Xuyong Li

Grain size distribution of road-deposited sediment and its contribution to heavy metal pollution in urban runoff in Beijing, China

Pollutant washoff from road-deposited sediment (RDS) is an increasing problem associated with the rapid urbanization of China that results in urban non-point source pollution. Here, we analyzed the RDS grain size distribution and its potential impact on heavy metal pollution in urban runoff from impervious surfaces of urban villages, colleges and residences, and main traffic roads in the Haidian District, Beijing, China. RDS with smaller grain size had a higher metal concentration. Specifically, particles with the smallest grain size (<44 μm) had the highest metal concentration in most areas (unit: mg/kg): Cd 0.28-1.31, Cr 57.9-154, Cu 68.1-142, Ni 25.8-78.0, Pb 73.1-222 and Zn 264-664. Particles with smaller grain size (<250 μm) contributed more than 80% of the total metal loads in RDS washoff, while suspended solids with a grain size <44 μm in runoff water accounted for greater than 70% of the metal mass in the total suspended solids (TSS). The heavy metal content in the TSS was 2.21-6.52% of that in the RDS. These findings will facilitate our understanding of the importance of RDS grain size distribution in heavy metal pollution caused by urban storm runoff.

Influence of earthworm invasion on soil microbial biomass and activity in a northern hardwood forest

Recent invasion and activity of exotic earthworms has profoundly altered the chemical and physical environment of surface soils in northern hardwood forests that previously had mor humus horizons. We investigated the influence of earthworm invasion on soil microbial biomass and activity in surface soils of Allegheny northern hardwood forests in central New York state. Earthworm activity in these sites had transformed surface soils with clear Oi, Oe, and Oa horizons (forest floor) overlying mineral soil, to more uniformly mixed organic-enriched A horizons. The highest concentrations of microbial biomass and activity occurred in the forest floor. Microbial biomass (assayed by chloroform fumigation-extraction) nearly doubled in surface (0-5 cm) mineral soils in response to earthworm activity, an effect that corresponded directly to redistribution of organic matter from forest floor into the mineral soil. Microbial activity in surface mineral soils was even more sensitive to the presence of earthworms than microbial biomass. For example, substrate-induced respiration (or maximum initial respiratory rate, MIRR) was 6.7-fold greater, basal respiration was 5-fold greater, and microbial respiration per unit microbial biomass (metabolic quotient, qCO(2)) was almost 3-fold greater in surface mineral soils where earthworms were present than in earthworm-free sites. Of the activity indices, only MIRR was higher when expressed on an organic matter basis. Surface mineral soils where earthworms were,9 present thus appear to retain a high proportion of the microbial biomass and activity found in mor organic horizons. Our findings suggest that earthworm activity stimulates the activity of soil microorganisms, probably by enhancing organic C availability via processing and mixing of litter. The relative pattern in microbial properties did not change over the growing season; however, there were some seasonal changes in the proportional differences between worm and no-worm soils. Our results indicate interactions among earthworms, organic matter, and soil microbial activity that should alter the carbon and nutrient balance of northern hardwood forest surface soils, relative to non-invaded soils

Risk assessment of metals in road-deposited sediment along an urban–rural gradient

We applied the traditional risk assessment methods originally designed for soils and river sediments to evaluation of risk associated with metals in road-deposited sediment (RDS) along an urban-rural gradient that included central urban (UCA), urban village (UVA), central suburban county (CSA), rural town (RTA), and rural village (RVA) areas in the Beijing metropolitan region. A new indicator RI(RDS) was developed which integrated the RDS characteristics of mobility, grain size and amount with the potential ecological risk index. The risk associated with metals in RDS in urban areas was generally higher than that in rural areas based on the assessment using traditional methods, but the risk was higher in urban and rural village areas than the areas with higher administration units based on the indicator RI(RDS). These findings implied that RDS characteristics variation with the urban-rural gradient must be considered in metal risk assessment and RDS washoff pollution control.

Heavy Metal Contents of Road-Deposited Sediment along the Urban-Rural Gradient around Beijing and its Potential Contribution to Runoff Pollution

Understanding the contribution of road-deposited sediment (RDS) and its washoff process is essential for controlling urban runoff pollution. Ninety-seven RDS samples were collected along the urban-suburban-rural gradient from areas of five administrative units in the Beijing metropolitan region, including central urban (UCA), urban village (UVA), central suburban county (CSA), rural town (RTA), and rural village (RVA) areas. RDS washoff was evaluated with different particle sizes using a rainfall simulator. Heavy metal elements (i.e., Cr, Cu, Ni, Pb, and Zn) were estimated in both RDS and runoff samples. The RDS mass per unit area increased in the order UCA (21 ± 24 g/m(2)) ≈ CSA (20 ± 16 g/m(2)) < RTA (59 ± 63 g/m(2)) < RVA (147 ± 112 g/m(2)) ≈ UVA (147 ± 198 g/m(2)). Compared to RDS from the other administrative units, RDS from the UCA and CSA had higher metal concentrations and higher proportions of smaller particles, whereas that from the RVA and UVA had larger quantities of metals per unit area. UCA and CSA had lower potential runoff pollution contributions per unit area. Our findings imply that controlling the first flush in the UCA and CSA, and improving existing street cleaning methods and road surface conditions in the TRA, UVA, and RVA will be appropriate strategies for controlling runoff pollution from RDS.

Understanding the relationship between heavy metals in road-deposited sediments and washoff particles in urban stormwater using simulated rainfall.

Understanding the relationship between heavy metals in road-deposited sediments (RDS) and washoff particles is essential to controlling urban runoff pollution. RDS and its associated heavy metals were investigated along the urban-suburban-rural gradient around Beijing, China. RDS washoff and its associated metal contaminants in different particle size fractions were evaluated using simulated rainfall. The results showed that the washoff percentage of RDS decreased along the urban-rural gradient. The Cu, Zn, Ni, Cr and Pb contents in washoff particles were 28.1%, 20.0%, 19.7%, 11.6%, and 11.4% higher than those in RDS, respectively. Additionally, different metals showed increases in different chemical fractions. RDS particles <105 μm accounted for 40% of the total mass in RDS, but 50%, 70% and 75% of the total metal content in RDS, washoff particle mass and washoff particle metal content, respectively. Rainfall intensity and duration affected the amount of metals in washoff, but did not lead to an increase in the percentage of metals with different chemical fractions in washoff particles. The findings presented herein will facilitate development of strategies for control of urban diffuse pollution from RDS.

Application of particle swarm optimization based on CHKS smoothing function for solving nonlinear bilevel programming problem

Particle Swarm Optimization (PSO) is a new optimization technique originating from artificial life and evolutionary computation. It completes optimization through following the personal best solution of each particle and the global best value of the whole swarm. PSO can be used to solve nonlinear programming problems for global optimal solutions efficiently, so a novel approach based on particle swarm optimization is proposed to solve nonlinear bilevel programming problem (NBLP). In the proposed approach, applying Karush-Kuhn-Tucker (KKT) condition to the lower level problem, we transform the NBLP into a regular nonlinear programming with complementary constraints, which is sequentially smoothed by Chen-Harker-Kanzow-Smale (CHKS) smoothing function. The PSO approach is then applied to solve the smoothed nonlinear programming for getting the approximate optimal solution of the NBLP problem. Simulations on 5 benchmark problems and practical example about watershed water trading decision-making problem are made and the results demonstrate the effectiveness of the proposed method for solving NBLP.

The potential impact of an inter-basin water transfer project on nutrients (nitrogen and phosphorous) and chlorophyll a of the receiving water system

Any inter-basin water transfer project would cause complex physical, chemical, hydrological and biological changes to the receiving system. The primary channel of the middle route of the South-to-North Water Transfer Project has a total length of 1267 km. There is a significant difference between the physical, chemical and biological characteristics of the originating and receiving drinking water conservation districts. To predict the impacts of this long-distance inter-basin water transfer project on the N&P (nitrogen and phosphorus) concentrations and eutrophication risk of the receiving system, an environmental fluid dynamics code (EFDC) model was applied. The calibrated model accurately reproduced the hydrodynamic, water quality and the entire algal bloom process. Thirteen scenarios were defined to fully understand the N&P and chlorophyll a (Chl a) variation among different hydrological years, different quantity and timing of water transfer, and different inflows of N&P concentrations. The results showed the following: (a) The water transfer project would not result in a substantial difference to the trophic state of the Miyun reservoir in any of the hydrological years. (b) The area affected by the water transfer did not involve the entire reservoir. To minimize the impact of water transfer on N&P nutrients and Chl a, water should be transferred as uniform as possible with small discharge. (c) The variation in Chl a was more sensitive to an increase in P than an increase in N for the transferred water. The increased percentages of the average Chl a concentration when water was transferred in the spring, summer and autumn were 7.76%, 16.67% and 16.45%. Our findings imply that special attention should be given to prevent P increment of the transferred water from May to October to prevent algal blooms. The results provide useful information for decision makers about the quantity and timing of water transfers.

Variations in source apportionments of nutrient load among seasons and hydrological years in a semi-arid watershed: GWLF model results

Quantifying source apportionments of nutrient load and their variations among seasons and hydrological years can provide useful information for watershed nutrient load reduction programs. There are large seasonal and inter-annual variations in nutrient loads and their sources in semi-arid watersheds that have a monsoon climate. The Generalized Watershed Loading Function model was used to simulate monthly nutrient loads from 2004 to 2011 in the Liu River watershed, Northern China. Model results were used to investigate nutrient load contributions from different sources, temporal variations of source apportionments and the differences in the behavior of total nitrogen (TN) and total phosphorus (TP). Examination of source apportionments for different seasons showed that point sources were the main source of TN and TP in the non-flood season, whereas contributions from diffuse sources, such as rural runoff, soil erosion, and urban areas, were much higher in the flood season. Furthermore, results for three typical hydrological years showed that the contribution ratios of nutrient loads from point sources increased as streamflow decreased, while contribution ratios from rural runoff and urban area increased as streamflow increased. Further, there were significant differences between TN and TP sources on different time scales. Our findings suggest that priority actions and management measures should be changed for different time periods and hydrological conditions, and that different strategies should be used to reduce loads of nitrogen and phosphorus effectively.

MODELING MINERAL NITROGEN EXPORT FROM A FOREST TERRESTRIAL ECOSYSTEM TO STREAMS

Terrestrial ecosystems are major sources of N pollution to aquatic ecosystems. Predicting N export to streams is a critical goal of nonpoint-source modeling. This study was conducted to assess the effect of terrestrial N cycling on stream N export using long-term monitoring data from Hubbard Brook Experimental Forest (HBEF) in New Hampshire. The field- scale DAYCENT model was used to quantify N pools and long-term annual streamflow and mineral N export for six subwatersheds at the HBEF. By combining DAYCENT with the Soil and Water Assessment Tool (SWAT) watershed model, mineral N export simulations were extended to the watershed scale. Our study indicated that only 13% of external N input was exported to streams during 1951-2000 at HBEF. As much as 4763 kg/ha of N was stored in forest litter, soil organic matter (SOM), and living plant biomass. Net N mineralization of SOM and forest litter contributed 93% of total available N for export within the HBEF ecosystem. The Nash-Sutcliffe coefficient (Ens) evaluating model performance of DAYCENT at six subwatersheds ranged from 0.72 to 0.82 for simulating annual streamflow (1964-2000) and from 0.48 to 0.67 for annual mineral N export (1971-1995), indicating reasonable simulated values. DAYCENT successfully predicted the effect of ecosystem disturbance such as forest cut and insect invasion on stream mineral N export. The watershed-scale simulation suggested that soil spatial variability affects stream N export in addition to the accepted controls of land cover, external N input, climate, and ecosystem disturbance.

Rainfall-runoff modeling, parameter estimation and sensitivity analysis in a semiarid catchment

We present a study on the Hydro-Informatic Modelling System (HIMS) rainfall-runoff model for a semiarid region. The model includes nine parameters in need of calibration. A master-slave swarms, shuffling evolution algorithm based on self-adaptive dynamic particle swarm optimization (MSSE-SDPSO) is proposed to derive model parameters. In comparison with SCE-UA, PSO, MSSE-PSO and MSSE-SPSO algorithms, MSSE-SDPSO has faster convergence and more stable performance. The model is used to simulate discharge in the Luanhe River basin, a semiarid region. Compared with the SimHyd and SMAR models, HIMS model has the highest Nash-Sutcliffe efficiencies (NSE) and smallest relative errors (RE) of volumetric fitness for the periods of calibration and verification. In addition, the studies indicate that the HIMS model with all-gauge data improves runoff prediction compared with single-gauge data. A distributed HIMS model performs better than a lumped one. Finally, the Morris method is used to analyze model parameters sensitivity for the objective functions NSE and RE. Present a MSSE-SDPSO method to select hydrologic model parameters.Check the suitability of HIMS model for streamflow simulation in semiarid catchment.In HIMS model, several rain gauges can predict runoff more accurately than a gauge.A distributed HIMS model performs better than a lumped HIMS model.Analyze the sensitivity of HIMS model parameters.