Dump on THEM
Unfortunately many slobs just ignore signs.
Most jurisdictions have a no-dumping ordinance which involve fines. Find out, and if yours does, make sure Grandfather will press charges.
Next, I'd add a No Dumping sign to the No Trespassing sign, preferably located where you can get a picture from cover of a vehicle entering which includes the signs.
Next I'd take a lot of pics of the cleaned up property, preferably with a camera that dates the pics. (The digital files contain the date & time created too; right-click/Properties).
Then I'd try to "ambush" & photograph a dumper passing the signs. Write down, date and time, the vehicle tag & description, and description of the passangers. Or ambush them as they leave, to avoid a confrontation. (Otherwise just have a shot of the signs, and try to photograph anyone you catch there).
Call 911 if they enter and park, and hope LEO arrive in time; if not, you will have all the evidence (the dated pics and their corresponding data files should nail them).
You might be able to get pics of them in the act (if you can do this safely/covertly), and their vehicle tag. (This is not something to get into an escalating physical confrontation over). Be careful, and do not display a weapon!
Anyway, you are going to have the before and after evidence, so this is not critical.
Take pics of the trash left, immediately after they leave, even if they get cited. (Extra evidence if it goes to court).
You may get them for dumping _and_ trespassing. Dumping is usually more serious. Trespass alone may not bring much of a punishment, if any: if you can show property damage (dumping, hacked trees), trespass will be taken more seriously.
Make sure the story gets into the local paper, but don't let them use your pic.
Good hunting!
C
