Salt, Grit and the Environment
At the Raynes Park Community Forum on 5 February 2009 a resident asked about the
environmental impact of salt and grit used on icy roads. I wrote to the Environment Agency and
the Highways Agency. Both sent very helpful replies. Since their views were very similar I have
reprinted the more detailed response from the Highways Agency below.
Bartle Sawbridge
Neighbourhood Governance Officer
Stronger Communities Team
Chief Exec’s Department
0208 545 4400
Grit or salt?
Although most of us call it gritting, there is in fact, no grit involved. The stuff spread on
the roads is rock salt, taken from an underground mine in Cheshire.
It is more or less the same as the rock salt you’d grind onto your food, other than it is
brown due to containing an impurity of Marl, which is a type of clay.
We spread salt onto the road, but it works best when it goes into a solution. We rely on
the tyres of cars passing over the top of it to crush the salt onto the road. This then
forms a solution with a higher de-icing capability.
Water freezes at 0C, but the salt stops the water from freezing until -6C to -8C.
If it snows, we spread 40g of salt per square metre on the roads before the snow
comes. Then as the snow falls we continue salting and we use snowplough attachments
if the snow accumulates.
Does the melting salt/marl have any adverse environmental impact?
Rock salt is the major de-icing agent used on roads in the UK and the HA has
undertaken some research into its potential impacts and alternative products and
application methods. It is comprised of > 90% Sodium Chloride , 2-4% water, anticaking agents. It can contribute to elevated levels of sodium and chloride in receiving
waters when applied to the road service and quickly followed by a rainfall event.
Laboratory toxicity tests have shown that high sodium and chloride levels could have a
potential impact on some invertebrates but that fish are quite tolerant for short durations
- this is not so surprising given that many fish species often live in brackish waters.
It is also recognised that a relatively low level of dilution is all that may be
required to protect aquatic communities. Any potential effect within receiving
watercourses is dictated by the amount of available dilution. As Rock Salt is only used
in the winter months it is unlikely that rivers and streams will be flowing at low flows
(typically experienced in summer months) when it is discharged. In fact the opposite is
more likely with winter rainfall events providing a higher degree of dilution in water
bodies. As all highway discharges are by their very nature intermittent and of a short
duration the exposure is likely to be low. Where highway discharges are treated by
balancing ponds a higher level of dilution and protection will be provided with a much
slower release to the receiving watercourse.
The Highways Agency have in recent years been looking at a number of ways of
reducing its rock salt usage and has a commitment with the Environment Agency to
work towards this without compromising road safety and operation of the Strategic Road
Network. Recent initiatives are;
1.
The investment in a new fleet of salters that use better technology to control the
application of salt more efficiently and effectively on roads surfaces. This minimises
the wastage and reduces the risk of over-salting the roads.
2. The HA is currently investigating alternatives to the use of rock salt on roads. One
product being explored is the use of Agricultural By-products which can be mixed with
Rock Salt thereby reducing the amount of salt used and the release of Sodium Chloride
into receiving waters. This is currently being trialled and its environmental and safety
benefits in comparison to Rock Salt will be evaluated.