Combustion is imperfect and it is occurring over metal which will erode into the mix of air and water. Then there is also whatever chemicals and material are on the ground prior to launch activities that get washed away with the deluge system

> Then there is also whatever chemicals and material are on the ground prior to launch activities that get washed away

Washed away after first being superheated by that water.

I would imagine most of the wash away is not close to the exhaust, but down slope, mainly consisting of construction, maintenance, and vehicle activities.

Though the area does receive many orders of magnitude more water from natural rainfall than the deluge system, so anything present in that water would have been washed away anyway.

Not according to the tests SpaceX submitted themselves. But if it were you would first show that to the EPA and then they would give you a permit to go ahead.