Not really related to your question but some experience with pumping lake
water...

My parents live on the lake and installed a water-source heat pump and
used lake water for the heat exchanger. The idea is great - exchange
heat with year round ~60 deg water rather than 90deg air in the summer and
30deg air in the winter.

The problem was that the pump would get clogged with silt fairly often
(even though there was rigging to keep the intake off the bottom of the
lake). In fact the pump had to be replaced about every 2 years!

That may have been an extreme case because I guess that pump is running
anytime the heat pump is running so maybe we just had more hours on the
pump then you will if you're just using it for irrigation or something.

Anyway, something to consider. When the heat pump needed to be replaced
he put in a standard (air cooled) high efficiency unit.

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Richard Thoms
Founder - Top Service Pros, Inc.
Connecting Homeowners and Local Service Professionals
http://www.TopServicePros.com