Study of water quality variation along a river

Study of water quality variation along a river 

The first example is the study of the water quality of the Gardon de Saint-Jean River located in the South of France in the Cevennes region. This 50-km-long river is of the Mediterranean type, with a very low flow rate in summer and frequent flash floods the rest of the time. From the pollution point of view, this river is characterised by the existence of two main villages (of about 2000-5000 inhabitants), with two wastewater treatment plants, but it is also the heart of a tourist area with an estimated overpopulation of 50,000 people in summertime. 

A sampling campaign has been carried out in summer 2000, with nine points located along the river (Fig. 4). For each station, the UV spectrum and the measurement of dissolved oxygen, conductivity and pH have been realised on fresh samples. A field-portable UV spectrophotometer (Pastel UV from Secomam) and handheld instruments for the other parameters have been used. 

The evolution of the physico-chemical parameters is presented in Fig. 5. Close to the source, the dilution of the river can be noted with water of low mineralisation. An increase of dissolved oxygen expressed as saturation percentage is observed, as well as an increase in the pH values downstream. 

The conclusion of this experiment is that, even if the pollution level of the river is low, due to high performances of the wastewater treatment plants, the use of UV spectrophotometry simplifies the field (and laboratory) work by bringing complementary information on the evolution of water quality. 

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