If you really want to tap there take a sample of the groundwater (dig a hole, let it fill up and fill a one cup specimin jar) and the sap from one of the trees to a lab for a full spectrum chemical analysis. They'll be able to tell you if anything exceeds allowable limits for human consumption. If you want to be double certain, boil down a few gallons of sap into syrup and have it tested for full spectrum as well - contaminants will be concentrated when you make sap into syrup.

Is the bush uphill or downhill from the dump? Obviously, groundwater won't flow uphill. Another thing to consider is most of the roots for maples are shallow so the chances of them picking up contaminants from groundwater are low.