Go to the town and look up your property deed and make sure that she does not have an easement on their for the well. The previous owner of your property may have given her permission to drill there, but if it;s not written into the deed then there's no proving it and it HAS TO GO!
I would send her a certified letter detailing (with pics) the surveyors report on lines and her trash being on your property, along with her garden AND well. If there is no easement in the deed, I would give her 30 days to Relocate the well, remove the trash and vacate the garden. (you cant take a chance on her getting hurt in the garden on YOUR property). Let her know that whatever course of action she is going to take, you NEED copies of everyone's INSURANCE BEFORE they are allowed on YOUR property to remove anything. An approved schedule of events to include proposed actions on removing trash, where it will be disposed of, how the land will be restored, how the approx 100' of well casing will be removed from the well and filled in, needs to be submitted so that people dont show up at 2am with equipment and workers.
Also let her know that you are truely sorry that the relationship has broken down to this point and you wish that it didnt have to come to this. You understand her concerns and although you made every attempt to accommodate her, it was the "going to the town" that took the issue out of your hands and are now FORCED to do things legally. During the course of trying to accomodate her you found the well, trash and garden is on your property and you are now forced to remove her from your property. She has 30 days to comply or you WILL remove the trash AND the well AND take her to court for the cost of everything, to INCLUDE your time for coordinating this and the surveyor to reaffirm the lines as well as restoring the land in the garden to it's original condition. Also, this cert'd letter serves as formal and final notice that nobody is allowed on YOUR property in the garden, or to address these written demands, WITHOUT formal written acceptance of her plan and schedule of events. I would also send a copy to the local health dept or whoever contacted you before, so they are aware of the situation and will ignore or dismiss any future calls from her.
This will send her over the edge but, the reality IS, that you really HAVE NO choice now. You have to do this legally now. She raised the bar and now you have to finish it. In the end, you "may" go back to being nice-nice but for now you HAVE to protect yourself.
As a side note, I would get a cost on removing the well and keep it. A new pump will run you about $300 and you'll have free water. Tell her the cost of removing the well is what the "rent" was for your land all this time.
Let us know how you make out