A while back as part of a stream gauging experiment (more of which in a later post) I did a literature review of the sub-lethal and toxic levels of salt in water for different classes of wildlife. This is one of those classic PhD exercises that is necessary for responsible science, but ordinarily would probably end up as an unread appendix tucked away in a dusty volume on a dusty shelf.
Given that cold weather results in a lot of salt spread on roads and paths I thought it would be useful for managers of rivers and ponds to understand at what level saline water becomes a problem for wildlife.
Salt is put onto roads as a chemical treatment for ice and snow; when absorbed by water it lowers the freezing point meaning ice doesn’t form as readily. However once the weather warms up this dissolved salt is washed into sewers and water courses and can build up to levels which are a problem for wildlife.
The table below shows concentrations I have identified from the scientific literature as confirmed to cause sub-lethal and toxic effects.
We can see from the table above that problems start to occur above 1g/l for amphibians, which are very poor osmotic regulators and are going to be the most vulnerable group to elevated salt concentrations.
An important context to this the amount of salt spread. The highways agency in the UK recommends a density of somewhere between 20-40 g/m (<40,000 mg/m) depending on ambient conditions – which of course could be spread ever day during cold weather. 1mm of rain (~=1cm of snow) amounts to 1litre of water over a square metre of ground surface, which in turn could equate to local spot salt concentrations of 40,000mg/l. It is important to emphasize that such concentrations would almost certainly be heavily diluted by mixing with rain/snowmelt from non-highway surfaces (quite probably to very low levels) once in main water courses, but serves to illustrate that locally runoff could be highly saline.
In flowing water salt concentration is likely to manifest as a short pulse moving through the system for a day or two and may not cause a problem. I can foresee problems occurring in slower moving water where concentrations could remain elevated for days. Ponds and lakes near to roads are particularly are at risk, especially if there are storm drains flowing into them. Stilling pools designed to filter highway run off will of course collect dissolved salt and this is their function, but if these areas are also wildlife havens salt laden runoff could cause issues.
If you are concerned about the salinity of water it is fairly easy to test a sample or use a hand-held probe. These can be bought for as little as £30. Typically they will read in “micro-siemens” where 2µm = 1mg/l.
The message here is don’t panic, in the main there are unlikely to be problems from highway salt runoff, but be aware of potentially vulnerable areas.References: Alvarado, R., 1979. Amphibians. In: G. Maloiy (Ed.), Comparative Physiology of Osmoregulation in Animals. Academic Press: London., pp. 261-303. Beadle, L., 1969. Osmotic regulation and the adaptation of freshwater animals to inland saline waters. Verh. Int. Ver. Limnol, 17, 421-429. Hart, B.T., Bailey, P., Edwards, R., Hortle, K., James, K., McMahon, A., Meredith, C., Swadling, K., 1991. A review of the salt sensitivity of the Australian freshwater biota. Hydrobiologia, 210(1), 105-144. Hogan, A., Nicholson, J., 1987. Sperm motility of sooty grunter, Hephaestus fuliginosus (Macleay), and jungle perch, Kuhlia rupestris (Lacepede), in different salinities. Marine and Freshwater Research, 38(4), 523-528. Moore, D.J., Reed, R.H., Stewart, W.D.P., 1985. Responses of cyanobacteria to low level osmotic stress: implications for the use of buffers. Journal of General Microbiology, 131(6), 1267. Padhye, A., Ghate, H., 1992. Sodium chloride and potassium chloride tolerance of different stages of the frog, Microhyla ornata. Herpetological journal, 2(1), 18-23. Parry, G., 1966. Osmotic adaptation in fishes. Biological Reviews, 41(3), 392-440. Reed, R.H., Richardson, D.L., Warr, S.R.C., Stewart, W.D.P., 1984. Carbohydrate accumulation and osmotic stress in cyanobacteria. Journal of General Microbiology, 130(1), 1. Sanzo, D., Hecnar, S.J., 2006. Effects of road de-icing salt (NaCl) on larval wood frogs (Rana sylvatica). Environmental Pollution, 140(2), 247-256.