Vinzer, most groundhog hunters keep their spots a little secret, I'm the same and will try to explain why. Groundhogs are really not as plentyful as gophers in the west. Don't expect to shoot 10 a day let alone 40 or 50. It's more of a hunt then a shoot. What you have to do is drive out of the city and get to farmland in areas where discharging a firearm is legal, in other areas a bow is fine and quite a lot of fun also. Just ask politely for permission. My experience is that most farmers will be willing to grant you access. You must show them that you are responsable, ask about where livestock is, buildings that may not be visable, hours that people work the fields. I always explain that I hunt alone or with my son, never show up with ten buddies and 10,000 rounds of ammo, you'll just blow it for everyone. I also ask about where to park my Jeep and if they allow trucks and ATV's on their land. Some do and some don't. Always keep their land clean and pick up not only your waste but anything else that you may come across like bailling wire. Always be prepared to give a helping hand if you notice that the need arises. Take 5 mins to engage in some small talk and a thankyou card with a bottle of wine or a basket of fine foods at Christmas is always appreciated. Remember, you may be 5kms from the farmstead but it's still their backyard, act as any guest would.
Next up is saftey. Odds are you will be sharing this land with others, kids playing in the fields, ATV's, hikers and birdwatchers and other homes in the area. Plan out your shooting angles and never give anyone the chance to complain. You don't have to deal with the upset neibours, the farmer has to.
Now that you have access take the time to walk about and spot where the hogs live, most farmers will direct you to them. Don't worry too much about getting one on the first day out. Just do a good recon of the area and it will help you plan you're next move. Hay fields are great as they are not plowed every year, the hogs dig thier homes and have some peace and quiet for a few years. Alfalfa and soy get a good plowing often and the hogs tend to give up and move to the ditches and wooded areas nearby. Railway lines are fantastic and full of hogs. They burrow into the built up sides and often sun themselves on the gravel, just be careful for ricochets. Don't leave dead animals littering around, my experience is that farmers are fanatical about keeping their land clean. Haul the carcases away( bring some bags) and dispose of them later.
Other things I have learned over the years:
Most farmers hunt big game. I ALWAYS explain that I have other areas for big game and I never walk their land during those seasons.
I have made buisness cards with my personal info to hand out. They then know where to contact me if the need arises.....like say another farmer asks if I can take care of their groundhogs also (Hint Hint)
Most farmer will also give you permission to hunt predators in the winter like coyotes and foxes. Some may give you the OK for migratory birds.
Good luck! Enjoy the hunt and things will come together for you. I've secured well over 2000 acres of farmland to hunt in just a few years. Most of it within 5 minutes of home. Granted I live in a rural area and you will no doubt have to travel a little further but it will be well worth it. You will see Deer and other critters, birds of prey and scenery that will blow your mind.
Bonne chance mon ami!