In the last few days, I have had reason to visit two highway outfalls. One is in Lancashire and discharges into a small stream which feeds a small ornamental lake. There is evidence of extensive pollution, including oily sediment along the banks of the stream and an oily film across the lake. This outfall receives no treatment and drains from a very busy stretch of the M6. This pollution has gone on for decades and there is no plan in place to install a treatment scheme anytime soon. But if we were to see a plan from National Highways to treat this pollution, what sort of treatment scheme would be appropriate? Maybe a large sediment separator and then a wetland located upstream of the lake to protect it from pollution.
The second site that I visited has just such a treatment scheme installed; two large built-in-situ sediment traps and then two treatment reedbeds have been installed to treat the runoff from the A38 into a lake in Stover Country Park. They cost £2.9million and they look impressive. Improvements in the quality of water in the lake has been observed and when I was on site there was a wide selection of waterfowl and more individuals of some species than have been there in previous years. However, there has been only limited monitoring of the water quality data to measure the effect of the new treatment scheme. We have no idea how much pollution the sediment traps and reedbeds are capturing. Nor do we know how long they will work for and when they will need maintenance.
The image at the top of this page shows one of the sediment traps at the site in Exeter.
So highway outfalls must have treatment in place to prevent pollution. But we must also measure the effectiveness of expensive treatment schemes to see if they are working, and then maintain them properly so that they can continue to work for decades, and to deliver improvements in river health.
This is the image of the outfalls from the Motorway in Lancashire:
