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AbstractDissolved oxygen (DO) in river systems is often depleted by polluting substances, sometimes rendering the water unfit for habitats and human use. The natural mass transfer of oxygen from the atmosphere can help to alleviate this. The reaeration coefficient, K2, which describes the rate of oxygen absorption, is an important parameter in water‐quality modelling. Owing to difficulties in the direct measurement of K2, values for use in water quality models are typically derived using predictive equations, but these are notoriously inaccurate. This paper presents a field method for the direct measurement of K2 by two alternative analysis methods and includes the results of eight tests conducted on a reach of the River Lagan, in Northern Ireland. The method is based on a long‐established protocol but replaces a now‐unacceptable radioactive tracer (85Kr) with stable noble gases. K2(20) values between 3 and 80/day are reported for the test reach.

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