Mixed Tracer Injection Method to Measure Reaeration Coefficients in Small Streams

Reaeration coefficient (k), the rate of oxygen exchange between the atmosphere and water surface, is an important parameter for understanding water quality impairment and stream metabolism. We modified the propane injection method to measure gas exchange coefficients and evaluated its application for small streams. The tracer solution was prepared by solubilizing propane directly in a conservative solute solution, and it was injected as a constant-rate injection, a single slug, or an extended slug. Water samples were taken at four to five sampling stations along the study reach at the tracer concentration peak, and propane and conductivity were measured. The propane exchange rate (k (propane)) was calculated using the regression method with the propane/conductivity ratio against solute travel time (in hours). The mixed tracer injection method was conducted in four streams, and all k (propane) measurements (n = 8) were statistically significant (p < 0.05). The short-duration constant rate injection and extended slug injection provided k (propane) estimates with higher r (2) than the single slug injection. The k (20) measured with propane injection ranged from 5.4 to 40.0 day(-1), and they were significantly correlated with empirically estimated k. The mixed tracer injection method with propane could potentially reduce field time, crew demands, and field equipment; thus, it would potentially lower the overall cost of gas exchange coefficient measurements and be an effective method in small, remote streams.