Evaluation Of A Novel Technique For Measuring Reaeration In Rivers
Free (open access)
P A Mackinnon, T Elliot, Y Q Zhao, J L Murphy & R M Kalin
Depletion of dissolved oxygen (DO) is one of the most common forms of river pollution. In extreme cases, it can result in fish kills, foul odours and can prohibit the use of water for drinking and other purposes. Regulatory authorities therefore place great importance on their ability to predict depletion of DO in waters which receive wastewater discharges and agricultural runoff, To date, predictive water quality models have relied almost totally on empirical methods to estimate the overall controlling factor for mass transfer of oxygen from the atmosphere into water bodies, the reaeration coefficient (K2). However, this coefficient varies from one river to another and changes with changing hydraulic conditions within a reach. Poor estimates have often led to inaccurate predictions of water quality. This paper describes the development of a novel method for the direct measurement of the reaeration coefficient in any river. This direct approach utilises two tracers – one volatile, one conservative – but, unlike previous tracer studies reported by other authors, it does not rely on the use of radioactive materials to achieve sensitive and precise measurement. Instead both tracers used are generally deemed ‘environmentally friendly’: the inert gas, Krypton, and the recognised fluorescent water tracer, Rhodamine WT, are used to quantify the reaeration coefficient. The adopted method is based on highly sensitive measurements of these substances using Gas Chromatography Mass Spectrometry (GC-MS) and fluorimetry techniques respectively. The outcome of field tests in a major river in Northern Ireland is reported. Issues affecting the precision of the method are addressed and the practicalities of applying it are discussed.