Reedmace / Bulrush as a filter

[LawtonWoods]

I have 2 ponds at the back of the house. The larger is about 40m x 40m, and is an old marl pit. It is within some ancient woodland, and is surrounded by trees. I am currently engaged in a project to improve it for wildlife. It is fed from groundwater – maintaining its level through the year, but with no obvious or external source.
The smaller pond is approx 8m x 8m, and was originally dug, I think, as a ‘sump’ for a highway surface water drain, which feeds into the area where the pond empties into a ditch / stream. I cleared the pond last year, on the recommendation of the highway people, as I was experiencing water-logging in that area of my garden. This has worked, with the added benefit that almost immediately some of the old pond plants returned. However, one of the problems I have is that when it rains heavily, the pond becomes contaminated – firstly with fine silt, which covers everything, and secondly with an oily scum.
Someone has suggested planting bulrush / reedmace across the pond, to form a channel such that in ordinary rainy weather, the highway water empties straight into the ditch, whilst in heavy rain, the excess water can still find its way into the pond, providing the required sump. The reedmace will also act as a biological filter, clearing the water which is in the pond.
I’m happy to do this, but don’t want to upset the ecological applecart. I suspect that the reedmace will quickly seed into the larger pond, and this is deep, as well as being wide. If it colonises areas beyond my reach, I will not be able to remove it easily. The ditch which drains the ponds feeds into a semi-natural ‘lake’ of about 10 acres, about 400m from the ponds. There is no reedmace in this lake currently, and I also suspect that seed will find its way into this water body, and establish itself there.
The advice was given by someone from one of the county ‘wildlife trusts’, so should be good, but has anyone else any comments or advice. If the advice is not to do it, are there any alternatives?