I live in rural Iowa and have been thinking about adding geothermal.  We are adding drainage tile to a tillable field that is 50 feet from my house.  The trench is 6-9 feet deep.  The trench is over 800 feet long.   Do you see any reason why I couldn't add horizontal wells in the trenches.  My house is in the middle so I plan on running 2 4-pipe trenches (header is in the basement) from the house to the drainage ditch (this will be filled in when I am finished).  Then where I intersect the drainage trench, I could send 2 loops in each direction.  In advice is appreciated.

The only thing that comes to mind is- is there a consequence if you freeze the drain? If the answer is yes, then I might look for a different plan.

To be sure, there would be that concern about freezing the drain in some circumstances. However, if there typically is ground water flowing into the drain all during heating season, however slowly, that flow constitutes a heat source, much like the use of "bleed" in a standing column well design for heat recovery. I suppose ocho's answer depends on how continuously the drain is expected to flow.