Snow pollution management in urban areas: an idea whose time has come?

Figures & data

Figure 1. PSMT accumulation module: snow and pollutant accumulation flow chart

Table 1. Input data required for running the PSMT accumulation module

Input data	Brief description
General parameters
Catchment area A [m^2]	Catchment (or site) area, over which snow accumulates
Duration of the simulation period [days]	Selected by the user; during this period, input process parameters remain quasi-constant
Water balance computations
Initial snow storage (SWE) [mm]	The initial snow depth (as SWE) at the start of simulation period
Snowfall (as SWE) [mm/day]	Average daily snowfall (SWE) during the simulation period
Snow input distribution [−]	Spatial distribution of snow in the catchment
Undisturbed snow [%]	The fraction of the catchment snowfall, which is not subject to mechanical handling or directly exposed to traffic (mostly parks and backyards in residential areas, also elements of green infrastructure and roofs)
Snow subject to clearance (ploughing) to road (street) sides [%]	The fraction of catchment snowfall subject to ploughing to roadside snowbanks
Snow stored in local storage (called a storage bin n) [%]	The fraction of catchment snowfall, which is locally stored (up to two such storage bins can be considered in PSMT)
Snow removed from the study area [%]	The fraction of the study area (catchment) snowfall that is removed and disposed of outside of the catchment
Pollution computations
Initial pollutant mass stored in snow deposits [mass/m^2]	The initial mass of individual pollutants (constituents) at the beginning of simulation period
Traffic pollutants – the production rate in mass/(m^2 of road. vehicle)	The daily release of specific pollutants (mass/day) by a single average vehicle normalized over the road surface area
Non-traffic pollutant releases (mass/m^2 . day)	Daily release rate of pollutants from other sources than traffic (e.g. atmospheric deposition, or applications of sand and road salts, normalized over a unit area)
Benzo(a)pyrene [%]	Simulated as a fraction of the sum of USEPA 16 PAHs
Snow and pollutant accumulation rates in individual snow deposits [−]	Different rates can be specified for up to three types of snow deposits (undisturbed snow, snow in snowbanks and snow stored at temporary sites (bins))
Roadside and street area [m^2]	The area where snow is ploughed to
Salt applications [g NaCl/m^2 . day]	Average mass of salt applied in one day
Strength of traffic pollutant sources [no. of vehicles . 10^3/day]	A parameter reflecting the effect of the number of vehicles passing in the immediate proximity of snow deposits
Snow depth attrition (during the simulation period) [%]	A factor accounting for snow loss during the snow/pollution accumulation period (due to sublimation, wind erosion, melting on roofs)
Module results
Snow deposit depth (as SWE) [mm]	Average snow cover depth (SWE) at the end of simulation period (= initial depth + depth accumulated during the simulation period – losses by attrition)
Mass of individual pollutants at the end of simulation period [mass/m^2]	Pollutant accumulations in snow deposits at the end of the simulation period (= the initial mass + accumulations during the simulation period- losses)
Pollutant concentrations in snow (SWE) [mass/L]	Pollutant concentrations in snow deposits at the end of simulation period

Figure 2. Snowmelt and pollutant transport from snow deposits simulated by the transport module of PSMT

Table 2. Input data for computing snowmelt and pollutant transport from snow deposits by the PSMT transport module

Input data	Brief description
General parameters
Duration of simulation period [days]	Selected by the user; during this period, input process parameters remain quasi-constant.
Snowmelt computations
Average air temperature [°C]	Average temperature during the simulation period
Rain on snow [mm]	Total amount of precipitation during the simulated snowmelt period
Temperature initiating snowmelt	Temperature, at which snow starts to melt (varies from 0°C for snow without chloride, to −4°C for snow exposed to salt applications)
Snowmelt coefficient (SWE) [mm/°C]	This coefficient indicates the daily snowmelt depth under various conditions per 1°C (for Swedish conditions, this coefficient was assumed to vary from 1.5 to 11 mm/1°C; for further information, see the Supplementary information)
Runoff coefficient [−]	A coefficient indicating what fraction of snowmelt runs off into the municipal drainage system
Pollution computations
Fraction of TSS leaving the snow deposit with snowmelt [%]	A coefficient indicating what fraction of TSS stored in the snow deposit leaves with snowmelt (those are typically fine solids; some literature indicates that TSS elution fraction can be as low as 0.01–0.11) (Westerlund and Viklander 2008; Westerlund et al. 2011)
Fraction of accumulated chemicals leaving the snow deposit with snowmelt [%]	These fractions are specified by the user, in the context of three classes of snow chemicals: (a) primarily dissolved, (b) partly dissolved, and (c) hydrophobic. The reported fractions widely vary, with some exceptions. Primarily dissolved are chloride and sodium; partly dissolved are some trace metal species in snowmelt (particularly in the presence of salt), with dissolved Cd, Cu and Zn ranging from 7–30%. Most PAHs (polycyclic aromatic hydrocarbons) are predominantly hydrophobic.
Fraction of chemicals leaving the catchment [%]	Similarly, as in the preceding point (elution from snow deposits), the pollutants leaving the catchment can be classified as: (a) primarily dissolved, (b) partly dissolved, and (c) hydrophobic. The dissolved chemicals cannot be treated in the PSMT, which considers only one treatment process – settling.
Transport Module results
Potential snowmelt depth [mm]	Maximum possible amount of snowmelt during the simulation period, for the given conditions (i.e. average temperature, snowmelt initiation temperature and a snowmelt coefficient)
Accumulated depth of snowmelt runoff [mm]	Total amount of runoff during the simulation period, taking into account the accumulated amount of snow and the runoff coefficient
Accumulated volume of snowmelt runoff [m^3]	Volume of snowmelt runoff leaving the respective snow deposit, which can be optionally diverted to the treatment module
Accumulated mass of individual transported pollutants [g]	Mass of pollutants transported by snowmelt and potentially reaching the receiving waters (via the municipal drainage system) or diverted to the treatment facility
Pollutant concentration [SEMC] in snowmelt runoff [mass/litre]	Concentration of simulated constituents in snowmelt runoff, averaged over the simulation period (hence, snowmelt Event Mean Concentration, SEMC)

Figure 3. Schematic presentation of hydraulic efficiencies (λ) of various settling basin layouts and inlet/outlet configurations (after Persson, Somes, and Wong 1999)

Figure 4. PSMT treatment module: treatment of snowmelt runoff by settling

Table 3. Input data required for running the PSMT treatment module

Input data	Brief description
Snowmelt particle characteristics
Median particle diameter [μm]	Median diameter (D50) of particles in snowmelt runoff (for calculating settling velocities).
Average particle mass density [kg/m^3]	The average density of particles for calculating settling velocities (reported range 1,500–2,650 kg/m^3; Ferreira and Stenstrom 2013).
Particle size distribution (fraction % by mass) [%]	Mass fractions can be entered for five fractions: (≤) 6, 25, 62.5, 125 and 250 µm.
Treatment facility characteristics
Average meltwater temperature [°C]	Needed for calculating dynamic viscosity
Surface area of the sedimentation facility [m^2]	Needed to calculate the overflow rate of the settling facility
Treatment facility surface area as a percentage of the catchment impervious area [%]	Some guidelines recommend to select a stormwater treatment pond surface area as a percentage (1–2%) of the catchment impervious area
Hydraulic efficiency parameter (λ) [−]	Parameter describing hydraulic efficiency of shallow settling basins (0 < ʎ < 1)
Results
Average runoff flow [m^3/d, m^3/s]	The average runoff flow rate during the snowmelt period
Density of water at average temperature of water in the settling basin [kg/m^3]	The density of water at the average meltwater temperature (used to calculate settling velocities of particles with different sizes)
Dynamic viscosity of water at average temperature of water in the settling basin [Pa . s]	The dynamic viscosity of water at the average meltwater temperature in the settling basin (used in calculations of settling velocities of different size particles)
Particle removal using the model for sedimentation ponds [%]	Percentage removal of particles of different sizes and total removal based on the equation presented by Pettersson (1999)

Westerlund, C., and M. Viklander. 2008. “Transport of Total Suspended Solids during Snowmelt – Influence by Road Salt, Temperature and Surface Slope.” Water, Air, and Soil Pollution 192 (1–4): 3–10. doi:https://doi.org/10.1007/s11270-007-9579-6.

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Westerlund, C., M. Viklander, K. Nordqvist, H. Galfi, and J. Marsalek, 2011. “Particle Pathways during Urban Snowmelt and Mass Balance of Selected Pollutants.” In 12th International Conference on Urban Drainage, Porto Alegre, Brazil, September 11–15.

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Persson, J., N. L. G. Somes, and T. H.F. Wong. 1999. “Hydraulics Efficiency of Constructed Wetlands and Ponds.” Water Science and Technology 40 (3): 291–300. doi:https://doi.org/10.2166/wst.1999.0174.

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Ferreira, M., and M.K. Stenstrom. 2013. “The Importance of Particle Characterization in Stormwater Runoff.” Water Environment Research 85 (9): 833–842. doi:https://doi.org/10.2175/106143013X13736496909103.

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Pettersson, T.J.R. 1999. “Stormwater Ponds for Pollution Reduction.” Ph.D. thesis no. 14, Department of Sanitary Engineering, Chalmers University of Technology, Gothenburg.

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