I’ve been putting together a few more graphs to try to understand a bit more about the relative impact of small sewage treatment works on Cinderella chalk streams: by that I mean the chalk streams not protected by either SAC or SSSI designation or the Urban Waste Water Treatment Directive.
The first chart shows the River Stiffkey, a small stream in my home county of Norfolk.
This data is taken from EA monthly readings, although as you can see the readings are not at all monthly (each vertical red or blue line is a reading / the tracker line is the rolling average). They are inconsistent and the data series vanishes here and there, which is a great shame. Nevertheless a picture emerges.
The chart for the upper River Stiffkey compares readings in the upper river at Great Snoring, with readings a few miles downstream at Wighton. Between the two Assessment Points there are four small STWs – Little Snoring, East Barsham, Houghton St Giles and Great Walsingham.
The Great Snoring data is inconsistent, but it is nevertheless clear that upstream readings are considerably lower and that the difference is more obviously attributable to P discharges from the STWs: the upstream spikes lag behind those downstream and coincide with winter rain. The downstream spikes rise through the summer when the difference between the two is at its greatest. A slight exception is late 2007, a very wet early autumn which followed a dry winter, which could explain high late summer levels from point-source discharges, immediately followed by high early autumn levels driven by diffuse run-off. Unfortunately the Great Snoring readings stop mid-winter 2007 and so the blue line tails off when it might actually have continued to be high.
The cumulative impact of four small sewage treatment works, none large enough to merit the investment of phosphate stripping, is only too obvious.