Dissertation: Year-round runoff processing necessary

The runoff from urban areas should be treated before letting it into other waterways, recommends Maija Taka in her doctoral dissertation.

Taka’s research monitored the water quality of the runoff in rainwater drains during different times of the year in different areas of Helsinki (Itä-Pasila, Pihlajamäki and Veräjämäki) as well as the seasonal variation in water quality in watersheds in southern Finland.

Peering under drain grates, Taka discovered an astounding amount of empty crisp packets, car licence plates and other objects that had no business being in a rainwater drain. The water samples showed alarming levels of nutrients, salts and metals.

Ground cover proves an important water quality regulator 

One of the research sites was the densely built Itä-Pasila area, in which all runoff is led directly into the sewage network. Much of the area is asphalt and concrete. When heavy rains pour down on such hard surfaces, the levels of harmful substances in the runoff are particularly high.

– The water has high levels of salts and potassium, for example, possibly dissolved from the concrete, as well as nitrates and metals, Taka explains.

However, it is difficult to identify the environmental factors regulating water quality because the substances derive from many different locations, they make their way into the water through many routes, and the whole process is highly complex.

This study was able to create a more comprehensive image, as water samples were taken over a longer period and also during winter.

Help clean your neighbourhood stream

While working in the field, Taka was delighted by the level of interest the locals showed in their waterways. Many would strike up conversations with her as she was taking samples.  There have been many positive experiences with communal projects to clean up stream banks.

– Urban streams are an important connection to nature in our cities, and we should take care of them, says Taka.

The research also studied regional variation in water quality by sampling water from different areas of southern Finland’s watersheds from a total of 90 sites.  Urbanisation places a burden on stream water, and densely populated areas showed more pollution than agricultural ones.

The research results show that the water quality of the runoff depends on the soil and the ground cover. The soil had more of an impact in agricultural areas, while in densely populated regions the quality of local streams and rivers was more closely connected to the ground cover. In urban areas, impermeable surfaces result in dry spells between rains, as no groundwater is generated or stored up in the area. In natural areas, groundwater maintains streams even during times of no rain.

In the samples taken in summer, the levels of harmful substances varied according to water quantity. Heavy rains meant more pollutants were flushed into the runoff. In winter, the water quality was less dependent on water quantity than it was on the sources of pollutants in the area.

Maija Taka began work as a researcher at Aalto University’s Department of the Built Environment at the beginning of this year, and intends to focus on water issues on a global scale in the future.

MSc Maija Taka will defend her doctoral dissertation entitled “Key drivers of stream water quality along an urban-rural transition – a watershed-scale perspective” on 31 March 2017 at 12.15 at the University of Helsinki’s Faculty of Science. The public defence will take place in the University’s Main Building, Auditorium XV, Fabianinkatu 33. Welcome.

The dissertation is also available in electronic format through the e-thesis service: Key drivers of stream water quality along an urban-rural transition – a watershed-scale perspective

More information:

Maija Taka University of Helsinki
+358 40 5185838

Science Communicator Riitta-Leena Inki
+358 50-448 5770
riitta-leena.inki@helsinki.fiTwitter: @inkiriitta