Density Currents in the Chicago River: Characterization, Effects on Water Quality, and Potential Sources

JACKSON, P.R; GARCÍA, C.M.; OBERG, K.,; JOHNSON K.,; GARCÍA, M.H.

SCIENCE OF THE TOTAL ENVIRONMENT

Elsevier Science

Año: 2008 vol. 401 p. 130 - 143

0048-9697

Bidirectional flows in a river system can occur under stratified flow conditions and in addition to creating significant errors in discharge estimates, the upstream propagating currents are capable of transporting contaminants and affecting water quality. Detailed field observations of bidirectional flows were made in the Chicago River in Chicago, Illinois in the winter of 2005–06. Using multiple acoustic Doppler current profilers simultaneously with a water-quality profiler, the formation of upstream propagating density currents within the Chicago River both as an underflowand an overflowwas observed on three occasions. Density differences driving the flow primarily arise from salinity differences between intersecting branches of theChicago River,whereaswater temperature is secondary in the creation of these currents. Deicing salts appear to be the primary source of salinity in the North Branch of the Chicago River, entering the waterway through direct runoff and effluent from a wastewatertreatment plant in a large metropolitan area primarily served by combined sewers. Waterquality assessments of theChicagoRivermayunderestimate (or overestimate) theimpairment of the river because standard water-quality monitoring practices do not account for densitydriven underflows (or overflows). Chloride concentrations near the riverbed can significantly exceed concentrations at the river surface during underflows indicating that full-depth parameter profiles are necessary for accurate water-quality assessments in urban environments where application of deicing salt is common.