Meeting with government representatives and civil engineers in about 1999 over the future of Kai Tak, we asked about the nulllah and how it could be cleaned up. They offered an oxygenation system of water pumps for starters, which I believe they installed, but when it came to dredging they said no.
Apparently as well as the pig farms and dye factories of Diamond Hill and the catchment area for the nullah, there had been many early electronics factories who washed their effluent straight into the drains. Core samples had shown levels of Cadmium, amongst other noxious metals that were so far in excess of world standards for mud extract that there was no technology in the world that could process it. So they left it there.
We didn't get our GA airport anyway...