Rain influence on river water quality

The river water during dry weather is characterised by the presence of nitrate (around 3 mg/L) and a low concentration of organic matter (around 1.8 mg/L of TOC). The filtration of the water is very easy (the TSS concentration being close to zero), and the spectrum of the filtrate (after filtration at 10 kD, i.e. about 0.001 pm) is very similar to that of raw water, indicating a very low quantity of colloids. 

During wet weather, the river water sample contains a TSS concentration of 276 mg/L (measured by a reference method), principally of mineral composition as organic fraction represents only 18%. The size distribution of solids, put in suspension in water by rain flow, is very wide. Settleable, supracolloidal and colloidal solids are present in water. Several filtration steps have been carried out, and the corresponding UV spectra of the 

The results show that, depending on fractions, several observations can be drawn. The different responses are related to the diffuse absorption response of solids and colloids. The general tendency is that the slope of the spectrum generally decreases as the particle size increases. Spectra corresponding to suspended solids are flat, confirming a mineral nature. Adsorption phenomenon can be seen mainly on fine colloids, and the shape of soluble fractions (artificially denitrified with the deconvolution method see Chapter 2) is related to the probable presence of humic-like substances (see Chapter 6). 

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