> Or do they discharge their effluence into the lake?

Given this is the Netherlands and this appears to be a wealthy area, I'm quite sure they don't. Guessing they either have septic tanks or holding tanks that need to be pumped regularly.

I have a degree which allows me to work in few fields, but wastewater management is one of those fields (which I have worked in, front-line (directly) >gross<): no water is "clean" — it's varying shades of "acceptable"

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There is a running joke (hundred+ years now) that the famous "Ruby Falls" on Lookout Mountain is in fact better named "Poopy Falls" – since the primary water source are the few hundred septic drainlines higher-up the mountain's-top.

<https://en.wikipedia.org/wiki/Ruby_Falls>

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If you have any open cuts/sores, do NOT swim in any outdoor water.