Daniel Viville

Hydrograph separation using isotopic, chemical and hydrological approaches (Strengbach catchment, France)

The streamflow components were determined in a small catchment located in Eastern France for a 40 mm rain event using isotopic and chemical tracing with particular focus on the spatial and temporal variations of catchment sources. Precipitation, soil solution, springwater and streamwaters were sampled and analysed for stable water isotopes (18O and 2H), major chemical parameters (SO42−, NO3−, Cl−, Na+, K+, Ca2+, Mg2+, NH4+, H+, H4SiO4, alkalinity and conductivity), dissolved organic carbon (DOC) and trace elements (Al, Rb, Sr, Ba, Pb and U). 18O, Si, DOC, Ba and U were finally selected to assess the different contributing sources using mass balance equations and end-member mixing diagrams. Isotopic hydrograph separation shows that the pre-event water only contributes to 2% at the beginning of the stormflow to 13% at the main peak flow. DOC associated to Si and U to Ba allow to identify the different contributing areas (upper layers of the saturated areas, deep layers of the hillslope and rainwater). The streamflow (70%) originates from the deep layers of the hillslope, the remaining being supplied by the small saturated areas. The combination of chemical (both trace and major elements) and isotopic tracers allows to identify the origin of water pathways. During the first stage of the storm event, a significant part of the runoff (30-39%) comes from the small extended saturated areas located down part of the basin (overland runoff then groundwater ridging). During the second stage, the contribution of waters from the deep layers of the hillslope in the upper subcatchment becomes more significant. The final state is characterised by a balanced contribution between aquifers located in moraine and downslopes. Indeed, this study demonstrates the interest of combining a variety of hydrometric data, geochemical and isotopic tracers to identify the components of the streamwater in such conditions

Influence of acid atmospheric inputs on surface water chemistry and mineral fluxes in a declining spruce stand within a small granitic catchment (Vosges Massif, France).

Abstract The characteristics of surface and soil waters draining an old declining spruce stand, and their chemical behaviour under acid input influence within a small catchment are described using open field precipitations, throughfalls, soil solutions, spring and streamwater data. Element fluxes are calculated in the different ecosystem compartments, and the proton mass balance has been assessed. The results show the importance of acid occult deposits and, more generally, of acid atmospheric inputs, in explaining present soil acidification. This acidification is characterized by the base cation deficiencies observed in spruce thereby showing symptoms of decline. However, stream waters and spring waters are not yet acidified owing to the neutralization of acid inputs by weathering in the regolith. Nevertheless, streamwater chemistry is dominated by strong acid anions which are not characteristic of waters draining granitic bedrock in pristine areas.

δ13C pattern of dissolved inorganic carbon in a small granitic catchment: the Strengbach case study (Vosges mountains, France)

Abstract The transfers and origins of dissolved inorganic carbon (DIC) were studied for a year in a soil–spring–stream system in the Strengbach catchment, Vosges mountains, France. This 80 ha experimental research basin is located on the eastern side of the mountains, at an altitude ranging from 883 to 1146 m.a.s.l. and is mainly covered by spruce (80%). Brown acid and podzolic soils developed on a granitic basement, and, as a result, the DIC originates solely from CO2 generated by oxidation of soil organic matter. The ( δ 13 C DIC ) in catchment waters is highly variable, from about −22‰ in the springs and piezometers to about −12‰ in the stream at the outlet of the catchment. In the springs, pronounced seasonal variations of δ 13 C DIC exist, with the DIC in isotopic equilibrium with the soil CO2 that has estimated δ 13 C of about −24‰ in winter and −20‰ in summer. These seasonal variations reflect an isotopic fractionation that seems only induced by molecular diffusion of soil CO2 in summer. In stream water, seasonal variations are small and the relatively heavy DIC (−12‰ on average) is a result of isotopic equilibration of the aqueous CO2 with atmospheric CO2.

Hydrochemical budgets of a small forested granitic catchment exposed to acid deposition: The strengbach catchment case study (Vosges massif, France)

Hydrochemical budgets have been obtained for the 3-yr period 1986–89 at Strengbach, a small granitic basin in the Vosges mountains (north-eastern France). Here, the spruce forest shows both yellowing and crown thinning, symptoms of forest decline. Water amount and surface water chemistry were monitored in each ecosystem compartment. Bulk precipitation is acidic. Some pollution episodes occur in winter and early spring, but the annual bulk deposition acidity is rather low. Throughfall however, is much more concentrated, particularly for H+ and associated strong acid anions. These inputs come as occult deposits which comprise major ecosystem inputs, as confirmed by the chloride balance for the catchment. Input-output budgets for the catchment indicate a net deficit of base cations, especially calcium. Sulfate also shows a net loss while N budget is well balanced. As the soil exchange capacity is nearly exhausted for base cations, and dominated by H+ and Al, the neutralization of incident acid inputs occurs mainly in the weathered bedrock. Silicate weathering processes lead to high losses of cations and of silica. Aluminium hydroxide is precipitated; however, bicarbonate remains very low indicating poorly-buffered surface water.

Interception in a mountainous declining spruce stand in the Strengbach catchment (Vosges, France)

In a over-mature (declining) 90-year-old Norway spruce stand (Picea abies) in the Vosges mountain area, gross precipitation, throughfall, stemflow and meteorological variables have been measured for three periods in the summers of 1988, 1989 and 1990; transpiration was measured from June to August 1989. Throughfall, interception and stemflow represent, respectively, 65.3%, 34.2% and 0.5% of the incident rainfall. A semi-logarithmic relationship between interception and gross precipitation is given. Transpiration of the stand determined by sap-flow measurements represents only 27% of the potential evapotranspiration.

Evidence of hydrological control of Sr behavior in stream water (Strengbach catchment, Vosges mountains, France)

Strontium and particularly 87Sr/86Sr ratios in stream water have often been used to calculate weathering rates in catchments. Nevertheless, in the literature, discharge variation effects on the geochemical behavior of Sr are often omitted or considered as negligible. A regular survey of both Sr concentrations and Sr isotope ratios of the Strengbach stream water draining a granite (Vosges mountains, France) has been performed during one year. The results indicate that during low water flow periods, waters contain lower Sr concentrations and less radiogenic Sr isotope ratios (Sr=11.6 ppb and 87Sr/86Sr=0.7246 as an average, respectively) than during high water flow periods (Sr= 13 ppb and 87Sr/86Sr=0.7252 as an average, respectively). This is contrary to expected dilution processes by meteoric waters which have comparatively lower Sr isotopic ratios and lower Sr concentrations. Furthermore, 87Sr/86Sr ratios in stream water behave in 3 different ways depending on moisture and on hydrological conditions prevailing in the catchment. During low water flow periods (discharge < 9 l/s), a positive linear relationship exists between Sr isotope ratio and discharge, indicating the influence of radiogenic waters draining the saturated area during storm events. During high water flow conditions, rising discharges are characterized by significantly less radiogenic waters than the recession stages of discharge. This suggests a large contribution of radiogenic waters draining the deep layers of the hillslopes during the recession stages, particularly those from the more radiogenic north-facing slopes. These results allow one to confirm the negligible instantaneous incidence of rainwater on stream water chemistry during flood events, as well as the existence in the catchment of distinct contributive areas and reservoirs. The influence of these areas or reservoirs on the fluctuations of Sr concentrations and on Sr isotopic variations in stream water depends on both moisture and hydrological conditions. Hence, on a same bedrock type, 87Sr/86Sr ratios in surface waters can be related to flow rate. Consequently, discharge variations must be considered as a pre-requisite when using Sr isotopes for calculating weathering rates in catchments, particularly to define the range of variations of the end-members.

Mid-Term Trends in Acid Precipitation, Streamwater Chemistry and Element Budgets in the Strengbach Catchment (Vosges Mountains, France)

In the Vosges Mountains (NE of France), integrated plot-catchment studies have been carried out since 1985 in the Strengbach basin to study the influence of acid atmospheric inputs on surface water quality and element budgets. In this paper, available mid-term time series (1985–1991) have been considered to detect obvious trends, if any, in surface water chemistry and element budgets. Air quality data showed a slight decline for SO2, whereas NO2 slightly increased over the period, but these trends are not very significant. This is in agreement with increased N concentration (mainly as NH4+) and with the stability of SO42− in open field precipitation. Because of a significant decrease in rainfall amount over the period, only inputs of NH4+ increased significantly whereas H+ and SO42+ inputs declined. In spring and streamwaters, pH and dissolved Si concentration increased mainly as a result of a reduced flow. Na+, K+, Cl− and HCO-3~− concentrations remained stable whereas Ca2+, Mg2+ and SO42+ concentrations declined significantly. Only NO3− concentration increased significantly in springwaters. The catchment budgets revealed significant losses of base cations, Si and SO42−. These losses decreased over the period. Nitrogen was retained in the ecosystem. However, a longer record is needed to determine whether or not changes in surface water chemistry have resulted from short-term flow reductions or long-term changes in input-output ion budgets. This is specially true with N because the decline in SO42− output was accompanied by N accumulation.

Evidence of Current Soil Acidification in Spruce Stands in the Vosges Mountains, North-Eastern France

To demonstrate directly soil acidification under spruce stands in the Strengbach catchment, soils from two adjacent stands aged 40 and 90 years were sampled intensively in 1990 and resampled in 1996. Soils already were very acid in 1990. Between 1990 and 1996, soils had experienced a significant decrease in exchangeable Ca2+ and Mg2+ at all depths at the two sites except in the 0-10 cm layer, for which base saturation remained constant. Losses of Ca2+ calculated from a budget study at the same sites and from the comparison of exchangeable stores were similar. In contrast, the loss of Mg2+ from the exchange complex was higher than that computed from the field budget. Various reasons, including most probably uncertainties linked to the extremely low levels of Mg in the ecosystem, may explain this discrepancy. Since 1987, a general decrease of the (Ca2+ + Mg2+)/Al3+ ratio in soil solution was observed at both sites. These results are consistent with present day acidification of soils poor in weatherable minerals under spruce stands in the Vosges Mountains.

Contribution des surfaces saturées et des versants aux flux d'eau et d'éléments exportés en période de crue: traçage à l'aide du carbone organique dissous et de la silice. Cas du petit bassin versant du Strengbach (Vosges, France)

Abstract Two contributing areas have been determined in the small forested Strengbach catchment by using dissolved organic carbon (DOC) and dissolved silica as the most efficient chemical tracers of stream water origin during a storm event. Dissolved silica represents the deep layers of the hillslopes which contributes 71–75 % of the total storm runoff and 85–99 % of the fluxes of major elements exported by the Strengbach. DOC corresponds to the saturated areas which occupy only 2 % of the total catchment area but which supply 25–29 % of the total streamflow and 56 % of the DOC stream flux. The specific fluxes of water (L·s−1·ha−1) and elements (kg·ha−1) are much more important in the saturared areas than in the rest of the catchment.

Comparative Hydrochemical Behaviour and Element Budgets of the Aubure (Vosges Massif) and Mont-Lozère (Southern Massif Central) Norway Spruce Forested Catchments

Since the late 1970s, small catchments have been extensively used all around the world to investigate the effect of land use and/or acid atmospheric deposition on surface water chemistry and ecosystem acidification. Most of the studies were undertaken in the USA (Likens et al. 1977), the Scandinavian countries (Wright and Johannessen 1980; Hultberg 1985), Canada (Thompson et al. 1980), Czechoslovakia (Paces 1985) and Germany (Hauhs and Wright 1986), where forest dieback and/or surface water acidification have been related to the atmospheric pollution.