The naturally oligotrophic rivers of northern Sweden are generally characterized by a low pollution level and high hygienic quality. E.coli, an indicator of fresh faecal contamination, shows however increasing trends and two recent outbreaks of the Cryptosporidium parasite in city drinking water supplies has caused much concern about drinking water security and ecological status of water bodies. There is also an increasing demand for scenic plots of land close to rivers and lakes for both permanent dwellings and holiday cottages and there is a need for better understanding of relationships between settlements within the riparian zone and river water quality.
Measurements of E.coli are made at fixed sampling points along the river. A transport model for E.coli in river water was developed based on literature data for E.coli inactivation rates, water temperature and hydrological conditions. The model was used to estimate the source distance upstream each sampling point in the investigated section of the river. This procedure enabled the establishment of sections of the river that could influence each sampling point by point or diffuse emissions of E.coli. Studies on human development within the riparian zone of each section could then be related to E.coli data at the sampling points.
Data on the latest 20 years of development within the riparian zone was gathered from the local municipality archive which contains information on all new and changed properties. The development data was compared to 18 years long time series of seasonal water quality measurement in the studied river. The analysis also considers the effects of known point sources of faecal pollution within the investigated sections such as discharge from municipal sewage treatment plants.
This study shows that E.coli is transported considerably longer during winter compared to summer in a regulated section of the river. Lower water temperatures in the winter increases the survival time of E.coli but there is also a strong effect of water regulation for hydro-electric power generation that completely changes the water flow pattern to relatively higher flow during the wintertime compared to non regulated parts of the river. Both these effects increases the transport distance of E.coli. The study also shows the importance of considering the features of the nearby water body when planning for new settlements.