The impact of river restoration on the diurnal and seasonal surface water quality changes in the Thur catchment in Switzerland

The importance of river restoration projects can only be realized upon evaluating their success or failure in a region mainly with regards to water quality, ecological adaptations and flood mitigation. The lack of temporal data in many restoration projects demand a spatial analysis of water quality parameters between the restored and unrestored parts of the river. Further, in large catchments there is a lack of water quality data in high frequency which are needed for the understanding of stream biogeochemical processes. This study addresses these challenges.The Thur catchment (1696 km²) in North eastern Switzerland is the chosen study area. The water quality along the lower part of the river reach has been analyzed with regard to the existing land use and a comparison is made with the water quality in the restored river sections of the river. A restored river section at Niederneunforn has been heavily monitored as part of the RECORD project (http://www.cces.ethz.ch/projects/nature/Record) and this data is vital for the present work. The water quality changes are to be observed by relating to some of the basic parameters like temperature, pH, alkalinity, electrical conductivity (EC), dissolved oxygen (DO), dissolved organic carbon (DOC), the concentration of ions like chloride, nitrate, nitrite, sulphate, ortho-phosphate, ammonium, magnesium, potassium and calcium (Ca). The diurnal and seasonal water quality changes are observed by conducting 24 hour sampling campaigns in summer (average flow 25 m³/s) and in winter (average flow 50 m³/s). Five monitoring stations are chosen in the lower part of the river which are situated two upstream of the monitored restored section and two downstream of it. The samples are collected at each station at an hourly interval for 24 hours using auto samplers. A new method of sampling is developed by calculating the average velocity of the river between the stations using a 1D hydraulic model (HEC-RAS) with the forecasted discharge on the day of sampling. This is done to sample at the time taken by the flow of a particular patch of water to cross each of the stations. Thereby, observing the changes in the water quality parameters in this patch of water as the local factors in each station will be the major influence on its water quality. In winter, diurnal oscillations of pH (nearly 2% increase in a day) and DO (increases from 12-19% in a day in each station) are observed. Already in winter, the DO concentration increased with increase in water temperature showing there is an influence of biological activity in the river. In winter, the diurnal oscillations in EC was not observed however this is expected in summer at low flow conditions. Further, the good correlation between the EC and Ca concentration in each of the stations gives a clear indication that the lower part of the river is a calcite dominated system. The five monitoring stations are interspersed with six waste water treatment plants of varying capacities. These are concluded to have a major influence on the changes in the water quality parameters due to the lateral inputs that occur in the vicinity of each station during the day.