Re. "water going right back into the environment"...
While that is true as a simple statement, the net local effect is usually nothing so simple. Ground water systems are almost always autonomous and utility companies rarely (probably close to never) put water back into the same systems that they are taking from. Too much risk, plus there is usually a shorter route to the 'dumping' area than the distance to pipe it back to the source.
It is very possible to entirely drain a localities water source as the water does not go full circle (evaporation, redistribution) in a timely manner. That is happening right now in the City where I work due to an unusually dry summer and combined with city planners who are approving record numbers of building permits (mostly for multi-unit dwellings) while still trying to run the city on it's old single reservoir.
This can be accentuated by looking at people who use wells in their own back yards for heat pumps. Some heat pumps operate on a dual well system where they draw from one well, use it for home heating, then dump the water back into another well. Even though both wells might be in your back yard, it is very possible that both wells touch entirely different underground water systems. It is possible to entirely drain your input well dry, even though you are dumping water back into the ground only 100' away.
I'm not preaching conservation here as I don't know your local situation. I just wanted to share a very tiny bit of what I've been learning lately about water tables.
Actually this can be true depending on your local situation Murph.
A city that draws water from a groundwater source (although alot less common b/c it would have to be one amazingly large and fast recharging aquifer for a large city), the water return would be to a surface source.
The key for such cities/houses is to know what their recharge rate is and not to exceed it, or even more conservatively, not exceed say 75% of its recharge rate. Many systems have not accounted for this rate (e.g. a typical farmer's plot out in rural areas) and just start drawing water, digging down deeper when the well runs dry after x many years.
In our city case, they draw from a lake and it goes back into a river which all follows the same watershed path. The water is simply taking a diverted course, so to speak.
Do any of the PEI towns/cities use desalination of ocean water for a source?
It has its inherit problems but would provide a lesser concern about drawing down freshwater aquifers on the island.
In re: to heat pumps, are you referring to the heat pump geothermal systems to heat a house?
If so, i've never heard of anyone using water in those systems (maybe this is an older design?). Typically it is propylene glycol in a sealed loop that runs through the earth under the frost line. Some friends of ours have a system that runs along a lake bottom using the lake water instead of earth as the moderating heat/cooling source.
