Applied Geochemistry (v.17, #9)

Role of sediment composition in trace metal distribution in lake sediments by L El Bilali; P.E Rasmussen; G.E.M Hall; D Fortin (1171-1181).
Sediment cores were collected from 20 lakes from the Muskoka region of Ontario, Canada, to study vertical changes in trace metal concentrations with depth and the distribution of metals amongst humic material, amorphous and crystalline Fe and Mn oxides, insoluble organics/sulphides, and silicates. Based on their total concentrations, trace elements displayed different degrees of affinity for the organic fraction (represented by organic C) and the mineral fraction (represented by Al). Certain elements (Hg, As, Sb, Pb, Cd, and Zn) displayed a positive correlation with organic C, a negative correlation with Al, and enrichment in surface sediments (with enrichment factors ranging from 2 to 24). Detailed speciation studies revealed that these elements were associated mainly with humic material and to a lesser extent with oxides in surface sediments. Other elements (Al, Cr, Co, Fe, and Mn) displayed a negative correlation with organic C, a positive correlation with Al, and no consistent enrichment in their total concentration at the surface. The speciation study revealed that metals of the latter group were mainly associated with the silicate fraction in both surface and deep sediments. This study shows that relative affinities for organic and mineral fractions play an important role in the distribution of trace metals during burial and diagenesis, and hence in the shape of their vertical profiles.

Watersheds in mineralized zones may contain many mines, each of which can contribute to acidity and the metal load of a stream. In this study the authors delineate hydrogeologic characteristics determining the transport of metals from the watershed to the stream in the watershed of Cement Creek, Colorado. Combining the injection of a chemical tracer, to determine a discharge, with synoptic sampling, to obtain chemistry of major ions and metals, spatially detailed load profiles are quantified. Using the discharge and load profiles, the authors (1) identified sampled inflow sources which emanate from undisturbed as well as previously mined areas; (2) demonstrate, based on simple hydrologic balance, that unsampled, likely dispersed subsurface, inflows are significant; and (3) estimate attenuation. For example, along the 12-km study reach, 108 kg per day of Zn were added to Cement Creek. Almost half of this load came from 10 well-defined areas that included both mined and non-mined parts of the watershed. However, the combined effect of many smaller inflows also contributed a substantial load that could limit the effectiveness of remediation. Of the total Zn load, 58.3 kg/day came from stream segments with no visible inflow, indicating the importance of contributions from dispersed subsurface inflow. The subsurface inflow mostly occurred in areas with substantial fracturing of the bedrock or in areas downstream from tributaries with large alluvial fans. Despite a pH generally less than 4.5, there was 58.4 kg/day of Zn attenuation that occurred in mixing zones downstream from inflows with high pH. Mixing zones can have local areas of pH that are high enough for sorption and precipitation reactions to have an effect. Principal component analysis classified inflows into 7 groups with distinct chemical signatures that represent water-rock interaction with different mineral-alteration suites in the watershed. The present approach provides a detailed snapshot of metal load for the watershed to support remediation decisions, and quantifies processes affecting metal transport.

Acid sulphate soils, common in the coastal areas of Finland, contribute strongly to high acid, S and metal loadings on adjacent surface waters. This, in turn, is causing significant harm to the aquatic ecology. There is, however, limited knowledge on the total amounts of acidity and chemical elements leached from these soils. The overall objective of this study was to determine geochemical patterns in acid sulphate soils and their parent sediments and, based on the identified patterns assess the extent, mechanisms and present state of leaching of major and trace elements from these soils. The distribution of pH, aqua regia extractable concentrations of P and metals (Al, Ba, Ca, Co, Cr, Cu, Fe, K, La, Mg, Mn, Na, Ni, Sr, Th, Ti, V, Zn) and total concentrations of S and C were determined in 30 vertical profiles collected in the 23 km2 large Rintala agricultural area (mid-western Finland) underlain largely with S-rich sediments. It was found that approximately 70% of the area consists of acid sulphate soils with a minimum pH<4.0, an average depth of 1.8 m, and S concentrations in the parent sediments varying from 0.24 to 1.04%. Acid sulphate soils have not developed where the S concentrations in the sediments are ⩽0.10% or where the concentrations of organic C in the soil zones are >4%. Four different methods were used to estimate the losses of chemical elements from the acid sulphate soils: (1) the concentrations in the soil were compared with those in the parent sediments, (2) due to indicated heterogeneities in several profiles, the vertical changes of the immobile Ti was used to re-calculate element losses, (3) element depletions in the acid sulphate soils (as compared to those in the parent sediments) were compared to the corresponding depletions in the non acid sulphate soils, (4) element concentrations in drainage waters were compared with those in the parent sediments. Based on these calculations, it was assessed that the percentual leaching of the aqua regia extractable fraction (total for S) has been as follows: S (40–50%), Na (30–50%), Mn (25–35%), Sr (15–20%), Ca–Ni–Co (approximately 10%), Mg–K–Zn (5–10%), Th–La–Cu–Al–P–Ti–Fe (<5%), and Ba–Cr–V (<1%). While it was possible to quite accurately estimate the percentages and thus the amounts of elements lost, it was not possible to estimate the rate of leaching as there is no available detailed information on dates when ditching activities and thus oxidation-acidification processes started. Other calculations indicated that the mobile S reservoir is still some 15 ton/hectare, which is huge but still smaller than the losses that have occurred since the area was drained (23–28 ton/hectare).

Three natural chlorites have been equilibrated with kaolinite (±quartz±gibbsite±hematite) in aqueous MgCl2 or NaCl solutions at saturated vapor conditions (P V=P H2 O ). The compositions of the chlorites, (Fe0.60 3+Fe5.43 2+Mg2.30Al2.98Mn0.05Ca0.03Zn0.010.60)(Si5.63Al2.37)O20(OH)16, (Al2.33Fe1.00 2+Fe0.14 3+Ca0.02Mn0.01Ni0.02Cr0.01Mg8.400.07)(Si5.66Al2.34)O20(OH)16 and (Si5.26Al2.74)(Al1.94Ti0.28Fe6.16 2+Fe0.56 3+Mn0.05Mg2.05Ca0.31P0.19V0.010.46)O20(OH)16 were determined from X-ray fluorescence (XRF) and Mössbauer spectroscopy. The experiments were conducted in sealed bottles (LDPE or PTFE) or Teflon-lined (PTFE) reaction vessels, and the starting solution compositions were constructed such that equilibrium boundaries were approached from high and low values of log a Mg 2+ 0.5 a H + , log aNa+ aH+ and log a SiO aq . The attainment of stable equilibrium is indicated by the reversibility of fluid-mineral equilibria, agreement of results obtained in the different aqueous media, and the variance of the 6-component system, MgO–Al2O3–SiO2-Fe2O3–FeO–H2O. To fully reflect the complexity of the equilibria being investigated, a number of reactions (with corresponding equilibrium constants) have been used to model the behavior of each chlorite under isothermal, isobaric conditions. These solution equilibration data also validate the applicability of solubility product conventions to chlorite solubility data and thus contradict the general presumption that the behaviors of complex layer silicates in aqueous solutions are not amenable to the law of mass action.

This study aimed to develop a methodology for assessing the hydrochemical evolution of a groundwater system, using fracture-filling and fracture-lining calcite. Fracture calcite in deep (to ca. 1000 m) granitic rocks of the Tono area, central Japan, was investigated by optical and electron microscopy, and chemical and isotopic analysis. Coupled with geological evidence, these new data imply 3 main origins for the waters that precipitated calcite: (1) relatively high-temperature hydrothermal solutions, precipitating calcite distinguished by δ18OSMOW from −3 to ca. 10‰, and with δ13CPDB from ca. −18 to −7‰; (2) seawater, probably partly of Miocene age, which precipitated calcite distinguished by δ13CPDB of ca. 0‰ and δ18OSMOW > ca. 20‰; (3) fresh water, with a variable δ13CPDB composition, but which precipitated calcite distinguished by δ13CPDB that was significantly < 0‰ and as low as ca. −29‰ and δ18OSMOW > ca. 17‰. Data for 14C suggest that at least some of the fresh-water calcite formed within the last 50 ka. The present day hydrogeological regime in the Tono area is also dominated by fresh groundwater. However, the marine calcite of probable Miocene age found at depth has shown no evidence for dissolution and many different calcite crystal forms have been preserved. Studies of other groundwater systems have correlated similar crystallographic variations with variations in the salinity of coexisting groundwaters. When this correlation is applied to the Tono observations, the calcite crystal forms imply a similar range of groundwater salinity to that inferred from the isotopic data. Thus, the present study suggests that even in presently low-salinity groundwater systems, calcite morphological variations may record the changing salinity of coexisting groundwaters. It is suggested that calcite morphological data, coupled with isotopic data, could provide a powerful palaeohydrogeological tool in such circumstances.

Origin of methane in quartz crystals from the Tertiary accretionary wedge and fore-arc basin of the Western Carpathians by Vratislav Hurai; Jan Kihle; Júlia Kotulová; Fero Marko; Anna Świerczewska (1259-1271).
Quartz crystals from mineralised joints of the Carpathian flysch contain fluid inclusions covering the compositional range from gaseous CH4±CO2±C2+ mixture, through CH4-saturated oil, to CH4-saturated aqueous solution. The C isotope composition of CH4 extracted from the fluid inclusions (−31 and −36‰ V-PDB) is indicative of a thermogenic dry gas released from overmature rocks, although the maturation level of the surrounding flysch corresponds to a wet-gas generating zone. Trapping PT conditions for the CH4-bearing fluids range between 130 and 205 °C and 0.5 and 3.7 kbar. The fluid inclusion-derived crystallisation temperatures are inconsistent with the vitrinite reflectance data. Strongly fluctuating fluid pressures during growth of the quartz crystals indicate the existence of a crack-seal mechanism with recurrent hydrostatic and lithostatic regimes. Fluid pressures exceeding the lithostatic load have been recorded in the Dukla nappe. The overpressures have been generated by fluid volume increase due to thermal decomposition of oil and kerogen to CH4 and pyrobitumen. Occurrence of oil and condensate in the CH4-bearing inclusion fluids trapped at temperatures considerably above oil window suggest a short-termed, transient post-orogenic heating, rather than a prolonged heating related to burial. The thermal event giving rise to the quartz crystallisation has been coincidental with regional collapse and uplift of the Western Carpathians.