Bertrand Fritz

Strontium as a tracer of weathering processes in a silicate catchment polluted by acid atmospheric inputs, Strengbach, France

This paper determines the weathering and atmospheric contributions of Ca in surface water from a small spruce forested silicate catchment (N–E France) receiving acid atmospheric inputs. The bedrock is a granite with K-feldspar and albite as dominant phases. The calcium content in plagioclase is low and the Ca/Na ratio in surface water is high, reflecting other sources of calcium from those expected from the weathering of major mineral phases. The biotite content is low. Only traces of apatite were detected while no calcite was found in spite of a major hydrothermal event having affected the granite. The strontium isotopic ratio 87Sr/86Sr and Sr content was used as a tracer of weathering and was determined in minerals and bulk bedrock, open field precipitation, throughfall, soil solution, spring and stream water. The Sr isotopic ratio of the reacting weathering end-member was predicted by simulating the alteration of the granite minerals by incorporating strontium into the water–rock interaction kinetic code KINDIS. In the early stages of water–rock interaction, K-feldspar and biotite strongly influence the isotopic composition of the weathering solution whereas, the Na-rich plagioclase appears to be the main long-term reactive weathering end-member. Approximately 50% of dissolved Sr in streamwater are atmospherically derived. The 87Sr/86Sr ratios of exchangeable Sr in the fine fraction at 1-m depth from a soil profile indicate that the amount of exchangeable Sr seems essentially controlled by atmospheric inputs. The exception is the deep saprolite where weathering processes could supply the Sr (and Ca). Na-Plagioclase weathering obviously control the chemistry and the isotopic composition of surface waters. The weathering of trace mineral plays a secondary role, the exception is for apatite when plagioclase is absent. Our hydrochemical, mineralogical and isotopic investigations show that a major part of the strong Ca losses detected in catchment hydrochemical budgets that result from the neutralization of acid precipitation has an atmospheric origin. Consequently, in the long term, in such areas, the availability of such an exchangeable base cation might be strongly limited and surface waters consequently acidified.

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.

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.

Dissolution of subglacial volcanic glasses from Iceland : laboratory study and modelling

Abstract Subglacial hyaloclastites from Iceland with ages ranging from 2 ka to 2.2 Ma have been studied from a mineralogical and geochemical standpoint. The chemical composition of palagonite (alteration crust formed on the surface of the glass) is almost identical with that of the clayey material filling the intergranular spaces of the rock. The clayey material is made up of two particle populations: the first is Si-, Mg- and Ca-rich with a smectite structure, while the second is amorphous, Fe−, Ti− and Al-rich, and has a smectite-like morphology. It is suggested that these two types of particles can be formed simultaneously, in the same solution, such that it is not necessary to explain their existence by local or temporary equilibria. The mineralogical sequences observed in natural samples were reproduced using the geochemical computer code DISSOL. The geochemical mass balances calculated with DISSOL also fit quite well with those calculated from Icelandic samples, illustrating the predictive capability of such a calculation code and gives us confidence in applying a similar approach to nuclear waste-form glass problems.

Geochemistry of Bolivian salars, Lipez, southern Altiplano: Origin of solutes and brine evolution

Abstract The southern Bolivian Altiplano is a volcanic area which contains numerous undrained basins occupied by playas and saline lakes, locally named salars. Solutes carried by springs and rivers into the salars originate mostly from the alteration of the volcanic rocks and the re-solution of ancient buried evaporites. Both weathering and hydrothermal alteration lead to Na-HCO 3 inflow waters of similar composition. High contents of lithium and boron are not specifically related to hydrothermal activity. Evaporite leaching leads to Na-Cl inflow waters of higher salinity. Atmospheric precipitation contributes only a small amount of Ca and SO 4 , and very little Na or Cl. The calculated evaporative evolution of the inflow waters shows that about half of them should become highly alkaline brines and the other half should end as near-neutral brines. However, alkaline soda lakes are rare in this region. Most lake brines are of the Na-Cl and Na-Cl-(SO 4 ) types. To understand this anomaly one characteristic sequence of evaporating waters was studied in detail. Precipitation of calcite and Mg-smectites accounts only partially for the reduction of alkalinity. A further loss of alkalinity is probably related to the oxidation of wind-blown native sulfur in peripheral ponds. Wind-blown sulfur is also deposited around the lakes, on the drainage basins. In the rainy season, ephemeral streams carry along sulfur particles in permanent pools of the preconcentration areas. A hydrologic control of brine chemistry is suspected. Inflow waters, that should become alkaline, appear to evolve to neutral brines if they remain in contact with atmospheric dust and peripheral colluvium for a long time. On the contrary, inflow waters to soda lakes are generally close to the shore and have only minimal contact with atmospheric dust and peripheral sediments. Presently, all lakes are at relatively low levels, and inflow waters often drain an important area of the drainage basin before reaching the topographic low. In the past, lakes were at higher levels and less interaction occurred between inflow waters, atmospheric dust, and peripheral sediments. As a result, the chemistry of some lakes may have changed during the Quaternary.

Chemical weathering of basaltic lava flows undergoing extreme climatic conditions: the water geochemistry record

This study was dedicated to the early stage of the weathering of historic basaltic flows located in Mount Cameroon. The combination of high relief (i.e. 0 to 4071 m) and high rainfall range (i.e. 1.8 to 12 m/year) lead to strong climatic contrast. Spring and rivers were sampled all around the volcano. We report here the basic chemistry of the waters as well as strontium and uranium isotopic ratios. The combination of the molar proportions of solute obtained with the modal amounts of the minerals in the basalts gives a prediction of what should be the relative molar concentrations of major compounds in the weathering waters issuing from Mount Cameroon. The measured Alkalinity/Si and Mg/Si ratios are higher than the calculated ones while the measured Ca/Si ratio is equal to the calculated value. We suggest that the Si-poor waters of Mount Cameroon are due to biological pumping, trapping of Si in Fe-silicate minerals such as Si containing ferrihydrite and Si interaction with bacterial cell wall leading to the formation of allophane type minerals which were observed in Mount Cameroon soil profiles. Calcium uptake by plants explains the lower Ca/alkalinity ratios measured in the water samples. The water–rock ratio (R) calculated from the strontium isotopic compositions of the water samples, ranges from 29,452 to 367,450. The calculated weathering rates (WR) range from 1 to 20 mm/ky and from 1 to 103 mm/ky for high and low elevations, respectively, and agree with both the thickness and the age of paleosoils found in the same area and with previously published estimates from coupled reaction-transport models. This difference emphasizes the role of vegetation and rainfall at lower elevations as compared to what happens at high elevations.

Geochemical behaviour of silica and magnesium during the evaporation of waters in Chad

Abstract In the Lake Chad system, most of the strongly evaporated waters yield high concentrations in HCO3CO3 and Na, but very low concentrations in Ca and Mg. An experimental evaporation of the diluted original water has shown a Ca depletion due to calcite precipitation and a Mg depletion due to a Mg-silicate formation. The geochemical balance of Mg in the Lake Chad system is discussed.

Modeling mineral/solution interactions: the thermodynamic and kinetic code KINDISP

Abstract The kinetic and thermodynamic geochemical model KINDISP (KINetics of DISsolution and Precipitation) describes the interactions between minerals and aqueous solutions, taking into account the irreversible dissolution of the reactants and the reversible precipitation of secondary products. The general laws included in the model are based on the theory of the Thermodynamics of Irreversible Processes. The water/rock interactions at low temperature are interpreted in a classical manner with the help of the Theory of the State of Transition and the chemistry of surface coordination. The mechanism which limits the rate of mineral dissolution or precipitation, the slowest one in successive irreversible reactions, is represented either by the aqueous molecular diffusion of an elementary entity (atom, molecule, etc.) or by the surface reaction in a broad sense. At each step of the calculation, KINDISP computes the reaction rates for each mineral reacting in the system and selects the slowest rate to represent the dissolution or precipitation law in this phase. The growth of secondary minerals is simulated in the domain of oversaturation (in nonequilibrium) after a nucleation step. The KINDISP model already has been used to simulate natural or induced water/rock interactions, not only at low temperatures, for example to study the effects of acid rain on surface weathering of a granite formation, estimate the formation time of a laterite layer, and the effects of pollution on the environment, but also at higher temperatures, for example to describe and account for diagenetic reactions in sedimentary basins for the purpose of exploiting the reservoirs as well as to study a system of hydrothermal reactions caused by heat storage or disposal of nuclear waste packages.

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.

Calcite veins of the Stripa granite (Sweden) as records of the origin of the groundwaters and their interactions with the granitic body

A Sr isotopic study combined with stable isotope determinations (δ18O and δ13C), petrographic observations and speciation calculations suggests that the Stripa granite (Sweden) contains at least three different types of calcite veins. One type with δ18O = −18 to −24%. (PDB) and 87Sr86Sr = 0.7814 to 1.0696 probably formed at temperatures above 200°C, together with chlorite and epidote, during one or two metamorphic events which are recorded in the Rb-Sr systematics of some minerals of the granite at 1.4 and 0.8 Ga. Another type with δ18O = −12 to −18%. (PDB) and 87Sr86Sr = 0.7406 to 0.7536 and mainly associated with chlorite, is most likely in equilibrium with the presentday groundwaters, which probably have reacted with the fracture minerals of the granitic body for a long time without any supply of external fluids. The third type of calcite with δ18O = −12 to −18%. (PDB), δ13C = −5 to −45%. (PDB) and 87Sr86Sr = 0.7266 to 0.7406, could have formed from reactions involving methane oxidation or sulphate reduction in the presence of bacteria.