Chemical and U–Sr isotopic variations in stream and source waters of the Strengbach watershed (Vosges mountains, France)

Abstract

Abstract. This is the first comprehensive study dealing with major and trace element data as well as 87Sr/86Sr isotope and (234U/238U) activity ratios (AR) determined on the totality of springs and brooks of the Strengbach catchment. It shows that the small and more or less monolithic catchment drains different sources and streamlets with very different isotopic and geochemical signatures. Different parameters control the diversity of the source characteristics. Of importance is especially the hydrothermal overprint of the granitic bedrock, which was stronger for the granite from the northern slope; also significant are the different meteoric alteration processes of the bedrock causing the formation of 0.5 to 9 m thick saprolite and above the formation of an up to 1m thick soil system. These processes mainly account for springs and brooks from the northern slope having higher Ca / Na, Mg / Na, and Sr / Na ratios, but lower 87Sr/86Sr isotopic ratios than those from the southern slope. The chemical compositions of the source waters in the Strengbach catchment are only to a small extent the result of alteration of primary bedrock minerals, and rather reflect dissolution/precipitation processes of secondary mineral phases like clay minerals. The (234U/238U) AR, however, are decoupled from the 87Sr/86Sr isotope system, and reflect to some extent the level of altitude of the source and, thus, the degree of alteration of the bedrock. The sources emerging at high altitudes have circulated through already weathered materials (saprolite and fractured bedrock depleted in 234U), implying (234U/238U) AR below 1, which is uncommon for surface waters. Preferential flow paths along constant fractures in the bedrocks might explain the – over time – homogeneous U AR of the different spring waters. However, the geochemical and isotopic variations of stream waters at the outlet of the catchment are controlled by variable contributions of different springs, depending on the hydrological conditions. It appears that the (234U/238U) AR are a very appropriate, important tracer for studying and deciphering the contribution of the different source fluxes at the catchment scale, because this unique geochemical parameter is different for each individual spring and at the same time remains unchanged for each of the springs with changing discharge and fluctuating hydrological conditions. This study further highlights the important impact of different and independent water pathways on fractured granite controlling the different geochemical and isotopic signatures of the waters. Despite the fact that soils and vegetation cover have a great influence on the water cycle balance (evapotranspiration, drainage, runoff), the chemical compositions of waters are strongly modified by processes occurring in deep saprolite and bedrock rather than in soils along the specific water pathways.

Geochemical tracing and modeling of surface and deep water–rock interactions in elementary granitic watersheds (Strengbach and Ringelbach CZOs, France)

“…1) is consistent with the long residence times deducted from the CFC and SF6 data. The occurrence of ( 234 U/ 238 U) ratios \1 in the Strengbach spring waters is common for the Strengbach catchment (Riotte and Chabaux 1999;Prunier 2008;Pierret et al 2014). Such values are interpreted in terms of U fluxes from previously weathered mineral phases, which agree with the idea that the water feeding these springs are supplied by subsurface waters.…”

“…u-strasbg.fr). More details can be found in Gangloff et al (2014), Pierret et al (2014), Prunier et al (2015) for the Strengbach CZO and in Schaffhauser et al (2014) Schaffhauser et al 2014). Both watersheds are also equipped with boreholes, which have confirmed the occurrence of deep circulations in the two watersheds along fractures concentrated in relatively narrow layers, several centimeters wide.…”

“…These analyzes were performed following the standard procedures commonly used in the laboratory (see details in Schaffhauser et al 2014 for summary). The data have been partly presented and discussed in previous works (Prunier 2008;Pierret et al 2014;Schaffhauser 2013;Schaffhauser et al 2014), which have emphasized the interest of coupling various geochemical tracers to constrain the main (bio)-geochemical processes involved in the CZ functioning. The multi-component reactive transport model KIRMAT (KInetic Reactions and MAss transport) has been also applied to constrain the main water-rock interactions controlling the chemical composition of the Strengbach and Ringelbach spring and borehole waters (Schaffhauser 2013;Ackerer 2017).…”

Hydrological and vegetation response to climate change in a forested mountainous catchment

“…The Strengbach watershed (80 ha) is a small granitic catchment located on the eastern side of the Vosges Mountains (Northeast France) where hydro-geochemical processes have been monitored since 1986 (Riotte and Chabaux 1999;Chabaux et al 2005;Rihs et al 2011;Gangloff et al 2014;Pierret et al 2014). The topography ranges from 883 m at the outlet to 1146 m at the catchment divide (Fig.…”

Factors controlling the chemical composition of colloidal and dissolved fractions in soil solutions and the mobility of trace elements in soils

“…The current study increases the understanding of the transport of major and trace elements by the colloidal and dissolved phases in the first meter of the soil, as well as their spatial and temporal displacement in the soil of a forested ecosystem. It was performed on soil solutions obtained by zero tension lysimeter plates installed in the forested Strengbach catchment (Riotte and Chabaux, 1999;Tricca et al, 1999;Schmitt et al, 2003;Stille et al, 2006;Cenki-Tok et al, 2009;Stille et al, 2009;Lemarchand et al, 2010;Rihs et al, 2011;Stille et al, 2011;Lemarchand et al, 2012;Viville et al, 2012;Pierret et al, 2014;Gangloff et al, 2014a;Gangloff et al, 2014b). The samples were collected during two periods (2009-2010 and 2012-2013) at different depths in the soil profile and ultra-filtered for the determination of the chemical composition of the colloidal and dissolved phases and, thus, leading to a better understanding of the spatial and temporal displacement of major and trace elements in the soil of a forested ecosystem.…”