The City of Ann Arbor has an FAQ on the Gelman dioxane plume heading towards the Huron River:
1) Where is the groundwater contamination?
The contaminated groundwater exists in several aquifers beneath the west side of Ann Arbor and Scio Township. For a map of the groundwater contamination, see www.srsw.org
2) What is the extent of the groundwater contamination?
Several aquifers have been contaminated. The unit E aquifer represents the largest local water body known to be contaminated with 1,4 dioxane. The contamination of this aquifer is known to extend east and north from the PLS Wagner Road facility, beneath the city of Ann Arbor. As of Jan 2004, the leading edge of the plume is thought to extend just to the east of the municipal drinking water well [closed] on Montgomery Street. The approximate dimensions of the 'plume', or contaminated area, are: 8500 feet long, 2000 feet wide.
The map, and a bunch of supporting information about the history of this pollution, is provided by Scio Residents for Safe Water.