Determining the Reaeration Coefficient and Hydrodynamic Properties of Rivers Using Inert Gas Tracers

Various contaminants which can be aerobically degraded find their way directly or indirectly into surface water bodies. The reaeration coefficient (K2) characterises the rate at which oxygen can transfer from the atmosphere across the air-water interface following oxygen depletion in a water body. Other mechanisms (like advection, dispersion and transient storage) determine how quickly the contaminants can spread in the water, affecting their spatial and temporal concentrations. Tracer methods involving injection of a gas into the water body have traditionally been used for direct (in-situ) measurement of K2 in a given reach. This paper shows how additional modelling of tracer test results can be used to quantify also hydrodynamic mechanisms (e.g. dispersion and storage exchange coefficients, etc.). Data from three tracer tests conducted in the River Lagan (Northern Ireland) using an inert gas (krypton, Kr) are re-analysed using two solute transport models (ADM, TSM) and an inverse-modelling framework (OTIS-P). Results for K2 are consistent with previously published values for this reach (K2(20)~10-40 d-1). The storage area constituted 30-60% of the main cross-section area and the storage exchange rate was between 2.5×10-3-3.2×10-3s-1. The additional hydrodynamic parameters obtained give insight into transport and dispersion mechanisms within the reach.