Applied Geochemistry (v.18, #1)

A combined approach involving evaluations of historical information, compositional trends, site mineralogy, and forward and inverse geochemical modeling was used to assess the effects of Au mining on ground water quality at the Cripple Creek Mining District. The District is located in a Tertiary volcanic diatreme complex surrounded by Precambrian granite. Historically, mining activity was underground whereas present-day mining occurs in surface mines. Between 1896 and 1941, a series of tunnels was excavated to drain the underground mining areas. The Carlton Tunnel, located about 900–950 m below the surface, is the primary ground water drain for the mining areas. Ground water flowing from the Carlton Tunnel has historically been of good quality. The geochemical processes controlling the quality of the Carlton Tunnel water were the focus of this study. Mineralogical and acid/base accounting data indicate that the diatreme is zoned vertically from an oxidized condition with acidic paste-pH, acidic ground water, and elevated metal concentrations near the surface to an alkaline condition with high pH, elevated SO4, and low metal concentrations at depth. The average travel time of water from the surface to the Carlton Tunnel is estimated to be at least 25a based on 3H determinations. Forward geochemical modeling results indicate that this travel time is sufficient for ground water to reach equilibrium with calcite, gypsum, and fluorite by the time it exits through the Carlton Tunnel. Equilibrium processes have effectively fixed the pH, alkalinity, and SO4 in the Carlton Tunnel water to near-constant levels for at least 24–70a based on comparisons to historically reported water compositions. Inverse geochemical modeling results indicate that there is sufficient neutralization capacity at depth in the diatreme to maintain the current good quality of the ground water flowing from the Carlton Tunnel for the forseeable future, assuming no significant changes in hydrogeochemical conditions.

The controls on the internal neutralization of low productivity, highly acidified waters by sulfide accumulation in sediments are yet poorly understood. It is demonstrated that the neutralization process is constrained by organic matter quality and thermodynamic effects which control the relative rates of SO4 and Fe reduction, and the fate of the reduced Fe and S in the sediments. The investigated sediments were rich in dissolved Fe(II) (0.005–12 mmol l−1) and SO4 (1.3–22 mmol l−1). The pH ranged from 3.0 to 6.8. Contents of reduced inorganic S (0.1–9.5%), molar C/N ratios of the organic matter (12–80) and metabolic turnover rates (1–110 μeq cm−3 a−1) varied strongly. Substantial amounts of Fe sulfides were only found at a simultaneous partial thermodynamic and solubility equilibrium of the involved biogeochemical processes. Sulfide oxidation was apparently inhibited, and SO4 and Fe reduction coexisted. In this type of sediment increases in C availability cause enhanced neutralization rates. In the absence of a partial equilibrium, the sediments were in a sulfide oxidizing and Fe reducing state, and did not accumulate Fe sulfides. The latter type of sediment will increase neutralization rates in response to decreasing deposition of reactive Fe oxides but not necessarily in response to increases in lake productivity by e.g. fertilization measures.

Based on a case study in which a single geochemical anomaly was located in the vicinity of an abandoned mine in Central Portugal, a recursive methodology for anomaly/background separation was developed. This methodology relies on the supplementary projection of each of the samples taken from a subset of ‘anomaly candidates’ onto the axes provided by Principal Components Analysis of the background subset. The concept of ‘anomaly intensity’, defined by the average of the distances from the original to the supplementary projections, is the basis for final anomaly identification.

Accumulation chamber measurements of methane fluxes: application to volcanic-geothermal areas and landfills by C Cardellini; G Chiodini; F Frondini; D Granieri; J Lewicki; L Peruzzi (45-54).
Direct measurement of present day CH4 diffuse degassing from the soil represents an effective tool to better estimate the degassing rate of individual sources and to calibrate global Earth degassing estimates. While many data exist on CH4 emissions from ecosystems, agricultural soils and landfills, few estimates of CH4 emissions from volcanic-geothermal areas have been performed. The authors report results and discuss applications of accumulation-chamber measurements of soil CH4 and CO2 flux from Solfatara of Pozuoli (Naples), Vulcano Island and Poggio dell'Olivo (Viterbo) volcanic-geothermal areas, and the Palma Campania landfill (Naples). Volcanic-geothermal study areas are characterised by vent discharges of fluids with different CH4/CO2 ratios (from 4.7×10−5 to 7.5×10−5, 4.7×10−4 and 2.5×10−3 by weight, for Solfatara of Pozzuoli, Vulcano island, and Poggio dell'Olivo areas, respectively). Soil CH4 fluxes range from 0.003 to 48 g m−2 day−1 in the volcanic-geothermal areas and from 0.0021 to 936 g m−2 day−1 in the landfill, with high spatial variability observed in all areas. Using statistical methods different flux populations were distinguished (i.e. background soil gases and deeply derived gases) and the total gas emissions from study sites calculated. The results of this work show that CH4/CO2 ratios of deep fluids, fumarolic fluids in the case of the volcanic-geothermal environment and biogas in landfills, are roughly maintained in the gas phase diffusely degassed by the soil. Due to high spatial variability, a large number of flux measurements and appropriate statistical methods are needed to estimate total gas discharge from study areas. Furthermore, the simultaneous measurement of diffuse CH4 and CO2 fluxes represents a strong constraint for interpretative models of deep processes associated with soil degassing.

Study of the pore water chemistry through an argillaceous formation: a paleohydrochemical approach by Claude Degueldre; Andreas Scholtis; Andreas Laube; Marı́a Jesús Turrero; Bertrand Thomas (55-73).
The spatial and temporal changes of the pore water along an argillaceous formation were studied on the basis of the chemistries of the fluids collected through the system. The study is based on chemical characterisation results of the pore water, which requires careful sampling and 4 years monitoring. The concept was applied to the water sampled from several boreholes drilled through the Opalinus Clay formation in the anticline at Mont Terri, Canton Jura, Switzerland. Asymmetrical gradients from the clay formation toward the limestone overburden are observed. In this upper formation, recharge with low mineralised water has caused the diffusion of species from the saline pore waters in the clay formation and their depletion. These species form concentration profiles from the underlying Jurensis Marl formation containing hydrophobic organic material toward the upper Lower Dogger Limestone formation. Non-sorbing species such as chlorides, bromides, iodides and sulphates as well as Na as a weakly sorbing cation form increasing concentration profiles from the Lower Dogger Limestone into the Opalinus Clay formation. Heavy water isotopes display similar profiles. The pH increases slightly downward through the system (from about 7 to 8) together with the total organic C (TOC) concentration, while the total inorganic C (TIC) concentration decreases from the Lower Dogger Limestone through the Opalinus Clay formation. A similar profile is observed for pe which decreases from the limestone groundwater (+2.5) toward the underlying marl water (−2.5). The water composition is discussed taking into account in-situ pore water dilution from recharge water by mean of a diffusion mechanism. Quantification of this transport process and of the consequent concentration profile is carried out through the 165 m thick Opalinus Clay layer, which was impregnated by fossil pore water derived from Tethys Ocean, considering a diffusion process that started about 10 Ma ago. The apparent diffusion coefficients estimated in the Opalinus Clay for Cl, Br, I and Na+, on one hand, and, for 2H and 18O on the other are 2.6±0.8 and 5.2±1.5·10−11 m2 s−1, respectively. These values are compared with data gained from other argillaceous systems. The Opalinus Clay formation is likely to have acted as a geological nanoporous barrier for 10 Ma.

The concentrations and δ13C values of atmospheric CO2 were measured in ∼150 air samples collected at 8 sites in the Dallas metropolitan area over the period August 1998 to December 1999. Measured concentrations (C) of atmospheric CO2 ranged from 369 to 475 ppm, while the δ13C values ranged from –12.0 to –8.1‰. These values contrast with a “global” concentration at the time of this study of approximately 367 ppm and a corresponding δ13C value of about –8.0‰. δ13C was linearly correlated with 1/C for samples collected at heights of ∼2 m at 3 sites adjacent to streets with significant automobile traffic. Extrapolation of this two-component mixing line to 1/C=0 yielded a δ13C value of about –27‰ for the CO2 input—i.e., the same as that of gasoline. A simple box model, incorporating photosynthesis, respiration, and anthropogenic addition of CO2, indicates that differences between downwind and upwind concentration-weighted δ13C values ( Δ[C δ13C]) of atmospheric CO2 may be linearly correlated with downwind and upwind differences in concentration ( C d −C u ), where C is reported as mol/m3. The model predicts that measurable effects of photosynthetic withdrawal of atmospheric CO2 are manifested by data arrays with slopes more positive than about –16. This effect of photosynthesis is evident in a linear array of “warm weather”, Dallas atmospheric CO2 data (slope of –12.7‰). Collectively, the data for all 8 sites exhibited considerable scatter about binary mixing lines that depict the addition of CO2 from combustion of natural gas and gasoline. However, when model slopes (m) were calculated for binary mixing between a “background” atmospheric CO2 and each individual sample, it was found that, in general, m increases with decreasing temperature. The effects of photosynthesis and respiration complicate this relationship, but the overall pattern suggests that, as temperature decreases, the proportion of anthropogenic CO2 derived from combustion of natural gas increases. This increase appears to reflect increased use of natural gas for home heating, etc., in cooler weather. Therefore, seasonally changing patterns of fossil fuel use are detectable in the atmospheric CO2 of this urban environment.

Aqueous geochemistry in the Udden pit lake, northern Sweden by Madeleine Ramstedt; Erik Carlsson; Lars Lövgren (97-108).
The Udden pit lake in northern Sweden was studied from June 1998 to February 1999 in order to increase knowledge of the geochemistry in lakes created as a result of decommissioning open pit mines. The vertical water profile in the lake was sampled on 4 different occasions, in June, August, September and February. Water samples were analysed for total concentrations of Fe, As, Cu, Cd, Zn, Pb, Al, Ca, K, Mg, Na, Mn, S, Cl, N and P. Temperature, concentration of dissolved O2, conductivity, pH, and redox potential were measured in situ at different depths. Four layers could be observed in the lake during summer, and 3 layers during winter. A thermocline was observed during summer at a depth of 5 m and on all 4 occasions a halocline was observed at a depth of ∼20 m, and a redoxcline at ∼35 m. Oxygen concentration decreased dramatically at a depth of 20 m. pH increased downwards in the lake from 4.8 at the surface to 6.4 at the bottom of the lake. Geochemical processes occurring in the lake, the origins of the layers, the metal concentrations and the anion concentrations are discussed in this article.

Preliminary data on the presence of Pt, Pd and Au in airborne particulate matter from the urban area of Palermo (Sicily, Italy) are presented. They were obtained by analysing 40 samples of pine needles (Pinus pinea L.) collected in and around the city. Observed concentrations range from 1 to 102 μg/kg for Pt, 1 to 45 μg/kg for Pd and 22 to 776 μg/kg for Au. Platinum and Pd concentrations in pine needles are up to two orders of magnitude higher than their crustal abundances. They exhibit a high statistical correlation (R 2=0.74) which suggests a common origin. Precious metal concentrations measured within the city centre are much higher than those occurring outside the town. The distribution patterns of Pt and Pd in the study area are compared to the distributions of Au and Pb. Gold is enriched at the same sites where Pt and Pd are enriched, while Pb shows some discrepancies. The most probable local source of all of these elements is traffic. Average Pt and Pd emissions in the city area are estimated to be about 136 and 273 g/a, respectively. This study supports the use of pine needles as biomonitors of PGE in the environment.

This paper describes the results of a study that was conducted to determine the relationship between hydrogeochemical composition and 87Sr/86Sr isotope ratios of the Mt. Vulture spring waters. Forty samples of spring waters were collected from local outcrops of Quaternary volcanites. Physico-chemical parameters were measured in the field and analyses completed for major and minor elements and 87Sr/86Sr isotopic ratios. A range of water types was distinguished varying from alkaline-earth bicarbonate waters, reflecting less intense water–rock interaction processes to alkali bicarbonate waters, probably representing interaction with volcanic rocks of Mt. Vulture and marine evaporites. The average 87Sr/86Sr isotope ratios suggest at least 3 different sources. However, some samples have average Sr isotope ratios (0.70704–0.70778) well above those of the volcanites. These ratios imply interaction with other rocks having higher 87Sr/86Sr ratios, probably Triassic evaporites, which is substantiated by their higher content of Na, SO4 and Cl. The Sr isotope ratios for some samples (e.g. Toka and Traficante) are intermediate between the value for the Vulture volcanites and that for the local Mesozoic rocks. The salt content of these samples also lies between the value for waters interacting solely with the volcanites and the value measured in the more saline samples. These waters are thus assumed to result from the mixing of waters circulating in volcanic rocks with waters presumably interacting with the sedimentary bedrock (marine evaporites).

Based on U-series disequilibrium arguments there is good evidence for the presence of U-rich accessory minerals in the outer mantle. The very large excesses of 226Ra and 231Pa activity relative to 230Th, 238U, and 235U in most mid-ocean ridge basalts and some non-divergent plate-margin basalts are inconsistent with prevailing incompatibility models of U-series fractionation. Application of a fundamental principle of equilibrium balance reveals that in these instances more than half of the original U and Th remains behind in the residual outer mantle when basaltic magmas separate. One is forced to conclude that, in the outer mantle, U and Th do not occur primarily in major silicate minerals, where they would indeed be incompatible. Rather, they must be occurring as high concentration components in refractory accessory minerals. The precipitous concentration gradients bounding such minerals would allow for the operation of physical processes, such as alpha recoil and daughter diffusion, to produce paired disequilibrated phases. Daughter deficiencies would develop in the high concentration minerals, and daughter excesses in the surrounding low concentration major silicates. This would constitute a steady-state condition existing in the mantle before the onset of melting. Subsequent preferential melting of the matrix silicates would readily result in the disequilibria observed in basalts.

Rare earth elements as indicators of groundwater environment changes in a fractured rock system: evidence from fracture-filling calcite by Seung-Gu Lee; Dae-Ha Lee; Yongje Kim; Byong-Gon Chae; Won-Young Kim; Nam-Chil Woo (135-143).
Rare earth element (REE) abundances in core samples from Precambrian crystalline rocks at the Samkwang Mine site provide evidence of the solution chemistry involved in precipitation of calcite on fractures. The rock types collected in core samples are mainly banded-gneisses, with mineral assemblages dominated by biotite, K-feldspar, quartz and plagioclase. Calcite, chlorite, muscovite and sericite occur as secondary minerals, with calcite being the main filling material in fractures. In general, the core samples from 4 boreholes are enriched in light REE (LREE) and depleted in heavy REE (HREE), with negative Eu anomalies. However, positive Eu anomalies also occur at specific depths within 3 boreholes. Variation of chondrite-normalized REE patterns results from the fracture-filling calcite in core samples. Calcite fracture fillings provide a record of paleo-hydrology, where Eu has been reduced and selectively concentrated in the solutions from which calcite has precipitated.

A karstic flow system in the upper Lost River drainage basin in south central Indiana, USA, was investigated using SO4 concentration and δ 34SSO4 as tracers. The flow system was characterized as vadose flow and phreatic diffuse flow. Vadose-flow samples were collected from 7 epikarstic outlets after storm events. Phreatic diffuse flow samples were collected from the Orangeville Rise, the major emergence point for the drainage basin, during the base flow periods. Discharge from the Orangeville Rise was constant during the base flow periods but showed large variations in flow rate (0.3–11.7 m3/s), SO4 concentration (11–220 mg/l), and δ 34SSO4 (+5.2 to +15.0‰) after storm events, due to the mixing of rain, vadose flow, and phreatic diffuse flow in the conduits that feed the Orangeville Rise. Sulphate concentrations and δ 34SSO4 were unique in vadose flow (SSO4 : 13–24 mg/l; δ 34SSO4 : +1.9 to +3.8‰) and phreatic diffuse flow (SO4: 220 mg/l; δ 34SSO4 : +15.0‰). Mean SO4 concentration of rainwater in the study area was measured as 1.8 mg/l. Using a 3-component mixing model for water in the karstic conduits, the mixing ratios of rain (16.5%), vadose flow (58.5%), and phreatic diffuse flow (25.0%) components were calculated in the Orangeville Rise discharge. These mixing ratios attained using SO4 concentration as a tracer indicated the important role of the vadose zone as a water storage area in karst aquifers.