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Relations between several stream processes and concentrations of dissolved oxygen and other constituents were evaluated for a 70.3-mile reach of the Du Page River in northeastern Illinois, by comparing measured data with computer-simulated data. Measurements, made during periods of low flow, were used to calibrate and verify the QUAL-II one-dimensional, steady-state, waterquality model (Southeast Michigan Council of Governments' version). Equations for prediction of reaeration rates and traveltimes were developed from values that were measured using a steady-state, gas tracer technique. Water samples were collected from the river and known inflows during two 24-hour (diel) periods in July and August 1983 and analyzed for up to 60 constituents. Diel dissolved-oxygen concentrations in the East Branch Du Page River were as low as 0.9 milligram per liter. During the two diel periods, the lower 8 miles of the East Branch had low dissolved-oxygen concentrations. Maximum diel dissolved-oxygen concentrations throughout this subreach were seldom above the State's minimum standard of 5 milligrams per liter. Model simulations indicated that, although ammonia oxidation played a role in dissolved-oxygen depletion, the most important factor in this subreach was sediment oxygen demand. In the East Branch, the maximum total iron concentration was 4,099 micrograms per liter compared to the State standard of 1,000 micrograms per liter, and the maximum total dissolved-solids concentration was 1,440 milligrams per liter compared to the State standard of 1,000 milligrams per liter. The maximum ammonia concentration for the two diel periods was 8.0 milligrams per liter as nitrogen, and the pH and water temperature of the stream were such that the calculated un-ionized ammonia concentration exceeded the State general use water-quality standard of 0.04 milligram per liter at three sites during both periods. Wastewater treatment facility effluent was the major input of ammonia to the river during these low-flow periods. Some subreaches of the West Branch and the Main Stem Du Page River had dissolved-oxygen concentrations as low as 2.1 milligrams per liter, but unlike the East Branch, this was only for a short period during the day. Model simulations indicated this was caused by inflows with low dissolved-oxygen concentrations and by algal respiration. High total dissolved-solids concentrations (maximum 1,190 milligrams per liter) were measured at most sites along the West Branch and Main Stem. Ammonia concentrations in the West Branch and Main Stem were very low; the maximum concentration was 1.0 milligram per liter as nitrogen. High iron concentrations (up to 2,175 micrograms per liter) were measured in samples from the upper West Branch.

Assessment of low-flow water quality in the Du Page River, Illinois