Since geothemal appears to be my area of expertice, I'll do my best to answer your questions. My particular installation is 5 units servicing an 8000 sqft home. It has been quite the experience over the last few years.
Pumping raw pond water to service the geo units will not work. If you use aeration to keep the pond circulating, the average temperature is in the 65 to 70F range. This water feed will do a poor job in cooling during the summer. There is not enough temperture difference to make it work.
You also have to consider the fouling of the coils. I use my well pump becuse it's clean filtered water, which I had to do anyway for the home. If the geo coils get fouled, you have a big problem. It involves flushing a strong acid and all sort of nasty things through the system.
At the end of the day, it was more cost effective to use the current well system to power the geo units than to set up a stand alone system to supply geo water. Double the equipment, double the cost!
