I searched and found lots of points and counterpoints on this but I'm not sure what's the best solution to ask for in my closed-loop system. I'll have a well in a neighborhood of wells so groundwater is a big concern. I think the installer said he uses a diluted methanol solution. Is that standard practice or should I ask for something else? Thanks and sorry if I'm reviving a dead topic.

Methanol is cheaper and has slightly better performance in terms of viscosity and heat transfer than the "environmentally-friendly" alternatives with the most common being propylene glycol. If you are below the water table you may want to consider propylene glycol. Here in PA, Centre County Council of Governments (COG) is working on a geothermal boring ordinance but so far it only deals with grouting methods and not the antifreeze.

-Adam

I take it the antifreeze is different from the stuff you put in your car. That stuff seems not so environmentally friendly. Some previous threads said that methanol evaporates quickly making it safe to use. Does that make sense at all?

Studies have shown that methanol degrades very quickly when exposed to water or air. If you start with a solution that is 20% or less to start with, the risk is very low. Local codes for your locality should be respected as not to run amuck with any inspectors. Eric Sackett weberwelldrilling.com

I've heard of tap water burning out the pump in certain situations. Should I ask for distilled water or anything?

If the leak is below ground it is not going to evaporate. It will begin to biodegrade as bacteria slowly eat it. Methanol does evaporate rapidly in air. However, it also causes blindness with just a few drops if accidently consumed (breath or drink). Ethylene glycol is the typical old style auto antifreeze and it works just fine. Propylene glycol is the newer environmentally friendly one (you can actually consume it) but is slightly less effective and more expensive - but like I said, you can drink the stuff.

No, you don't need to request distilled water. Regular tap water is fine. In fact this is the first I have ever heard of anyone saying they had heard of tap water burning out a pump made to pump water.

On site will be well water with a fair amount of sulfur. But those in the area with wells don't have premature pump problems. Here is the article I got that from. Not sure if it's legit:

http://hvacrdistributionbusiness.com/mag/increase-profits-warranty-0409/index1.html

If the water meets drinking water standards (i.e. is tap water) then it is not going to cause a problem. Yes you could use river water or surficial aquifer water and cause a problem (most likely by it having too much particulate matter) but why would you do that? Good old drinking water (i.e. tap water) is absolutely fine for this application.

This would not necessarily be the case for wet/evaporative cooling systems whereby you might encourage biuld up but even on those units I don't think they typically use distilled water.

In the fine print, many of the geo glycol manufacturers want you to use mixing water that is <100 ppm total hardness which is reasonably soft water.  Distilled water can have its own problems since the pH can be pretty low. 

As long as the water doesn't have a really high hardness, it's probably not a problem.

-Adam