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Unfortunately you cannot "opt out" of this matter. These administrative matters tend to "mushroom" rather than shrink.
I grew up in, and work in, agricultural communities and I understand the importance of having access to functioning well water. I would highly recommend making a cost/benefit analysis here: how much is it going to cost you to be without water (and or pay the fine - depending on which way the DNR is headed here), then compare that to the cost of hiring a local ag lawyer (look for an attorney that actually understands agricultural law - you may have to search around a bit, don't settle for a general practitioner that has to bring themselves up to speed on your dime).
You can usually push back against these "fishing expeditions" but in my experience you have to be aggressive up front, it is very hard to do so on the back end (once the administrators have made their findings that your well or water usage is at issue, it is very hard to disprove that finding - you are then fighting an uphill battle).
While it appears you are already in a good position (they made a finding against the 4 neighboring farms), anticipate that costs may eventually include additional testing (if you have to disprove something the DNR finds later).
Again, this is all cost/benefit analysis - don't spend more than you are going to benefit - but do understand these findings are hard to change later on.
I truly do wish you the best of luck with this matter.