Ball Park is on old garbage dump( says this guy that was born and raised here back in the 1950's) but any leaching from this site has probably occurred decades ago if it was going to happen. Best to not disturb existing site to prevent exposure of other unknown products. Good news is - back then it was mostly metal, glass, paper, and organic matter. Plastic wasn't common back in the good old days.
Two more likely sources are - -
Revelstoke Dam. Those monster turbines don't just run water through them, they also have bearings and seals that aren't perfect and constantly leak oil. Ask BC Hydro how much oil they use at the Dam and you'll appreciate how much leakage there is into the river.
Downie Sawmill - -Piles of Muck (aka - rotting bark and wood) produces leachate that will float on the water and look like oil ( because it is oil from the trees) Check out the channel leading to the pumping station on the Illecillewaet Dyke - black muck - leachate - All from Downie - very toxic to fish and all aquatic environment. Check out the mess of wood and bark next to the Greenway. All leaching into the reservoir and river.