Say one uses a SCW or a SCW/open loop hybrid and say it averages 25% bleed.   Ie, 75% of the water goes back into the well and 25% is dumped.   Does this eliminate 75% of the water quality problems?  It would seem that after the water has been through the system once, there should be some reduction in hardness and reusing it again should leave less hardness available to precipitate out.  Or is the amount that precipitates out so small that the hardness doesn't change and stays supersaturated?

If this were true, it would make cost effective open loop systems available to more people.

Also, has anyone seen information on open loop water quality requirements for heating only systems?  In theory, hardness precipitating out is caused primarily by heating of the water.  Cooling the well water (like a geothermal system does in Winter) increases the water's ability to keep hardness dissolved (ie, precipitation is unlikely).   So in climates where a geothermal system could be used only for heating (use an air source AC), water quality should be much less of an issue. I would guess that iron bacteria growth is also less of a problem at low temperatures.

I don't think water is significantly softened by a trip thru the coax. IMO the amount of scale buildup it takes to cause a problem is dwarfed by the amount of minerals available for potential scaling in the quantities of water used in geo

That could be - although the other argument would be that it takes very little to bump it from slightly supersaturated to slightly unsaturated. In other words, perhaps once it goes a hair below saturation, all deposits stop - even though the hardness may still be quite high.

How about this? With a typical SCW doing no bleed (5% only when demand is high), it is feasible (ie, doesn't cost much) to just soften the water (ie, remove the hardness). Ground water flow would cause some minor replacement of the water.

That might work if there was little to no underground water movement.

I would have guessed that solubility would decrease as water temp decreases. For nearly any substance heating the solvent increases solubility. Why would it be different for water hardness?

Don't know, but it definitely is.

"In practice, water with an LSI between -0.5 and +0.5 will not display enhanced mineral dissolving or scale forming properties. Water with an LSI below -0.5 tends to exhibit noticeably increased dissolving abilities while water with an LSI above +0.5 tends to exhibit noticeably increased scale forming properties".

"It is also worth noting that the Langelier Saturation Index is temperature sensitive. The LSI becomes more positive as the water temperature increases. This has particular implications in situations where well water is used. The temperature of the water when it first exits the well is often significantly lower than the temperature inside the building served by the well or at the laboratory where the LSI measurement is made. This increase in temperature can cause scaling, especially in cases such as hot water heaters."

Now you've got me googling. I found a LSI calculator online. I'm familiar with 3 of the measurements (temp, pH and hardness) but not the alkalinity measurement. Not sure how that is measured but hopefully it's not hard to find someone who can do that.

I played around with the calculator and it looks like you can overcome high hardness and get a balanced LSI by increasing alkalinity. Seems like a simple injector to increase alkalinity might solve a lot of scaling problems. I suppose on an open loop pump and dump that could get pricy but perhaps on a SCW it would take very little to balance out the LSI?

Agreed, treatments other than softening (even hardness sequestering)  are worth considering.  Some treatments are at the 1 quart per 10,000 gallons rate (sounds affordable but I don't have details).

The cost of closed loop systems (to avoid corrosion) keeps many people from adopting geothermal.

Pool and spa companies sell test kits that include alkalinity tests. pH is also easy to change.