On December 9, 2011, the U.S. Environmental Protection Agency issued an agreed Clean Air Act Section 303 Order to the Detroit Water and Sewerage Department (DWSD) alleging that a pollution source is presenting an imminent and substantial endangerment to public health or welfare, or to the environment, for high levels of hydrogen sulfide (H2S) from an abandoned tunnel access shaft, the Detroit River Outfall (DRO-2) tunnel access shaft, a concrete extension about 34 feet in diameter and 12 feet above grade that has been used to contain groundwater. On September 20, 2011, using a Cavity Ringdown Spectrometer (CRDS), EPA measured ambient H2S concentrations in excess of 2,300 parts per billion (ppb) in the vicinity of the DRO-2 tunnel shaft. On October 5, 2011, EPA�s CRDS detected H2S levels in the vicinity of the shaft that were more than 12,100 ppb. EPA has provided the Agency for Toxic Substances and Disease Registry (ATSDR) with the CRDS monitoring data. After review of this data, ATSDR has advised EPA that exposure to the levels of H2S found on public areas near the vent pipe are an �urgent public health hazard� for even relatively short periods of exposure. In reaching this conclusion, ATSDR relied on the health-based screening level for acute exposures to H2S of 70 ppb. In compliance with EPA's CAA 303 Order, DWSD has extended the height of the DR)-2 shaft collar to prevent water from overflowing, covered the opening of the shaft, monitors water levels on a dail