it can be done. it just won't work very well.
to just dump an air-style heat exchanger in the ground (low thermal conductivity), you wind up exhausting the available heat around the heat exchanger pretty fast. you also wind up with corrosion problems, etc.
placing an air heat exchanger in river water doesn't solve the corrosion problem, but the heat exchange might work a little better, until the fins filled with ice.
(no joke; I've seen fully iced-up fins in foggy 60'F weather) flow velocity through the fins is critical.
if you wanna do the earth-source thing, you're better off getting a LONG line, and making a big loop out in the yard someplace.
for water use, you'd do better to get a purpose-built heat exchanger and pump water through it at a sufficiently high rate to prevent freezing. you'd also have to keep the pump running at all times in a climate that has freezing air temps for more than an hour in any given day. . . so the pump would probably eat up a large portion of your potential energy savings.
I'd say that by the time you were done with the conversion, you might be money and agravation ahead to just go ahead and get a commercially engineered soil-source unit.
-Dan