This study was implemented for the environmental and land use department of the city of Hämeenlinna. The project began in March 2014. The original aim was to find out the influence of the expanding urban area to the runoff generation and quality of the Myllyoja stream. Myllyoja runs across the west part of the city down to the lake Lehijärvi.

The goal was to collect and process the results of the student studies 2008–2009 and old monitoring results 1963–2005 as well as perform field measurements to research the quality of the water, functionality of Vuorentaka wetland-pond and identify possible point sources of pollution. The increased amount of the storm water due to the expanding urban area was also meant to study. Storm water has caused downstream flooding and possible solutions were planned to suggest especially inside the city planning area. The nutrient flow was calculated based on the measurements as well as using VEMALA model.

During the master plan work of Sampo-Alajärvi in spring 2014, the consultant surveys of storm water management and constructability were ordered by the city. These surveys answered some of the research questions, so the study was redefined. These surveys were included in the theory part. Other studies from the same area were used as background information. Myllyoja was also viewed as a part of the green network of the city providing both ecological connections and recreational areas.

Three measurement rounds were performed along the stream, one when the snow was melting in March and the other two during the rainfall in June and August. Water samples were also analysed during the latter two rounds. The concentration of bacteria was considerably high (6000–20000 pmy/100ml) indicating the influence of the sewage water. More research was found necessary in order to find out the point source of pollution. The results of the functionality of Vuorentaka wetland-pond were not unambiguous. The nutrients and the suspended sol-ids were somewhat absorbed, but in some cases the nutrients were also released during rain. The wetland-pond area was overgrown and in need of restoration.

Total nitrogen concentration in Myllyoja was 500–2700 μg/l (often <1500 μg/l). Total phosphorus concentration was 30–90 μg/l (max 170 μg/l). VEMALA-model was used to evaluate the load to the Lehijärvi. Total phosphorus was 1818 kg/year, total nitrogen 43,5 tons/year and total suspended solids 538,6 tons/year. The load was also calculated using measurement results, the catchment area (30 km2) and runoff (8,3 l/s/km2). In this case the phosphorus load was 386 kg/year, nitrogen 17,0 tons/year and total suspended solids 62,3 tons/year.