Wash-off Process Research Articles

Leaching characteristics of nutrients in food waste digestate-derived biochar

Food waste anaerobic digestion requires proper utilization of solid digestate, and pyrolysis emerges as an effective method to produce nutrient-rich biochar. This study investigated the leaching characteristics and speciation changes of nutrients in food waste digestate (FWD)-derived biochar pyrolyzed at 350 °C (BC350), 450 °C (BC450), and 550 °C (BC550). BC350 featured inorganic nitrogen, while BC450 and BC550 contained elevated organic nitrogen. Nitrogen, potassium, and dissolved organic carbon were released via a quick surface wash-off process. Polyphosphates prevailed in BC350 and leached through a fast diffusion-controlled process. BC450 and BC550 were dominated by Ca/Mg orthophosphates and released via a slow dissolution-controlled process. Leachates from BC450 and BC550 stimulated the shoot length of wheat seeds. After 5 leaching cycles, there were more aromatic dissolved organics, and BC450 and BC550 exhibited higher abundance of C-N and O-P-O. Overall, pyrolysis of FWD at 450 °C and 550 °C shows potential in producing slow-release biochar fertilizers for resource recycling.

Characterization of road-deposited sediment wash-off and accurate splitting of initial runoff pollution in heterogeneous urban spaces

Particulate materials arising from road-deposited sediments (RDS) are an essential target for the control and management of surface runoff pollution. However, the heterogeneity of urban spaces hinders the identification and quantification of particulate pollution, which is challenging when formulating precise control measures. To elucidate the factors that drive particulate pollution in heterogeneous urban spaces, the accumulation of RDS on dry days and the total suspended solids during six natural rainfall events were investigated across three urban–rural spatial units (central urban, central suburban, and remote suburban). The underlying surface type (asphalt or cement roads) and particle size composition jointly determined the spatial heterogeneity in the static accumulation and dynamic output loads of RDS during rainfall. These two factors explained 59.6% and 18.9% of the spatial heterogeneity, respectively, according to principal component analysis. A novel CPSI exponential wash-off equation that incorporates particle size composition and underlying surface type was applied. It precisely described the spatial heterogeneity of RDS wash-off loads, the estimated values exhibiting event mean concentration errors of 10.8–18.2%. When coupled with the M(V) curve, this CPSI exponential wash-off equation more precisely split the initial volume of runoff: a lower total volume (17.6–38.0%) was shown to carry a higher proportion of the load (70.0–93.7%) compared to the traditional coupled exponential wash-off equation (volume: 31.6–49.0%, load: 37–90%). This study provides a new approach to characterizing RDS wash-off processes and splitting initial runoff in heterogeneous spaces.

Air pollution removal with urban greenery – Introducing the Vegetation Impact Dynamic Assessment model (VIDA)

Urban greenery is identified as a potential tool in air pollution mitigation. However, the impact is still debated. This paper introduces the innovative VIDA (Vegetation Impact Dynamic Assessment) model, specifically designed to quantify air pollution removal through deposition on vegetation. The VIDA model offers an advanced representation of vegetation that could be integrated into urban air quality dispersion models in the future. Furthermore, the model serves as a valuable tool for exploring the intricate interactions among deposition, resuspension, and wash-off processes, as well as understanding how meteorological conditions and various leaf traits influence these processes.The current version of the model focuses on particulate matter (PM) and encompasses a range of processes, including deposition on vegetation surfaces, encapsulation within the waxy cuticle, wind-driven resuspension, and wash-off. Additionally, the model takes into consideration dynamic changes in PM concentrations on the leaf surface over time, incorporating factors such as PM size fractions, meteorological conditions, and leaf characteristics. This comprehensive approach allows for the evaluation of various species or species groups based on their distinct traits. The VIDA model effectively reproduces measured data, yet continued evaluation remains crucial as new data emerges. Notably, challenges were encountered due to data scarcity and the absence of standardized methods for characterizing vegetation traits. Addressing these challenges and refining the representation of wash-off process will enhance the VIDA model's utility in predicting the dynamic relationship between vegetation and air quality.The introduction of VIDA provides a significant advancement in modelling air pollution removal by deposition to vegetation at a relevant local scale and enables inclusion of urban greenery as tool in urban planning for air pollution mitigation.

Analysis of suspended solids emissions from a combined sewage system using the Stormwater Management Model (SWMM)

The protection of water bodies requires the reduction of pollutant emissions from all major sources. In urbanized areas, these include: wastewater treatment plants (WWTPs) and (depending on the type of sewage system) combined sewer overflows (CSOs) and stormwater drainage outlets. WWTPs are usually monitored and emitted pollutant loads are known, but it is more difficult to assess the pollutant load discharged by CSOs and stormwater drainage systems. The article attempts to use the Stormwater Management Model (SWMM) to assess emissions of suspended solids from a large urban combined catchment. Suspended solids are the main pollutant of stormwater runoff in urban areas, and the dynamics of their emission from catchments is very diverse. The amount of suspended solids discharged by CSOs functioning in the given city was assessed in comparison with emissions from a wastewater treatment plant. The results show that CSOs discharge a pollutant load to the receiver which is comparable to WWTPs, but in a much shorter time and in a violent manner which can lead to the severe deterioration of receiving water quality. The modelling took into account the quality of dry weather sewage, the build-up of suspended solids, wash-off processes in the catchment area, and local precipitation characteristics. Factors affecting the quality of the obtained model and the accuracy of the emission level assessment were analysed.

Synergy of different leaf traits determines the particulate matter retention capacity and its susceptibility to rain wash-off

Rainfall plays a crucial role in the removal of particulate matter (PM) from plant leaves, influencing PM retention and the environmental behaviour of harmful substances that accumulate in PM. This study examined the PM retention capacity, particle size distributions, and wash-off rates of leaf surface PM from three common green tree species in northern China during two natural rainfall events (light rain: 8.3 mm; heavy rain: 54.2 mm), to investigate the relationship between the leaf traits, PM retention capacity, and PM wash-off process. Our results found that leaf morphometric characteristics, such as leaf size, length, width, and aspect ratio (length-to-width), had a negative and significant correlations with the PM retention capacity, but had no significant correlation with the leaf surface PM wash-off rate. Smaller leaves with low aspect ratios exhibited greater stability under external disturbances than large leaves with high aspect ratios, resulting in a higher PM retention capacity and lower wash-off rate. Ridges and grooves enhanced the PM retention capacity by increasing the leaf roughness. Rainfall could wash off all particle size ranges of leaf surface PM without altering their mechanical composition. Larger particles were more easily washed off. Euonymus japonicus, with its small leaf size and low aspect ratio, exhibited the highest PM retention capacity. Its curled leaf shape also hindered light rain from washing off leaf surface PM. Forsythia suspensa, with denser grooves and ridges compared with Prunus serrulata, exhibited a rougher leaf surface and higher PM retention capacity. However, this roughness may reduce wettability, making it easier for runoff to form on the leaf surface and dislodge leaf surface PM, resulting in F. suspensa having the highest wash-off rate. Our results highlight the synergy of different leaf traits on PM retention capacity and the PM stability after rainfall.

NUMERICAL SIMULATION OF DRIFTWOOD TRANSPORT BY WAVES IN A LABORATORY BASIN

Driftwood plays an important role in coastal ecosystems but, in large accumulations, can have negative impacts and pose hazards to navigation, infrastructure and communities (Murphy et al. 2021). The ability to accurately predict the fate and transport of coastal driftwood is central to informing sustainable management practices. However, there is a paucity of numerical modelling studies focused on driftwood transport in coastal waters (Murphy et al. 2021). Models developed for rivers (e.g. Ruiz-Villanueva et al. 2014) lack consideration of processes affecting driftwood transport and accumulation in coastal environments, such as wind waves. Murphy et al. (2020) conducted experiments in a 50-m by 30-m wave basin, wherein model driftwood was released and tracked within a 1/30 scale physical model of a sandy beach system with various coastal structures. The tests revealed that several factors influence the mobility of coastal driftwood exposed to waves, beaching and washoff processes in particular.

Modeling the wash-off of road-deposited sediments with different particle-sizes on asphalt and concrete roads

Road-Deposited Sediments (RDS) modeling of wash-off is essential for dealing with the total suspended solids (TSS) contributed by RDS. The exponential wash-off equation is commonly used for RDS wash-off, but it does not fully account for the influence of particle size and road surface characteristics (such as roughness, texture, and depression depth). We studied the RDS wash-off with 7 particle sizes and 6 rainfall intensities on asphalt and concrete roads. Particle size plays a key role in RDS wash-off and shows a negative exponential relationship between RDS wash-off fraction and particle size. The difference in RDS wash-off fraction between asphalt and concrete roads shows that road surface characteristics have an important influence on RDS wash-off. To quantify the effects of particle size and road surface characteristics on RDS wash-off, a new coefficient (CPSI) is added to the original exponential wash-off equation. The coefficient of determination (R2) of the modified exponential wash-off equation increases from 0.7039 to 0.8677. In addition, an empirical curve of CPSI facilitates the estimation of RDS wash-off. The new method makes the estimation of the RDS wash-off process more accurate and convenient.

Experimental study of sediment wash-off process over urban road and its dependence on particle size distribution.

Sediment originating from the urban road runoff is a main contributor to water pollution in urban areas. The size of the road sediment varies significantly, but its influence on sediment wash-off process has not been well investigated. In this study, sediments with different particle size distributions have been used in rainfall-runoff experiments over idealized urban road surface. The results show that, under the same experimental conditions, the capacity factor CF increases with the decrease of the median particle diameter D50, which is the dominant influencing factor on CF. The wash-off coefficient k is affected by both D50 and the grading of sediment. During the wash-off process, D50 of the sediment collected at the outlet increases with time. Such a grain coarsening phenomenon is particularly apparent when the road is originally covered with very fine sediments. Furthermore, the presence of coarse grains slows down the transport of fine sediment whose size is smaller than 14 μm. This shielding effect significantly affects the sediment wash-off process in the early stage of a rainfall event, while later on the interaction between particles of different sizes becomes unimportant. This study advances the understanding of sediment wash-off mechanism on urban road surface.

Modelling for the Efficient Effluent Dye Removal to Reuse Water and Salt

The objective of this work was to determine the optimal conditions for the electrooxidation treatment in order to decolourise the effluents that contain reactive dyes. According to the results, when Na2SO4 is used as an electrolyte, the decolouration reactions follow first-order kinetics. However, when NaCl is present in the effluent, the first-order kinetics is stabilised after applying a minimal electric current value. The models obtained from the results show that the higher the concentration of NaCl, the lower the energy consumption. On the other hand, an increase in dye concentration leads to an increase in electrical consumption. In relation to the pH, the results show that it is not a key factor in the decolouration efficiency. Finally, the obtained model was applied to two real effluents. The feasibility of individually treating the effluents from the dyeing process and those from the subsequent wash-off process was evaluated. From an industrial application point of view, it is recommended to mix both effluents before treatment, especially when the dye concentration is high.

Examination of the exponential wash-off function using experimental data

The standard exponential wash-off function has been widely adopted in urban stormwater quality models and tested in many studies of sediment wash-off experiments. There is neither sufficient information on the wash-off coefficients for other types of pollutant nor a good mathematical interpretation that accounts for the effects of surface features on wash-off processes. This study assesses the standard exponential wash-off function using experimental data on the wash-off of various types of pollutant. In-situ wash-off experiments using simulated rainfalls were conducted in typical urban areas with various landuse. Experimental data from the literature were also analysed to support the examination. Mass balance requirement was always applied as a criterion in the experimental data analysis and wash-off model examination. The experimental results indicate that wash-off processes of organic pollutants (BOD5), nitrate (NO3), total oil and grease (TOG), total petroleum hydrocarbon (TPH) and coliform stochastically comply with the standard exponential function.

Influence of particle size on diffuse particulate pollutants in combined sewer systems

Since combined sewer systems are still considered as a threat to urban water environment, understand their pollution transport process and load distribution characteristics during rainfall-runoff events would assist to mitigate urban stormwater pollution. In this study, built-up and wash-off processes of particulate pollutants on road surface and in sewer were synchronously observed during eight natural rainfall events in the combined sewer system in Zhuhai, Southern China. Field observation results showed the significant influence of particles size distribution characteristics on TSS concentration. High TSS concentrations (Road runoff, >200 mg/L; combined sewer runoff, >150 mg/L) usually contained relatively coarse particles (>100 μm) during the eight rainfall events, but the counter argument. Combined sewer runoff had a particle size coarser than road runoff. The relationship between the event means concentration (EMC) of TSS from road and combined sewer runoff varied with rainfall. EMC of TSS in road runoff were higher than that in sewer runoff during small rainfall, while that in combined sewer runoff were higher during large rainfall. The results also showed that 65.2 % of total nitrogen (TN) loadings and 62.3 % of total phosphorous (TP) loadings in the catchment came from combined sewer sediments, while 58.6 % of chemical oxygen demand (COD) loadings came from road runoff, which were mainly determined by the content of each pollutant in the sewer sediments and road-deposited sediments (RDS). Additionally, high pollutant loads phases (drainage of pollutant loads faster than runoff at a given incremental time during an event) of road runoff occurred earlier than that of combined sewer runoff. These findings can provide a basis for the type selection and scale determination of stormwater control measures in combined sewer systems.

Development of a New Dislodgeable Foliar Residue Analytical Laboratory Method for Pesticides.

The dislodgeable foliar residue (DFR) is the amount of pesticide that exists on foliage after the pesticide has dried and which could dislodge to the skin or clothes of workers and is a key parameter for non-dietary risk assessments required to demonstrate safe use for pesticide registration. DFR data in the literature are described as insufficiently reliable, limited, and encompasses considerable statistical uncertainties. The purpose of this article is to describe a newly developed laboratory method for the quantification of DFR with an illustrative example. The laboratory method reflected available field DFR methodology but involved controlled application of droplets to leaves and validation of the wash-off process used to remove the residue from the leaf surface before the analytical quantification. A very high level of accuracy (99.7-102.1%) and precision (±1.5%) was achieved. Residue data generated from the illustrated application of the method showed a robust normal distribution, unlike field studies. The method is deemed to be controllable, cost-efficient, and time-saving, taking hours rather than days. This enables the generation of more data to allow extrapolation between the generated data by investigating multiple factors that may influence DFR. An improved understanding of DFR could save time, money, and resources.

Predicting stream water quality under different urban development pattern scenarios with an interpretable machine learning approach

Urban development pattern significantly impacts stream water quality by influencing pollutant generation, build-up, and wash-off processes. It is thus necessary to understand and predict stream water quality in accordance with different urban development patterns to effectively advise urban growth planning and policies. To do so, we collected pollutant concentration data on nitrate (NO3−-N), total phosphate (TP), and Escherichia coli (E. coli) from 1047 sampling stations in the Texas Gulf Region. We utilized a Random Forest (RF) machine learning model to predict stream water quality under four planning scenarios with different urban densities and configurations. SHapley Additive exPlanations (SHAP) was used to prove the importance of urban development pattern in influencing stream water quality. The spatial variations of the impact of these patterns were explored with Geographically Weighted Regression (GWR). SHAP results indicated that Largest Patch Index (LPI), Patch Cohesion Index (COHESION), Splitting Index (SPLIT), and Landscape Division Index (DIVISION) were the most important urban development pattern metrics affecting stream water quality. The spatial variations of such patterns were shown to impact stream water quality depending on pollutants, seasonality, climate, and urbanization level. RF prediction results suggested that high density aggregated development was more effective in reducing TP and NO3−-N concentrations than the current sprawl development, but had the potential risk of increasing E. coli pollution in the wet season. The results of this study provide empirical evidence and a potential mechanistic explanation that stream water quality degradation is a consequence of urban sprawl. Lastly, machine learning is a powerful tool for scenario prediction in land use planning to forecast environmental impacts under different urban development pattern scenarios.

Transition of Particulate Pollutant in the Parcel-based Catchment of Sponge City

Most sponge city constructions in China are carried out in urban parcel-based catchments, and the quality and quantity of surface runoff can be controlled by several low impact development (LID) facilities. However, there are few reports on the generation and control of urban diffuse pollution. In this study, two areas with different hardening rates were compared to analyze the load conditions during the accumulation-wash-off-transport process of particulate pollutants. The results showed that the road surface in the catchment was the main underlying surface that the particulate pollutants contributed to. The road dust accumulation in the medium hardening rate (61.1%) and high hardening rate (73.6%) plots accounted for 88.4% (2.22-12.51 g ·m-2) and 90.1% (4.99-33.43 g ·m-2) of the catchment area unit, respectively. The contribution to the suspended solids (SS) load of runoff was 91.7% (0.97-7.34 g ·m-2) and 90.5% (0.92-18.77 g ·m-2), respectively. The SS load of road runoff accounted for approximately 95.2% and 83.1%, respectively. The pollution load (SS) after treatment by the LID facilities was approximately 24.0% and 40.2% of the surface runoff, respectively. The particle size distribution of road dust during the accumulation and wash-off processes was>150 μm, while that in surface and output runoff was <50 μm. With the increase in the impervious area, the distribution of finer particles (<105 μm) in the process of accumulation and wash-off increased (24.4%, 106.4%), while the distribution of particles <50 μm in road runoff decreased (12.4%). The particle size distribution of the accumulated, washed dust, and the rain runoff on the roof were roughly similar to those on the road. However, compared to the particle size distribution of road dust, in the accumulation and wash-off processes, the coarser particles (>1000 μm) of the medium hardening rate plot and the particles of size 250-450 μm and <45 μm of the high hardening rate plot increased significantly (>1000 μm: 58.1%, 108.5%; 250-450 μm: 72.9%, 41.8%; <45 μm: 59.2%, 64.8%). The results revealed the entire distribution process (accumulation-wash-off-transport) of particulate pollutants and the effect of LID facilities on the total SS pollution load of the catchment, which can provide an important reference for the scientific assessment of the project performance of LID installation in urban parcel-based catchments.

An original urban land cover representation and its effects on rain event-based runoff and TSS modelling

During rainfall events, a wide range of contaminants generated by urban areas are transferred into rivers. Modelling tools are increasingly required by decision makers to assess the effects of urban development and mitigation measures on urban runoff quality. However, representing the spatial heterogeneity of urban land cover in water quality modelling is a real challenge. This study presents a methodology to analyze and characterize the land cover of an urban catchment within a water quality model. The application site is the urban river Verdanson located in Montpellier city (France), where the climate is Mediterranean. Firstly, a time effective original method of identifying homogeneous urban surfaces in terms of build-up/wash-off processes from an existing land-use map and aerial photo interpretation is presented. Secondly, the sensitivity of model outputs to the discretization of a catchment into homogeneous land cover types, called Water quality Response Units (WQRUs), is tested. The results reveal that discretizing the catchment into WQRUs enhances the relevance of hydrological outputs and provide more flexibility for a calibration of Total Suspended Solids wash-off parameters. The findings open interesting opportunities to integrate catchment-scale land cover data into the modelling of wash-off processes and allow urban runoff managers to assess the influence of urban land use planning scenarios on water quality.

Removal of Reactive Dyes in Textile Effluents by Catalytic Ozonation Pursuing on-Site Effluent Recycling.

The textile wash-off process consumes substantial amounts of water, which generates large volumes of wastewater that pose potential pollution issues for the environment. In the present study, catalytic ozonation was applied to degrade residual dyes present in rinsing effluents from wash-off processes towards the aim of recycling the waste effluents. A magnetic catalyst was prepared for promoting dye degradation by catalytic ozonation. Via a hydrothermal reaction, highly magnetic manganese ferrite (MnFe2O4) particles were successfully loaded on carbon aerogel (CA) materials (MnFe2O4@CA). The results showed that the developed catalyst strikingly promoted the degradation of dye contaminants by catalytic ozonation, in terms of color removal and reduction of chemical oxidation demand (COD) in rinsing effluents. COD removal efficiency in catalytic ozonation was enhanced by 25% when compared with that achieved by ozonation alone under the same treatment conditions. Moreover, we confirmed that after catalytic ozonation, the rinsing effluents could be recycled to replace fresh water without any evident compromise in the color quality of fabrics. The color difference (ΔEcmc(2:1)) between fabrics treated with recycled effluents and water was not more than 1.0, suggesting that the fabrics treated with recycled effluents displayed acceptable color reproducibility. Although colorfastness and color evenness of fabrics treated with recycled effluents were slightly poorer than those of fabrics treated with water, they were still within the acceptable tolerance. Therefore, the present study validated that catalytic ozonation was a promising technology for saving water and wastewater elimination in textile dyeing. It provides a feasibility assessment of catalytic ozonation for recycling waste effluents to reduce water dependence in textile production. Furthermore, we show a new perspective in on-site recycling waste effluents by catalytic ozonation and enrich the knowledge on feasible approaches for water management in textile production.

Uncertainty-based parameter estimation for urban pollutant buildup and washoff simulation using a multiple pattern inverse modeling approach

Reliable model parameter estimation for stormwater runoff water quality modeling has always been a challenge, especially in ungauged areas. Insufficient local observations can increase parameter uncertainty, leading to modeling outputs of limited credibility. This paper proposes an event-based multiple pattern inverse modeling approach to estimate regional-scale parameters characterizing the buildup and washoff processes in urban watersheds. The approach explicitly accounts for system uncertainties by simultaneously using a wide range of storm events. A genetic algorithm was employed to solve the inverse models. The solution populations that minimize the error between the observed and predicted values were then filtered into distinct parameter groups using a K-means clustering algorithm. Each identified parameter group may be used to represent the buildup and washoff processes.The proposed approach was applied for the New England region (United States) using two study sites with impervious land cover: Massachusetts and New Hampshire. Stormwater monitoring data of total nitrogen (TN) and total phosphorus (TP) in small to medium size rainfall events were used to identify representative parameter sets for the region. Of the 21 identified candidate parameter combinations, 13 were found to have acceptable Root Mean Square Error for both TN and TP.The 13 parameter sets were grouped into four distinct patterns to represent different potential buildup and washoff mechanisms. The results demonstrate that the approach is capable of estimating robust sets of buildup and washoff parameters at a regional scale. These can be used in a continuous simulation model to predict TN and TP loads in ungauged areas in the region.

Understanding the washoff processes of PM2.5 from leaf surfaces during rainfall events

The PM on plant surface being washed into soil during the rain is one of the key processes for plants to reduce atmospheric PM. This study attempted to explore the PM2.5 washoff processes from different tree leaves during rain events (<16 mm/h) using an artificial rainfall simulation experiment. We aimed to improve our knowledge of processes associated with PM2.5 reduction and to provide a basis for accurately evaluating the ability of plants to reduce PM2.5. This is the first study showing that the PM2.5 washoff processes from leaves follow quartic functions and 4 pattern curves under different conditions were categorized. They respectively explained the PM2.5 washoff processes in broad-leaved trees with large leaves and simple crowns (bimodal curved), in conifer species with small leaves and complex crowns under high rainfall intensity (unimodal curved), in trees with extremely complex crowns under high rainfall intensity (continually-rising curved), and in conditions under which extremely high water interception efficiency but rather low water storage capacity occur (U-shaped curved). These curves indicate that the amount of PM2.5 on leaves was not necessarily reduced in rainfall events. The general ranking of the average values of PM2.5 number on leaves surface during rain events was Cedrus Deodara (40.3 × 103/cm2), Japanese Maple (33.0 × 103/cm2) > Dragon Juniper (14.7 × 103/cm2), Dawn Redwood (12.6 × 103/cm2) > Common Boxwood (6.4 × 103/cm2), Lotus Magnolia (4.1 × 103/cm2). The cycles of PM2.5 accumulation and removal on broad-leaved trees might be shorter than that of conifers, meaning that they may have a better PM2.5 washoff efficiency during rain, which is opposite to the PM2.5 deposition efficiency. Higher rainfall intensity can reduce the cycle length and enhance the PM2.5 washoff efficiency.

Land use types control solid wash-off rate and entrainment coefficient of Fukushima-derived 137Cs, and their time dependence

Understanding solid 137Cs wash-off in sediment sources is important for predicting radiological risks in zones contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident. Yoshimura et al. (2015) studied solid 137Cs wash-off using soil erosion plots representing different land uses in Fukushima. However, temporal trends of 137Cs activity concentration in sediments remained unclear owing to the short duration of their observations. This study is a follow-up to that of Yoshimura et al. (2015) and provides additional observations that test the dependency of temporal variations in 137Cs activity concentration in sediment and solid wash-off processes of 137Cs on land use types. Eight soil erosion plots were observed, and data from 2011 to 2014 were analyzed. A normalized 137Cs solid entrainment coefficient, defined as Sc (m2 kg−1), was calculated by dividing 137Cs activity concentration in sediment by initial 137Cs deposition. A particle size-corrected Sc, defined as Sccorrect (m2 kg−1), was also calculated based on granulometry. Sediment quantity-weighted mean values of Sc and Sccorrect, ranged from 0.0072 to 0.084 m2 kg−1 and 0.0052–0.078 m2 kg−1, respectively. Annual wash-off rates of solid 137Cs were 0.0029–12% year−1. There was no significant decreasing trend in Sc or Sccorrect on most of the plots due to its huge variability. However, on an uncultivated farmland after the removal of surface vegetation, marked surface erosion including formation of rill network was found, and Sccorrect significantly decreased as the cumulative sediment discharge increased. Our follow-up observations suggest that temporal changes in 137Cs activity concentration in sediment at the sediment source should be controlled by soil erosion processes and their intensity.

Pollutant Transport Analysis and Source Apportionment of the Entire Non-point Source Pollution Process in Combined Sewer Systems

A comprehensive and scientific understanding of non-point source pollutant transport pathways and source apportionment in combined sewer systems is essential for managing and improving the urban water environment. This study analyzed build-up and wash-off processes of pollutants on road surfaces and in sewers within a catchment of combined sewer systems in a typical old district in Zhuhai. Besides, source apportionment of the entire urban non-point source pollution was investigated by using the mass conservation method. The outcomes revealed that the build-up load of road deposited sediments in the study area was (28.81±10.69) g·m-2. The average wash-off load of road deposited sediments during five different rainfall events was (19.27±10.90) g·m-2 and the wash-off percentage was (52.69±13.3)%. The event mean concentrations of suspended solids (SS) in road runoff were 52-109 mg·L-1, and the event mean concentrations of SS in sewer runoff were 68-158 mg·L-1. Source apportionment analysis showed that road runoff, domestic wastewater, and sewer sediments contributed 39%-72%, <20%, and 13%-56% to SS, respectively. The thickness of sewer sediments increased by 1-14 cm during light and moderate rains, and the thickness decreased by 7-17 cm during heavy rains. It was found that rainfall characteristics affected the contribution percentages of pollution sources. The contribution of pollution from road runoff, domestic sewage, and sewer sediments in combined sewer systems were 2%-52%, 9%-65%, and 8%-81%, respectively. The derived outcomes should be useful for developing recommendations to control non-point source pollution in combined sewer systems and improve urban receiving water quality in China.