Let me first say, I'm no where near qulaified to answer this question. I assume you want to use lake water for your geothermal system, that is draw water in from the lake for the geothemal system and discharge it into the same lake. Instead of running a closed loop line into the lake. That's the only way I could see the geothermal system and Sprinkler system being on the same line, the same water lines that feeds the geothermal system would be pressurizing the Splinkler system.
1. Can't say I've even heard of a Pond being used for an open loop system, usually systems that utilize ponds are closed loop systems. I would be interested to know what kind of filtering method would be used to keep impurities from getting sucked into the intake line, as well as keep them from clogging. I hate to imagine the expanse of having to hire divers in the middle of winter to dive in 10 feet of water to clear a clogged intake pipe.
2. Is untreated lake water safe to drink? I for one wouldn't want untreated water from sprinklers raining down on me. Not that Sprlinkler water is all the sterile to begin with. The water in pressurize sprinkler pipes sits for years collecting rust in the water, but once the lines are flushed out after a minute or so, the water is clean enough after that.
3. What are you really saving here? You still have to pipe drinkable water into the facility for the sinks and showers. Are you saving all that much be not using city water to pressurize the sprinkler system?