IDENTIFICATION OF THE SOURCES OF CHLORIDE FROM ROAD SALT AND TREATED WASTE WATER IN THE ILLINOIS RIVER USING HALIDES

The Illinois River is one of the main tributaries of the upper Mississippi River, draining a watershed of about 78,000 km2. The water quality of the Illinois River is heavily influenced by urban activities in its headwaters (particularly in the Chicago area), including road-salt affected runoff and groundwater discharge, and discharge of treated wastewater (TWW) directly into the river. Both TWW and road salt have led to increased levels of Cl- in the river which can be harmful to aquatic biota. In order to examine the fate of TWW and road salt runoff in the Illinois River Basin, samples were collected from the Illinois River and selected tributaries from Chicago to its confluence with the Mississippi River over a 2 year period. Samples of TWW, road salt runoff, precipitation, groundwater and tile drain water were also collected. Chloride concentrations in the Illinois River ranged from 40 to 488 mg/L; base flow Cl- concentrations in uncontaminated areas are expected to be °Ü 15 mg/L for groundwater and runoff. Chloride concentrations in river water spiked during the late winter and early spring as a result of road salt runoff, primarily in the Chicago region. A large component of Cl- in the Illinois River throughout the year was attributed to TWW from the Chicago area. Agricultural activities, which dominate land use in the watershed downstream from Chicago, imparted little Cl- to the river resulting in dilution of road salt and TWW. During periods of low flow, elevated Cl- levels were detected all the way to the confluence with the Mississippi River, demonstrating the role of TWW from Chicago in affecting Illinois River water quality when there is minimal dilution downstream.

Chloride concentrations in the Illinois River have also been increasing with time. The annual median concentration at Peoria increased from about 20 mg/L in 1946 to near 100 mg/L in 2005, and concentrations have increased throughout the year, suggesting increased concentrations in groundwater. While median Cl- concentrations are well below EPA's secondary standard for drinking water (250 mg/L), periodic spikes (maximum measured in 2003 was 904 mg/L) may be harmful to freshwater biota.