Surface Water Levels Research Articles

Lake Chad vegetation cover and surface water variations in response to rainfall fluctuations under recent climate conditions (2000−2020)

Monitoring the evolution of the Sahelian environment is a major challenge because the great Sahelian droughts, marked by significant environmental consequences and social impacts, contributed, for example, to the drying up of Lake Chad. We combined remote sensing images with a water level database from the Hydroweb project to determine the response of Lake Chad vegetation cover and surface water variations to rainfall fluctuations in the Lake Chad watershed under recent climate conditions. The variance in lake surface water levels was determined by computing the monthly anomaly time series of surface water height and area from the Hydroweb datasets. The spatiotemporal variability of watershed rainfall and vegetation cover of Lake Chad was highlighted through multivariate statistical analysis. The spatial distribution of correlations between watershed rainfall and Lake Chad vegetation cover was investigated. The results show an increase in watershed rainfall, vegetation cover, and surface water area and height, as their slopes were all positive i.e., 5.1 10−4 (mm/day); 4.26 10−6 (ndvi unit/day); 1.2 10−3 (km2/day) and 6 10−5 (m/day), respectively. The rainfall variations in the watershed drive those of Lake Chad vegetation cover and surface water, as the rainfall trend was strongly and positively correlated with those of vegetation cover (0.79), surface water height (0.57), and area (0.53). The time lag between the watershed rainfall fluctuations and lake surface water variations corresponded to approximately ∼112 days. Between rainfall variations and vegetation cover changes, the spatial distribution of the time lag showed a response time of <16 days in the western shores of the lake and on both sides of the great barrier, about 16 days in the bare soils of the northern basin and the eastern part of the south basin, and >64 days in the marshlands of the southern basin. For the analysis of lakes around the world, this research provides a robust method that computes the spatiotemporal variances of their trends and seasonality and correlates these with the spatiotemporal variances of climate changes. The correlations obtained have strong potential for predicting future changes in lake surface water worldwide.

Elevated manganese concentrations in groundwater wells after longtime abstraction with bank filtration: developing and testing of a sorption model for Ringerike waterworks, Norway

Groundwater abstraction wells located in aquifers hydraulically connected with rivers or lakes will induce bank filtration if the water level in the aquifer is lowered below the surface-water level. Many groundwater works utilize this induced infiltration to increase the abstraction capacity of their wells. Unfortunately, infiltration of organic-rich surface water may alter the aquifer’s redox geochemistry, and mobilize iron and/or manganese. Ringerike groundwater works in southeastern Norway operates with bank filtration. Manganese concentrations in the abstracted water began to increase 19 years after the start of operations, and stabilized at an elevated level 8 years later. The estimated manganese retardation (Rsorp = 45±18) from a simplified advection-sorption model is comparable to the observed manganese retardation range 36 < Robs < 50, assuming a groundwater residence time tgw ~200 days. The advection-sorption model is based on limited sediment composition and hydrochemical data, which introduces significant uncertainty to Rsorp. However, Rsorp and Robs in the same range, and the good fit between PHREEQC simulated and observed data, indicates that sorption is a possible explanation for the delayed breakthrough of elevated manganese concentrations in Ringerike waterworks’ production wells. Comparable bank filtration schemes are advised to establish observation well(s) between the bank filtration and production well field for early detection of sorption fronts. This would facilitate (1) longer time available for remediation measures and their evaluation, and (2) less manganese to be desorbed during the remediation stage.

Does Snowfall Introduce Disinfection By-product Precursors to Surface Water?

Snow with large specific surface area and strong adsorption capacity can effectively adsorb atmospheric pollutants, which could/might lead to the increase of disinfection by-product (DBP) precursors in surface water. In this study, the contents and characteristics of dissolved organic matter (DOM) in meltwater were investigated, and DBP formation and the DBP-associated cytotoxicity index during chlorination of meltwater was first explored. Overall, meltwater exhibited high nitrogen contents. Meltwater-derived DOM was mainly composed of organics with low molecular weights, low aromaticity, and high unsaturated degrees. DBP formation potentials and cytotoxicity indexes in chlorinated meltwater were positively correlated with air quality index and were significantly impacted by snowfall stages. The trihalomethane and haloacetic acid yields from meltwater were relatively low, while yields of highly cytotoxic DBPs, especially halonitromethanes (6.3-10.8 μg-HNMs/mg-DOC), were significantly higher than those of surface water (1.7 μg-HNMs/mg-DOC). Notably, unsaturated nonaromatic organic nitrates in meltwater were important precursors of halonitromethanes. The actual monitoring results showed that snowfall significant increased the haloacetaldehydes and nitrogenous DBP formation levels of surface water. Considering increased DBP formation and DBP-associated toxicity, it was demonstrated that DOM derived from snowfall in atmosphere-polluted areas could deteriorate surface water quality and pose potential risks to drinking water.

Water pollution and its impact on the Blue Economy initiative: A lesson learned from the Kenyan Coast

AbstractWater, sediment and biota samples were collected from selected locations along the Kenyan Coast. The nutrient levels in surface water were relatively elevated as a result of sewage and riverine inputs. As, Cd, Pb, Hg and Ag levels in sediment from Kilindini Harbor pose minimal threat to marine life whereas Cu, Zn and Ni in the sediment pose a moderate threat to marine life while Cr levels in the sediment may have a severe impact on marine life. The concentrations of trace metals, PCBs, and DDT in the surface sediment were lower than the ERL thus posing a low threat to marine organisms. DDD+DDE/DDT ratios were above 0.5 suggesting historical input of DDT from rivers Tana, Sabaki and Ramisi. Trace metal concentrations in biota from Mtwapa, Makupa and Tudor creeks were below the WHO permissible levels of heavy metals in fish. The study concludes that sound management of marine environment is crucial for promoting fisheries as one of the sectors of the Blue Economy.

A first continuous and distributed satellite-based mapping of river discharge over the Amazon

River discharge integrates many water-related processes over land, it is crucial for understanding inland water. Unfortunately, in situ measurements are very sparse at the global scale. This paper presents a totally new approach for the mapping (i.e. spatially continuous estimate) of the river discharge based on satellite observation of hydrological variables and the water budget balance. First continuous satellite estimate of three water components (precipitation, evapotranspiration, and total water storage change) are corrected at basin scale using river discharge from a few gauge measurements. Secondly, the water budget is balanced at the grid level using flow direction for horizontal water exchange. This new approach is therefore based solely on satellite products and in situ measurements without the use of any dynamical model (except river map). The methodology is evaluated in interpolation mode between successive gauges (median KGE is 0.8) and extrapolation mode over small confluent rivers (mean KGE is 0.6), to be compared to 0.6 and 0.4 for a river dynamic model such as CaMa-flood. The spatially continuous river discharge shows a good agreement with altimetric water surface level (WSL, median correlation of anomalies is 0.6) and surface water extent satellite estimate (correlation of anomalies is up to 0.7 over rivers with floodplains). This suggests that timing of flood and low flow is fine. A comparison with the hydrological model CaMa-flood shows the benefit of such a continuous product to investigate model differences at high or low flow, as well as early flow peak modeling. While the spatial pattern of extreme events cannot be well represented only by in situ gauges information, our study shows the added value of the mapping to better describe these events. As hydrological application, our method can be used in synergy with all the altimetric stations to create discharge-WSL pair data, which will benefit to advanced applications.

Changes in the salt concentration distribution in surface water (seawater) due to seawater recirculation in a porous medium under tidal fluctuations

Many studies have used numerical simulations and experiments to simulate saltwater intrusion in a nearshore aquifer. Thus, it is known that seawater recirculation occurs in a porous medium just below a sloping beach, and a saltwater wedge occurs in the medium below where the slope intersects the water level at low tide. This study investigated the salt distribution in the surface water when subterranean recirculation of seawater is occurring and there is a saltwater wedge in an underlying porous medium, along with the relationships between the seawater recirculation and the tidal amplitude and beach gradient. The salt distribution in a subterranean aquifer and in surface seawater where the shore sloped gently seaward with a constant slope of 1/10 or 1/5 under tidal amplitudes of 0.5, 1.0, or 2.0 m was numerically simulated by the ASGMF method, which couples water pressure and water flow in a porous medium with those in the overlying surface water and can simulate variable-density flows and the salt concentration distribution in both the porous medium and the surface water. The results showed that the recirculation of seawater depended on the tidal amplitude, being greater when the amplitude exceeded 1.0 m, but that it was unlikely to occur when the beach gradient was steep. Thus, the aspect ratio (width to depth) of the seawater recirculation decreased as the tidal amplitude increased above 1.0 m. Furthermore, the simulated surface water level was often lower than the tidal water level; thus, the surface water level was not consistent with the tidal water level imposed at the right boundary of the simulation domain but varied slightly under the influence of water flow in the surface water and gas flow in the atmosphere. The salt concentration in the surface water was not always the same as the seawater salt concentration because a thin freshwater layer and a brackish water layer of mixed freshwater and saltwater overlay the seawater.

Expansion of typical lakes in Xinjiang under the combined effects of climate change and human activities

Lakes are important guarantees of regional economic development and ecological security. Previous studies focused on trends of surface area and water level of lakes in Xinjiang, but paid less attention to nonlinear change and driving mechanisms of lake areas at annual and monthly scales. To address this issue, this study used a remote-sensing Big Data cloud platform and mathematical statistical methods to investigate the change in typical lakes and its attribution in Xinjiang during 1986–2020. The results showed that: 1) there was a significant linear trend in Xinjiang lakes: except for Jili Lake, the plains lakes showed an insignificant (p &amp;gt; 0.1) expansion trend, while the mountain lakes showed an significant expansion trend (p &amp;lt; 0.01). 2) There was a significant nonlinear trend in Xinjiang lakes: most of the plains lakes showed periodicity at 14 and 21 years, however most of the mountain lakes showed periodicity at 17 and 21 years. Most of the mutation trends of plains lakes were not significant, yet the mutation trends of mountain lakes showed significant expansion. 3) Human activities were the dominant factor leading to changes in the plains lakes: among the anthropogenic factors, farmland area, GDP, and population had significant effects on lake area (p &amp;lt; 0.1), and lake expansion was closely related to population and farmland area. Among climatic elements, precipitation mainly affected the changes in plains lakes. 4) Climate change was the dominant factor leading to changes in mountain lakes. The effects of temperature change on mountain lakes were all significantly positive (p &amp;lt; 0.05). In the future, it will be necessary to build lake protection schemes that adapt to climate change and human disturbances. This study can provide an important scientific basis for the rational development and utilization of lakes in Xinjiang.

Apparent shifts in the microplankton community in response to zinc and iron enrichments in the Northeastern Subarctic Pacific

The offshore Gulf of Alaska—in the northeastern subarctic Pacific—is a high-nitrate, low-chlorophyll (HNLC) oceanic region where concentrations of dissolved iron (DFe) and zinc (DZn) are of the order ~0.1 nmol kg−1 in surface waters. At the average winter mixed layer depth of ~120 m, DFe and DZn are of the orders ~0.1–0.2 nmol kg−1 and ~1–2 nmol kg−1, respectively. Vertical supply of Fe to surface waters is therefore limited, with phytoplankton blooms driven by episodic atmospheric and lateral inputs of Fe. In contrast, DZn is presumably replenished through seasonal vertical mixing, though maintained at low levels in surface waters, even in winter, implying that removal processes are in operation. Previous supplementation experiments have demonstrated that phytoplankton biomass (chl-a), growth, and drawdown of nitrate (NO3-) are stimulated strongly by Fe but only minimally, if at all, by Zn alone. Using on-deck bioassay incubations of surface waters, we confirm that the addition of Fe stimulates significant increases in chl-a and large diatoms, and drawdown of NO3- and silicic acid [Si(OH)4]. Associated drawdown of DZn and dissolved cadmium (DCd) indicated Zn stress in the control and +Fe treatment. Supplementation with Zn alone had no significant impact on NO3- and commonly monitored pigments such as chl-a and fucoxanthin. However, in the +Zn treatment, DFe and coccolithophore abundance were significantly lower, and the concentration of particulate organic carbon (POC) and that of the pigments alloxanthin and chlorophyll c1+2 were significantly higher than those in control incubations. Our experiment corroborates previously observed relationships between DZn and alloxanthin and chlorophyll c across the subarctic north Pacific and Bering Sea and could help to explain the low or undetectable concentrations of alloxanthin frequently observed in this region. Alloxanthin is a pigment specific to the cryptophytes, either within free-living cells or within functional plastids retained by mixotrophic protists. Both cryptophytes and mixotrophic ciliates, such as Laboea sp. and Strombidium spp., make a variable but significant contribution to phytoplankton biomass in this area. In the absence of Fe supply to surface waters, Zn could play a hitherto unsuspected role in shaping plankton communities in the HNLC Gulf of Alaska.

Coupled modelling of storm surge, circulation and surface waves in a large stratified lake

The wind-driven nature of large lakes suggests that the accuracy of meteorological inputs is essential for hydrodynamic modelling. Moreover, coupling between the meteorological inputs and density stratification may also influence the simulated lake behavior. To investigate wind-driven large lake processes, a high-resolution coupled Delft3D-SWAN model was applied to Lake Ontario to simulate storm surges, surface waves, and circulation during two recent storm events. In both events, the sustained wind speeds approached 20 m s−1; however, variations in wind direction and duration altered the lake's surface wave and storm surge responses. The influence of different atmospheric inputs was investigated by comparing results from two spatially varied atmospheric models: the Rapid Refresh (RAP) and the High-Resolution Deterministic Prediction System (HRDPS). Hydrodynamic simulations using HRDPS were marginally better, with maximum root mean squared errors (RMSE) between modelled and observed water levels of 0.07 m, compared to 0.08 m with RAP. Predictions of the magnitude and timing of the maximum wave heights varied based on wind fields, with differences between predicted peak wave heights of up to 0.4 m. Both events occurred during a stratified period, allowing for a comparative evaluation of the influence of baroclinic and barotropic processes on the simulated surface wave and storm surge results. Simulations including the vertical density gradient gave a better representation of current velocities with depth and resulted in an improved prediction of peak storm surge magnitudes and surface water level behavior following the storms, reducing the RMSE by up to 12%.

Pesticide concentration monitoring: Investigating spatio‐temporal patterns in left censored data

AbstractMonitoring pesticide concentration is very important for public authorities given the major concerns for environmental safety and the likelihood for increased public health risks. An important aspect of this process consists in locating abnormal signals, from a large amount of collected data. This kind of data is usually complex since it suffers from limits of quantification leading to left censored observations, and from the sampling procedure which is irregular in time and space across measuring stations. The present manuscript tackles precisely the issue of detecting spatio‐temporal collective anomalies in pesticide concentration levels, and introduces a novel methodology for dealing with spatio‐temporal heterogeneity. The latter combines a change‐point detection procedure applied to the series of maximum daily values across all stations, and a clustering step aimed at a spatial segmentation of the stations. Limits of quantification are handled in the change‐point procedure, by supposing an underlying left‐censored parametric model, piece‐wise stationary. Spatial segmentation takes into account the geographical conditions, and may be based on river network, wind directions and so forth. Conditionally to the temporal segment and the spatial cluster, one may eventually analyze the data and identify contextual anomalies. The proposed procedure is illustrated in detail on a data set containing the prosulfocarb concentration levels in surface waters in Centre‐Val de Loire region.

IMPACT OF SURFACE CURRENTS ON VARIATIONS IN LEVELS AND MEAN ICE THICKNESS OF THE ARCTIC SEAS

The impact of surface currents on variations in levels and mean ice thickness of the Arctic seas is considered in the paper. On the example of sites of the Greenland Sea and the Baffin Sea water areas, the hypothesis of a significant correlation between changes in the topography of the water surface and the average ice thickness in areas under the influence of warm and cold surface currents is tested. Similar, although less expressed, patterns of currents also take place in other Arctic seas, including the Barents and Kara seas, which are characterized by intensive navigation in the waters of the Northern Sea Route. The results of the GLORYS12v1 reanalysis, available for every day of a period of more than 10 years with a coordinate step of 5 arc minutes, are used as initial data on the water surface level and mean ice thickness in the study area. For each node of the coordinate grid for the months from November to March, time series of data on the average monthly values of the indicated characteristics are formed. It has been established that at the sites through which warm currents pass, carrying more saline water, such a correlation exists in most areas and it is negative. At the sites through which cold currents pass, carrying water with lower salinity, the correlation of the same processes can be significant and positive. It is assumed that the identified patterns are also manifested in other areas of the Arctic seas with similar currents systems, including those located along the Northern Sea Route and characterized by active navigation. The expediency of taking into account sea level changes in such areas in the course of medium- and long-term forecasting of the average ice thickness in them in order to plan year-round navigation, including during the most difficult winter-spring navigation period, is noted.

Spatiotemporal water dynamic modelling of Ramsar-listed lakes on the Victorian Volcanic Plains using Landsat, ICESat-2 and airborne LiDAR data

Spatiotemporal dynamic information on surface water area and level is a prerequisite for effective wetland conservation and management. However, such information is either unavailable or difficult to obtain. In this study, for the first time, we leverage Landsat imagery, ICESat-2 and airborne LiDAR data to develop time series of water body dynamics over the last 35 years (1987–2021) using machine learning method on a cloud computing platform for lakes identified as international importance in the Western District Lakes Ramsar site in Victoria, Australia. Our results reveal distinct seasonal (dry and wet) variation patterns and long-term changes in trends of lake water areas and levels in response to seasonal rainfall variations and regional climate changes for the periods of before, during and after the Millennium Drought when southeast Australia experienced unprecedented dry conditions. Lake water bodies have not recovered to the status of pre-Millennium Drought, and many permanent Ramsar-listed lakes in the region have become to ephemeral lakes due to climate change. The outcome of this study provides a baseline to help understand the historical and ongoing status of the Ramsar-listed lakes in a warming and drying climate in support of the development of strategic plan to implement international obligations for wetlands protection under the Ramsar Convention.

Monitoring Lake Volume Variation from Space Using Satellite Observations—A Case Study in Thac Mo Reservoir (Vietnam)

This study estimates monthly variation of surface water volume of Thac Mo hydroelectric reservoir (located in South Vietnam), during the 2016–2021 period. Variation of surface water volume is estimated based on variation of surface water extent, derived from Sentinel-1 observations, and variation of surface water level, derived from Jason-3 altimetry data. Except for drought years in 2019 and 2020, surface water extent of Thac Mo reservoir varies in the range 50–100 km2, while its water level varies in the range 202–217 m. Correlation between these two components is high (R = 0.948), as well as correlation between surface water maps derived from Sentinel-1 and free-cloud Sentinel-2 observations (R = 0.98), and correlation between surface water level derived from Jason-3 altimetry data and from in situ measurement (R = 0.99; RMSE = 0.86 m). We showed that water volume of Thac Mo reservoir varies between −0.3 and 0.4 km3 month−1, and it is in a very good agreement with in situ measurement (R = 0.95; RMSE = 0.0682 km3 month−1). This study highlights the advantages in using different types of satellite observations and data for monitoring variation of lakes’ water storage, which is very important for regional hydrological models. Similar research can be applied to monitor lakes in remote areas where in situ measurements are not available, or cannot be accessed freely.

Enrichment of poly- and perfluoroalkyl substances (PFAS) in the surface microlayer and foam in synthetic and natural waters

Bench-scale experiments were performed to interrogate poly- and perfluoroalkyl substance (PFAS) enrichment in the water surface microlayer (SML). In initial experiments using electrolyte-only solutions, the perfluorooctane sulfonate (PFOS) and perfluorooctane carboxylate (PFOA) enrichment in the SML were reasonably (with a factor of 2) described by the Gibbs adsorption equation coupled with a Freundlich-based interfacial adsorption model. Enrichment in the SML among perfluorinated sulfonates and perfluorinated carboxylates of varying chain lengths was proportional to their surface activity. The PFOS enrichment factor (EF), defined as the PFAS concentration in the SML divided by the concentration in the bulk water, was 18 in a 200 mg/l NaCl solution. The presence of elevated organic carbon levels in synthetic surface waters inhibited PFAS accumulation in the SML, with resulting EF values of approximately 1 for all PFAS. However, in the presence of elevated organic levels coupled with foam, PFAS enrichment in the foam was observed, with a foam EF of 25 measured for PFOS in synthetic surface waters. PFAS EF values measured in several natural surface waters without foam showed little variation among the waters tested, with PFOS EF values ranging between 6 and 10. Together, these results suggest that PFAS accumulation in the SML is largely controlled by PFAS sorption at the air-water interface for the conditions examined in this study, and the presence of foam with natural organics enhances PFAS uptake at the water surface.

Response of nir-type rhizosphere denitrifier communities to cold stress in constructed wetlands with different water levels

Constructed wetlands (CWs) are used to remove nitrogen-containing organic pollutants based on the roles of plant and microbial communities. However, the rhizosphere nitrogen removal process and microbial response to cold stress in CWs in winter remain unclear. In this study, we investigated the potential denitrification rate and microbial mechanisms of two CWs with different water levels in response to decreasing temperatures in winter. Our study sites were located at Beijing Wildlife Rescue and Rehabilitation Center, the low surface water level site (LCW, 0–10 cm) and Cuihu Wetland Park, the high surface water level site (HCW, 40–50 cm). Denitrifying enzyme activity (DEA) and functional microbial diversity were significantly reduced at LCW, but at HCW, the decreases were not as significant as that in LCW due to the insulating effect of ice. There was no significant difference in the composition of functional microbial communities in the rhizosphere of plants from these sites. We observed changes further in the co-occurrence networks of denitrifiers with decreasing temperature. With increasing cold stress, the co-presence links between the same nir gene (nirS-nirS or nirK-nirK) and the mutual exclusion nirS-nirK links gradually increased, indicating that similar denitrifiers tend to accumulate in response to cold stress. In addition, Bradyrhizobium (nirS/nirK), Azoarcus (nirS), Azospira (nirS), Pseudomonads (nirS) and Rhizobium (nirK) as the keystone species are the primary contributors to the denitrification process in winter. Overall, denitrification was stronger in CWs with high water levels (HCW, 40–50 cm) during the freezing period in winter, and the more similar nirK- or nirS- denitrifiers collaborated to maintain denitrification under cold stress.

A Model Assessment of the Occurrence and Reactivity of the Nitrating/Nitrosating Agent Nitrogen Dioxide (•NO2) in Sunlit Natural Waters.

Nitrogen dioxide (•NO2) is produced in sunlit natural surface waters by the direct photolysis of nitrate, together with •OH, and upon the oxidation of nitrite by •OH itself. •NO2 is mainly scavenged by dissolved organic matter, and here, it is shown that •NO2 levels in sunlit surface waters are enhanced by high concentrations of nitrate and nitrite, and depressed by high values of the dissolved organic carbon. The dimer of nitrogen dioxide (N2O4) is also formed in the pathway of •NO2 hydrolysis, but with a very low concentration, i.e., several orders of magnitude below •NO2, and even below •OH. Therefore, at most, N2O4 would only be involved in the transformation (nitration/nitrosation) of electron-poor compounds, which would not react with •NO2. Although it is known that nitrite oxidation by CO3•− in high-alkalinity surface waters gives a minor-to-negligible contribution to •NO2 formation, it is shown here that NO2− oxidation by Br2•− can be a significant source of •NO2 in saline waters (saltwater, brackish waters, seawater, and brines), which offsets the scavenging of •OH by bromide. As an example, the anti-oxidant tripeptide glutathione undergoes nitrosation by •NO2 preferentially in saltwater, thanks to the inhibition of the degradation of glutathione itself by •OH, which is scavenged by bromide in saltwater. The enhancement of •NO2 reactions in saltwater could explain the literature findings, that several phenolic nitroderivatives are formed in shallow (i.e., thoroughly sunlit) and brackish lagoons in the Rhône river delta (S. France), and that the laboratory irradiation of phenol-spiked seawater yields nitrophenols in a significant amount.

Occurrence and Distribution of Per- and Polyfluoroalkyl Substances from Multi-Industry Sources to Water, Sediments and Plants along Nairobi River Basin, Kenya.

Per- and polyfluoroalkyl substances (PFAS) are ever-present pollutants in the environment. They are persistent and bio-accumulative with deleterious health effects on biota. This study assesses the levels of PFAS in environmental matrices along the Nairobi River, Kenya. An aggregate of 30 PFAS were determined in water, while 28 PFAS were detected in sediments and plants using solid phase extraction then liquid chromatography–mass spectrometric techniques. In water, higher levels of perfluoroundecanoic acids of up to 39.2 ng L−1 were observed. Sediment and plant samples obtained in the midstream and downstream contained higher levels of perfluorooctanoic acid of up to 39.62 and 29.33 ng g−1, respectively. Comparably, levels of long-chain PFAS were higher in water and sediments than in plants. Sediment/water log distribution of selected PFAS ranged between 2.5 (perfluoroundecanoic acid) and 4.9 (perfluorooctane sulfonate). The level of perfluorooctane sulfonate (1.83 ng L−1) in water is above the acceptable level in surface water posing high human health and ecological risks. The observed PFAS concentrations and distribution were attributed mainly to multi-industries located along the river, among other sources. The knowledge of PFAS occurrence and distribution in Nairobi River, Kenya, provides important information to local regulatory agencies for PFAS pollution control.

ICE COVER OF THE EUROPEAN SECTOR OF THE ARCTIC AND SURFACE HEIGHTS OF THE ATLANTIC INNER SEAS (ON THE EXAMPLE OF THE BLACK SEA)

ICE COVER OF THE EUROPEAN SECTOR OF THE ARCTIC AND SURFACE HEIGHTS OF THE ATLANTIC INNER SEAS (ON THE EXAMPLE OF THE BLACK SEA)

A unified surface-gradient and hydrostatic reconstruction scheme for the shallow water equations

We propose a new second-order accurate hydrostatic reconstruction scheme for the Saint-Venant system. Such a scheme needs to overcome several difficulties: besides the well-known issues of positivity and well-balancing there is also the difficulty of unphysical reflections from non-monotone bottom reconstructions. We address all of these problems at once by changing the logic of the reconstruction of the bottom, the water depth and the water surface level. The scheme gives excellent results in one and two space dimensions. To highlight the novel reconstruction of bottom and water surface, we call the scheme bottom-surface-gradient method (BSGM).

Poyang and Dongting Lakes, Yangtze River: tributary lakes blocked by main-stem aggradation

During its 6,300-km course from the Tibetan Plateau to the ocean, the Yangtze River is joined by two large lakes: Dongting Lake and Poyang Lake. We explain why these lakes exist. Deglaciation forced the ocean adjacent to the Yangtze mouth to rise ∼120 m. This forced a wave of rising water surface elevation and concomitant bed aggradation upstream. While aggradation attenuated upstream, the low bed slope of the Middle-Lower Yangtze River (∼2 × 10-5 near Wuhan) made it susceptible to sea level rise. The main stem, sourced at 5,054 m above sea level, had a substantial sediment load to "fight" against water surface level rise by means of bed aggradation. The tributaries of the Middle-Lower Yangtze have reliefs of approximately hundreds of meters, and did not have enough sediment supply to fill the tributary accommodation space created by main-stem aggradation. We show that the resulting tributary blockage likely gave rise to the lakes. We justify this using field data and numerical modeling, and derive a dimensionless number capturing the critical rate of water surface rise for blockage versus nonblockage.