Summary
In order to better document the limestone powder dissolution in rivers NIVA suggested to calculateannual limestone-related flux of calcium (Ca) in representative salmon rivers where different limestone powder products have been used. In this project we used water chemistry and water flow data from six rivers for the 4-year period 2012-2015 to estimate the annual limestone related Ca flux and compared these data with the annual consumption of Ca in the lime dosers. Ca data were collected from unlimed but also lime influenced reference stations and from the main station in the lowermost part of each river. Emphasis was put on getting representative background Ca-concentrations. These were subtracted from the measured and sea salt corrected Ca concentrations in the target part of the river. Together with the sea salt correction, acid rain dependent time trends (decrease) in Ca and the general increase in Ca downstream in rivers were corrected for. The largest uncertainties are probably related to this last factor, but estimates based on long term data (from 1980 until onset of liming) for the two already mentioned rivers showed that the selected factors were sound. All estimates showed annual dissolution rates of around 100 % of added Ca. This number applies both for the most finely ground limestone powder (0-0.02 mm; 50 % < 0.005 mm) and for the somewhat courser powders (0-0.1 mm; 50 % < 0.02 mm). The finely ground powder probably dissolves quickly, regardless of resuspension, mechanical tear and further dissolution, whereas the courser ground powders are dependent on these processes to become fully dissolved over time. With only one lime doser and no pH regulation downstream there are limited possibilities to take long term dissolution into consideration. A pH regulated doser will adjust the doses based on the total limestone dissolution upstream. It may therefore be assumed that 100 % dissolution may be accounted for in large rivers with several dosers if the lowermost is pH regulated. Particles of the somewhat courser powders sediment during low flow periods, dissolves and may produce higher than target pH. However, in periods of increasing flow resuspension and further dissolution may serve as a buffer and protect against rapid pH decrease. This should be considered when choosing limestone powder qualities for river liming.