The effect of chamber mixing velocity on bias in measurement of sediment oxygen demand rates in the Tualatin River basin, Oregon

Three sediment oxygen demand (SOD) measurement chambers were deployed in the Tualatin River near Tigard, Oregon, at river mile 10 in August 2000. SOD rates were calculated for three different circulation velocities during each chamber deployment. The SOD rate at each velocity was calculated from a graph of dissolved oxygen concentration versus elapsed time. An acoustic doppler current profiler (ADCP) was used to measure stream discharge and near-bottom water velocities in the Tualatin at river mile 10 and at two upstream locations. Measured river and chamber velocities were similar, indicating that results from the chambers were representative of instream effects.

At low to moderate chamber circulation velocities (less than about 7.5 centimeters per second), the measured SOD rate appeared to be only slightly affected by the circulation velocity, indicating that the measured rates reflect the rate of oxygen utilization by chemical and biological reactions in the sediment rather than the rate of physical transport of oxygen to the sediment-water interface. Above about 7.5 centimeters per second, however, the measured oxygen depletion rate was affected by the circulation velocity, as sufficient energy was generated within the chamber to resuspend bottom sediment, as evidenced by increased turbidity. The resuspended sediment particles contributed to the measured oxygen loss rate by increasing the surface area of decomposing material in contact with the water column, resulting in a measured SOD rate that was anomalously high. Two different alignments of the chamber circulation diffusers were tested. With both diffuser alignments, SOD rates were similar at circulation velocities low enough to avoid sediment resuspension.

The same resuspension effect probably exists in the Tualatin River during storm-runoff events following prolonged periods of low flow, when increased stream velocity may result in the resuspension of bottom sediments. The resuspension causes increased turbidity and increased oxygen demand, resulting in lower instream dissolved oxygen concentrations.