Computational fluid dynamics modelling of flow and particulate contaminants sedimentation in an urban stormwater detention and settling basin

Sedimentation is a common but complex phenomenon in the urban drainage system. The settling mechanisms involved in detention basins are still not well understood. The lack of knowledge on sediment transport and settling processes in actual detention basins is still an obstacle to the optimization of the design and the management of the stormwater detention basins. In order to well understand the sedimentation processes, in this paper, a new boundary condition as an attempt to represent the sedimentation processes based on particle tracking approach is presented. The proposed boundary condition is based on the assumption that the flow turbulent kinetic energy near the bottom plays an important role on the sedimentation processes. The simulated results show that the proposed boundary condition appears as a potential capability to identify the preferential sediment zones and to predict the trapping efficiency of the basin during storm events.     

Accumulated sediments in a detention basin: chemical and microbial hazard assessment linked to hydrological processes

Accumulated sediments in a 32,000-m³ detention basin linked to a separate stormwater system were characterized in order to infer their health hazards. A sampling scheme of 15 points was defined according to the hydrological behaviour of the basin. Physical parameters (particle size and volatile organic matter content) were in the range of those previously reported for stormwater sediments. Chemical analyses on hydrocarbons, PAHs, PCBs and heavy metals showed high pollutant concentrations. Microbiological analyses of these points highlighted the presence of faecal indicator bacteria (Escherichia coli and intestinal enterococci) and actinomycetes of the genus Nocardia. These are indicative of the presence of human pathogens. E. coli and enterococcal numbers in the sediments were higher at the proximity of the low-flow gutter receiving waters from the catchment. These bacteria appeared to persist over time among urban sediments. Samples highly contaminated by hydrocarbons were also shown to be heavily contaminated by these bacteria. These results demonstrated for the first time the presence of Nocardial actinomycetes in such an urban context with concentrations as high as 11,400 cfu g^?1.     

Impact of land use changes on water quality in headwaters of the Three Gorges Reservoir

The assessment of spatial and temporal variation of water quality influenced by land use is necessary to manage the environment sustainably in basin scales. Understanding the correlations between land use and different formats of nonpoint source nutrients pollutants is a priority in order to assess pollutants loading and predicting the impact on surface water quality. Forest, upland, paddy field, and pasture are the dominant land use in the study area, and their land use pattern status has direct connection with nonpoint source (NPS) pollutant loading. In this study, two land use scenarios (1995 and 2010) were used to evaluate the impact of land use changes on NPS pollutants loading in basins upstream of Three Gorges Reservoir (TGR), using a calibrated and validated version of the soil and water assessment tool (SWAT) model. The Pengxi River is one of the largest tributaries of the Yangtze River upstream of the TGR, and the study area included the basins of the Dong and Puli Rivers, two major tributaries of the Pengxi River. The results indicated that the calibrated SWAT model could successfully reproduce the loading of NPS pollutants in the basins of the Dong and Puli Rivers. During the 16-year study period, the land use changed markedly with obvious increase of water body and construction. Average distance was used to measure relative distribution patterns of land use types to basin outlets. Forest was mainly distributed in upstream areas whereas other land use types, in particular, water bodies and construction areas were mainly distributed in downstream areas. The precipitation showed a non-significant influence on NPS pollutants loading; to the contrary, interaction between precipitation and land use were significant sources of variation. The different types of land use change were sensitive to NPS pollutants as well as land use pattern. The influence of background value of soil nutrient on NPS pollutants loading was evaluated in upland and paddy field. It was found that total nitrogen (TN) and total phosphorous (TP) in upland were more sensitive to NPS pollutants loading than in paddy fields. The results of this study have implications for management of the TGR to reduce the loading of NPS pollutants into downstream water bodies.     

Water quality performance of a batch-type stormwater detention basin.

The objective of this research was to modify an extended detention basin to provide batch treatment of stormwater runoff. An automated valve/controller was developed and placed on the outlet of a detention basin in Austin, Texas, which allowed the water quality volume to be retained in the basin for a preset length of time. The influent and effluent of the modified basin were monitored for total suspended solids (TSS), nutrients, chemical oxygen demand (COD), and total and dissolved metals. Statistically significant removal of total metals, COD, total nitrogen, total phosphorus, and TSS was observed, with a discharge event mean TSS concentration of 7 mg/L and a TSS removal efficiency of 91%. The modified basin has substantially better pollutant removal than conventional extended detention basins and is comparable with that of Austin sand filters, which are a common structural stormwater treatment system in the Austin area. The valve also can be used to isolate hazardous material spills.     

Copper mobilization affected by weather conditions in a stormwater detention system receiving runoff waters from vineyard soils (Champagne, France)

Copper, a priority substance on the EU-Water Framework Directive list, is widely used to protect grapevines against fungus diseases. Many vineyards being located on steep slopes, large amounts of Cu could be discharged in downstream systems by runoff water. The efficiency of stormwater detention basins to retain copper in a vineyard catchment was estimated. Suspended solids, dissolved (Cu_diss) and total Cu (Cu_tot) concentrations were monitored in runoff water, upstream, into and downstream from a detention pond. Mean Cu_tot concentrations in entering water was 53.6 μg/L whereas it never exceeded 2.4 μg/L in seepage. Cu_tot concentrations in basin water (>100 μg/L in 24% of the samples) exceeded LC₅₀ values for several aquatic animals. Copper was principally sequestered by reduced compounds in the basin sediments (2/3 of Cu_tot). Metal sequestration was reversible since sediment resuspension resulted in Cu remobilization. Wind velocity controlled resuspension, explained 70% of Cu_diss variability and could help predicting Cu mobilization.     

Storm water runoff concentration matrix for urban areas

The infrastructure (roads, sidewalk, commercial and residential structures) added during the land development and urbanisation process is designed to collect precipitation and convey it out of the watershed, typically in existing surface water channels, such as streams and rivers. The quality of surface water, seepage water and ground water is influenced by pollutants that collect on impervious surfaces and that are carried by urban storm water runoff. Heavy metals, e.g. lead (Pb), zinc (Zn), copper (Cu), cadmium (Cd), polycyclic aromatic hydrocarbons (PAH), mineral oil hydrocarbons (MOH) and readily soluble salts in runoff, contribute to the degradation of water. An intensive literature search on the distribution and concentration of the surface-dependent runoff water has been compiled. Concentration variations of several pollutants derived from different surfaces have been averaged. More than 300 references providing about 1300 data for different pollutants culminate in a representative concentration matrix consisting of medians and extreme values. This matrix can be applied to long-term valuations and numerical modelling of storm water treatment facilities.     

Characterization of trace element geochemistry in continuous flow-through microcosms: a preliminary step to environmentally meaningful ecotoxicological experiments

The increasing use of freshwater/sediment microcosms in geochemical and ecotoxicological studies requires additional efforts to characterize and understand their functioning and the main parameters that can influence the pollutants' behavior and bioavailability inside the microcosms themselves. In this study, we investigated the geochemical behavior of four elements (Cr(III), Cu, Cd, and Pb) in microcosms containing one type of natural water and sediment. The microcosms were operated under flow-through conditions with continuous metal spiking (2.5-40 μgL(-1) for Cr(III) and Pb; 1.25-20 μgL(-1) for Cu and Cd) over a period of 1 month. During this period, metal concentrations and partitioning between colloidal and truly dissolved phases in the microcosm water columns showed very little variability indicating that the system rapidly reached and maintained a steady state. Metal concentrations in pore waters also showed little variability, while Cd, Cr, and Pb levels in the top layer of sediments increased linearly from day 0 to day 28 (no significant variations in sedimentary Cu levels were observed). These features make this type of microcosms particularly suitable for ecotoxicological studies with zooplanktonic or epibenthic organisms. Characterizing the geochemical properties of microcosms provides critical information for properly interpreting microcosms-based ecotoxicity data and for reducing uncertainty in laboratory-to-field extrapolation.     

Pharmaceuticals on a sewage impacted section of a Mediterranean River (Llobregat River,NE Spain) and their relationship with hydrological conditions

Introduction  

Mediterranean rivers are characterized by a high flow variability, which is strongly influenced by the seasonal rainfall. When water scarcity periods occur, water flow, and dilution capacity of the river is reduced, increasing the potential environmental risk of pollutants. On the other hand, floods contribute to remobilization of pollutants from sediments. Contamination levels in Mediterranean rivers are frequently higher than in other European river basins, including pollution by pharmaceutical residues. Little attention has been paid to the transport behavior of emerging contaminants in surface waters once they are discharged from WWTP into a river. In this context, this work aimed to relate presence and fate of emerging contaminants with hydrological conditions of a typical Mediterranean River (Llobregat, NE Spain).     

Potential impacts of water injection dredging on water quality and ecotoxicity in Limehouse Basin, River Thames, SE England, UK

The use of water injection dredging (WID) is increasing in the UK's inland waterways and marinas. Jets of water are injected under low pressure directly into bottom sediment creating a turbulent water-sediment mixture that flows under the influence of gravity. Many of these sediments are highly contaminated and little is known of the effects of contaminant release on water quality or the risk to biota living in both the sediment and the water column. Sediment cores were collected from Limehouse Basin, a proposed WID site in SE England and current sediment toxicity was assessed using a number of techniques. Comparison of metal data to US sediment quality guidelines indicated intermediate levels of toxicity while, calculation of acid volatile sulphide to simultaneously extracted metal ratios underestimated the potential toxicity to sediment dwelling organisms. In contrast, porewater ammonia concentrations were in excess of all published ecotoxicological guidelines and indicate serious risk to biota. Re-suspension experiments were used to mimic the effects of WID on overlying water quality and ecotoxicity tests were carried out on elutriates using Daphnia magna to examine the impacts on biota. Concentrations of a range of metals in the elutriates predict that adverse biological effects would be observed during WID, however only 10% of the elutriate samples caused an adverse effect on Daphnia. Limehouse Basin is a complex aquatic environment receiving predominantly fresh waters while the sediments have high porewater chloride concentrations reminiscent of previous tidal inputs to the basin, making the choice of test organism problematic.     

Review on physical and chemical characterizations of contaminated sediments from urban stormwater infiltration basins within the framework of the French observatory for urban hydrology (SOERE URBIS)

Urban stormwater infiltration basins are designed to hold runoff from impervious surfaces and allow the settling of sediments and associated pollutants. However concerns have been expressed about the environmental impacts that may be exerted by the trapped pollutants on groundwater, soils and ecosystems. In this context, sediment characterization represents a key issue for local authorities in terms of management strategies. During the last two decades, several studies were launched including either physical or chemical characterization of stormwater sediments but without real synthesis of data and methods used. Consequently, there is an important need for reviewing the current experimental techniques devoted to the physico-chemical characterization of sediment. The review is based on the outcomes of two experimental sites for which long term monitoring and data collection have been done: the Cheviré basin (near Nantes) and the Django Reinhardt basin (near Lyon). The authors summarize the studies dealing with bulk properties, pollutant contents, their potential mobility and speciation. This paper aims at promoting the significant progresses that were made through a multidisciplinary approach involving multi-scaled and combined experimental techniques.     

Effects of untreated hospital effluents on the accumulation of toxic metals in sediments of receiving system under tropical conditions: Case of South India and Democratic Republic of Congo

Physicochemical and ecotoxicological analyses have been performed to assess the quality of sediments receiving untreated hospital effluents from Indian and Democratic Republic of Congo (DRC) hospitals. The sediments were collected monthly and characterized for grain size, organic matter, total organic carbon, total carbon, nitrogen, phosphorus, toxic metals and ecotoxicity. The results highlight the high concentration of toxic metals from the Indian hospital effluent receiving systems, especially for Cr, Cu, As, Zn and Hg. On the other hand, the metal concentrations in the sediment receiving system from DRC are low (e.g. maximum Hg and Zn concentration were 0.46 and 48.84 mg kg⁻¹ respectively). Ostracods exposed to sediment samples H2 (September month sample) and H3 (June and September month samples) were found dead after 6 d of exposure whereas the higher mortality rate for Congo sediments was 23% but was accompanied with 33 ± 7% of growth inhibition. The results of this study show the variation of sediment composition on toxic metal levels as well as toxicity related to both, the type of hospitals and the sampling period. Additionally, hospital effluent disposal practices at the study sites can lead to the pollution of water resources and may generate risks for aquatic organisms and human health.     

Solution by dilution?—A review on the pollution status of the Yangtze River

The Yangtze River has been a source of life and prosperity for the Chinese people for centuries and is a habitat for a remarkable variety of aquatic species. But the river suffers from huge amounts of urban sewage, agricultural effluents, and industrial wastewater as well as ship navigation wastes along its course. With respect to the vast amounts of water and sediments discharged by the Yangtze River, it is reasonable to ask whether the pollution problem may be solved by simple dilution. This article reviews the past two decades of published research on organic pollutants in the Yangtze River and several adjacent water bodies connected to the main stream, according to a holistic approach. Organic pollutant levels and potential effects of water and sediments on wildlife and humans, measured in vitro, in vivo, and in situ, were critically reviewed. The contamination with organic pollutants, including polycyclic aromatic hydrocarbons, polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs), polychlorinated dibenzo-p-dioxins/polychlorinated dibenzofurans, polybrominated diphenyl ethers (PBDEs), perfluorinated compounds (PFCs), and others, of water and sediment along the river was described. Especially Wuhan section and the Yangtze Estuary exhibited stronger pollution than other sections. Bioassays, displaying predominantly the endpoints mutagenicity and endocrine disruption, applied at sediments, drinking water, and surface water indicated a potential health risk in several areas. Aquatic organisms exhibited detectable concentrations of toxic compounds like PCBs, OCPs, PBDEs, and PFCs. Genotoxic effects could also be assessed in situ in fish. To summarize, it can be stated that dilution reduces the ecotoxicological risk in the Yangtze River, but does not eliminate it. Keeping in mind an approximately 14 times greater water discharge compared to the major European river Rhine, the absolute pollution mass transfer of the Yangtze River is of severe concern for the environmental quality of its estuary and the East China Sea. Based on the review, further research needs have been identified.     

New methodological improvements in the Microtox® solid phase assay

The classic Microtox® solid phase assay (MSPA) based on the inhibition of light production of the marine bacteria recently renamed Aliivibrio fischeri suffers from various bias and interferences, mainly due to physico-chemical characteristics of the tested solid phase. To precisely assess ecotoxicity of sediments, we have developed an alternative method, named Microtox® leachate phase assay (MLPA), in order to measure the action of dissolved pollutants in the aqueous phase. Two hypotheses were formulated to explain the observed difference between MSPA and MLPA results: a real ecotoxicity of the solid phase or the fixation of bacteria to fine particles and/or organic matter. To estimate the latter, flow cytometry analyses were performed with two fluorochromes (known for their ability to stain bacterial DNA), allowing correction of MSPA measurements and generation of new (corrected) IC50. Comparison of results of MLPA with the new IC50 MSPA allows differentiating real ecotoxic and fixation effect in classic MSPA especially for samples with high amount of fines and/or organic matter.     

Chemical fractionation of Cu and Zn in stormwater, roadway dust and stormwater pond sediments

This study evaluated the chemical fractionation of Cu and Zn from source to deposition in a stormwater system. Cu and Zn concentrations and chemical fractionation were determined for roadway dust, roadway runoff and pond sediments. Stormwater Cu and Zn concentrations were used to generate cumulative frequency distributions to characterize potential exposure to pond-dwelling organisms. Dissolved stormwater Zn exceeded USEPA acute and chronic water quality criteria in approximately 20% of storm samples and 20% of the storm duration sampled. Dissolved Cu exceeded the previously published chronic criterion in 75% of storm samples and duration and exceeded the acute criterion in 45% of samples and duration. The majority of sediment Cu (92-98%) occurred in the most recalcitrant phase, suggesting low bioavailability; Zn was substantially more available (39-62% recalcitrant). Most sediment concentrations for Cu and Zn exceeded published threshold effect concentrations and Zn often exceeded probable effect concentrations in surface sediments.     

Seasonal variation of sediment toxicity in the Rivers Dommel and Elbe

Contaminated sediment in the river basin has become a source of pollution with increasing importance to the aquatic ecosystem downstream. To monitor the temporal changes of the sediment bound contaminants in the River Elbe and the River Dommel monthly toxicity tests were applied to layered sediment and river water samples over the course of 10 months. There is an indication that contaminated sediments upstream adversely affected sediments downstream, but this process did not cause a continuous increase of sediment toxicity. A clear decrease of toxic effects in water and upper layer sediment was observed at the River Elbe station in spring related to high water discharge and algal blooms. The less obvious variation of sediment toxicity in the River Dommel could be explained by stable hydrological conditions. Future monitoring programmes should promote a more frequent and intensive sampling regime during these particular events for ecotoxicological evaluation.     

Terrestrial and aquatic ecotoxicity assessment of Cr(VI) by the ReCiPe method calculation (LCIA): application on an old industrial contaminated site

The most stable forms of chromium in the environment are chromium (III) and chromium (VI), the former being relatively immobile and necessary for organisms, and the latter being highly soluble and toxic. It is thus important to characterise ecotoxicological impacts of Cr(VI). However, there are still some important uncertainties in the calculation of ecotoxicological impacts of heavy metals in the LCIA global approach. The aim of this paper is to understand how the spatial and dynamic characterization of life cycle inventory (LCI) data can be exploited in life cycle impact assessment and particularly for the evaluation of the aquatic and terrestrial ecotoxicity of Cr(VI). To quantify these impacts, we studied an industrial waste landfill in the North of France that was contaminated with chromium. On the polluted area, the aquatic contamination is due to the slag heap as well as to chromium spots in soil. The soil contamination is mainly due to infiltration of chromium from the infill. The concentration of Cr(VI) in soil and water varies according to seasonal climatic variations and groundwater level. These variations have an effect on the Cr(VI) fate factor, in particular on transfer and residence time of the substance. This study underlines the spatial distribution of aquatic ecotoxicity and the temporal variation of freshwater ecotoxicity. We analysed the correlation between precipitation, temperature, concentration and ecotoxicity impact. With regards to the terrestrial ecotoxicity, the study focused on the vertical variation of the ecotoxicity and the major role of the soil layer composition into terrestrial pollution.     

Effect of bubble-induced surface turbulence on gas-liquid mass transfer in diffused aeration systems.

Models that predict volatilization of organic compounds from wastewater treatment basins may underestimate emission rates if the surfaces are considered as quiescent. In reality, the water surface may be agitated by subsurface aeration, increasing mass transfer across the tank surface air-water interface. This study investigated the effect of turbulence, induced by diffused bubble aeration, on mass transfer at the water surface of a pilot aeration basin. The mass transfer of ammonia from an enclosed headspace over the basin to acidified water was measured when different diffuser types and airflow rates were applied. Oxygen-transfer tests were conducted immediately following each ammonia-transfer test. Increasing airflow rates through fine- and coarse-bubble diffusers had a significant effect on the ammonia mass-transfer rate. Experimental mass-transfer parameters (K(L)a's) for surface volatilization derived with aeration present were up to 48% higher than the K(L)a values for quiescent conditions over the range of conditions tested. No effect of diffuser type on ammonia transfer could be determined. The study results infer an effect on oxygen transfer into the water at the surface and potential transfer of volatile organic compounds, if present, from the water. The results of the ammonia mass-transfer experiments suggest that adjustments to the existing mass transfer correlations for surface volatilization from aeration basins may be in order. Such adjustments will have the greatest effect on predictions for the less volatile compounds, under conditions of low airflow rates.     

Deriving environmental quality standards in European surface waters: when are there too few data?

Recent technical guidance has been published by the European Commission that outlines methodologies for the derivation of Environmental Quality Standards (EQS) in European surface waters under the Water Framework Directive (WFD). The guidance allows the derivation of a long-term EQS from a small dataset. Specifically an EQS can be derived from just three acute data points, although the safety factors built into such an EQS are large (e.g. up to a factor of 1,000). Large safety factors make such EQS uncertain, and often difficult to achieve in practice. We examine dataset requirements for the derivation of EQS and specifically the minimum number of tests needed for setting EQS for long-term chemical exposures that result in reduced relative uncertainty, as assessed simply through the reduction in standard deviation of the means of the values derived. Using ecotoxicity datasets for four example chemicals, for which EQS have been derived in many jurisdictions, we show that variation in the EQS is greatest when using the minimum dataset allowable under the WFD guidance, but decreases rapidly when seven or more datapoints are available. Increasing the minimum number of ecotoxicity data in deriving an EQS results in a greater understanding of ecotoxicological effects. With this knowledge, the mitigating effects of water chemistry can be accounted for in deriving an EQS, even with relatively limited datasets. The new guidance suggests “simplistic” approaches to account for chemical availability, but does not detail how this might be undertaken. We provide examples of ways by which water chemistry effects can be included in deriving implementable EQS for metals with relatively few reliable and relevant data.     

The water-soluble fraction of potentially toxic elements in contaminated soils: relationships between ecotoxicity, solubility and geochemical reactivity

To better understand the impacts posed by soil contamination to aquatic ecosystems it is crucial to characterise the links between ecotoxicity, chemical availability and geochemical reactivity of potentially toxic elements (PTE’s) in soils. We evaluated the adverse effects of water extracts obtained from soils contaminated by chemical industry and mining, using a test battery including organisms from different trophic levels (bacteria, algae and daphnids). These tests provided a quick assessment of the ecotoxicity of soils with respect to possible adverse effects on aquatic organisms although the ecotoxicological responses could be related to the solubility of PTE’s only to a limited extent.The analysis of results of bioassays together with the chemical characterisation of water extracts provided additional relevant insight into the role of conductivity, pH, Al, Fe, and Mn of soil extracts on toxicity to organisms. Furthermore, an important conclusion of this study was that the toxicity of extracts to the aquatic organisms could also be related to the soil properties (pH, Org C and Fe_ox) and to the reactivity of PTE’s in soils which in fact control the soluble fraction of the contaminants.The combined assessment of ecotoxicity in water fractions, solubility and geochemical reactivity of PTE’s in soils provided a more comprehensive understanding of the bioavailability of inorganic contaminants than ecotoxicological or chemical studies alone and can therefore be most useful for environmental risks assessment of contaminated soils.     

Persistent organic pollutants in water and surface sediments of Taihu Lake,China and risk assessment

Organic pollutants, especially persistent organic pollutants were examined in the water and surface sediments of Taihu Lake, China. Both 12 water and 12 sediment samples were collected over the lake. C-18 solid-phase extraction technique was applied to extract organic pollutants in collected water samples. Soxhlet extraction procedure was used to extract organic pollutants in sediment samples. The analysis was performed by GC-MS controlled by a Hewlett Packard chemstation. Two hundred and seventy-three kinds of organic chemicals in water were examined, 200 more than that detected in 1985; 188 kinds of chemicals in sediments were detected as well. Among them 21 kinds of chemicals belong to priority pollutants as well as 17 kinds to be the endocrine disruptors. The concentrations of the pollutants were more than 2 times higher than that in 1985. The possible source and relation to anthropogenic activity were discussed.