Salt

Overview

Each year, Canadians spend over $1 billion on public and private roads, parking lots and sidewalks (Hossain et al., 2015). While the use of salt is essential to ensure public safety, there is a growing concern regarding the large quantities of salt (mainly chloride ions), being released to the environment.

In their 2001 assessment under the Canadian Environmental Protection Act, it was found that high releases of road salts from winter maintenance activities were having an adverse effect on freshwater ecosystems, soil, vegetation, and wildlife (Environment Canada, 2001). Based on this conclusion, Environment Canada developed its “Code of Practice the Environmental Management of Road Salts,” which focuses mainly on municipal and provincial road maintenance. This code, which requires the development of Salt Management Plans for those organizations using more than 500 tonnes of road salt annually, was released by Environment Canada in 2004 (Environment Canada, 2004). While the uptake of these guidelines has been successful, and many agencies have adopted best practices because of it, it is not generally applicable for companies that apply salt to private roads, parking lots, and roadways; and additional measures are needed to realize reductions in these areas.

There are studies and training programs that address these issues, which aim to educate private contractors about best practices, and how these can improve level of service, protect the environment, and reduce costs. However, uptake of these programs has been low.

To address this gap, LSRCA and its partner agencies identified a need for a guideline document that could be used by designers, regulatory agencies, owners, contractors, and others to consider design elements in the design and layout of parking lots and related infrastructure that can help to reduce the requirement for salt application. This effort culminated in the development of the Parking Lot Guidelines to Promote Salt Reduction.

The LSRCA commissioned study into salt management design strategies for parking lots can be read here: LSRCA salt guide. The report identified four key design strategies. They can be see summarized below:

Effective Grading

• Proper grading can minimize the freezing of wet pavement surfaces and prevent melt water from ponding and re-freezing, reducing the need for re-application of salt.
• Areas for vehicular and pedestrian traffic should be graded between 2 - 4 % to reduce the chances of depressions forming over time (maximum permitted 5% for AODA). Small depressions can result in ponded water icing over in the winter.
• Subbase should be well compacted for the same reason.
• In winter months efficient salt application should be made along the top of slopes; melting snow will carry the salt solution down-gradient.
• Effective grading can also direct melt water towards strategically placed stormwater collection infrastructure (i.e. catch basins, vegetated swales, bioretention features, landscaped areas), preventing salt application in heavy traffic areas that are also pathways for runoff.
• The key to effective stormwater collection during winter is to ensure that melt water from high traffic areas or snow piles does not have to travel great distances to a collection point.

Snow Pile Location

Not quite well graded enough; the puddle in the foreground will refreeze overnight.

• Snow piles should be strategically located to minimize the risk of melt water draining across high traffic areas and refreezing.
• Storage locations for snow piles should be around the outer edges of parking lots and downgradient from high traffic areas, in sunny areas where possible to accelerate melting.
• Consider grading the storage location to distribute the melt-water as sheet flow over a grass filter strip into an adjacent BMP, such as a bioretention cell or infiltration trench. In some cases, with careful vegetation selection and adequate drainage, the BMP itself can serve as a snow storage location. Designing specific drainage collection features for snow piles can ensure that melt water is quickly collected in the vicinity of the pile to reduce the opportunity for refreezing.
• Snow storage areas should be clearly marked for seasonal maintenance staff.

Sidewalk Design and Pedestrian Flow

• The design process should consider that pedestrians typically follow the path of shortest distance and do not necessarily use designed walkways. Walkways from adjacent residential areas and transit stops should be prioritized, and unnecessary walkways should be avoided.
• Sidewalks that receive infrequent use could be closed for the winter season.
• Maintained sidewalks should be ≥ 1.5 m wide to accommodate plowing and minimize the salting required.
• Using textured pavers can improve grip for pedestrians, again reducing the salt required.
• In busy areas around building entrances, covered walkways and heated mats also reduce salt requirements.

Landscaping Features

Trees

• Landscaping features (i.e. vegetated swales or landscaped islands) can lead to a reduced requirement of salt application by reducing the amount of paved surface.
• Specifying deciduous trees along walkways and near snow piles will maximize winter sunlight penetration. This will naturally enhance the melting of frozen surfaces, limiting the need for winter maintenance.
• Coniferous trees can be used to create treed wind breaks along the site perimeter to avoid snow drifts.

Other vegetation

Vegetation varies in its reaction to soils with high salinity:

• Salt in soil water generally makes it more difficult for roots to take up water. This phenomenon mimics drought conditions for the plant.
• If passing traffic sprays salty water onto plants it can reduce cold hardiness in buds and new twigs. These then become more susceptible to freezing, mortality or deformation.
• In high enough concentrations, sodium and chloride can also be directly toxic to plants. In some species the ions are absorbed by the plant and build up in the leaves causing them to die.

Generally, the vegetation growing closest to the source will be most strongly affected by salt. Plants actively growing in late winter (when salt levels are highest) are also more significantly affected. So, warm season grasses offer an advantage over cool season grasses, because they emerge later in the spring when excess salt has been flushed away. Resilient turf grasses are particularly useful in the design of vegetated filter strips, dry ponds and enhanced grass swales. The Ministry of Transportation have standardized a number of grass mixes. The 'Salt Tolerant Mix' is of particular value for low impact development applications alongside asphalt roadways and paved walkways.

Canada #1 Ground Cover (salt tolerant mix)

Common name	Scientific name	Proportion
Tall Fescue	Festuca arundinacea	25 %
Fults Alkali Grass	Puccinellia distans	20 %
Creeping Red Fescue	Festuca rubra	25 %
Perennial ryegrass	Lolium perrenne	20 %
Hard Fescue	Festuca trachyphylla	10 %

Other Design Features

Other options that can be considered to reduce the amount of salt that needs to be applied in a parking lot include:

• The use of permeable pavers: these improve drainage and prevent melt water from ponding and refreezing.
• Seasonally closing parking areas: many parking lots have areas that are infrequently used outside of the holiday shopping period. These areas can be closed and not maintained through much of the winter season, reducing both the effort and amount of salt required.

Additional Work

Parking Lot Friction Testing

Two of the main considerations contractors face in maintaining parking lots in winter are: what application rate should be used; and what is the level of service expected by the property owner, for which the bare pavement return time is a common measure (this is the amount of time it takes after treatment to achieve a bare surface). To better understand these questions in 2017 the LSRCA obtained a friction tester, with a goal of quantifying the effectiveness of various practices and salt application rates. Here we present some of the findings of this study.

This image demonstrates the two extremes of LSRCA’s friction testing: a perfectly clear and dry surface, with a µ value of 0.9 and the same surface covered in a light layer of snow, with a µ of only 0.11.

As can be seen in the inset table, the unit for measuring friction is ‘µ’, and the closer to 1.00 the µ value, the safer the surface. A high µ, however, is not the only measure of safety – many smooth indoor floors will have low µ values, in the range of 0.3 to 0.4, and they are generally not considered unsafe. Through this study, we measured the friction of several different surfaces, which received varying treatments.

Friction Values and Related Road Surface Conditions

Measured Friction Value (µ)	Road Surface Condition
0.8 - 1.00	Dry, New Asphalt
0.50 - 0.80	Wet Asphalt
0.30 - 0.50	Wet Sand on Ice
0.25 - 0.30	Dry Sand on Ice
0.25 - 0.25	Dry Ice
0.05 - 0.15	Wet Ice

Friction values for a properly treated surface (left) with a small amount of residue (µ =0.63) and an over salted surface right, which has a much lower friction value (µ =0.26).

High volumes of salt are often applied because contractors, property managers, and parking lot users feel that the more salt there is, the safer the surface is to walk or drive on. However, a surface that has been treated at an appropriate rate, (which is slightly wet with a small amount of salt residue) has a much higher friction value (μ); with the level of service achieved far more efficiently, than when compared to the same surface where rock salt has been heavily applied (over salted).

The picture above shows the same walkway where more than 10 times the generally recommended amount of salt was applied in the photo on the left, and only shoveling was done in the photo on the right, and both µ values were in the low 0.20s.

Through this work, as referenced above - LSRCA staff documented higher friction values on untreated surfaces than on surfaces with large volumes of product; the µ value of a surface may remain low if it has only been shoveled or plowed. While shoveling is an important part of the winter maintenance process, practitioners need to consider the site and predicted conditions on a day-to-day basis to determine how to attain the safest surface for vehicle and foot traffic. In many cases the sun or traffic may melt the residual snow on a shoveled or plowed surface without any further treatment being necessary (saving both time and money); while in other cases, some salt, applied at an appropriate rate, may be necessary.

Higher Costs, Little Benefit

Friction testing has demonstrated that bare pavement is safest, as it has the highest friction value, and that the over-application of salt does not always translate to safer conditions. Simply put, applying salt at the prescribed rate for the conditions and shoveling or plowing where appropriate will attain a higher friction rate than an overapplication of salt.

What is a reasonable amount of time to achieve the desired level of service?:

• Depending on the operating hours of the property being maintained, it may be possible to reduce the salt application rate without sacrificing the desired level of service.
  • For example, many commercial properties keep hours between 9:00 am and 9:00 pm, which would mean that the lot does not need to be clear until shortly before 9:00 am. The table below demonstrates the time it would take to reach bare pavement at typical industry-recommended application rates, in a situation where the temperature is between -7 and -9 °C, with between 0.5 and 1.5 cm of snow on the ground. The rate may need to be increased or decreased slightly to achieve the desired level of service depending on varying factors related to traffic, sunlight, type of snow, and/or pavement type.

Time to Reach Bare Pavement Scenario Comparison

Difference Among Scenarios	Time to bare pavement (hrs)	Application Rate (g/m2)	Volume of salt used for each application (kg)*	Total salt applied/season** (tonnes)	Material costs/season (assuming $100/tonne)
Scenario 1	2	87	13,050	913	$91,300
Scenario 2	3	58	8,700	609	$60,900
Difference	1	29	4,350	304	$30,400

External links

• Smart about Salt

---

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.

Lake Simcoe Region Conservation Authority

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

a gently sloping, densely vegetated areas that treat runoff as sheet flow from adjacent impervious areas. They function by slowing runoff velocity and filtering out suspended sediment and associated pollutants, and by providing some infiltration into underlying soils.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

Natural or artificial means of intercepting and removing surface or subsurface water (usually by gravity).

A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

Mineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.