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Water Quality

IDOT maintains approximately 60,000 bridges over streams and rivers. During the construction or rehabilitation of bridge structures, IDOT must comply with the Clean Water Act, the National Environmental Policy Act (NEPA), the Fish and Wildlife Coordination Act, and other related laws and regulations. To comply with these regulations IDOT conducts environmental surveys. These field surveys range from the identification of federal and state listed endangered and threatened species to the characterization of the physical (substrate), biological (fish, mussels, aquatic macroinvertebrates), and chemical (concentrations of heavy metals, chloride, dissolved oxygen) components of streams. The results of these surveys are used to obtain permits (section 404 and 401 of the Clean Water Act) from the Corps of Engineers, Illinois Environmental Protection Agency (IEPA) and/or to meet the requirements of NEPA.

IDOT looks at rivers on the National Wild and Scenic River System and the Nationwide Rivers Inventory and coordination may occur for work on these streams with the National Park Service. IDOT uses IEPA Illinois Integrated Water Quality Report and Section 303(d) List to determine streams or lakes water quality standards, the designated uses of these water bodies, and the IEPA assessment of these uses.

Highways may impact water quality through storm water runoff. IDOT is involved in activities to control and reduce contamination from highway runoff (see Storm Water Management Program on this website). Deicing materials used for snow and ice removal flow into area streams via roadside ditches and/or storm sewers. In addition, a portion of road salt that remains on the pavement may be dispersed into the air due to the motion of traffic. In order to analyze atmospheric dispersal of salt, IDOT, in conjunction with the Illinois State Tollway Authority and the Federal Highway Administration initiated studies that were performed by the Illinois Water Survey.

The 1987 Illinois Groundwater Protection Act was implemented to protect groundwater resources from degradation. The Act allows for the establishment of both regulated groundwater recharge areas and groundwater protection areas. For transportation projects groundwater is assessed in terms of recharge areas to identify the potential for proposed highway corridors to contaminate shallow aquifers. Groundwater studies using monitoring wells are often performed in areas that are considered sensitive because they support plant communities containing endangered or threatened species or land areas that have been designated as Natural Areas or Nature Preserves. Potential impacts to these resources can involve interruption and/or change in groundwater flow patterns. Areas featuring karst topography are especially subject to groundwater contamination because these areas do not contain the usual filtering layers provided by soil so that surface water contamination can flow directly into the groundwater.

An aquifer is an area containing saturated (with groundwater) soils and geologic materials that are sufficiently permeable to readily yield economically useful quantities of water to wells, springs, or streams under ordinary hydraulic gradients. A sole-source aquifer is one which supplies at least 50 percent of the drinking water consumed in the overlying area with no practicable alternative drinking water source. While Illinois has no designated sole-source aquifer at the present time, it has many shallow (sand and gravel) and deep (bedrock) aquifers. Public well water supplies are regulated through set back zones established by IEPA.

Every two years the IEPA issues an “Illinois Integrated Water Quality Report and Section 303(d) List”. Information on groundwater standards and groundwater quality can be found in this report.

Floodplains are areas of land that could be inundated by floodwaters from any source. Floodplains usually occur adjacent to streams, rivers, and lakes. Transportation projects are assessed for the potential for impacts to floodplains, more specifically encroachments into the 100-year floodplain. The 100-year floodplain is defined as an area that has a 1 percent or 1 chance in 100 of experiencing a flood condition that exceeds any previous 1 year period. Coordination with the U.S. Army Corp of Engineers and the Department of Natural Resources Office of Water Resources (IDNR-OWR) is required to obtain the necessary permits for work within floodplains. A regulated floodway is the channel of the stream plus any adjacent floodplain areas. These areas must be kept free of encroachment so that the 1 percent chance annual flood can occur without substantial increases in flood heights. Depending upon the location of the regulated floodplain, the regulatory authority may be the community within which the floodway is located, FEMA or IDNR-OWR. Bridge structures located over major rivers such as the Mississippi must be coordinated with the U.S. Coast Guard to ensure that pier placement does not affect shipping channels.