The impact of a Sewage Treatment Works on phosphate levels in a chalk stream

I’ve been making charts of phosphate readings on chalk streams to try and get some sense of the comparative scale of point-source to diffuse pollution on different chalk streams. The Misbourne tells a clear story of this relative impact in a very populated part of the country. On other rivers, especially the chalk streams protected by designation, the picture is more nuanced. But the charts below speak for themselves: the sewage treatment works at Gerrard’s Cross on the River Misbourne (a chalk stream which also suffers from low flows) does not (to my knowledge) feature a phosphate stripper.

Not far upstream of the sewage works the EA Assessment point data shows consistently low phosphate readings, while downstream of the STW the readings are very high.

There are also some clear, and revealing patterns. Upstream the peaks, such as they are, clearly coincide with high flows in the mid-winter, whereas downstream the peaks coincide with low flows in late summer, indicating that the primary source of phosphate upstream is from diffuse, landscape sources while downstream the primary source is quite obviously the sewage treatment works.

The phosphate concentrations go up and down in inverse relationship to each other. At the upstream assessment point increasing flow leads to increasing amounts and concentration of phosphate, whereas downstream increasing flow decreases phosphate concentrations through the effect of dilution.

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