Elsevier

Journal of Hydrology

Volumes 472–473, 23 November 2012, Pages 126-147
Journal of Hydrology

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)

https://doi.org/10.1016/j.jhydrol.2012.09.023Get rights and content

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 SO42-, 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 NO3- and Br contents. Br is suspected to originate from methyl bromide fertiliser. In conglomerate aquifers, NO3- 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.

Highlights

► Groundwater and surface water are studied in the Low Var Valley. ► Trace and major elements, organic carbon and Pb–Sr–O isotopes are investigated. ► Concentrations are close to the natural baseline quality. ► Pb-isotopes give unclear results with primitive signature. ► Agricultural contamination is evidenced with nitrates and Br, maybe from pesticides.

Introduction

In Mediterranean areas, water is becoming one of the main environmental issues because of drought sensitivity and increasing consumption. Groundwater provides a large part of drinking water and its quality must be strictly monitored in order to prevent and detect both accidental and chronic contaminations. Detection of anomalous concentrations requires the knowledge of a previously well-established natural baseline. Moreover, this baseline must concern a large range of chemical elements including trace elements, in order to detect low-level pollutions. The notion of baseline is discussed in details by Reimann and Garrett (2005) who insist on the fact that background remains specific to a restricted area. The definition used for this study will be “the range of concentrations of a given element […] in solution, derived entirely from natural, geological, biological or atmospheric sources, under conditions not perturbed by anthropogenic activity” (Edmunds and Shand, 2008). The knowledge of natural groundwater quality is necessary for long-term quality management, to achieve the purposes of the European Community Water Framework Directive (European Council, 2000) and its daughter Groundwater Directive (European Council, 2006).

The Var River is 114 km long and has a drainage area of 2819 km2. With an average water discharge of 50.9 m3 s−1 at Nice, it is the second river (after the Rhône River) reaching the Mediterranean Sea in Southern France. Its alluvial aquifer in the lower valley supplies drinking water to more than 600,000 inhabitants of the Côte d’Azur. Like many Mediterranean coastal sites, geography of the Var alluvial valley makes it really attractive for urbanisation because of the lack of flat space in surrounding areas. Groundwater resources are therefore often located below inhabited and industrial zones. This is a reason why many Mediterranean water resources are vulnerable and must be carefully monitored. Moreover, the studied watershed has representative lithologies of many catchments around the Mediterranean Sea, with crystalline rocks in the hinterland, and mostly marls, limestones, sandstones and evaporites in the foreland.

The aim of this work is (1) to define the main chemical characteristics of the different aquifers of the Lower Var Valley (LVV) and (2) to evaluate their relative anthropogenic and natural geological contributions. If the anthropogenic contribution is sufficiently low, we shall be able to use the obtained data as a baseline. In this paper, we shall report chemical data of major, trace elements and Sr–Pb–O isotopes in surface and ground waters of the LVV in order to explain which are the main factors driving hydrochemistry. This study is useful not only in a local scale for water resource management, but also in a larger scale because of the representativeness of this watershed in the Mediterranean Basin.

Section snippets

Geology

The study area is located in the Southern Alps (south-eastern France), in the 25 km long alluvial valley of the Var River. The whole watershed of the Var River is characterised by a large variety of lithologies (Fig. 1) (Féraud et al., 2009). (1) Crystalline rocks (migmatites, gneisses, micaschists and granites) of the Argentera–Mercantour massif belong to the Hercynian chain, at the north-eastern edge of the basin. (2) Continental Carboniferous (lignite) and Permian sediments, essentially red

Sampling and analytical methods

Concentrations of dissolved major elements (major ions and dissolved silica called hereafter major elements even thought it is a misuse of language), trace elements and organic carbon have been investigated. 453 Samples of the Var River and the groundwater of alluvial, limestone and conglomerate aquifers were collected during 6 complete campaigns in different hydrological periods. Moreover, a weekly monitoring was carried out on 5 selected sites: 3 alluvial, 1 conglomerate and 1 river samples,

Hydrochemical data

All data are given for each class of water: “alluvial GW” is groundwater (GW) sampled in the alluvial aquifer, “limestone GW” is groundwater from the aquifer of karstified limestone, “conglomerate GW” define samples from the aquifer of Pliocene conglomerate and “Var River” is the surface water from the river. Table 3 shows the range of variation and the median values for pH, electrical conductivity (EC), temperature, dissolved oxygen content, isotopic signature of oxygen (δ18O) and Dissolved

Rainfall

Rainwater can be regarded as the initial input to explain baseline quality. The Lower Var Valley aquifer recharges provided by snowmelt in the highland and rainfall on the overall basin can be characterized by Oxygen isotopic signature. As generally observed, rainwater in high elevation area shows more depleted values than coastal precipitations (Ingraham, 1998): δ18O weighted mean value is −4.2‰ in Monaco (altitude 2 m) and −6.5‰ in Malaussène (359 m) near the confluence of the Tinée and Var

Natural baseline

Most major and trace elements of groundwaters and surface waters in the Low Var Valley originate from natural processes related to water/rock interactions. Concentration ranges can thus be used as reference baseline for future quality monitoring. In the future, analysing trace element besides will help to detect early contamination. The Low Var Valley is representative of many alluvial valleys of Mediterranean catchments, for geological features such as crystalline rocks in the headwater and

Acknowledgments

This study was financially supported by: Conseil Général des Alpes Maritimes, Agence de l’Eau RMC, Conseil Régional PACA, Syndicat Mixte d’Etudes de la Basse Vallée du Var, PPF grant and Véolia-eau. We are also very indebted to J.P. Goudour for his help in Thermal Ion Mass-spectrometry. We are grateful to Philippe Abela for figure drawing, Vittorio and Geneviève Barci for help in water sampling, and Marc Fiquet, Christian Mangan, Alexandre Emily and Guillaume Tennevin for fruitful discussions.

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