Applied Geochemistry (v.22, #1)

This paper reports detailed O2 measurements of pyrite bearing sediments in a column study and their interpretation based on a hydrogeochemical modelling approach. The research focuses on the quantitative effects of effective diffusion and microbiologic activity on pyrite weathering and acidification. A column experiment was set up and O2 saturation and moisture contents were monitored over 100 days. The anoxic material used for the column experiment was taken from a sediment core of a mining waste dump in the southern periphery of the Lohsa storage system in the Lusatia region of Germany. The measured O2 breakthrough curves were modelled using the simulator SAPY, a one-dimensional reactive transport code which considers the kinetics of chemical reactions and the delivery of O2 into the sediment. The simulation yielded a strong dependence of pyrite oxidation on the moisture content which was quantified by an empirical equation. It was shown that the oxidation rate was catalysed by microbial activity exceeding the rate of diffusive O2 delivery. In order to develop a management tool for predictive issues the results have already been applied to natural environments in another study using the adapted model.

Within the framework of the HPF project (Hyperalkaline Plume in Fractured Rock) at the Grimsel Test Site (Switzerland), a small scale core infiltration experiment was performed at the University of Bern. A high-pH solution was continuously injected, under a constant pressure gradient, into a cylindrical core of granite containing a fracture. This high-pH solution was a synthetic version of solutions characteristic of early stages in the degradation of cement. The interaction between the rock and the solutions was reflected by significant changes in the composition of the injected solution and a decrease in the permeability of the rock. Changes in the mineralogy and porosity of the fault gouge filling the fracture were only minor. One-dimensional reactive transport modeling, using a modified version of the GIMRT code, was used to interpret the results of the experiment. Dispersive and advective solute transport, mineral reaction kinetics and a coupling between porosity and permeability changes were taken into account. In order to obtain a reasonable agreement between models and experimental results, reactive surface areas of the order of 105  m2/m3 rock had to be used. These values are much smaller than the values measured for the fault gouge filling the fracture, which are in the order of 106–107  m2/m3 rock. However, the results could be improved by adding a small fraction of fine grained mineral, which could explain the high initial peaks in Al and Si concentration. With the inclusion of this fine grained fraction, the initial surface areas in the model were within the range of the measured specific surface areas of the fault gouge. The fact that the decrease in permeability was significant despite the minor changes in mineralogy, suggests that permeability may be controlled by changes in the structure of the rock (pore geometries) rather than by only the bulk volumetric (porosity) changes.

The cycling of Ni, Zn, Cu in the system “mine tailings–ground water–plants”: A case study by Nikolay V. Sidenko; Elena I. Khozhina; Barbara L. Sherriff (30-52).
The comparative behaviour of Ni, Cu and Zn in the system “mine tailings–ground water–plants” has been investigated at the Ni–Cu mine site operated by INCO Ltd. Thompson Operations, Thompson, Manitoba. Oxidation of sulphide minerals causes the release of metals from exposed tailings containing Ni ∼2000 ppm, Cu ∼150 ppm and Zn ∼100 ppm to the ground water, which contains 350 mg/L Ni, 0.007 mg/L Cu, and 1.6 mg/L Zn. The metal concentration in the ground water is affected by the relative proportions of sulfide minerals, the rate of oxidation of sulphide minerals (Ni-bearing pyrrhotite > sphalerite > chalcopyrite), and the affinity of the metals for secondary Fe-phases (Ni > Zn > Cu).Metals bound to Fe-phases are unavailable for plants, whereas water-soluble, exchangeable and carbonate bound metals in the tailings are considered to be available. The available Ni (340 ppm) was found to be significantly greater than available Cu (2.3 ppm) and Zn (2.5 ppm). The concentration of Ni (290 ppm), in the root system of water sedge (Carex aquatilis Wahl.) is about the same as in the available fraction, whereas Cu and Zn in the roots are higher (42 and 21 ppm, respectively). Therefore, Cu and Zn seem to be actively absorbed by the plants, while Ni diffuses passively into the roots. Nickel and Zn penetrate from the root into the shoot system more readily than Cu. After death, the shoots of the plants from the tailings still accumulate metals to give concentrations of Ni (850 ppm), Cu (97 ppm), and Zn (23 ppm) which were greater than those in the roots and living shoots. In an uncontaminated area, shoots do not absorb metals after death.

The objective of this study was to experimentally simulate interactions between liquid manure and soil in diffusion-dominant areas beneath earthen manure storage (EMS). A previous radial diffusion cell method was modified to include an anaerobic chamber that employed a plastic glove bag supplied with inert Ar gas. Anaerobic conditions were maintained during the entire run time of the experiment. Little oxidation of NH 4 + occurred; consequently, NO 3 - and NO 2 - concentrations were lower than the detection limit. Chloride played a key role in redistribution of major cations and anions resulting from the NH 4 + diffusion. Linear NH 4 + and K+ adsorption isotherms were obtained. The resulting distribution coefficients, K d, for NH 4 + ranged from 0.3 to 0.4 L/kg. Significant NH 4 + exchange reactions led to an average increase in hardness of 137% in the reservoirs, due to extraction of exchangeable Ca and Mg. Geochemical mixing modeling using PHREEQC adequately simulated the linear NH 4 + adsorption at low dissolved NH 4 + concentrations (<30 mM). The predicted manure volumes to cause NH 4 + saturation were 1.0–1.4 mL/g for the glacial clay soil samples.

A system of connected lignite mining pits (part of the former Goitsche mining complex, Germany) was flooded with river water between 1999 and 2002. A considerable accumulation of acid associated with oxidized sulfides in sediments was seen as a critical point for the development of the lake water. To characterize the components contributing to the supply of dissolved lake water SO4 hydro-chemical and isotope investigations with respect to groundwater, pore water in the sulfide bearing sediments, river water and lake water were performed. δ 34S of pore water SO4 that was dominated by oxidized pyrites ranges around −25‰ VCDT and differs strongly from river water SO4 with about +4.4‰. Thus, interactions between lake water and sediments were particularly pronounced during the first phase of flooding. For this period, a more quantitative estimation of the SO4 components in the lake water was difficult because of the heterogeneous SO4 distributions between the different sub-basins of the lake and according to the flooding process itself. Later, a component separation was attempted following mixing of the whole lake, which first occurred in spring 2002. A very heterogeneous groundwater environment with respect to highly variable SO4 concentrations and δ 34S values and changing interaction with the forming lakes proved to be one of the most important limitations in the calculations of the mixing.

The strongly reducing nature of permeable reactive barrier (PRB) treatment materials can lead to gas production, potentially resulting in the formation of gas bubbles and ebullition. Degassing in organic C based PRB systems due to the production of gases (primarily CO2 and CH4) is investigated using the depletion of naturally occurring non-reactive gases Ar and N2, to identify, confirm, and quantify chemical and physical processes. Sampling and analysis of dissolved gases were performed at the Nickel Rim Mine Organic Carbon PRB, which was designed for the treatment of groundwater contaminated by low quality mine drainage characterized by slightly acidic pH, and elevated Fe(II) and SO4 concentrations. A simple 4-gas degassing model was used to analyze the dissolved gas data, and the results indicate that SO4 reduction is by far the dominant process of organic C consumption within the barrier. The data provided additional information to delineate rates of microbially mediated SO4 reduction and confirm the presence of slow and fast flow zones within the barrier. Degassing was incorporated into multicomponent reactive transport simulations for the barrier and the simulations were successful in reproducing observed dissolved gas trends.

Detailed mineralogical and geochemical investigations were conducted within abandoned Pb and Zn flotation tailings at “El Fraile” impoundments in Taxco, Guerrero, central-southern México. These tailings are divided into an active oxidation zone near the surface, an underlying transition zone and an unoxidized zone. Although these tailings have undergone 30 a of sulfide oxidation, the active oxidation zone has only penetrated to a depth of 0.2 m in the settling pond, and to 0.6–1.2 m in the dam. The oxidation of sulfide minerals and the insufficiency of pH-buffering minerals have produced low-pH conditions (pH = 1.9–4.4) and high concentrations of dissolved SO 4 2 - , As and heavy metals: SO 4 2 - (1534–10086 mg L−1), Fe (1.5–2568 mg L−1), Zn (36.7–2435 mg L−1), Cd (0.4–30.6 mg L−1), Pb (<0.01–0.6 mg L−1), Cu (0.5–38.2 mg L−1) and As (0.01–164 mg L−1). These concentrations of dissolved constituents are attenuated by a series of precipitation and sorption reactions. Precipitation of secondary phases, gypsum, goethite, hematite and K-jarosite has led to the formation of cemented layers within the active oxidation zone in the tailings dam. These cemented layers act as a trap for released, potentially toxic elements from the overlying oxidized tailings. Adsorption and coprecipitation on Fe-precipitates play an important role in the mobilization and attenuation of Zn, Cd, Cu and As within the “El Fraile” tailings. Additionally to the well-known ability of Fe-precipitates to strongly trap As and heavy metals, this study shows that precipitation of beudantite (PbFe3AsO4SO4(OH)6) appears to be one of the solid-phase controls on the natural attenuation of As and Pb and other heavy metals in these tailings.

Characteristics of deep groundwater flow in a basin marginal setting at Sellafield, Northwest England: 36Cl and halide evidence by Richard Metcalfe; Mark B. Crawford; Adrian H. Bath; Anna K. Littleboy; Paul J. Degnan; Hugh G. Richards (128-151).
A number of chemical and physical processes inside and outside a sedimentary basin (e.g. evaporite dissolution and topographic drive, respectively) affect groundwater flow near the basin’s margin. Contrasting formations at the margin, typically basinal sedimentary rocks and basement, are host to the interplay between these processes so that groundwater flows and compositions change within a relatively small volume. To interpret how groundwater flow and geochemistry have evolved, interactions between these processes must be understood. Such interactions were investigated near the margin of the East Irish Sea Basin in NW England, by sampling deep groundwaters (to 1500 m below sea level) from Ordovician volcanic basement rocks and Carboniferous to Triassic sedimentary cover rocks. Variable Br/Cl ratios and Cl concentrations in deep saline waters and brines indicate mixing patterns. Variations in 36Cl/Cl constrain the timing of mixing. Relatively low Br/Cl ratios (ca. 1 × 10−3 by mass) characterise brine from the western sedimentary cover and reflect halite dissolution further west. Saline water with relatively high Br/Cl ratios (ca. 2 × 10−3 by mass) of uncertain origin occupies the eastern basement. These two waters mix across the area. However, mixing alone cannot explain variable 36Cl/Cl ratios, which partly reflect differing in situ 36Cl production rates in different rock formations. Most 36Cl/Cl ratios in groundwater sampled from the eastern metavolcanic basement (mean = 25 × 10−15) and western sedimentary cover (mean = 10 × 10−15) are at or close to equilibrium with in situ 36Cl production. These variations in 36Cl/Cl across the site possibly took >1.5 Ma to be attained, implying that deep groundwater flow responded only slowly to the Quaternary glaciation of the site. Interplay between varied processes in basin marginal settings does not necessarily imply flow instability.

Effects of surface-water irrigation on an alluvial aquifer were evaluated using chemical and isotopic data including δ 2H, δ 18O, 3H, δ 3He, Ar, Ne, N2, δ 15N, and 234U/238U activity ratios in a transect of nested wells in the North Platte River valley in western Nebraska, USA. The data were used to evaluate sources and fluxes of H2O, NO 3 - and U, all of which were strongly affected by irrigated agriculture. Combined results indicate that the alluvial aquifer was dominated by irrigation water that had distinctive chemical and isotopic features that were inherited from the North Platte River or acquired from agricultural soils or recharge processes. Values of δ 2H, δ 18O, Ar and Ne indicate that most of the ground water in the alluvial aquifer was irrigation water that was derived from the North Platte River and distributed during the growing season. The irrigation water was identified by an evaporated isotopic signature that was acquired by the river in major upstream reservoirs in Wyoming, and by relatively warm gas-equilibration temperatures related to warm-season recharge. Apparent 3H–3He ages indicate that the ground water in the alluvium was stratified and mostly <30 a old, with apparent recharge rates varying widely from about 0.2 to >3.0 m/a. Age gradients and NO 3 - concentrations indicate that recharge occurred by a combination of focused leakage from irrigation canals (rapid local recharge, low NO 3 - ) and distributed infiltration beneath the irrigated agricultural fields (lower recharge, high NO 3 - ). Large amounts of U with relatively low 234U/238U activity ratios were present in the alluvial aquifer as a result of irrigation with U-bearing river water, and minor amounts of U with higher 234U/238U activity ratios were added locally from basal and underlying volcanic-rich sediments. Distributions of NO 3 - , δ 15 N [ NO 3 - ] , dissolved gases, and ground-water ages indicate that NO 3 - concentrations increased and δ 15 N [ NO 3 - ] values decreased in distributed recharge in the last few decades, possibly in relation to a documented increase in the agricultural use of artificial fertilizers. Canal leakage caused substantial dilution of NO 3 - within the alluvial aquifer, whereas denitrification occurred mainly near the bottom of the alluvium. The average residence time of the irrigation water within the aquifer was relatively short (about 9 a) and reactions such as respiration, denitrification and U exchange in the saturated zone had relatively little effect on the overall composition of the alluvial ground water in comparison to what they might have had in the absence of irrigation recharge.

A review of land–sea coupling by groundwater discharge of nitrogen to New England estuaries: Mechanisms and effects by J.L. Bowen; K.D. Kroeger; G. Tomasky; W.J. Pabich; M.L. Cole; R.H. Carmichael; I. Valiela (175-191).
Hydrologists have long been concerned with the interface of groundwater flow into estuaries, but not until the end of the last century did other disciplines realize the major role played by groundwater transport of nutrients to estuaries. Mass balance and stable isotopic data suggest that land-derived NO3, NH4, and dissolved organic N do enter estuaries in amounts likely to affect the function of the receiving ecosystem. Because of increasing human occupancy of the coastal zone, the nutrient loads borne by groundwater have increased in recent decades, in spite of substantial interception of nutrients within the land and aquifer components of watersheds. Groundwater-borne nutrient loads have increased the N content of receiving estuaries, increased phytoplankton and macroalgal production and biomass, decreased the area of seagrasses, and created a cascade of associated ecological changes. This linkage between land use and eutrophication of estuaries occurs in spite of mechanisms, including uptake of land-derived N by riparian vegetation and fringing wetlands, “unloading” by rapid water removal, and direct N inputs to estuaries, that tend to uncouple the effects of land use on receiving estuaries. It can be expected that as human activity on coastal watersheds continues to increase, the role of groundwater-borne nutrients to the receiving estuary will also increase.

Persistence of lutetium disilicate by María D. Alba; Pablo Chaín (192-201).
RE2Si2O7 is one of the final products of the chemical interaction of radioactive cation simulators and the silicates used in the engineered barrier of deep geological repositories. The aim of this paper is to study the chemical and structural behaviour of Lu2Si2O7 (Lu as actinide tripositive state simulator) in several different chemical systems. The conditions expected at the repositories are simulated in the laboratory with solutions of different compositions and pH. Both the solid products and remnant solutions were analyzed using techniques that give information on the short- and long-range structural order. The results show good stability of Lu2Si2O7, under the studied conditions, and, therefore, encapsulation of the radioactive actinide cations as a disilicate phase could be considered a viable method to ensure the long-term safety of the repositories.

Initial behavior of granite in response to injection of CO2-saturated fluid by Yuko Suto; Lihui Liu; Nakamichi Yamasaki; Toshiyuki Hashida (202-218).
To understand the initial reactions of granite in a CO2-saturated hydrothermal system, experiments were conducted using a batch-type autoclave over a temperature range of 100–350 °C at up to 250 bar and numerical computations of phase equilibria based on the experimental results were carried out. The experiments showed that the dissolution of granite and the deposition of secondary minerals were encouraged by the addition of CO2. Solution chemistry and examination of the granite’s surface texture suggested that its initial dissolution is characterized by the release of Na and Ca (from the dissolution of plagioclase) and that initial precipitation occurs by deposition of some secondary minerals on to plagioclase and/or biotite in the CO2-saturated system. However, the effect of CO2 was small at 350 °C owing to the low activity of H2CO3. According to EDX analysis and numerical phase equilibrium calculations, the secondary minerals formed might be kaolinite, muscovite, smectite and calcite. That is, the granite as a whole might have the potential to take-up dissolved CO2. The results suggest that the alteration of granite under CO2-saturated hydrothermal conditions has the potential to capture CO2 when it is injected at moderate temperatures (150–250 °C) into granite-hosted rock masses.

Measuring the specific caesium sorption capacity of soils, sediments and clay minerals by A. de Koning; A.V. Konoplev; R.N.J. Comans (219-229).
Two methods to quantify the specific Cs sorption capacity of soils and sediments, which is generally believed to be associated with the Frayed Edge Sites (FES) of illitic clay minerals, are described in detail and are critically reviewed. The first method is a direct measurement of the FES capacity, while the second quantifies the combined parameter K D Cs × [ K + ] ( = K C ( K → Cs ) × [ FES ] ) , i.e. the product of the FES capacity and the affinity of these sites for Cs. Both methods use the bulky AgTU-complex to mask non-specific sorption sites for Cs and are applied to a number of different soils and pure minerals. Measurement of the FES capacity of pure illite is straightforward. It is shown that the measured capacity is independent of the saturating ion, but does depend on particle size. This method could not be successfully applied to a peat bog soil with 90% organic matter, because the necessary correction for non-specific Cs sorption by the large pool of organic exchange sites overpasses the capacity of the small FES fraction. Measurement of the combined parameter K D Cs × [ K + ] is shown to be more appropriate in such cases. Application of the FES capacity method to the hydrous aluminosilicate mineral allophane, an important soil constituent of Andisols, shows that the AgTU-complex is unable to block all non-specific sorption sites for Cs on this mineral. The K D Cs × [ K + ] measurements show evidence of a very small number of specific Cs sorption sites on allophane, much smaller than inferred from the FES capacity measurement. The FES capacity of the clay mineral vermiculite is difficult to quantify because the high Cs concentrations that are needed to measure the FES capacity probably cause a collapse of the vermiculite interlayers, thereby creating more high-affinity sites for Cs. The K D Cs × [ K + ] method, in which only trace concentrations of Cs are used, is shown to be more appropriate for soils containing substantial amounts of vermiculite. It is concluded that both the direct FES capacity measurement and the measurement of the combined parameter K D Cs × [ K + ] can be very useful methods to isolate and characterise Cs-selective sorption sites in soils and sediments, but that results should be interpreted with great care.

Predicting the impact of land use on the major element and nutrient fluxes in coastal Mediterranean rivers: The case of the Têt River (Southern France) by Javier Garcia-Esteves; Wolfgang Ludwig; Philippe Kerhervé; Jean-Luc Probst; Franck Lespinas (230-248).
This study presents a detailed discrimination between the natural and anthropogenic sources of dissolved major elements in the Têt River, a typical small coastal river in the south of France. The main objectives were to quantify the materials that were released by human activities in the basin, and to determine the specific element inputs for the major land use forms. The dissolved material fluxes were estimated by weekly monitoring over a hydrological year (2000–2001) along the major water gauging stations, and the flux relationships were examined in the context of anthropogenic and natural basin characteristics as determined by a Geographical Information System (GIS). Intensive agricultural land use in the form of fruit tree cultures and vineyards has a strong control on the dissolved element fluxes in the river. Area specific element releases for these cultures are greatest for SO4, with an estimated average of about 430 ± 18 keq km−2  a−1. This is ⩾11 times the natural SO4 release by rock weathering. Also for K, NO3, PO4 and Mg, the specific releases were ⩾6 times the natural weathering rates (respectively about 44, 60, 4 and 265 keq km−2  a−1). Waste-waters are the other major source of anthropogenic elements in the river. They have an important role for the fluxes of inorganic P and N, but they are also a considerable source of Cl and Na to the river. For example, the average annual release of Cl is around 150 moles/inhabitant in the rural basin parts. Further downstream, however, where population density strongly increases, industrial effluents can enhance this value (>300 moles/inhabitant). The waste-waters contribute more than 70% of the dissolved inorganic N export to the sea, although their contribution to the average DOC export is almost negligible (3%).