In 2002, application of potassium formate as a sole deicing agent was started on a stretch of a highway in Finland. Prior to that, from 1977 to 2002, the stretch had been deiced with sodium chloride. Microbial degradation of formate at low temperatures in the top soil in the vicinity of the highway results in a rapid removal of formate in the environment. Up to 96 % of ¹⁴C-formate was degraded to ¹⁴CO₂ within 1 day in the road side soil samples at -2 ^oC to +6 ^oC. Consequently, no formate has been detected in the groundwater by September 2007. Chloride concentrations in the groundwater underlying the highway show mostly decreasing trends. However, in the wells with the highest chloride concentrations (up to 320 mg/L, which exceeds the natural background chloride concentration by a factor of 100), chloride concentrations increased or remained high until 2005-2006. In conclusion, potassium formate can be applied in winter road deicing at areas where the impacts of deicing on groundwater quality need to minimized.