Groundwater and river baseline quality using major, trace elements, organic carbon and Sr–Pb–O isotopes in a Mediterranean catchment: The case of the Lower Var Valley (south-eastern France)

Abstract

Summary Dissolved trace and major elements, organic carbon and Pb–Sr–O isotopes have been investigated in surface and groundwater of the Var River Valley (SE France), including alluvial, conglomerate and limestone aquifers, as well as surface water. Boxplots and cumulative frequency distribution diagrams define chemical characteristics of each water group and distinguish between natural and anthropogenic range of concentrations. Low concentration of trace elements, statistical analysis of data and Pb isotopic ratios demonstrate that the measured baseline quality is close to the natural background, mainly influenced by water/rock interaction. Pb and Sr isotopes evidence specific primitive end members, buffering these two elements through leaching late Paleozoic rocks, strongly depleted in U and Rb since their formation. Arsenic undergoes geochemical processes such as sorption on clay minerals of the alluvial deposits. Sr isotopic ratios and high SO 4 2 - , Sr and Li contents show that Permo–Triassic sediments including evaporites strongly imprint surface waters and alluvial groundwaters. Limestone and conglomerate aquifers are mainly influenced by carbonate minerals, but may be locally affected by evaporite dissolution. High dissolved silica is also specific to conglomerate groundwaters. Limestone and conglomerate groundwaters are characterised by low and heterogeneous trace element compositions that may result from various residence times and rock compositions related to different sampling depth. Even if water quality is generally good for most of the investigated elements, pollution by agricultural activity (fertilisers and pesticides) in the alluvial and some limestone groundwaters has been demonstrated by high NO 3 - and Br− contents. Br− is suspected to originate from methyl bromide fertiliser. In conglomerate aquifers, NO 3 - contents may reach high levels, probably due to both agriculture and residential waste pollution. Despite the industrialization in the Low Var Valley, no significant metallic contamination could be detected and therefore, for trace elements, given parameters (min, max and median values) can be used as a baseline guide to detect future contamination.