Applied Geochemistry (v.19, #4)

Geochemistry applied to the watershed survey by Yves Tardy; Vincent Bustillo; Jean-Loup Boeglin (469-518).
A periodic sampling (85 samples) collected fortnightly along 3 annual hydrological cycles (1990–1993) in the Niger River basin, at the outlet of Bamako (Mali), allowed the calibration of an hydrochemical model based on the hydrograph separation. As a first step, 5 reservoirs are identified: Rr the rapid runoff, Rs the superficial runoff, Rh the hypodermic or differred runoff, Ns the superficial ground water, and Np the deep ground water, also called base flow. In each reservoir, the physico-chemical composition of water is supposed to be constant with time. Along the hydrograph, or total discharge curves, and during 3 hydrological cycles, the relative proportions of each of the reservoirs fluctuate continuously. The methodology, processing step by step, is first calibrated, by using concentrations of specific tracers: dissolved Na+, HCO3 , and suspended sediments (TSS). Then, the proportions of Rr, Rs, Rh, Ns, and Np, contributing to the total flow measured at each instant of sampling, are calculated. As a second step, pH and the concentrations of aqueous species (K+, Ca2+, Mg2+, Cl, SO4 2−, DOC, SiO2), are in turn calculated by regression analysis in each reservoir. Finally a test of validity of the method is presented as a very close correlation between measured and predicted fluctuating concentrations of each of the elements. From that satisfactory stage, the geochemical budget of weathering is conducted, with particular attention to SiO2 release and CO2 consumption. Parent-rock mineralogical compositions contributing to dissolved species in the total discharge as well as in the individual flow components have been reconstituted. Rates of both chemical erosion (soil profile formation by weathering), and mechanical erosion (soil profile and landscape denudation) have been evaluated. The methodology, successfully applied to the Niger basin, is proposed as a strategic tool for studying the watershed dynamics for any time and space scales. The model revealed its ability to extrapolate and predict the geochemical or the environmental behaviour of such basins, naturally submitted to large secular or annual, regular or even catastrophic climatic oscillations.

The hydrogeochemistry of the Lac du Bonnet granitic batholith has been determined for the region of the Whiteshell Research Area (WRA) in southeastern Manitoba, Canada. This work forms part of the geosciences studies performed for the Canadian Nuclear Fuel Waste Management Program over the period 1980–1995 by Atomic Energy of Canada Limited (AECL). Knowledge of the variation of groundwater chemistry and its causes is useful in assessing the performance and safety of a nuclear fuel waste vault located at depths of up to 1000 m in a crystalline rock formation of the Canadian Shield. Groundwaters and matrix pore fluids have been obtained by standard sampling methods from shallow piezometers in clay-rich overburden, from packer-isolated borehole zones intersecting fractures or fault zones in the bedrock, and from boreholes in unfractured rock in AECL's Underground Research Laboratory (URL). Eighty-six individual fracture groundwaters have been sampled and analysed from permeable zones in 53 boreholes drilled to depths of up to 1000 m in the Lac du Bonnet batholith. In addition, 28 groundwaters from piezometers in a large wetland area near the URL have been sampled and analysed to determine the influence of clay-rich overburden on the bedrock hydrogeochemistry. Analyses have been made for major and minor ions, pH, Eh, trace metals, and stable and radioactive isotopes, to characterise these groundwaters and relate them to their hydrogeologic regimes. Shallow groundwaters in the fractured bedrock are generally dilute (TDS <0.3 g/l), Ca–Na–HCO3 waters and show little indication of mixing with Ca–Mg–HCO3–SO4 groundwater from overburden sediments. The near-modern levels of 3H and 14C, and a warm-climate 2H/18O signature in these groundwaters, indicates that the upper ∼200 m of fractured bedrock contains an active groundwater circulation system with a residence time of tens to hundreds of years. Deeper fracture groundwaters (200–400 m depth) in recharge areas, are more alkaline, Na–Ca–HCO3 waters and evolve to Na–Ca–HCO3–Cl–SO4 waters with increasing distance along the flow path. Isotopic data indicate the presence of a glacial melt-water component suggesting that the residence times of these waters are 103–105 a. These waters form a transition zone between the upper, advective flow regime and a deeper regime in sparsely fractured rock where groundwater in fractures and fracture zones is largely stagnant. At these depths (> 500 m), Na–Ca–Cl–SO4 waters of increasing salinity (up to 50 g/l) with depth are found and in some fractures the waters have evolved to a Ca–Na–Cl composition. Isotopic data indicate that these waters are warm-climate and pre-glacial in origin, with residence times of over 1 Ma. Pore fluids observed to drain from the unfractured rock matrix in the URL facility are almost pure Ca–Cl in composition, ∼90 g/l salinity, and have a 2H/18O composition displaced well to the left of the global meteoric water line, about which all other WRA groundwaters lie. This information indicates that these pore fluids have undergone prolonged water-rock interaction and have residence times of 101–103 Ma. Most of the deeper fracture groundwaters and pore fluids have low Br/Cl ratios and moderate to high δ 34S values of dissolved SO4 which indicates that their salinity could be derived from a marine source such as the basinal sedimentary brines and evaporites to the west of the batholith. These fluids may have entered the batholith during early Paleozoic times when sedimentary rocks were deposited over the granite and were driven by a hydraulic gradient resulting from higher ground in western Canada. The hydrogeochemical data and interpretations show that below ∼500 m in the WRA, fracture-hosted groundwaters are very saline, reducing and old, and are, therefore, indicative of stagnant conditions over the period of concern for nuclear waste disposal (1 Ma). The intact rock matrix at these depths is extremely impermeable as indicated by the presence of pore fluids with unusual geochemical and isotopic characteristics. The pore fluids may represent basinal brines that have evolved geochemically and isotopically to their current composition over periods as long as 103 Ma.

The isotopic composition (18O and 2H) of rainwater and groundwater was determined in the Yunquera-Nieves hydrogeological unit, a karstic massif in S Spain with complex orography and hydrogeology. The aims were to identify: (1) the source of the water, (2) the main factors determining the isotopic content, (3) the catchment area of the springs and (4) the aquifer behaviour. The isotopic content of the sampled waters is of mixed Atlantic–Mediterranean origin and is opposite to the quantity of rainwater distribution, both in space and time. Spatially, water is isotopically less depleted toward the eastern sector, where rainwater quantity is lower (which is locally affected by the orography): thus, it is possible to differentiate the catchment areas of the main springs. Aquifer recharge is produced by winter precipitation, when rainfall water is isotopically more negative and evaporation is lower. The decrease in rainfall quantity during the study period provoked an increase in δ18O and δ2H values, both in rainwater and in groundwater. The recorded variations of the isotopic content of the rainfall are rapidly evident in the spring waters, which demonstrates a well developed karst network inside the carbonate aquifers, and thus the existence of conduit flow systems.

Weathering rates and anthropogenic influences in a sedimentary basin, São Paulo State, Brazil by Fabiano Tomazini da Conceição; Daniel Marcos Bonotto (575-591).
The weathering rate of rocks and chemical dynamics of the Corumbataı́ River basin, São Paulo State, Brazil, were evaluated using major elements as natural tracers. This basin has serious environmental problems in terms of quality of surface and rainwater, which affect the determination of weathering rate. The Corumbataı́ River, downstream from Rio Claro City, receives several elements/compounds through anthropogenic activities, with only K, SO4 2− and alkalinity yielding positive flux values. The negative flux of some anions/cations can be attributed to atmospheric loading mainly related to anthropogenic inputs, providing K a value of 16.7 ton/km−2a−1 for the material removed by weathering in the Corumbataı́ River basin. This is equivalent to 26×106 kg of rock being removed each year by the Corumbataı́ River. The instantaneous flux was found to be a function of discharge, with the majority of dry residue (dissolved load) being transported during the summer (wet) months. The removed material in Corumbataı́ River basin derives mainly from two sub-basins (Cabeças River and Passa Cinco River), where the sandstones weather more easily than siltstones and claystones in the basin.

In July 2001, samples of surface suspended particulate material (SPM) of the Irtysh river in its middle and lower reaches (from Omsk City to the confluence with the Ob river) and its main tributaries were collected (18 stations along 1834 km). The SPM samples were analyzed for major and trace element composition. The results show that the geochemistry of Irtysh river SPM is related to landscape and geochemical peculiarities of the river basin on one hand and to industrial activities within the drainage area on the other hand. In the upper basin polymetallic and cinnabar deposits and phosphorite deposits with high As content are widespread. The open-cut mining and developed oil-refining, power plants and other industries lead to the contamination of the environment by heavy metals and other contaminants. The territory of the West Siberian lowland, especially the Ob-Irtysh interfluve, is characterized by the occurrence of swamps and peat-bogs. Tributaries of the Irtysh river originating in this region, have a brown color and the chemical composition of the SPM is specific for stagnant water. In the first 500–700 km downstream from Omsk City the Irtysh river has the typical Al–Si-rich suspended matter composition. After the inflow of the tributaries with brown water the SPM composition is significantly changed: an increase of POC, Fe, P, Ca, Sr, Ba and As concentrations and a strong decrease of the lithogenic elements Al, Mg, K, Na, Ti, Zr can be observed. The data show that Fe-organic components (Fe-humic amorphous compounds, which contribute ca. 75–85% to the total Fe) play a very important role in SPM of the tributaries with brown water and in the Irtysh river in its lower reaches. Among the trace metals significant enrichments relative to the average for global river SPM could only be observed for As and Cd (coefficient of enrichment up to 16 for As and 3–3.5 for Cd). It can be shown that the enrichment of As in the SPM is related to natural processes, i.e. the weathering of phosphate containing deposits with high As concentrations in the upper Irtysh basin and the high As–P affinity in the swamp peaty soil. Dissolved P and As are absorbed by amorphous organic C/Fe oxyhydroxide components which act as carriers during the transport to the main stream of the Irtysh river. The role of anthropogenic factors is probably insignificant for As. In contrast, the enrichment of Cd is mainly related to anthropogenic input. The threefold enrichment of Cd in the SPM just below Omsk City and its continuous decrease down to background level at a distance of 500–700 km downstream points quite definitely to the municipal and industrial sewage of Omsk City as the main source of Cd in the SPM of the Irtysh river.

Subsurface aeration is the in situ oxidation of Fe from groundwater that is used to make drinking water potable. When subsurface aeration is applied to an anaerobic groundwater system with pH>7, Fe(II) is oxidised heterogeneously. The heterogeneous oxidation of Fe(II) can result in the in situ formation of Fe colloids. To study this, the effect of substances commonly found in groundwater (e.g. PO4, Mn, silicate and fulvic acid) on the heterogeneous oxidation process was measured. The heterogeneous oxidation of Fe(II) becomes retarded when PO4, Mn, silicate or fulvic acid is present in the groundwater in addition to Fe(II). Phosphate and fulvic acid retarded the oxidation process most. The heterogeneous oxidation was described using a model with a homogeneous (k 1) and an autocatalytic oxidation rate constant (k2). From the modelling it followed that the homogeneous oxidation rate constant was not affected or even slightly elevated whereas the autocatalytic oxidation rate constant decreased remarkably by the addition of PO4, Mn, silicate or fulvic acid. From speciation calculations it followed that the decreased availability of the Fe(II) species can only explain a small part of the retarded autocatalytic oxidation process. Therefore exploratory calculations were performed to gain insight into whether the adsorption of PO4 or fulvic acid could explain the retarded autocatalytic oxidation. These calculations showed that the adsorption of fulvic acid could explain the retarded autocatalytic oxidation process. In contrast the adsorption of PO4 only partly explained the retarded autocatalytic oxidation process. In terms of colloid formation this study shows that the heterogeneous oxidation of Fe(II) in presence of PO4, Mn, silicate or fulvic acid leads to the formation of Fe colloids.

This study concerns the application of multiple correspondence analysis and factorial kriging analysis to soil data, and aims to identify spatial patterns and superficial soil anomalies of the Au and Ag deposit at Marrancos, Vila Verde. The mineral deposit can be described as a quartz auriferous shear-zone, consisting of a quartz breccia of tectonic origin hosted by metamorphic rocks (hornfels). Gold is associated with arsenopyrite and pyrite, and Ag with galena and galenobismuthite. A total of 286 soil samples were analysed for Fe, Cu, Zn, Pb, Co, Ni, Mn, Ag and Bi by atomic absorption spectrometry, As, Se, Te and Sb by atomic absorption spectrometry–hydride generation system and Au by inductively coupled plasma–atomic emission spectroscopy after extraction of the metal by an organic solvent (methyl-isobutylketone). The methodology used included (a) multiple correspondence analysis applied to soil data to obtain some factors that summarize geochemical information, (b) a structural analysis (variography) in order to account for spatial variability of these factors, and (c) factorial kriging analysis used to split these factors into their spatial components. This methodology allowed an efficient multi-element characterization of the spatial patterns as well as the identification and interpretation of significant anomalies, not always associated to Au-bearing geological structures.

Mobilization of aluminium and magnesium by roots of banana (Musa spp.) from kaolinite and smectite clay minerals by Gervais Rufyikiri; Didier Nootens; Joseph E. Dufey; Bruno Delvaux (633-643).
Banana plants (Musa spp.) are very sensitive to Al, which is mobilized in acid soil conditions. These plants may, however, contribute to their own intoxication because their roots can excrete protons in large quantities. The authors studied the mobilization of Al by banana roots from clay minerals in experimental designs exacerbating the root–mineral contact. The plants were grown on agarose-gel or sand substrates previously mixed with smectite (montmorillonite) and kaolinite as sole source of Al. The pH and the ion concentrations in the aqueous and exchangeable phases of the substrates were determined as concentrations of Al, Ca, Mg and K in plants. In both agarose and sand substrates, pH significantly decreased in the close vicinity of roots, relative to the bulk substrate. This root-induced acidification involved a preferential mobilization of Al in kaolinite substrates and of Mg in smectite substrates, and thereby a significant plant uptake of Al and Mg from these respective substrates. Root-induced weathering of kaolinite and montmorillonite thus suggests that the mobilization of, respectively, Al and Mg are the limiting steps in the dissolution of these respective minerals, just as demonstrated in previous chemical weathering studies.