Influence of the Chicago Metropolitan Area on Elevated CO2 and CH4 Concentrations in the Upper Illinois River Waterway

Streams and rivers receive organic and inorganic carbon (C) from their surrounding landscapes and are sites of active biogeochemical processing leading to variations in carbon dioxide (CO2) and methane (CH4) concentrations. Supersaturation of these gases with respect to the atmosphere results in their emission across the water-air interface. Estimates of inland water carbon fluxes for North America identify CO2 emission across water surfaces as the dominant flux pathway. It is therefore important to quantify the variability of these gas concentrations and fluxes, but currently there are large uncertainties in CO2 and CH4 emission estimates due to spare gas concentration measurements and large variation at a range of spatial and temporal scales. The United States Geological Survey (USGS) is monitoring CO2 and CH4 concentrations in the Illinois River waterway using multiple approaches. Continuous measurements of CO2 are being collected with fixed sensors at three sites (two in the upper and one in the lower Illinois River waterway). These sites also have paired monthly CO2 and CH4 sampling. In addition, two mapping campaigns of CO2 and CH4 were conducted at whole river scale using a boat-mounted flow-through system and gas analyzer from Lake Michigan to the Mississippi River. Preliminary results from the mapping campaigns indicated highly elevated CO2 and CH4 concentrations in the Chicago Sanitary and Ship Canal that range from approximately 10 to 23 times atmospheric for CO2 and from 70 to >680 times atmospheric for CH4. Concentrations of CO2 and CH4 decrease downstream as the river widens and its watershed transitions from predominantly urban to increasingly agricultural. CO2 and CH4 remain supersaturated along the entire course from Lake Michigan to the confluence with the Mississippi River. Continuous sensor records of CO2 concentrations and monthly CO2 and CH4 samples at the three gaging stations confirm these patterns and provide insight into seasonal and diel patterns. For example, CO2 concentrations in the Des Plaines River range from about 2.5 times atmospheric in winter to about 32 times atmospheric in summer through fall. Such results showcase the temporal and spatial variation in CO2 and CH4 in rivers and aid in reducing uncertainty in modeling greenhouse gas emissions from surface waters to the atmosphere.