Mapped: Aerial imagery shows M25 runoff ponds have not been maintained for decades | New Civil Engineer

Not only are they likely full of sediment, they are probably overfull preventing the ponds from working. Sediments in the water can now run into streams, the water also has less time to cool (the roads heat rain runoff) and that warm water flowing into streams can disrupt the ecosystem as well.

The fact the ponds are full of sediment also reduces their ability to store water in the event of flooding conditions.