Brettski:

My understanding is that surface discharging includes any system that discharges ANY effluent to the ground's surface. The top of the following links states 140K units; but article is dated 2004. The IEPA rep stated 150K last week. I have heard the 150K number from other IDPH and IEPA reps in the past year.

http://www.aces.uiuc.edu/news/stories/news2956.html

This is more interesting reading that the public notices.

http://www.epa.state.il.us/water/conservation-2000/lake-notes/septic-systems/septic-systems.pdf

http://www.epa.gov/safewater/protect/pdfs/septic.pdf

As Mark pointed out, illnesses related to septic systems are rare. Very rare. If memory serves, at a meeting I attended a couple years ago IEPA and the Prairie Rivers Network were challenged to provide one documented case of a fatality or serious illness and did not. My understanding is that the City of Chicago is occasionally allowed to release untreated sewage into a river up there. I don't know if that is true, but seems like I heard it from some health official.

All that said, I'm not about to drink out of one of those systems which discharges a "clean" effluent like I've heard some claim you can and I'm not about to eat fish caught out of that Chicago river. ;\)

Pragmatically, we should probably all worry more about that extra Twinkie we eat and the exercise we don't get than the septic system discharges.

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