Abstract
The study presents an alternative strategy to conventional spot sampling for monitoring metals and polycyclic aromatic hydrocarbons (PAHs) at an upstream, rural and karstic catchment in the north eastern part of France, in order to get insight into their spatial and temporal variability. Passive samplers, as diffusive gradients in thin films (DGT) and semipermeable membrane device (SPMD), are monthly deployed from August 2012 to March 2016 at five of the catchment monitoring stations located on the Saulx and Ornain Rivers. An improvement of the frequency of quantification (by a factor 2 to 8, depending on the targeted compound) is observed allowing us to better identify spatial and temporal variability. For instance, the upstream monitoring station on the Saulx River is characterized by high concentrations of Ni and Mn whereas the upstream monitoring station on the Ornain River is enriched in Cu and Zn. Furthermore, five metals (Al, Fe, Mn, Ni, Zn) and three PAHs (fluoranthene, pyrene and chrysene) show significant variations with water levels when grouped in three categories (low, medium and high water levels) in relation with hydrological and climatic patterns. This study leads to a more accurate assessment of the background pollution in metals and PAHs within surface waters than when based on spot sampling data.
Highlights
Anthropogenic sources as well as natural processes greatly influence water quality of surface and ground waters
Comparing time weighted average (TWA) concentration of labile compounds with dissolved (< 0.45 μm) for metals or total concentration for polycyclic aromatic hydrocarbons (PAHs) in spot samples is rather inappropriate, as underlined by several authors [12, 18], considering that the measurements do not reflect the same fraction of matrix and the same time scale
If compared with the frequency of quantification in spot samples collected every two months from 2012 to 2016 at the five monitoring stations, it was obvious that metals such as Zn, Ni, Cr and priority substances lead (Pb) were more quantified in diffusive gradients in thin films (DGT), with 40% to 98% of quantification in DGT versus 5% to 27% of quantification in spot samples (Table 1)
Summary
Anthropogenic sources (urban, industrial and agricultural activities) as well as natural processes (precipitation inputs, erosion and chemical weathering) greatly influence water quality of surface and ground waters. Due to spatial and temporal variations in water chemistry, a monitoring program providing a representative and reliable estimation of the quality of surface and ground waters is necessary to support decision making, in particular whether to implement a cost-effective set of measures to achieve load reduction of targeted substances. Water quality monitoring programs have been designed and regularly updated to take into account regulatory requirements (e.g. Water Framework Directive, WFD [1]). Most water quality monitoring programs are classically based on spot samples collection at a stated frequency, followed by laboratory analysis. To set the sampling frequency, the expected fluctuation with time of each parameter to be monitored should be considered. The number of monitoring stations and their location should reflect the spatial variability of water quality parameters of interest [2, 5]
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