Modified Tracer Gas Injection for Measuring Stream Gas Exchange Velocity in the Presence of Significant Temperature Variation

Gas exchange between streams and overlying air is an important physical-chemical environmental process that is typically determined by injecting a tracer gas into a stream at a steady rate and sampling steady-state tracer gas concentrations in the stream water. Previous modes of tracer gas injection allow gas-water partitioning of the tracer gas, making the rate of gas injection and thus the measured gas transfer velocity potentially sensitive to temperature variation. Presented here is a modification to the tracer solution injection method in which a tracer gas solution was prepared in Tedlar((R)) bags from which all headspace was removed before injecting the solution into the stream. Along with four other strategies to prevent a headspace from forming in the bags during tracer injection in the field, this zero-headspace tracer solution method prevents gas-water partitioning anywhere in the injection system, allowing a steady delivery of tracer gas to the stream even in the presence of variation in air and/or stream water temperature. A field test of the method in Nebraska yielded a gas transfer velocity of 4.1 m/day, within the range found in the literature for similarly-sized streams. Plain Language Summary This paper reports on the development and testing of a method that allows injection of tracer gases into streams and rivers at a constant rate even if the temperature of the air or water is changing over time. The new method allows accurate estimation of gas transfer velocities from injected tracer gases at times and places with significant temporal variation in temperature.