I do not know if your part of the world uses bug sprays and weed killers...if so, there is a good chance the park uses those items as well...and rinsing will not get the poisons "out of" the plant....
You can try to find the supervisor/manager of your area's park and recreation department, call them and ask if they've sprayed the area you want to harvest from. I just use water from the tap to rinse off my veggies and greens.
I wouldn't. If you are not sure that its safe, its not worth the risk.
And I wouldn't take the park managers word for it either. I asked the landscaping manager at my mother in laws condo complex if they sprayed any pesticides or herbicides and he told me no with a smile. I even explained that I was going to feed it to animals and they could die if there were chemicals on them and he said no, because of people's pets. Days later I saw one of his guys walking around with one of those backpack sprayers, spraying chemicals on all the weeds. Either the manager doesn't know what his guys are doing, or he lied. Either way it could have killed my herds had I fed that stuff to them.