentration in the ice reflect the distribution of trapped bubbles. The mean CH4 concentration in the ice of the bubble area (1674.8 ± 3926.8 nmol L-1) was 2 orders of magnitude higher than that of no-bubble area (53.7 ± 9.2 nmol L-1). Moreover, a large amount of CH4 accumulated under the ice in the bubble area. These findings suggest that bubbles determine the CH4 storage in ice and CH4 accumulation in the underlying water. Ice growth increases CH4 storage in ice and the underlying water because of the entrapment and re-dissolution of CH4 bubbles. However, ice melting releases the CH4 accumulated in the ice and underlying water. A comparison of the field and incubation experiments indicated that the deep-water environment of the reservoir had a CH4 burial effect. Stepwise regression analysis revealed that higher sediment organic matter content, median particle size, and porosity increased the production and release of CH4 bubbles, trapping more CH4 bubbles in ice. Overall, this study improves the mechanistic understanding of CH4 dynamics and predictability of CH4 emissions during ice-covered periods.

Multiple anthropogenic influences in the Pará River (Amazonia, Brazil): A spatial-temporal ecotoxicological monitoring in abiotic and biotic compartments

The mineral wealth of the Amazon region is prone to intense exploration with consequent metals mobilization in ecosystems. Besides that, a number of other anthropogenic activities contribute to the imbalance of this important environment. The Pará River is an important water body in the Amazon basin and is under multiple anthropogenic influences, including disorderly urbanization, port activities and processing of minerals such as bauxite and kaolin. In this study, metals concentrations (Al, Cr, Pb, Ba, Ni, and Mn) in water, sediment and organisms (the fish Cheirocerus goeldii and the shrimp Macrobrachium amazonicum) and biochemical biomarkers (total antioxidant capacity, ACAP and lipid peroxidation, LPO) were analyzed along five points in the Para River with different distances to a center of anthropogenic activity, in three seasonal periods (rainy, transition and dry). Metals concentrations were similar among sites in each seasonal period but Aluminium (Al) presented the highest concentrations among all analyzed metals both in sediment and water considering all sampling points and all seasonal periods. In the dissolved fraction, Al had values above those established by the local environmental agency, especially during the rainy season. In the biotic compartment, both fish and shrimps showed higher concentrations of metals (Al and Ba) in the rainy season compared to the other seasons. Shrimp was more responsive to metal accumulation than fish, showing an adaptive response of biomarkers. Fish showed an increase of LPO in gills for individuals from the point of greater anthropogenic interference during the rainy season, but no differences in metal accumulation. We conclude that there is a seasonal pattern of metals concentration in different environmental compartments. Metal concentration in organisms and biomarkers responses, showed the effect of anthropogenic influences, which was not evident in results from chemical analyses alone, due to the intensive hydrodynamics in the region.

Microbial controls on heavy metals and nutrients simultaneous release in a seasonally stratified reservoir

The eutrophication of reservoirs can change the physicochemical parameters of water, thus affecting the migration and transformation of heavy metals. At present, there is insufficient research on the coupling mechanisms between nutrients and heavy metals, especially between heavy metals in suspended particles. In this paper, spatial and temporal distribution characteristics of nutrients dissolved heavy metals, and heavy metals in suspended particles were analyzed in a seasonally stratified reservoir. Combined with the nitrogen and phosphorus biogeochemical process, the coupling mechanisms between heavy metals and nutrients were discussed. The results showed that the Aha Reservoir had temperature and dissolved oxygen stratification in April and July. The reduction and dissolution of Fe and Mn oxide/hydroxide and the resuspension of sediments might result in a simultaneous increase in the concentrations of nutrients, dissolved heavy metals and heavy metals in suspended particles in hypolimnion in July and October. In the presence of dissimilatory iron-reducing bacteria (DRIB), the dissolution of iron-bound phosphorus in sediments and suspended particulate matter (SPM) might lead to the simultaneous release of iron and phosphorus into the water. The dissolution of metal sulfides in the sediments and SPM under the action of dissimilatory nitrate reduction to ammonium (DNRA) bacteria might lead to the simultaneous release of ammonia nitrogen and heavy metals into the water. Due to the coupling between nitrogen and phosphorus and heavy metals, seasonal stratified reservoir may face the risk of periodic simultaneous pollution of eutrophication and heavy metals in summer and autumn. This research provides theoretical support for the treatment of heavy metal and eutrophication combined pollution in karst areas.

Flood impact on the transport, transition, and accumulation of phosphorus in a reservoir: A case study of the Biliuhe Reservoir of Northeast China

Stormflow runoff is the most important agent for phosphorus (P) input to reservoirs, as the particulates contained in runoff carry a substantial amount of P. The settling process of particulates affects the P content of water, and the distribution of particulates determines the P distribution in reservoir sediment. An understanding of flood impacts on the transport, transition, and accumulation of P in a reservoir is critical to reservoir management. In this study, water samples before and after flooding and sediment samples after flooding were collected from Biliuhe Reservoir in Northeast China. P content and load in the water and P-fractions and particle sizes of the sediments were analyzed. Results showed that total particulate P (TPP) increased sharply from 1.56 to 26.72 t after flooding, whereas dissolved organic P (DOP) decreased markedly from 3.24 to 1.17 t, which was largely caused by biological uptake directly or indirectly before flooding. Orthophosphate (PO₄^3-) shared a similar trend with TPP, indicating that PO₄^3- could be adsorbed onto settling particulates, helping to reduce the reactive P introduced by flooding. Reservoir sediment showed a fining trend downstream and the clay fraction exhibited an obvious correlation with P-fractions, demonstrating that the distribution of particulate matter determined P distribution in the sediment. This study also found that particulates from the largest tributary (Biliu River) were only minimally transported from its reservoir entrance to the dam because of a longer travel distance, while contrastingly, particulates from a smaller tributary (Bajia River) were maximally carried to the dam because of a shorter distance. Our fundings suggests that surface water in the reservoir should be released prior to flooding in order to mitigate control of P in the water, moreover, it is necessary to strengthen the effectiveness of pollutant control projects at the reservoir entrance of the Bajia River.

Hydrodynamic impact on trace metals in sediments in the cascade reservoirs, North China

Cascade reservoirs facilitate the effective use of water resources and help to alleviate existing problems of water shortage in drought-prone regions. However, the geochemical behavior and controlling mechanisms of trace metals in response to the operation of cascade reservoirs are relatively unknown. Here, trace metals (As, Cr, Cu, Li, Ni, Pb and Zn) from thirty sediment cores from cascade reservoirs (Panjiakou and Daheiting Reservoirs) in China were evaluated. Multiple methods including geochemical baseline, geostatistical analysis, factor analysis (FA), and positive matrix factorization (PMF), were combined to assess pollution status, identify and quantify potential anthropogenic sources, and determine the influence of hydrodynamic conditions on trace metals distribution. The results indicate that minor enrichment of trace metals appeared in both cascade reservoirs. However, trace metal concentrations exhibited spatial heterogeneity between two cascade reservoirs, and diverse hotspots of different metals were unexpectedly observed. This can be explained by the following three aspects: (1) Metal hotspots were detected upstream of the cascade dams via geostatistical analysis and FA, particularly for naturally sourced metals (As and Li) where dam interception resulted in higher concentrations in the upstream reservoir. (2) PMF analysis identified agricultural, industrial, and natural sources to account for 23.44%, 41.61%, and 34.95%, respectively, to the metal concentrations in the downstream reservoir. Anthropogenic emissions were the dominant factors influencing the spatial variability of Cu, Pb, and Zn between the cascade reservoirs, with higher concentrations observed in the downstream reservoir. (3) The hydrological regime also influenced the redistribution of human-derived metals, where slower flow velocities at river bends resulted in higher deposition of metal-bearing particles. This study shed light on the spatial distribution of trace metals in response to the construction and operation of cascade reservoirs, and it suggests that trace metal hotspots should be monitored to prevent potential contamination in sediments.