We did a lot of research before we put in our geothermal system. We went with two 300-foot sealed wells.
Using the pond is not really advantageous for a number of reasons. One is that during the summer, your pond water is warm. In the winter your pond water is cold -- just the opposite of what you want. It may be better than a standard heat pump working with outside air temperatures, but you lose the efficiency of the earth's rather large heat sinking characteristics. Also, there apparently is a tendency for the pond elements to freeze up during winter, thus trying to float up to the top, and damaging them through stress.
We've had our system for a little less than a year. I'm impressed. Generally, our electric bill this time of year would be around $200-$250 per month. It is running about $65 per month. Our summer propane bill has been cut considerably also, as the geothermal system provides most of our hot water in the process. The propane usage was really cut during the winter heating season.
By the way, wells for geothermal are not anywhere near as expensive as water wells. The biggest difference is that no casing is needed. Our two 300 foot wells were drilled and plumbed within about six hours of the time the well drilling truck backed into the yard. They came back the next day and filled the holes with bentonite. I don't remember the cost, but it was under $2000 for the drilling.
