Experimental study of two-dimensional mixing of natural tracers in rivers

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Il Won Seo, Sung Hyun Jung, Inhwan Park, Young Do Kim

Wednesday 1 july 2015

9:45 - 10:00h at Asia (level 0)

Themes: (T) Hydro-environment, (ST) Impacts of pollutants on the water environment

Parallel session: 8G. Environment - Impact

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In large rivers in Korea, since pollution sources and intakes for the water supply exist very closely in the same reach, the two-dimensional analysis of the pollutant mixing is required for the safe operation of the water intake facilities. To investigate the pollutant mixing in rivers, tracer studies were usually conducted for the collection of the concentration and dispersion data. Most of them were done using the artificial tracers, such as fluorescence dye, radioactive tracer, etc. The natural tracers, such as water temperature, electrical conductivity (EC), can be a good alternative in case tracer tests using the artificial tracer are not viable in large rivers due to financial and environmental problems. In this research, tracing study was conducted to investigate the transverse mixing characteristics at the downstream of the tributary confluence in the middle reach of Nakdong River in Korea. Two tributaries of Keumho River and Jincheon Creek, merging to the left side of Nakdong River near Gangjeung Weir, contain various pollutants from point and non-point sources such as wastewater and sewage treatment plants of nearby cities and industrial complexes. EC was selected as a natural tracer because EC levels were high in those tributaries. In the tributaries, sensors of EC and temperature were installed at the fixed locations, while in the Nakdong River, data collection was made with the sensors attached onto the moving boat. In the tributaries of Keumho River and Jincheon, the conductivity concentration showed uniform distribution to the transverse direction, and average concentration levels are higher than the background concentration of the Nakdong River at the upstream of the confluence. In the Nakdong River, after two tributaries merged at the left side, non-uniform distribution of the EC concentration was observed at the downstream sections due to transverse mixing of EC cloud introduced from the tributary channels. However, as EC cloud was moving further downstream, the transverse concentration gradient was decreasing. Transverse dispersion coefficient was calculated using the moment-based methods. The calculated values of dispersion coefficients were larger than the values suggested by Rutherford (1994) for curved channels.