Residents report declining water quality in Lake Mattilanlampi after Highway 4 reconstruction in Finland

The reconstruction of Highway 4 between Hartola and Joutsa concluded just over a year ago, but local residents around Lake Mattilanlampi continue to feel its impacts. Previously a clear-water lake, residents now report that it has become murky. In summer 2023, blue-green algae was observed in the lake for the first time in 30 years.

The highway project aimed to enhance safety and traffic flow and required a permit under water legislation, as it traverses a groundwater area. However, runoff from the highway now drains approximately four kilometers directly into Lake Mattilanlampi via three new ditches, preventing water from absorbing into the ground due to stricter isolation of groundwater.

Measurements taken in spring 2023 indicated a clear deterioration in the lake’s water quality. Samples taken in August confirmed the water was still murky and contained small particles, possibly algae, along with elevated nutrient levels. However, winter samples showed a decrease in concentrations, and the monitoring indicated no significant changes in nutrient levels.

The Hämeen ELY Centre received three nuisance reports regarding the declining water quality in August 2023. Consequently, it was agreed that the condition of Lake Mattilanlampi would be monitored more closely for a year. The monitoring concluded this summer, and according to ELY Centre’s senior inspector Helka Sillfors, no substantial changes in nutrient levels have been detected.

Residents, concerned about increased traffic and accidents, argue that while groundwater has been protected, surface water quality has suffered due to the runoff. They believe that the impacts of directing runoff to the lake were overlooked during the project’s planning stages, asserting that the project has ultimately harmed the lake’s ecosystem.