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Priority Substances List Assessment Report for Road Salts
2.0 Summary of Information Critical to Assessment of "Toxic" Under CEPA 1999
2.1 Identity, properties, production and sources
2.1.1 Substance characterization
Road salts can refer to any salt applied to roadways for roadway maintenance. As outlined below, road salts are mainly used in Canada as de-icing and anti-icing agents during winter road maintenance, but smaller quantities are also used as dust suppressants. "Salt" can refer to any compound consisting of the cation from a base and the anion from an acid and which is readily dissociated in water. While sodium chloride (NaCl) is by far the most frequently used road salt in Canada, other inorganic salts used in Canada include calcium chloride (CaCl2), magnesium chloride (MgCl2) and potassium chloride (KCl).
Few additives are used in Canada as part of road salt formulations; sodium ferrocyanide (Na4Fe(CN)6·10H2O) is the only product regularly added as an anti-caking agent in Canada. Organic salts and a few other products are essentially used in Canada for maintenance of airports and for plane de-icing or limited roadway trials.
Abrasives such as sand are used for winter roadway maintenance; while these are not salts, chloride salts are frequently mixed with abrasives. Accordingly, abrasives blended with salts can be a source of chloride salts to the environment.
This assessment focuses on the inorganic chloride salts used for roadway maintenance. This is based on the following considerations:
- The original recommendations of the Ministers' Expert Advisory Panel called for an ecological assessment of the most commonly used de-icing chloride salts.
- Sodium chloride and, to a lesser extent, calcium chloride are by far the most commonly used salts on roadways in Canada.
- All inorganic chloride salts have broadly similar behaviour and effects in the environment. Notably, effects related to the toxicity of chloride can be considered to depend on the cumulative input of all chloride salts.
In addition, public concerns have been expressed regarding the use of ferrocyanide salts in formulations of road salts, notably given that, in solution, they can photolyse to yield free cyanide ions, which are highly toxic to aquatic organisms. Since ferrocyanides are commonly used in road salt formulations, their entry, exposure and effects are also considered as part of the assessment of road salts.
Organic salts are not used in Canada or are used in specific circumstances, such as at airports (rather than on roadways), and they are not assessed in this report. While abrasives are used in large quantities in Canada, the nature and potential for environmental effects of these compounds are distinct from those of road salts and are not considered in this assessment.
Limited data were available for parking lots, industrial, commercial and other private properties. However, the assessment was based essentially on uses and releases from public roadways, snow disposal sites and patrol yards.
2.1.2 Physical and chemical properties
The Chemical Abstracts Service (CAS) registry numbers and physical-chemical properties of four inorganic salts (sodium chloride, calcium chloride, magnesium chloride and potassium chloride) and ferrocyanides used as road salts are given in Table 1.
Table 1 Physical-chemical properties of road salts
The eutectic temperature is the lowest freezing temperature that can be achieved for water by adding a given salt to it. The greater the difference between the ambient temperature and eutectic temperature, the higher the rate of melting (OECD Scientific Expert Group, 1989). Thus, from the salts presented in Table 1, calcium chloride would produce the highest melting rate. The rate of reaction is observed to be approximately the same for sodium chloride and calcium chloride at temperatures between -1 and -4°C; from -5°C downwards, however, sodium chloride acts more slowly than calcium chloride when equal quantities are applied (OECD Scientific Expert Group, 1989).
Salinity is defined as the total dissolved solids in water after all carbonates have been converted to oxides, all bromides and iodides have been replaced by chlorides and all organic matter has been oxidized (Stumm and Morgan, 1981). Since road salts in this assessment are chloride salts, chloride (Cl-) is the principal contributing anion to salinity resulting from the application of these salts, but other contributing anions in the environment include bicarbonate (HCO3-), carbonate (CO32-) and sulphate (SO42-). Cations that contribute significantly to salinity include calcium (Ca2+), magnesium (Mg2+), sodium (Na+) and potassium (K+). Salinity is closely related to the total halide concentration, which is often called chlorinity. The relation between the two is described by the empirically derived Knudson equation (Mayer et al., 1999).
Ferrocyanide is a very complex anion of limited solubility composed of a central iron atom surrounded by an octahedral configuration of cyanide ligands (Letts, 2000a).
2.1.3 Production and use
Road salts (mostly sodium chloride) have been used as ice-disbonding and ice-melting agents in Canada since the 1940s (Perchanok et al., 1991).
2.1.3.1 Sodium chloride
The predominant chloride salt used as a de-icer in North America is sodium chloride, which is composed of about 40% sodium and 60% chloride by weight. Trace elements, including trace metals, may represent up to 5% of the total salt weight. Substances potentially present include phosphorus (14-26 mg/kg), sulphur (6.78-4200 mg/kg), nitrogen (6.78-4200 mg/kg), copper (0-14 mg/kg) and zinc (0.02-0.68 mg/kg) (MDOT, 1993).
World production of sodium chloride totalled 189 000 kilotonnes in 1995. The largest producers are the United States (21%), China (15%), Germany (8%) and Canada (7%) (Natural Resources Canada, 1998). The largest global market for sodium chloride is the chemical industry (60%), followed by table salt (20%) and road de-icing (10%) (CIS, 1994).
As of 1993, there were 12 manufacturers of sodium chloride in Canada, with 24 plants in seven provinces. Total nameplate capacity was 13 645 kilotonnes per year, and total domestic production was 10 895 kilotonnes. A further 1010 kilotonnes were imported from the United States and Mexico, making a total supply of 11 905 kilotonnes. Of this, Canada exported 3106 kilotonnes, mainly to the United States. Canadian domestic demand was 8799 kilotonnes (CIS, 1994). The largest market for sodium chloride in Canada is snow and ice control, which accounts for about half the domestic demand (4240 kilotonnes in 1993). Total amounts used for de-icing fluctuate from year to year, depending on weather conditions (CIS, 1994).
2.1.3.2 Calcium chloride
Calcium chloride is the second most commonly used road salt in North America. Calcium chloride is the leading chemical used for dust suppression in Canada. Liquid calcium chloride is applied primarily to gravel roads to consolidate aggregates and control dust. Calcium chloride is also used to pre-wet salt or sand in winter highway maintenance and to stabilize road base mixtures after pulverization. Of these three uses, dust control accounts for approximately 97% of total use (Morin and Perchanok, 2000). The quantity of calcium chloride used for winter road maintenance may increase as different agencies start experimenting with pre-wetting techniques.
In 1995, there were three calcium chloride producers in Canada, with one plant in Ontario and four brine wells in Alberta. Total nameplate capacity for calcium chloride in Canada for that year was 629 kilotonnes. At that time, domestic production totalled 399 kilotonnes and 25 kilotonnes were imported, creating a total supply of 424 kilotonnes. Of this, 156 kilotonnes were exported, while Canadian domestic demand was 268 kilotonnes. The total amount used in 1995 in road dust suppression and in road construction was reported at 201 kilotonnes (CIS, 1996).
2.1.3.3 Mixture of sodium chloride and calcium chloride
Sodium chloride pre-wetted with calcium chloride brine has been recommended for reducing total salt applications (Gooding and Bodnarchuk, 1994). The total amount used for roads in Canada is included in the above estimates for sodium chloride and calcium chloride.
2.1.3.4 Magnesium chloride
Although not recommended for dust control on roads, magnesium chloride is an alternative dust suppressant for use on material piles, road shoulders or material transfer ponds (OMEE, 1993). Using data provided by industry, Morin and Perchanok (2000) estimated that approximately 25 000-35 000 tonnes of magnesium chloride are used annually for roadway de-icing activities in Canada. No information was found concerning magnesium chloride production volumes in Canada, but volumes are likely quite low.
2.1.3.5 Mixture of sodium chloride and magnesium chloride
While sodium chloride-based de-icing products can contain magnesium chloride as well as some corrosion-inhibiting additives (MDOT, 1993), there is no evidence of their current use in Canada.
2.1.3.6 Potassium chloride
There is limited use of potassium chloride in road de-icing chemicals; however, potash mine tailings containing 0.5-2% potassium chloride (the rest is primarily sodium chloride) are applied to some Canadian roads for de-icing (CFI, 1997). The total amount of potassium chloride used annually for roadway de-icing in Canada has been estimated to be 2000 tonnes (Morin and Perchanok, 2000) and 3300 tonnes (CFI, 1997). It is estimated that total Canadian production of potassium chloride in 1999 was 13.6 million tonnes, and the potash producing industry operated at 61% of production capacity (Prud'homme, 2000). Most of the potash produced was used for agricultural purposes.
2.1.3.7 Brines
Oil field brine is a traditional dust suppressant containing calcium, magnesium, sodium and chloride. Oil field brine is obtained as formation water generated from oil exploration operations at several locations in southwestern Ontario (OMEE, 1993).
2.1.3.8 Ferrocyanide
To prevent the clumping of chloride salts during storage and de-icing operations, sodium ferrocyanide and ferric ferrocyanide can be added to these salts. In Canada, sodium ferrocyanide is added to sodium chloride in amounts of 30-124 mg/kg (Letts, 2000a).
Sodium ferrocyanide is not currently produced in Canada. However, approximately 300-350 tonnes per year are imported from European and Asian manufacturers for use as an anti-caking agent. Road salts containing sodium ferrocyanide are used in Ontario, Quebec and the Atlantic provinces. From Manitoba to the interior of British Columbia, the by-product salt from Saskatchewan potash mines that is used as road salt is not treated with sodium ferrocyanide. On the west coast, rock salt imported from Chile is treated with sodium ferrocyanide at a rate of 60 mg/kg. The quantity of ferrocyanide used in various jurisdictions can be estimated from the amount of salt used and its average ferrocyanide content.
2.1.4 Sources and releases
2.1.4.1 Natural sources
Natural sources of sodium chloride, calcium chloride, magnesium chloride and potassium chloride salts in the environment include weathering and erosion of rocks and soils, atmospheric precipitation and groundwater discharge. The composition of the rock material, relief and climate are important factors that largely control the rates of weathering and dissolution. In aquatic ecosystems, cations such as calcium, magnesium and potassium are not uniquely associated with chlorides but can be derived from natural sources such as carbonates from soils and bedrock in the watershed (Mayer et al., 1999). Igneous rocks and associated soils have generally lower salt contents than shales and limestones (Pringle et al., 1981). The contribution of salinity from atmospheric sources is particularly important in the coastal maritime regions. Areas that are dry with low rates of precipitation, such as the Canadian Prairies, have landscapes that tend to concentrate salts in surface soils because of high net evapotranspiration from the land. Natural sources of salts in aquatic ecosystems are reviewed by Evans and Frick (2001).
Sodium and chloride are stored in vegetation, but they are also easily lost by ion uptake, leaf washing and litter decomposition. Chloride is an important inorganic solute of osmotic regulation. The chloride concentration in the environment is not affected by chemical or biological reactions. Chloride does not adsorb onto particulate matter. Potassium ion tends to be the most constant quantity in animal cells. Calcium is the most reactive ion of all the major cations contributing to salinity; it is required as a nutrient in higher plants and is one of the basic inorganic elements present in algae. Magnesium is important in enzymatic transformations and is required by chlorophyll-bearing plants.
While ferrocyanides do not occur naturally in the environment, cyanide compounds are naturally occurring substances reported from over 2000 plant species (Conn, 1980). Sodium ferrocyanide and ferric ferrocyanide are highly stable and relatively immobile in the environment. When they are exposed to sunlight, the ferrocyanide complex decomposes and releases cyanide ions (CN-) (Meeussen et al., 1992a). Under natural conditions, the cyanide ion hydrolyses to the volatile hydrogen cyanide (HCN) molecule (Shifrin et al., 1996).
2.1.4.2 Anthropogenic sources
Anthropogenic sources of inorganic chloride salts include domestic sewage and industrial processes (Sonzogni et al., 1983), such as effluent from the chemical and petrochemical industry (Johnson and Kauss, 1991), gas manufacturing and acid mine drainage (U.S. EPA, 1973; Pringle et al., 1981). Once these substances enter the environment, they remain in aqueous phase until their concentrations exceed their solubility products, when crystallization and subsequent sedimentation of mineral salts might occur.
Salts may be released to the environment during their production, mining, mixing, and bulk transportation and storage. Since these industrial sources are not all limited to ultimate use as road salts, they are not considered further in this assessment.
Ultimately, all road salts enter the environment as a result of:
- storage at patrol yards (including losses from storage piles and during handling);
- roadway application (at the time of application as well as subsequent movement of the salts off the roadways); and
- disposal of waste snow.
Releases are therefore associated with both point sources (storage and snow disposal) and line sources (roadway application). The following sections review material pertinent to entry into the environment, notably loadings (amounts applied to roadways), disposal of waste snow and patrol yards.
2.2 Road salt loadings
This section summarizes findings in a report prepared by Morin and Perchanok (2000), which characterizes the use of road salts in Canada. Information describing application rates and annual mass loadings per highway length and per geographic district area is provided. These detailed, geographically based data were used to calculate exposure and to support subsequent sections of the assessment. Data in this section focus on the use of sodium chloride and calcium chloride salts. Magnesium chloride and potassium chloride salts are used to a lesser extent. Industry representatives estimated that approximately 25 000-35 000 tonnes of magnesium chloride and 2000 tonnes of potassium chloride are used for roadway de-icing in Canada annually.
2.2.1 Surveys
Information on the use of road salts in Canada was obtained from surveys conducted by the Transportation Association of Canada, the Ontario Ministry of Transportation and Environment Canada. The Environment Canada survey included data from provinces, municipalities and the private sector. Details on the methodology used in the surveys are presented in Morin and Perchanok (2000).
2.2.1.1 Provinces
Information on salt loadings and road network length was obtained from all provincial and territorial departments of transportation. The 1997-98 winter had the most complete data nationwide; these data were used to estimate total salt use. Comparisons between data from salt suppliers and data from provincial agencies suggest that the quantities reported are reliable (Morin and Perchanok, 2000).
2.2.1.2 Municipalities and regional governments
Information on salt use and road network length was obtained from 104 municipalities across Canada. Data on salt use were also obtained from all regional municipalities and counties in Ontario. Some respondents provided information on salt use and road network length for each of the past 5 years; others provided information on salt use for the 1997-98 winter only.
2.2.1.3 Salt industry data
Salt suppliers provided purchase and tender data for 500 municipalities in Canada. A comparison between tender data and purchase data indicates a strong concordance between the two.
2.2.1.4 Municipal estimates
Information on salt use by road type is available only for 104 municipalities. Even by combining data obtained from municipalities (104) and salt suppliers (500 municipalities), this does not represent total municipal salt use. Estimates were made to account for salt use by municipalities where no data were submitted.
Population was chosen as the variable to predict municipal salt use. This was done by deriving an average annual loading per person for municipalities with survey or purchase/tender data and multiplying this by the population of all other municipalities in a maintenance district. By summing known municipal salt use and estimated municipal salt use, it was possible to estimate total municipal salt use by maintenance district. After estimates were made, salt use was mapped by maintenance district.
Figure 1 Provincial road network area fraction (from Morin and Perchanok, 2000)
2.2.1.5 Provincial and municipal use of dust suppressant salts
The major producer of calcium chloride provided market estimates of calcium chloride use on roadways by province. Recommended application rates for calcium chloride were obtained from industry. Rates vary depending on the type of gravel road, traffic and policies regarding roadway maintenance. Minimum and maximum loadings for a gravel road 7.4 m wide were calculated.
2.2.2 Results
Detailed results are presented in Morin and Perchanok (2000).
Using survey information, maps depicting the distribution of provincial roads were created. A provincial road network area fraction was calculated and mapped (Figure 1 indicates the area of saltable roads maintained by provincial agencies per district area). This map can be used to estimate salt loadings to surface water in drainage areas of a similar size and shape. Districts with the highest proportion of saltable road to district area are in parts of the Atlantic provinces, southern Quebec, southern Ontario and parts of British Columbia.
Figure 2 Recommended provincial application rates for sodium chloride road salts, 1998 (from Morin and Perchanok, 2000)
2.2.2.1 Sodium chloride
2.2.2.1.1 Recommended application rates
Information on recommended provincial application rates is depicted in Figure 2; rates for select municipalities are presented in Figure 3. Application rates cannot be used to quantify annual loadings; they provide information on the quantity of salt applied with each application, not the number of applications and resulting total mass applied. Loadings refer to average total annual mass, not application rates.
2.2.2.1.2 Loadings
Detailed data were analysed separately for provinces, municipalities and regional governments, including calculations of mass of salts per length and surface area of two-lane saltable roads and per area of maintenance district (Morin and Perchanok, 2000). Areas with highest loadings were in central and southern Ontario and Quebec, followed by the Atlantic provinces; lowest loadings were in the Prairies.
Municipal loadings were combined with provincial and territorial loadings to estimate total salt use by maintenance district for the 1997-98 winter season (Figure 4); it is estimated that 4 418 462 tonnes of sodium chloride were used as de-icers on roadways in Canada.
The above estimates of total loading include data on the use of road salts by municipal, provincial, territorial and regional governments only. Private contractors, industry and agencies such as port and transit authorities also use road salts. Cheminfo (1999) estimated that the total use by commercial and industrial customers is 5-10% of the road salt market. A midpoint value of 7.5% was used to estimate the quantity of road salts used by commercial and industrial sources in Canada (Table 2). If these quantities are included, it is estimated that 4 749 847 tonnes of sodium chloride road salts were used during the 1997-98 winter season.
Figure 3 Average recommended municipal application rates for sodium chloride road salts, 1998 (from Morin and Perchanok, 2000)
Figure 4 Total amount of sodium chloride used, by maintenance district (from Morin and Perchanok, 2000)
- Commercial and industrial road salt use is assumed to be at 7.5% of provincial and municipal use (7.5% based on estimate by Cheminfo, 1999).
- Including Nunavut.
2.2.2.2 Calcium chloride
The quantity of calcium chloride used by province and territory is listed in Table 3. While Ontario is the province where the largest quantity of calcium chloride is used on roadways, analyses by Morin and Perchanok (2000) indicate that Nova Scotia is the province with the highest usage per unit area of land.
2.2.2.3 Total chloride loadings
Estimates of the quantity of sodium chloride and calcium chloride used on roadways in Canada are combined to estimate total chloride use on roadways by province. Table 4 presents total chloride use by province, based on total sodium chloride loadings for the 1997-98 winter season and the estimated use of calcium chloride in a typical year. Mass of chloride used per area of province was calculated to provide a basis on which to compare loadings. Thus, while Ontario and Quebec are the provinces where the most chloride is used on roadways, Nova Scotia has the highest loading per unit area of province (Morin and Perchanok, 2000).
2.2.2.4 Historic trends
Changes in road salt loadings over time were considered. A few agencies that maintain roadways provided long-term information on total salt use. While these data suggest that total road salt loadings have generally increased over time, they also indicate that there are annual fluctuations. Furthermore, it is difficult to conclude that loadings have increased without information on the length of the road network that is maintained.
1 Including Nunavut.
1 Including Nunavut.
One way to roughly ascertain if annual road salt loadings have changed is by comparing survey data compiled by the Salt Institute from the mid 1960s to early 1980s (Salt Institute, 1964-1983) with data collected for the 5-year survey period (1993-94 to 1997-98). This comparison suggests that road salt loadings per two-lane-kilometre have not decreased.
While some agencies used similar quantities of salt per two-lane-kilometre over both time periods, road salt loadings by other agencies increased slightly from the early 1980s to mid 1990s. Figure 5 shows average provincial loadings per two-lane-kilometre of provincial road for Ontario, Quebec, New Brunswick and Nova Scotia.
Figure 5 Historical salt use by provincial agencies (based on surveys conducted by the Salt Institute, 1964-1983, and Morin and Perchanok, 2000)
The comparison and merger of data from different sources for different time periods were assessed by comparing the correspondence of different data sets for provincial agencies in Ontario and Nova Scotia. This type of comparison could not be done for the other provinces, because overlapping data were not available. When data could be compared, however, there was a concurrence between the quantities reported. For example, data obtained from the Salt Institute indicate that the Ontario Ministry of Transportation used 418 997, 486 648 and 402 346 tonnes of sodium chloride for the winters of 1978-79, 1981-82 and 1982-83, respectively (Salt Institute, 1964-1983). Data obtained from the Ontario Ministry of Transportation indicate that it used 415 381, 486 648 and 402 346 tonnes of sodium chloride for the same years, respectively. Data obtained from the Salt Institute and Nova Scotia Department of Transportation and Public Works both indicate that 201 989 and 129 453 tonnes of sodium chloride were used during the winters of 1981-82 and 1982-83, respectively. While this comparison is limited in scope, it suggests that data obtained from the Salt Institute and provincial agencies concur.
2.2.3 Reliability of loadings data
Although a concerted effort was made to obtain data from the most accurate sources, errors in data and calculations are possible. Quantities of materials applied may have been inaccurately recorded by agencies, or there may be non-reported salt usage. There could also be inconsistencies in the data that were provided. For example, some provinces purchase salts for use by municipalities or maintain municipal roads. Hence, municipal data may be included in the provincial loadings for certain districts. Another problem could be that survey data generally refer to salt purchased and not salt used. While all the salts purchased will eventually be used, there is no way of determining the quantity of salt that is applied each year. Potential errors with data from salt suppliers were assessed by correlating provincial bids and usage. Overall, both data sets are highly correlated.
One possible source of error could be the use of 1997-98 data to characterize salt loadings. While these quantities are a best estimate, available provincial data indicate that loadings varied by year. A comparison of the total salt loadings per unit length of road for all agencies that provided 5 years of data suggests that the 1997-98 winter season is not atypical (Figure 5). To validate assumptions that the 1997-98 winter season was representative, two statistical analyses were done (Collins, 2000). Data were analysed using an analysis of variance (ANOVA), and, since data were not normally distributed, they were also analysed using the non-parametric Friedman's two-way analysis by ranks. Whenever there was a significant difference among years, Ryan's step-down procedure tested for differences among years. Results of these analyses indicate that the loadings were somewhat lower for the 1997-98 period (Collins, 2000).
A potential limitation of analyses may be the use of a "saltable" category to characterize road salt loadings. Since highways generally receive higher loadings, it could be argued that the saltable category underestimates salt loadings on high-volume roads. Another limitation may be the use of population to estimate salt loadings for municipalities with no data. While there is a good correlation between municipal population and salt use at the provincial level, the strength of this relation varies by maintenance district.
Despite these potential limitations, data presented are good indicators of the quantities of salt used on roadways in Canada. This is particularly so when considering that there is a difference of 9.4% (417 501 tonnes) between known and estimated usage of sodium chloride by municipal, county and provincial agencies for the 1997-98 winter.
2.2.4 Summary
The estimated annual use of sodium chloride and calcium chloride road salts in Canada was determined from surveys of provincial, territorial and municipal agencies, industry data and estimates based on population. It is estimated that approximately 4.75 million tonnes of sodium chloride de-icers were used in the 1997-98 winter and that 110 000 tonnes of calcium chloride are used on roadways in a typical year. When combined, it is estimated that 2.95 million tonnes of chloride were used on roads in Canada during the 1997-98 year. It is recognized that amounts used will vary on a yearly basis, notably because of variation in climatic conditions.
Annual loadings vary geographically, with the highest loadings on a length of road basis occurring in Ontario and Quebec, intermediate loadings in the Atlantic provinces and lowest loadings in the western provinces. Overall, data presented in this section indicate that road salt loadings have not decreased over the past 20 years. Furthermore, data for some provinces suggest that loadings have increased since the late 1970s (Figure 5).
2.3 Snow disposal
Delisle and Dériger (2000) reviewed the physicochemical and ecotoxicological characteristics of roadside snow and the different methods used for removing snow from roads and sidewalks.
2.3.1 Characterization of snow from roadways
Road salts contribute to the presence of sodium, chloride and cyanide ions in snow. Other major contaminants also found in urban snow include debris, suspended solids, oil and grease, and metals (lead, manganese, iron, chromium) (Gouvernement du Québec, 1997). Average concentrations of chlorides in roadside snow or its meltwater for certain municipalities in Quebec ranged from 3.8 to 5689 mg/L (see Delisle and Dériger, 2000).
Chenevier (1997) analysed the physicochemical parameters of roadside snow from Montréal's primary and secondary streets in 1997. Concentrations for all parameters were higher for the primary streets. Average concentrations of chloride were 3115 mg/L for secondary streets and 5066 mg/L for primary streets (Delisle and Dériger, 2000). This difference is probably due to the higher frequency or rates of application of road salts. A similar study by Delisle et al. (1997) monitored the physicochemical characteristics of roadside snow for two snowstorms in January 1997. Mean chloride concentrations for samples taken during the first and second storms were 7716 and 3663 mg/L, respectively. Chloride concentrations for the 24 samples taken during both storms ranged between 1366 and 18 230 mg/L.
The characteristics of roadside snow depend on a wide range of factors, including community size, the land occupancy factor, traffic density, quantities of de-icing salt and abrasives used, and duration of snow clearing (Malmqvist, 1985; Delisle et al., 1995). Contaminant content generally changes with time. For example, concentrations of chloride were 530 mg/L after 72 hours for snow from residential city streets and 7496 mg/L after 72 hours for snow from commercial city streets (Delisle and Leduc, 1987). Concentrations subsequently diminished to 590 mg/L after more than 1 week, presumably due to loss of ions though meltwater.
2.3.2 Volumes of snow in Canadian cities
Removal and disposal of snow may be required when the accumulation of snow on or along roadways may hamper traffic or safety. As such, the quantity of snow to be removed and disposed of depends on the volume of snow and the extent of urban development (Table 5). Given the large amount of snowfall in Montréal (annual snowfall in southern Quebec is between 200 and 350 cm) and the size and density of the city, about 11.258 million cubic metres of snow were brought to snow disposal sites during the winter of 1997-98. This quantity can be compared with 1.5 million cubic metres for Toronto (before municipal amalgamation) and a total of about 30 million cubic metres of snow for all municipalities in Quebec.
2.3.3 Snow clearing and snow disposal methods
Clearing of snow generally involves plowing snow to the side of the roadway. In cities, snow clearing can begin as snowfall reaches 2.5 cm, with the snow being pushed to the sides of streets and onto sidewalks. However, when there is considerable accumulation (more than 10 cm) or when necessary, snow is cleared and transported to various disposal sites (City of Montréal, 1998). The various snow disposal methods have been reviewed by Delisle and Dériger (2000) and can be grouped into three categories, as described below.
2.3.3.1 Methods not involving snow removal
Methods for clearing snow from the roadway without transporting it to snow disposal sites typically involve plowing snow to the side of the road or blowing it onto land adjacent to the roadway. These methods are generally not effective in areas with a high land occupancy factor (Delisle, 1994). These methods are, however, the most prevalent approach to the clearing of snow from roadways in non-urban areas. This type of approach contributes to potential impacts of salts on roadside soils, vegetation, surface water and groundwater. Data collected by Watson (2000) characterized chloride concentrations in ponds and wetlands adjacent to roadways in southern Ontario. Results of this study indicate that the chloride concentrations are variable, but concentrations greater than 4000 mg/L were observed. This study indicates that runoff from roadways can adversely affect aquatic environments adjacent to roadways.
A study by Delisle (1999), which associated elevated chloride concentrations in municipal wells with the use of road salts, also indicated that high chloride concentrations were observed in an area where ditch waters were left to percolate through the soils. Concentrations of chloride at three municipal production wells in the study area increased by 102, 116 and 145 mg/L between 1983 and 1994.
- These quantities are approximate and vary yearly with snowfall amount. N/A = not available.
- Plus 100 000 tonnes of a sand/salt mix containing 5% NaCl.
- Plus 39 000 tonnes of a sand/gravel mix (1:9 for sidewalks and 5:5 for streets) during freezing rain.
2.3.3.2 Methods involving release into waterways after processing or treating meltwater
Roadway snow can be transported to snow disposal sites where snow melts and the meltwater is treated. This typically involves dumping snow at surface sites or in quarries where runoff is channelled to treatment facilities. Generally, snow disposal sites are located on impermeable or slightly permeable ground or must be equipped with a geotextile membrane. Some sites are also equipped with sedimentation facilities or are designed to direct the meltwater towards a wastewater treatment system (Gouvernement du Québec, 1991). These sites should not be located next to watercourses that could be affected by runoff.
Pinard et al. (1989) characterized chloride concentrations in runoff from snow disposal sites. This study indicated that only 2% of the salt spread on city streets was present in meltwater from snow disposal sites, with most of the salt likely released to the environment from the roadway or roadside. This concurs with Delisle and Leduc (1987), who indicated that chloride concentrations in roadside snow initially increase then decrease with increasing time. The concentration of chloride in snow removed from roadways will, however, vary by street type; concentrations in snow collected from primary streets can be an order of magnitude higher than concentrations in snow from secondary streets.
While the percentage of salt present in snow transferred to snow depots may be low (e.g., 2%), the concentration of chloride in runoff is still elevated. A study by Péloquin (1993) indicated that the average chloride concentration in meltwater from a snow disposal site was 414 mg/L. Pinard et al. (1989) monitored chloride concentrations in runoff from a Québec City snow disposal site from April 18 to the end of June 1988. Concentrations in runoff ranged from approximately 100 to 1100 mg/L. Concentrations were highest in the early sampling and gradually decreased throughout the spring. It is not known if more elevated chloride concentrations were present in runoff prior to the monitoring program or if the concentrations increased throughout the summer due to a decrease in the volume of runoff water.
The potential impact of snowmelt on groundwater quality will vary by disposal site. A study described in Morin (2000) indicates that chloride concentrations between 233 and 1820 mg/L were measured at monitoring wells installed to assess the impact of a snow disposal site on shallow groundwater quality.
Other agencies (e.g., City of Montréal) dump snow through chutes linked to municipal sewer networks (Godbout, 1996; Couture, 1997). Certain agencies use snow melters. The underlying principle of melters is to dump water rather than snow into the sewer system. Melters can be stationary or mobile. Snow is dumped by trucks into preheated water tanks equipped with oil or gas burners. The City of Toronto uses this technique occasionally. So-called "geothermal" snow melting, using geological formations as a natural storage reservoir for water during the summer and keeping the water sufficiently hot to melt the snow during the winter, has been used since January 1998 in Cap-Rouge, Quebec (Bilodeau, 1999).
2.3.3.3 Methods involving release to surface water without water treatment
This method of disposing of snow involves dumping snow directly into a waterway or onto its banks. Snow can also be dumped down sewer chutes that are not linked to treatment plants. This results in the release of waste snow and any contaminants to surface water directly (dumping into rivers or into the ocean), with some removal of large debris (dumping onto banks) or with possible dilution by stormwater (dumping into sewer chutes not linked to treatment plants).
2.3.4 Use of disposal methods by Canadian municipalities
The use of snow disposal approaches by selected Canadian municipalities is shown in Table 6. The City of Montréal uses the greatest variety of disposal methods. In 1997-98, the City of Montréal used surface sites (7 sites), sewer chutes (11 sites), dumping into the St. Lawrence River (3 sites) and quarry dumps (2 sites) (City of Montréal, 1998).
Surface sites are clearly the most common method of disposal in Canada. For example, Regina uses between two and five surface disposal sites, depending on the year (City of Regina, 1997). In the case of the Regional Municipality of Ottawa-Carleton, several studies have been conducted to determine the number of surface sites needed to accommodate snow removal in that region (McNeely Engineering, 1990; McNeely-Tunnock Ltd., 1995). A few municipalities (e.g., St. John's, Halifax and Vancouver) dump snow into the sea at harbours. The direct dumping of snow to fresh surface water is restricted and will not be permitted in Quebec as of 2002 (Gouvernement du Québec, 2001).
In the past, municipalities such as Montréal blew snow onto private property and occasionally used snow melters. Blowing onto private land was virtually abandoned by Montréal because of social and political pressure, and snow melters proved to be too expensive to operate because of high fuel costs (City of Montréal, 1998). In certain cities like Halifax, large quantities of de-icing salt are used, but little snow is removed; roadway meltwater is channelled to storm sewer systems (Delisle and Dériger, 2000).
2.3.5 Summary
In addition to inorganic ions from road salts, waste snow can contain a broad range of physical and chemical contaminants. Chloride ions are dissolved and ultimately transported in meltwater and are largely not affected by physical or biological water treatment. Impacts depend on total amounts and concentrations of the ions at the point of release of meltwater into surface water or into soil and groundwater. While chloride concentrations in waste snow and its meltwater can be quite variable, they can be as high as 18 000 mg/L. Average chloride concentrations in snow from streets in Montréal were approximately 3000 and 5000 mg/L for secondary and primary streets, respectively. Canadian municipalities use a variety of snow disposal techniques, but surface sites are the most common. While there is uncertainty regarding the portion of salt that may be transported to snow disposal sites, it is clear that runoff from these sites has elevated chloride concentrations.
2.4 Patrol yards
Patrol yards (also referred to as storage yards or maintenance yards) are used to store road maintenance materials before their application to roadways. The following sections describe and characterize these facilities, outline environmental exposure pathways and present measured concentrations from specific points around patrol yards. Most of the data are based on pre-1998 standards for patrol yard design and the storage of road salts. While more effective designs are currently being promoted to reduce salt loss (TAC, 1999), current design and management are largely similar to those considered in this assessment. Data in the following sections were summarized from a report prepared by Snodgrass and Morin (2000).
2.4.1 Number of patrol yards and quantity of materials stored
Data compiled by Morin and Perchanok (2000) were used to estimate the number of provincial and territorial patrol yards and the quantity of road salts and abrasives stored at these yards. There is no definite information on the numbers of patrol yards in Canada, but it is estimated that there are 1300 provincial patrol yards. Assuming that total salt and abrasive use by jurisdictions is distributed equally among patrol yards, the average quantities used at patrol yards were estimated (Table 7). This table does not include municipal and county yards and storage facilities used by private contractors and hence significantly underestimates the number of patrol yards in Canada. The average amount of salts and abrasives stored at patrol yards varies considerably across Canada. Table 7 suggests that the average amount of salts stored at patrol yards in Ontario, Quebec and the Maritime provinces ranges between 1300 and 3800 tonnes. The average amount of abrasives for these provinces ranges between 1000 and 8600 tonnes. The number of patrol yards by district and the average tonnage of sodium chloride and abrasives that would be used by patrol yards for selected provinces are presented in Snodgrass and Morin (2000).
1 Including Nunavut.
Data for the winter of 1997-98 from 117 patrol yards administered by the Ontario Ministry of Transportation indicate that the quantity of sodium chloride stored at these patrol yards ranges between 45 and 21 400 tonnes per yard; the quantity of abrasives stored ranges between 10 and 13 200 tonnes per yard.
2.4.2 Patrol yard design and management
Patrol yards can be located in a variety of settings, and patrol yard design and standards vary substantially across Canada. Accordingly, the degree of protection against weathering varies considerably. Covered facilities used to store road salts can include domes/igloos, sheds and lean-tos. Doors or walls may or may not be present, and storage under an overpass can be considered as covered by some agencies. Salt may be stored on asphalt or concrete pads or outside on a thick plastic tarp and covered by another tarp. While storage in covered facilities can protect against precipitation, which leaches the salt, there is no universal standard design for storage facilities. Use of best management practices that promote good housekeeping at patrol yards is equally important to reduce salt loss. The effectiveness of best practices, however, depends on the rigour with which they are implemented.
Salts can be released from mixed abrasive/salt piles as well as from salt piles. A mixed abrasive/salt pile consists of either sand or gravel mixed with salt to prevent the abrasive from freezing and to keep the abrasive in a flowing state (NB DOE and DOT, 1978). The percentage of salt varies - most agencies have 5% mixtures, but this can range between 2.5 and 15% salt. A New Brunswick study (NB DOE and DOT, 1978) monitored the quantity and quality of leachate from a 2000-tonne abrasive pile with a 2.5% salt content. During the first year, 420 m3 of leachate passed through the monitoring system. Sodium and chloride concentrations in the leachate are depicted in Figure 6. The maximum concentrations for sodium and chloride were 37 000 and 66 000 mg/L, respectively. It was estimated that total salt loss through leaching during the first year was 18.2 tonnes, or over a third of total salt added to the pile, even though 80% of the abrasive pile had already been removed for sanding operations by the end of January. Results of the second year also indicated high concentrations in leachate from the pile. A total of 255 m3 of leachate was measured. The maximum concentrations for sodium and chloride were 49 000 and 82 000 mg/L, respectively. Considering these concentrations and the results of a survey suggesting that few provincial agencies cover mixed abrasive/salt piles (Snodgrass and Morin, 2000), the potential for salt loss from mixed abrasive/salt piles is high.
Spillage during stockpiling and loading of spreaders is a major source of salt loss (TAC, 1999), as confirmed by a review of electromagnetic surveys done at patrol yards (Snodgrass and Morin, 2000). These surveys, which typically identify subsurface materials and groundwater (within 6 m of the surface) that are highly conductive (e.g., soils and groundwater with elevated levels of salts), indicated high conductivity in areas adjacent to driveways and around storage facilities where salts are handled.
Another potential source of contamination is from washwater used to wash winter road maintenance vehicles. At patrol yards in urban areas, the washwater is likely released to a municipal sewage treatment system; in rural areas, discharge is probably to a dry well and left to percolate through the soil. Washwater samples from seven patrol yards had chloride concentrations between 3500 and 37 000 mg/L; the average concentration was 16 000 mg/L (Beck et al., 1994). This study also estimated that between one-third and two-thirds of patrol yards in Ontario discharge washwater to dry wells. Chloride concentrations in washwater can also be estimated by analysing chloride concentrations in oil/water interceptors. These devices have been installed at some patrol yard garages to remove oil, grease and similar compounds from washwater waste streams. Since oil/water interceptors do not remove salt, concentrations in discharges will be the same as in the influent. A study by INTERA Consultants (1996) indicated that chloride concentrations in oil/water interceptors at three patrol yards ranged between 1100 and 35 400 mg/L. These concentrations roughly concur with those monitored by Beck et al. (1994).
2.4.3 Estimation of releases of salts from patrol yards
Estimates of releases of salts from patrol yards are addressed in this section; details of the assumptions used to calculate these releases are in Snodgrass and Morin (2000). Estimates of releases to the environment were made by considering a hypothetical scenario of releases from 190 active yards. Total estimated releases from patrol yards were then compared with a total roadway application of approximately 590 000 tonnes annually. This hypothetical scenario corresponds roughly to a major province or several smaller provinces as the aggregate basis for the calculations.
Figure 6 Average daily concentration of leachate from salt-treated sand pile, 1975-76 (from NB DOE and DOT, 1978)
Table 8 provides three estimates based on the following scenarios: (1) a patrol yard with best management practices where salt and mixed abrasive piles are stored indoors; (2) a patrol yard where salt piles are stored indoors and abrasive piles are stored outdoors; and (3) a patrol yard where neither abrasive piles nor salt piles are stored indoors. Of the three scenarios, options 1 and 2 are probably most representative of patrol yards. The last column of Table 8 estimates the percentage of total road salt use that may be lost at patrol yards. Depending on the facility and salt management options, 0.2-20% of total salt use can be lost at patrol yards.
2.4.4 Salt concentrations at different areas around patrol yards
The following sections provide an overview of concentrations observed at surface and subsurface endpoints at patrol yards.
2.4.4.1 Concentrations at surface points around patrol yards
A limited database for surface drainage systems around patrol yards is available from environmental site assessments (ESAs) done at some Ontario Ministry of Transportation patrol yards (MTO, 1993-1999). Chloride concentrations at 29 yards where samples were taken ranged between 4 and 4880 mg/L; the mean chloride concentration was 659 mg/L.
A similar type of study was done for a yard in Alberta. A sample taken from the evaporation pond located on this property had a chloride concentration of 32 100 mg/L (Alberta Infrastructure, 2000). A study by the Nova Scotia Department of Transportation and Public Works indicated that runoff and shallow groundwater flowing from a patrol yard to a small brook increased chloride concentrations in a pond located approximately 250 m downgradient from less than 75 mg/L to approximately 1000 mg/L (Rushton, 1999).
2.4.4.2 Concentrations in patrol yard well water
A study by the Ontario Ministry of Transportation (MTO, 1997) graphed chloride concentrations from over 300 patrol yards in Ontario over a 20- to 30-year period ending in 1988. A 5-year period, 1983-1988, was selected to characterize the range of concentrations that could be observed in patrol yard wells. Of the yards in the analysis, most (229 yards, 74%) had chloride concentrations below 200 mg/L. A smaller number of yards (82 yards, 26%) had concentrations above 200 mg/L, including 30 yards (9%) with concentrations greater than 500 mg/L and 14 yards (4%) with concentrations greater than 1000 mg/L.
ESAs done for patrol yards in the Ontario Ministry of Transportation's Eastern Region tested the quality of water in potable water wells (MTO, 1993-1999). Concentrations of chloride in samples from 36 wells ranged between 1.5 and 5050 mg/L; the mean concentration was 722 mg/L. Sixty-four percent of the samples had chloride concentrations greater than 100 mg/L, and 50% of the samples had chloride concentrations that exceeded 250 mg/L. Thirty-five percent of the samples had concentrations greater than 500 mg/L. The depth of the wells sampled ranged from 12 to 113 m, with an average depth of 39.2 m. Elevated chloride concentrations at wells do not necessarily indicate that the entire aquifer is contaminated. For example, faulty well construction can cause salts to flow down around the well casing to the groundwater.
2.4.4.3 Concentrations in shallow groundwater
The same set of ESAs (MTO, 1993-1999) was also used to assess the quality of shallow groundwater. At several patrol yards, monitoring wells were installed to depths between 0.26 and 5.25 m and groundwater was sampled. Chloride concentrations ranged between 1 and 24 000 mg/L; the mean chloride concentration for the 102 samples was 2600 mg/L. A summary of the data indicated that 75% of the samples had chloride concentrations greater than 100 mg/L and 69% of the samples had chloride concentrations that exceeded 250 mg/L.
In similar types of analyses, chloride concentrations in groundwater at three Alberta patrol yards ranged between 26 and 26 400 mg/L (Alberta Infrastructure, 2000). Chloride concentrations in shallow groundwater at three patrol yards managed by the Nova Scotia Department of Transportation and Public Works ranged between 254 and 38 600 mg/L (Rushton, 1999).
2.4.4.4 Concentrations in patrol yard soils
At several patrol yards subjected to ESAs, test pits and bore holes were dug and soil samples were analysed (MTO, 1993-1999). Chloride concentrations from 53 test pits ranged between 5 and 14 500 µg/g soil; sodium concentrations for 46 samples ranged between 39 and 13 100 µg/g soil. The average concentrations for chloride and sodium were 2100 and 2600 µg/g soil, respectively.
Chloride concentrations in soil samples from 46 bore holes ranged between 2 and 13 300 µg/g soil. The average chloride concentration was 1500 µg/g soil. Sodium concentrations for 98 samples ranged between 86 and 6720 µg/g soil; the average sodium concentration was 870 µg/g. Bore holes were between 0.1 and 5 m deep, with an average depth of 1.6 m.
