Interesting. I'm considering some significant landscaping, adding an additional, walled "tier" to that back slope -- it already has one, lower down.

I wasn't aware that it could -- on its own, depending upon design -- provide significant filtration of such contaminants, although I am considering how it could also help divert rainwater to the side property lines, aiding its flow past the house to the front yard and sewer.

However, the best place for the garden -- best sun, and out of the way of other activity -- would still catch a lot of runoff.