Determination of gas exchange rate constants for a small stream on Walker Branch Watershed, Tennessee

Abstract

The steady state tracer gas method was used to determine gas exchange rate constants (k) for a small stream (annual average flow ≤1 m 3/min) draining the West Fork of Walker Branch watershed in eastern Tennessee. Chloride was used as a conservative tracer to account for dilution by lateral inflow, and propane and ethane were used as volatile tracers. Gas exchange rate constants for propane (kp) were about 100 d−1 over a wide range of flow conditions, while those for ethane (k e) were about 117 d 1; an equivalent rate constant for O 2 (KO 2) would be about 118–139 d−1, depending on the method used for its calculation. These rate constants are much larger than those typically found in rivers and large streams. Much lower k p values (about 50d−1) were found during one experiment conducted at low flow with much of the stream surface covered with floating leaves. Nineteen previously published empirical equations were used to predict kO2 values for one 72m stream reach; agreement between the predicted and measured values was generally very poor, underscoring the importance of field‐measured gas exchange rates in studies of the transport and fate of volatile compounds. Because ethane and propane have similar gas exchange rates and similar aqueous diffusion coefficients (k e/k p and D e/D p are both close to 1, where D is the compound's diffusion coefficient), accurate determination of the exponent n in the relationship ke/Kp− (D e/D p)n was not possible. The ratio k e/k p (1.17) is much closer to D e/D p (1.24) than to H e/H p (0.82, where H is the compound's Henry's law constant), suggesting that stripping of dissolved volatiles by air bubbles was not a dominant mode of gas exchange for the study stream.