Comparison of the radioactive and modified techniques for measurement of stream reaeration coefficients

The radioactive and modified tracer techniques were used to measure the reaeration coefficients of two reaches each of Black Earth Creek and the Madison Effluent Channel near Madison, Wis. Comparison of the results showed that coefficients measured with the modified technique ranged from -8.96 to +3.61 and from +15.7 to +32.2 percent different from the coefficient measured with the radioactive tracer technique on the two reaches of Black Earth Creek. The larger coefficients measured with the modified technique on the second reach were attributed to increased wind conditions during the latter part of the modified-technique experiment. Interpretation of the results of the Madison Effluent Channel study was complicated by an unsteady flow condition during the modified-technique experiment. It was necessary to estimate the part of the reduction in the area under the dye concentration-versus-time curve that was the result of dye loss and the part that was the result of the increase in water discharge. Using these estimated values, the coefficients measured with the modified technique ranged from +25.3 to +57.9 and from -4.74 to +2.94 percent different from the coefficient measured with the radioactive technique on the two reaches of the Madison Effluent Channel.

Reaeration coefficients were predicted for the 4 stream reaches with 19 predictive equations from the literature. The range of the predicted coefficients for each of the reaches varied from about a 6-fold range for the first reach of the Madison Effluent Channel to an almost 11-fold range for the second reach of Black Earth Creek.

There are advantages and disadvantages to both the radioactive and modified tracer techniques. The main advantage of the radioactive technique is that the tracer gas is chemically inert; the main disadvantage is that a radioactive isotope of the gas must be used to obtain the necessary analytical sensitivity. The main advantage of the modified technique is that radioactive tracers are not necessary; the main disadvantage is that the hydrocarbon tracer gases may be subject to biological degradation and sorption losses. Results of this comparison study suggest that the modified technique is a promising alternative to the use of radioactive tracers.