Urban stormwater quality: A review of methods for continuous field monitoring.

Urban stormwater is contaminated by a wide range of substances whose concentrations vary greatly between locations, as well as between and during rain events. This literature review evaluates advantages and limitations of current methods for using continuous water quality monitoring for stormwater characterization and control. High-temporal-resolution measurements have been used to improve the understanding of stormwater quality dynamics and pollutant pathways, facilitate the performance evaluation of stormwater control measures and improve operation of the urban drainage system with real-time control. However, most sensors used to study stormwater were developed for either centralized water treatment or natural water contexts and adaptation is necessary. At present, the primary application of interest in stormwater - characterization of pollutant concentrations - can only be achieved through the use of indirect measurements with site-specific relationships of pollutants to basic physical-chemical parameters. In addition, various problems arise in the field context, associated with intermittent or variable flow rates, the accumulation of debris and sediment, adverse conditions for electrical equipment and human factors. Obtaining reliable continuous stormwater quality data requires the adoption of best practices, including the calibration and regular maintenance of sensors, verification of data and accounting for the considerable uncertainties in data; however, the literature review showed that improvement is needed among the scientific community in implementing and documenting these practices.

Variation in urban snow quality indicated by three seasonal sampling surveys conducted in Luleå (Sweden) within a span of 27 years.

Concentrations of total suspended solids (TSS), trace metals (Cu, Cd, Cr, Zn, Cd, Pb), Na and 16 US EPA priority PAHs in urban snow were studied in the City of Luleå in Northern Sweden. Snow was sampled at six central urban and suburban sites with various traffic intensities, in three sampling surveys (1994-95, 2002-03, 2020-21), repeated for three ages of the urban snow cover of 40, 80, and 120 days, respectively. The older data, from the 1994-95 and 2002-03 surveys, were obtained from the existing literature. The concentrations and mass loads of TSS and most trace metals studied (Zn, Cu, Pb, and Cd) varied with time. TSS, Zn, and Cu showed slightly higher concentrations and mass loads in the 2003 (TSS avg = 2300 µg/L, Zn avg = 620 µg/L and Cu avg = 250 µg/L) and 2021 (TSS avg = 1500 µg/L, Zn avg = 530 µg/L and Cu avg = 220 µg/L) sampling surveys, compared to the 1995 survey (TSS avg = 620 µg/L, Zn avg = 240 µg/L and Cu avg = 97 µg/L). However, no evident trend was observed between the 2003 and 2021 sampling surveys. The highest concentrations of Pb and Cd were observed in snow samples from the 1994-95 sampling survey (Pb max = 570 µg/L, Cd max = 4.6 µg/L). Results indicated higher concentrations of the pollutants studied in the city centre, compared to the residential suburbs, and in areas with heavier traffic, where concentrations of metals correlated well with traffic intensity. Fractionation analysis of trace metals indicated that Zn, Cu and Pb occurred mostly in the particulate-bound phase (>0.45 µm) containing the most of Zn, Cu, and Pb mass, at 80, 84 and 94% of the total, respectively. Over 50% of the dissolved phase of Zn and Cu was in the truly dissolved fraction (<3000 MWCO). Concentrations of PAHs also increased with traffic intensity, with pyrene being the most frequently detected PAH, likely because of the strength of sources and various physical processes influencing the snowbanks development and causing spatial and temporal variations in pollutant concentrations.

Spatial heterogeneity assessment of factors affecting sewer pipe blockages and predictions.

Efficient management of sewer blockages requires increased preventive maintenance planning. Conventional approaches to the management of blockages in sewer pipe networks constitute largely unplanned maintenance stemming from a lack of adequate information and diagnosis of blockage causative mechanisms. This study mainly investigated a spatial statistical approach to determine the influence of explanatory factors on increased blockage propensity in sewers based on spatial heterogeneity. The approach consisted of the network K-function analysis, which provided an understanding of the significance of the spatial variation of blockages. A geographically-weighted Poisson regression then showed the degree of influence that explanatory factors had on increased blockage propensity in differentiated segments of the sewer pipe network. Lastly, blockage recurrence predictions were carried out with Random Forest ensembles. This approach was applied to three municipalities. Explanatory factors such as material type, number of service connections, self-cleaning velocity, sagging pipes, root intrusion risk, closed-circuit television inspection grade and distance to restaurants showed significant spatial heterogeneity and varying impacts on blockage propensity. The Random Forest ensemble predicted blockage recurrence with 60-80% accuracy for data from two municipalities and below 50% for the last. This approach provides knowledge that supports proactive maintenance planning in the management of blockages in sewer pipe networks.

Treatment of road runoff by coagulation/flocculation and sedimentation.

A laboratory investigation of the treatment potential of a coagulation process in the context of stormwater treatment was undertaken. The initial 25 L road runoff generated from four rain events was collected and subjected to a jar-testing regime with two commercial coagulants. The treatment effect was assessed by analysing the runoff before and after treatment for turbidity, suspended solids and metal content. The coagulation process resulted in particle and total metal reduction of more than 90% compared to 40% for only sedimentation. Up to 40% reduction of dissolved Cr, Cu and Pb was also observed compared to 0% for sedimentation. This study shows that coagulation may be a useful process for stormwater treatment systems when the treatment requirements are high.

Mineral and Anthropogenic Indicator Inorganics in Urban Stormwater and Snowmelt Runoff: Sources and Mobility Patterns.

Inorganic chemicals in urban stormwater and snowmelt runoff originate from catchment geology and anthropogenic activities. The occurrence, partitioning and mobility of six minerals and six trace metal (TM) indicators of anthropogenic activities were studied in stormwater, snowmelt and baseflow in four urban catchments, and the sampling of inorganics was supplemented by measurements of electrical conductivity (EC), pH and total suspended solids (TSSs). Minerals occurred at concentrations several orders of magnitude higher (1-102 mg/L) than those of TMs (10-2-102 µg/L) and reflected the composition of local groundwater seeping into sewers. Concentrations of Ca, K, Mg and Na were enhanced by baseflow contributions and followed closely the electrical conductivity. Al and Fe minerals occurred in insoluble forms, and their pollutographs were similar to those of TMs, whose concentrations mimicked, to some extent, the flux of TSS. The TMs with the highest and lowest particulate fractions were Cr&Pb and Cu&Zn, respectively. The concentrations of total TMs in snowmelt were two to four times higher than those in stormwater, and both sources likely exceeded some of the stormwater effluent limits (for Cd, Cu and Zn) proposed in Sweden. Where such concentrations depended on water hardness, the risk of toxicity might be reduced by elevated hardness of the monitored snowmelt and stormwater. Recognizing the good ecological status of the study area receiving water, Lake Storsjön, some protection against polluted runoff and snowmelt may be needed and could be achieved by implementing stormwater management measures controlling TSS and TMs.

Evaluation of factors affecting soil carbon sequestration services of stormwater wet retention ponds in varying climate zones.

The carbon sequestration services of stormwater wet retention ponds were investigated in four different climates: U.S., Northern Sweden, Southern Sweden, and Singapore, representing a range of annual mean temperatures, growing season lengths and rainfall depths: geographic factors that were not statistically compared, but have great effect on carbon (C) accumulation. A chronosequence was used to estimate C accumulations rates; C accumulation and decomposition rates were not directly measured. C accumulated significantly over time in vegetated shallow water areas (0-30cm) in the USA (78.4gCm-2yr-1), in vegetated temporary inundation zones in Sweden (75.8gCm-2yr-1), and in all ponds in Singapore (135gCm-2yr-1). Vegetative production appeared to exert a stronger influence on relative C accumulation rates than decomposition. Comparing among the four climatic zones, the effects of increasing rainfall and growing season lengths (vegetative production) outweighed the effects of higher temperature on decomposition rates. Littoral vegetation was a significant source to the soil C pool relative to C sources draining from watersheds. Establishment of vegetation in the shallow water zones of retention ponds is vital to providing a C source to the soil. Thus, the width of littoral shelves containing this vegetation along the perimeter may be increased if C sequestration is a design goal. This assessment establishes that stormwater wet retention ponds can sequester C across different climate zones with generally annual rainfall and lengths of growing season being important general factors for C accumulation.

An evaluation of applying existing bioretention sizing methods to cold climates with snow storage conditions.

Eight of the current sizing and design methods proposed for bioretention facilities were evaluated for rainfall runoff and snow storage volumes for a costal cold climate in Trondheim, Norway. The RECARGA bioretention infiltration model was used to compare the performance of the methods using 30 months of observed data from a pilot scale bioretention box. The surface areas, total ponding time, number and duration of overflow events, and snow storage volumes were compared. It was found that even in a costal cold climate with several intermittent melt cycles die snow storage requirements were an important design parameter, and if more than 25% of the total snow volume should stored this became the deciding design parameter.

The influence of temperature on nutrient treatment efficiency in stormwater biofilter systems.

Nutrients can cause eutrophication of natural water bodies. Thus, urban stormwater which is an important nutrient source in urbanised areas has to be treated in order to reduce its nutrient loads. Biofilters which use soil filter media, biofilms and plants, are a good treatment option for nutrients. This paper presents the results of a biofilter column study in cold temperatures (+2 degrees C, +8 degrees C, control at +20 degrees C) which may cause special problems regarding biofilter performance. It was shown that particle-bound pollutants as TSS and a high fraction of phosphorus were reduced well without being negatively influenced by cold temperatures. Nitrogen, however, was not reduced; especially NO(x) was produced in the columns. This behaviour can be explained with both insufficient denitrification and high leaching from the columns.

Polycyclic aromatic hydrocarbons and metals in snow along a highway.

Snow quality and its variations due to distance from the road were studied. Also, how the snow quality changes over time during the melting period was discussed. Snow samples were collected at three occasions during the winter of 2004. The samples were taken along a highway in the Luleå region, Sweden, with an average daily traffic load of 9200 vehicles. Snow samples were taken perpendicular to the road and at different distances. The snow samples were analysed for metals and polycyclic aromatic hydrocarbons (PAH). Also, weather parameters such as temperature, precipitation, and wind speed and wind direction were measured. The highest total metal and PAH concentrations were found at the sample site closest to the road and at the end of the season. Before the melting period started, 42-57% of the total amount of metals and PAH were found in the first 1 m of the snow pack. This information could be valuable when one is discussing how to achieve sustainable snow-handling management.

Urban drainage and highway runoff in cold climates: conference overview.

This overview of research findings presented at the conference on urban drainage and highway runoff in cold climates starts with generation of urban runoff and snowmelt, followed by snowmelt and winter runoff quality, best management practices for urban snowmelt and winter runoff, and snow management in urban areas. Research on the urban hydrological cycle is lagging behind the needs in this field, particularly in terms of data availability. The current studies of winter urban runoff quality focus on road salts in the urban environment and their environmental effects. The needs for better source controls in salt applications, improved management of chloride-laden runoff, and selective adoption of environmentally safer alternative de-icers were reported. Adaptation of the conventional stormwater best management practices (BMPs) for winter operation remains a challenge. The first step in refining the existing BMPs for winter operation is to advance the understanding of their operation, as reported for some cases at the conference. Finally, snow management in urban areas may require local storage of fresh (unpolluted) snow and disposal of more polluted snow at central snow disposal sites.

Review of operation of urban drainage systems in cold weather: water quality considerations.

Cold climate imposes special requirements on urban drainage systems, arising from extended storage of precipitation and pollutants in the catchment snowpack, processes occurring in the snowpack, and changes in catchment surface and transport network by snow and ice. Consequently, the resulting catchment response and runoff quantity differ from those experienced in snow- and ice-free seasons. Sources of pollutants entering urban snowpacks include airborne fallout, pavement and roadside deposits, and applications of de-icing and anti-skid agents. In the snowpack, snow, water and chemicals are subject to various processes, which affect their movement through the pack and eventual release during the melting process. Soluble constituents are flushed from the snowpack early during the melt; hydrophobic substances generally stay in the pack until the very end of melt and coarse solids with adsorbed pollutants stay on the ground after the melt is finished. The impacts of snowmelt on receiving waters have been measured mostly by the snowmelt chemical composition and inferences about its environmental significance. Recently, snowmelt has been tested by standard bioassays and often found toxic. Toxicity was attributed mostly to chloride and trace metals, and contributed to reduced diversity of benthic and plant communities. Thus, snowmelt and winter runoff discharged from urban drainage threaten aquatic ecosystems in many locations and require further studies with respect to advancing their understanding and development of best management practices.

Comparison of the effects of using local and central snow deposits: a case study in Luleå.

The aim of the study was to determine if an increased use of local land-based snow deposits would be more sustainable than the use of a central snow deposit. The study focused on transport related emissions, costs for transporting the snow, technical attendance, local effects, public acceptance, land use, effects on the recipient environmental control and potential for accidents. General information was obtained from an inventory regarding snow handling that was made in 14, geographically spread, Swedish municipalities during 2001. The comparison of costs for transporting snow and transport-related emissions was based on information gathered from the municipality of Luleå. The study showed that using local land-based snow deposits would decrease traffic-related emissions such as CO2, CO and NO(x) by 40% annually and would decrease the annual cost for transporting snow by nearly 80%. On the other hand local snow deposits may lead to an increased risk of accidents and to negative local effects such as delayed growing season, flooding and drainage problems. Available land for local snow deposits in the cities is hard to find, and is usually expensive. Therefore a combination of local and central snow deposits is likely to be the most realistic option.

Seasonal variations in road runoff quality in Luleå, Sweden.

In regions with cold climate the urban drainage and highway runoff processes become much more complex, compared to temperate regions. Therefore, climatic conditions should be taken into account in planning and design of BMPs and snow handling strategies. In order to increase the knowledge of road runoff quality during melt and rain periods, respectively, measurements were carried out at a field site during a two-month period. The field site was situated at Södra Hamnleden, a road with 7,400 vehicles/day, in the central part of Luleå. Runoff samples were analysed for suspended solids and heavy metals (Pb, Cu, Cd, Ni and Zn). The results showed that the concentrations of suspended solids, lead, copper and cadmium were higher for the melt period, compared to rain generated runoff on the catchment without snow, and the highest concentrations were found during the rain-on-snow events. The results indicate a flow dependent increase in the concentration of suspended solids during the melt period. A comparison of the total mass of suspended solids over a one-month period showed that the melt period produced about 3 times more suspended solids. Metal elements during melt period were more particulate bound as compared to the rain period characterised by a higher percentage of the dissolved fraction.

Stormwater management in a catchbasin perspective--best practices or sustainable strategies?

A strategy for sustainable stormwater management is needed. This study has focused on the relative importance of stormwater as a pollutant source in a catchbasin, if Best Management Practices (BMPs) result in pollutant removal or pollutant redistribution, and methods for screening of stormwater strategies. Stormwater is most likely an important pathway for pollutants in a catchbasin perspective. True pollutant removal can only be achieved if the pollutant sources are eliminated. Until that is reached, we should have the best possible control of the pollutant fluxes in the watershed. This study indicates that the search for a sustainable stormwater strategy could be easier to handle if different "screens" could be used. The Swedish environmental objectives, which try to encapsulate all aspects of sustainability, may be used as a foundation for a "sustainability screen". By using this screen, the "unsustainable" features of different stormwater strategies could be pointed out. A "standards and legislation screen" will be based on the EU Water Framework Directive. As this study has shown, it is doubtful whether the conventional BMPs, such as stormwater ponds and infiltration facilities, produce a sufficient pollutant control.

Snow quality in the city of Luleå, Sweden--time-variation of lead, zinc, copper and phosphorus.

The quality of urban snow was studied with regard to how the concentrations of selected metals and phosphorus vary over time. The area studied is situated in northern Sweden (65 degrees 36' N; 22 degrees 13' E). Snow samples were taken at three different locations in the city centre, approximately every second week, throughout winter. This study has clearly shown the importance of local conditions and snow clearance operations on snow quality. The study also shows that it is difficult to interpret and predict the concentrations of lead, zinc, copper and phosphorus in the snow. However, it should be possible to predict the mass loads of these substances from the deposition velocities, if the snow handling methods and routines, as well as the local circumstances are known.