Testing of conductivity/calcium and rubidium/strontium ratios as indicators of the chemical stability of a river: comparison with a biological indicator

Abstract

It is customary to detect pollution in a water flow by monitoring the increase of sensitive elements concentrations (NH4+, PO4(3-), NO3-...). However, concentrations are dependent on the flow rate and these compounds are not conservative, implying a concentration decrease downstream leading to false negative diagnosis of pollution impact. The use of elemental ratios of conservative compounds should diminish these pitfalls. We then thought of the chi/Ca (conductivity/calcium) and Rb/Sr (rubidium/strontium) ratios as water chemical stability indicators to clearly identify and discriminate point from diffuse pollutions. This hypothesis has been tested on 12 brooks located in the basin of Lake Geneva, during 2 hydrological years. The results were compared to the observed land use of the watershed and a biological indicator: the Pollution Sensitivity Index (PSI). The PSI is calculated from diatom taxonomy and evaluates biological quality with a grade ranging from 0 to 20 (bad to excellent). The main results of the research can be summarized as follows. The pollution signal is observable far downstream of the pollution site. Both chi/Ca and Rb/Sr ratios are water quality indicators expressing the stability of water chemistry. They can both be used to detect diffuse and point pollution impact. These indicators provide complementary information: chi/Ca variations increase in case of point pollution; Rb/Sr variations increase when diffuse pollutions occur. The results obtained with the indicators chi/Ca and Rb/Sr agree with biological indicator and observation of the land use. chi/Ca and Rb/Sr ratios represent important tools to identify and discriminate point source pollution from diffuse pollution.