Jean-Pierre Vernet

Transport of the environmental radionuclides in an alpine watershed

Abstract The radionuclides137Cs,210Pb and7Be have been examined in the alpine Rhoˆne watershed (Switzerland) during a period of two years in order to evaluate their usefulness as tracers of the removal and transport rate of top-soil particles and particle-reactive contaminants of atmospheric origin. The specific activities of the radionuclides in fluvial suspension show a distinct seasonal pattern which depends on the hydrologic regime of the stream and the sources of the suspended matter. Input-output budgets based on the atmospheric deposition and fluvial removal of three radionuclides in the alpine Rhoˆne watershed are used to estimate their erosional residence times. The simplest one box model yields mean residence times of about 800 and 1400 years for137Cs and210Pb, respectively. The removal rate of short-lived7Be suggests that a part of the watershed (0.6–2.3% of the total surface) is exposed to a rapid erosion, in which the mean residence time of the radionuclides is in the range of 1–220 days. This has little influence on the calculated residence time of137Cs but increases the estimated residence time of210Pb in soil to over 1800 years. The use of210Pb-7Be pair in fluvial output may be very helpful in the assessment of the impact of atmospheric pollutants on the water quality in rivers and lakes.

Nutrient, heavy metal and organic pollutant composition of suspended and bed sediments in the Rhone river

The environmental quality of the Rhone River (Switzerland-France) has been assessed with a geochemical survey of the pollutants bound to suspended sediments. Ten samples were collected between Lake Geneva and the Mediterranean Sea in Nobember 1989 by continuous flow centrifugation and analysed for grain size distribution, carbonate, organic C, N, forms of particulate P, trace metals, and organic compounds (chlorobenzenes, organochlorine pesticides, PCBs, and PAHs). Four bed sediment samples were also studied for comparative purposes. The suspended solids provide lower variance by parameter than the bed sediments and are clearly most suitable for synoptic monitoring.The Upper Rhone River carries a glacial derived sediment with a low nutrient content, the stretch from Geneva to Lyon provides a sediment dominated by carbonate, and in the Lower Rhone the organic matter and phosphorus are relatively increased, mainly due to wastewater effluents and to an industrial P source. High concentrations of metals and organic micropollutants downstream of Lyon indicate a multiple contamination in the Lower Rhone, whereas more specific inputs are located downstream of Geneva and Arles.The comparison with data from other polluted major systems, the Rhine, the Niagara and the Detroit rivers, shows on overall similarity confirming that the Rhone quality is degraded downstream of Lyon. The levels of particular concern are for Hg, DDT metabolites which reveal a recent release in the basin, PCBs with a likely high chlorine content, and PAHs.The statistical evaluation of the compositional variables indicates a limited number of well defined associations, suggesting that the contamination of the suspended sediments results from the combination of numerous and intermittent point and diffuse sources in the Rhone River basin.

The manganese cycle in Lac Léman, Switzerland: the role of Metallogenium

The manganese pathways in Lac Leman have been investigated on the basis of chemical analyses undertaken on water, suspended solids, bottom sediments and interstitial water samples.

Texture and sedimentation rates in Lake Geneva

Thirty two cores were collected from Lake Geneva sediments along one longitudinal and eight transverse profiles. Rates of sedimentation determined by137Cs vary from 0.01 to 1.86 g cm−2 y−1. The average deposition rates in coastal and slope areas amounts to 0.37 g cm−2 y−1 in the Upper Lake (Grand Lac) and 0.12 g cm−2 y−1 in the Lower Lake (Petit Lac). In the deep basins, average rates of 0.13 and 0.05 g cm−2 y−1 were found for the Grand Lac and Petit Lac, respectively.The estimated mass of sediment deposited yearly outside of the principal deltas and turbidity current depositional areas is about 1.0 million tons (about 13% of the estimated total river load). One turbidite is clearly identified in the deepest, central lake area.There is little variation of surface sediment texture (mean grain size about 8–9µm) with the exception of delta areas. Since the beginning of the twentieth century, both carbonate and organic matter have increased as a result of lake eutrophication.

Sediment Core Correlation and Mapping of Sediment Accumulation Rates in Lake Geneva (Switzerland, France) Using Volume Magnetic Susceptibility

Studies of the relation between a lake and its watershed often need an accurate knowledge of sediment accumulation rates over large areas. We report here the combined method of sediment core correlation through Volume Magnetic Susceptibility profiles and sediment absolute dating, which permits drawing sediment accumulation rate maps. The Rhone River is the principal tributary to Lake Geneva, both in terms of water discharge and sediment load. Due principally to the construction of hydroelectric dams on major tributaries of the river, there has been an important modification to the yearly distribution of discharge. Although the total annual discharge is not affected, the sediment load should be reduced because of the lower summer flows and sediment entrapment in dam reservoirs. To confirm this hypothesis, we analyze 82 sediment cores, recovered in the Rhone delta region in Lake Geneva. Volume Magnetic Susceptibility (VMS) profiles are used to successfully correlate 52 cores, using a “shifting and stretching” method. Two VMS peaks are dated as 1961 and 1943 using time markers (137Cs and Hg contamination) in seven cores. The numerous correlated VMS profiles enable maps of sediment accumulation rate to be constructed for two different periods. These maps show a clear decrease in sedimentation after 1961. From these maps, we estimate the sediment deficit to be 250,000 t y−1. This value is comparable to the mass of sediment trapped in reservoirs located in the watershed.

Seasonal delivery of the particulate forms of phosphorus to Lake Geneva from the upper Rhone river

Large volume water samples were taken at Porte du Scex and Bouveret at the mouth of the Upper Rhone River as it enters Lake Geneva. Samples were taken every two weeks during 1982 up until August 1983. Water samples were analyzed for total phosphorus (TP), and soluble reactive phosphorus (SRP) and were centrifuged in the field using a continuous flow centrifuge to recover the suspended solids following sieving at 63 µm. The < 63 µm solids were analyzed for total particulate phosphorus (TPP), organic phosphorus (OP), apatite phosphorus (AP) and non apatite inorganic phosphorus (NAIP). The > 63 µm were similarly analyzed and the weight of total solids in both size fractions recorded. Results were compared throughout the period of record to the hydrograph situated at Porte de Scex. The annual cycle of the Rhone can be divided into a low turbidity, low flow winter period (SED 1) and high flow, high turbidity summer season designated (SED 2). Turbidity is well related to discharge. The > 63 µm sediment is mobilized at 200 m3 s−1 and thereafter increases in concert with, though at a faster rate than, the < 63 µm fraction. The coarse fraction contains significant quantities of phosphorus and in 1982 accounted for 26% of the TPP loading. OP and NAIP are higher in SED 1 than in SED 2 though AP remains constant throughout the year. OP is believed to be driven primarily from point sources whereas NAIP, in addition to point sources, has secondary sources in spring and summer due to sheet erosion from the agricultural soils of the valley. Loadings of phosphorus were calculated by four methods which showed internal consistency though they were higher than previous estimates. Bioavailable phosphorus (BAP) was estimated to account for some 20% of the total phosphorus loading of some 1500 tonnes; BAP is here taken to be the sum of SRP and NAIP and is that portion of the phosphorus load believed to be available to generate phytoplankton growth.

ΣDDT, PCBs, and HCB in the sediments of Lake Geneva and the upper Rhône River

Eighty surface sediment samples taken in 1978 from Lake Geneva, together with two cores sampled in 1976 and 1980, and 30 samples (1978) from the Rhone River were analyzed for organochlorine compounds. DDT, its metabolites (DDE, TDE), HCB, and PCB were found. No other organochlorine compound was identified at the detection level of 0.5 ppb. DDT metabolites were widespread and indicated residual DDT sources in the Rhone watershed and other minor lake tributaries, particularly on the Swiss coast. DDT levels have declined since an apparent peak usage in 1957. HCB was found in almost all samples. It first appeared about 1962 with consistent usage to the present. Use is widespread with the major source being the Rhône River. PCB was found in all samples at levels comparable to levels in the North American Great Lakes. Sources appear to be diffuse urban/industrial, with major inputs on the Swiss shore between Vevey and Morges. Peak use of PCB occurred in 1972 and was followed by a decline to 1978, with some evidence for a second increase to 1980, the latest sample available. Both HCB and PCB were found at high levels in the most upstream samples of the Rhône River which may be indicative of substantial atmospheric loadings of these two compounds to the Lake Geneva Basin.

Variation of nutrient stocks in the superficial sediments of Lake Geneva from 1978 to 1988

Two sediment sampling campaigns were conducted in 1978 and 1988 in Lake Geneva (Switzerland). Organic carbon, total nitrogen, total phosphorus and its various forms were analyzed. Results indicate a stability of organic carbon and nitrogen mass, and a significant increase of phosphorus. The variation of phosphorus mass is related to the increase of nonapatite inorganic phosphorus. This study attempts to quantify the phosphorus exchanges at the water sediment interface. The dissolved oxygen level in the bottom water determines the exchange direction. In aerobic conditions, sediments accumulate the excess of phosphorus, while in anaerobic conditions, they constitute an internal source.

Phosphorus trapping by turbidites in deep-lake sediments

Deposition of detrital material of turbiditic origin can modify the cycling of phosphorus in freshwater sediments. It may preserve mobile P from being released to the water column and maintain P at the ancient sediment-water interface. A conceptual model is proposed to explain the trapping of P in sediments. For sediments having a large inventory of mobilizable P, immobilization in sediments by turbidites or even thin detrital layers may positively influence the water quality of the hypolimnion. Examples of this phenomenon are presented for four sites in two Swiss lakes

The role of sediments in the phosphorus cycle in Lake Lugano. I. Geochemical approach

The bottom sediment from three coring stations in Lake Lugano (Lago di Lugano) show major differences in their role in the P-cycles related to their geochemistry and characteristics of sedimentation. In the northern basin, the deepest sediment may be considered practically “inactive”, due to a permanently reduced condition at the sediment-water interface. In the southern basin, the sediments are “active” with respect to P-recycling with strong seasonal variations. One of the sites (Figino) behaves as a sink for P due to a high iron content and an important rate of detrital sedimentation.