Black Ice: BMPs Come of Age to Address the Grits That Grip

The sight of the gravel spreader is reassuring to those who travel in snow country. It’s good to know that when temperatures drop in the mountains, radios crackle at road maintenance stations and trucks roll. Equipment first hits the high-risk areas and emergency routes, then connects the road segments between them, spreading little shards of crushed rock that provide a temporary medium to help tires make contact with frozen, icy, or snow-packed roads. But with traffic, time, warming, and runoff, the crushed rock, which so recently came dancing out of the hopper, is washed off the road or kicked off by the buffeting of traffic and the spin of wheels. Throughout the winter, human life and safety will be guarded by repeated applications of road-sanding grits each time the radios crackle.

Most people don’t have a reason to think about the ultimate fate of the crushed rock spread on roads and highways to increase winter driving safety. If they travel in snow country, they’re glad to see the gravel truck and pleased that a mechanical substitute for salt is being applied on mountain roads. But with the National Pollutant Discharge Elimination System (NPDES) and the Endangered Species Act pressing state and municipal road departments to assess maintenance and operations, the paths of road-sanding grits are getting some serious attention. Where sanded roadways drain to wetlands, streams, or sensitive habitats, concerns about cumulative sediment loadings are beginning to arise.

Assess Modes of Transport

It comes as no surprise that the vectors of these sediments-wind, creep, ravel, and overland flow-are the same familiar agents of erosion and transport that we face in every erosion analysis. So the first step of an assessment is to look at how these sediments go into transport.

Road-sanding grits leave the traveled surface by means of both active and passive mechanisms. Gravels are flicked to the roadside by the spin of tires and by the turbulent cells of air that large vehicles push ahead of themselves and drag behind-the same ones that in summer stir the roadside grasses, swoosh other roadside particles into the air, and make you wish you’d remembered your goggles for that bike ride. Rainfall sheeting off crowned or sloped road surfaces also transports road-sanding grits. The wider and steeper the road, the greater the energy that runoff is capable of gathering to transport loose materials in its exit path from the paved surface.

In road clearing operations, ice, snow, and gravel most commonly are pushed or blown into areas adjacent the road. When snow is pushed, residual gravels become concentrated near the road, usually in the right of way. Blowing scatters the gravels over a wider road corridor zone. In some areas, such as resorts and city centers, snow is loaded and hauled to disposal sites where the gravels are recovered. A few areas scoop snow into an ambulatory snow-melting machine that retains the gravels and sends the melt water to the storm sewer. But for the most part, pushing and blowing remain the order of the day when it comes to snow removal practices.

If we follow the pathways of sanding grit after it has left the road, we will find that much of it comes to a more-or-less permanent halt fairly nearby. Much reposes on road embankments but can be gullied when concentrated road runoff runs over the embankment on the way to the roadside ditch. Some becomes entrained in roadside drainage systems, where it can be transported quite efficiently before it comes to rest in wetlands or is delivered to streams.

Look at Impacts on Receiving-Water Resources

Ask any wetlands ecologist what the effects will be of incremental filling of a wetland that is hydrologically connected to a small stream system. You will hear that watershed-regulating functions will be degraded in many ways. The capability of the wetland to store runoff for later release to base flows will be lessened. As a result, the receiving stream might have higher flows during the wet part of the year, but lower flows during the dry season. Channel responses will result, depending on channel materials and their influence on the ability of the stream to shift vertically and laterally. A widening stream might get warmer as it gets shallower and receives greater solar exposure. The pace of both chemical and biological activities will increase in the warming system. These will influence both water chemistry and biota. A deepening stream might grind away at its bed and banks, producing sediments that can become a chief factor in downstream water-quality impairment and simplifying aquatic habitats.

There’s more: The wetland’s ability to absorb background sediments might be reduced as wetland volume is taken up by the road-sanding grits. As the weight of these sediments displaces both air and water from the soil, plant species can change. The wetland may develop a distinct channel capable of passing background sediment loads right on through the system. The ability of the wetland to recycle inflowing nutrient loads might diminish. The changed nutrient balance of the receiving stream may be reflected by shifts in species of aquatic organisms, which in turn affect the species that eat them.

From the point of view of a Coho salmon that has swum upstream more than 100 mi. from the ocean to a cool, flashing mountain stream, the steady delivery of exogenous (from elsewhere), finely crushed rock to its spawning habitat is not just a housekeeping nightmare; it is more like a foreclosure. Just as space becomes unavailable in a living room piled high with boxes, stream habitat simply becomes unavailable where gravels settle out and stream pockets become packed with deposited gravels. As the small gravels migrate down into the spaces between larger alluvium of the streambed, the larger rock becomes “locked” in place, making it difficult for anadromous fish to build spawning redds or for eggs in the redds to receive enough oxygen.

One study estimated that 15 yd.³ of sanding grits were introduced annually into a half-mile segment of stream so small that a person could jump over it. In another, residual sanding grits that were blown onto a steep highway embankment were creeping back down after the snow melted, directly entering a salmon-bearing stream.

Fit the Solution to the Problem

Such concerns have prompted the development of a fresh batch of best management practices (BMPs) for managing roads treated with sanding grits, particularly in watersheds where threatened and endangered cold-water fish and other sensitive species are present. The solutions are as familiar as the agents of transport. They are prevention, interruption of overland flow, containment, and maintenance. As is the case with most BMPs, the practices are not rocket science but straightforward, commonsense, good-housekeeping actions that for the most part are affordable (see Table 1).

But there’s a hitch: Wherever the new practices take a snow disposal site out of service, a compensatory area must be found. Snow managers have long appreciated the assistance they get from streams and wetlands-whose snowmelt capabilities can be impressive-when it comes to disposing snow. Large volumes of snow can be melted and dispersed by these natural systems, and thus, when wetlands and live waters are removed from snow management systems, there is an immediate need for an appropriate nearby snow disposal site capable of handling the volumes of snow delivered to it.

Determining what makes an appropriate snow disposal site can be a challenge. Most states and municipalities do not regulate snow disposal per se, so assessing the impacts of snow disposal depends in part on how federal, state, and local water-quality regulations are interpreted, carried out, and enforced at the local level. Stormwater research is beginning to look at the pollutants present in discarded snow and the likely impacts of these pollutants on aquatic environments and living things. A few studies have indicated that snow-particularly that which has been plowed from highways having high trip numbers, from high-volume parking lots, or into a huge pile to await the spring thaw-can have surprisingly high concentrations of pollutants. Likely contaminants in managed snow are listed in Table 2.

Table 2. Likely Contaminants in Managed Snow

According to some studies, however, the range of expected contaminants in snow varies according to where it falls, how long it stays before it melts, what atmospheric pollutants are or might become associated with the snow, whether other snow is added to it (such as at a snow disposal site), and whether runoff-particularly from snow storage or disposal sites-infiltrates or flows to wetlands or streams. Groundwater contamination by snowmelt might be a concern in wellhead areas for drinking water. Other local considerations may come into play in determining how clean or dirty snow is. These can include the acidity of snow and the potential for pollutants associated with melting snow to be adsorbed by local soils. For all of these reasons, it might be important to characterize levels of snow cleanliness in developing a local snow management plan. Table 3 provides a sample classification scheme and sample disposal BMPs for disposing of each “level” of snow. The scheme is offered with the caveat that some municipalities no longer dispose of snow in streams-no matter how clean the snow is considered to be. Others allow snow to be disposed in streams only if discharge is considered high enough for dilution.

For the most part, evaluation of snow disposal sites has not received a lot of attention, yet the potential exists that such sites could come to be commonly regarded as stormwater facilities under many municipal and state NPDES and total maximum daily load permits. Especially in settings near wetlands, drinking-water wells, or surface waters, where high groundwater or threatened, endangered, or sensitive aquatic species are present, snow disposal sites might need to be selected on the basis of assessment of potential environmental impacts. Public safety, noise, aesthetics, and economics will also figure in site selection. Table 4 lists selection criteria for snow disposal sites. Many of the measures listed are derived from South Dakota Department of Environment and Natural Resources, Watershed Protection (www.state.sd.us/denr/DFTA/WatershedProtection/snow.htm) and other sources.

Learn From Others’ Actions

Thousands of municipalities in snow zones now face stormwater permitting under NPDES II. Good resources for managers looking for ideas about how to deal with road-sanding grits are the annual NPDES reports of their state’s department of transportation and their region’s most populous cities. Many such reports contain BMPs for planning, operating, maintaining, and upgrading drainage systems associated with roads, and they contain sections on road-sanding grits.

A Web search will quickly reveal that control of residual gravels is but one of a nested set of related stormwater issues concerning snow management. As a result of rising concerns about toxic pulses associated with early-season melting of large snow-storage piles, some municipalities are managing snow as part of their solid waste and stormwater programs. Others have adopted detailed codes and standards to ensure that the relatively clean snow of residential areas is retained on the structures and properties where it falls in order to avoid contamination and costs associated with movement, storage, and runoff.

Although shouldering additional road maintenance BMPs under NPDES might seem onerous, some snow managers have remarked that the mandatory yearly analysis of maintenance and operations has been eye-opening. They have been able to identify opportunities to save energy, train workers, and conserve and recycle road-sanding grits. “We really have a better handle on our entire operations,” remarks a colleague who is the only engineer of small town listed as a partner in a larger municipal NPDES permit. “Our housekeeping has gotten better. We understand where we are making progress and where we need to focus on improvements.”