Applied Geochemistry (v.20, #9)

Secondary mineral–fluid equilibria in the Krafla and Námafjall geothermal systems, Iceland by Björn Thór Gudmundsson; Stefán Arnórsson (1607-1625).
Aquifer fluid compositions and aqueous species distribution have been calculated for 22 samples collected from producing wells in the Krafla and Námafjall geothermal areas, Iceland. Overall mineral–solution equilibrium is rather closely approached in the aquifer beyond the depressurization zone around wells for all major components entering hydrothermal minerals. The minerals involved are in alphabetical order: albite, calcite, chlorite, epidote, K-feldspar (adularia), prehnite, pyrite, pyrrhotite, quartz and wairakite. Calculated saturation indices for OH-bearing Al–silicates show more scatter than for minerals of simpler composition. A large scatter is also observed for pyrite and pyrrhotite. The main uncertainty involved in calculating mineral saturation indices, particularly in the case of “excess” enthalpy well discharges, lies in the model adopted to calculate the aquifer water composition and its aqueous species distribution and not in the quality of the thermodynamic data on the aqueous species and the minerals with the possible exception of epidote and Fe2+.

Isotopic composition of NO3 (δ 15NNO3 and δ 18ONO3 ) and B (δ 11B) were used to evaluate NO3 contamination and identify geochemical processes occurring in a hydrologically complex Basin and Range valley in northern Nevada with multiple potential sources of NO3. Combined use of these isotopes may be a useful tool in identifying NO3 sources because NO3 and B co-migrate in many environmental settings, their isotopes are fractionated by different environmental processes, and because wastewater and fertilizers may have distinct isotopic signatures for N and B. The principal cause of elevated NO3 concentrations in residential parts of the study area is wastewater and not natural NO3 or fertilizers. This is indicated by some samples with elevated NO3 concentrations plotting along δ 15NNO3 and NO3 mixing lines between natural NO3 from the study area and theoretical septic-system effluent. This conclusion is supported by the presence of caffeine in one sample and the absence of samples with elevated NO3 concentrations that fall along mixing lines between natural NO3 and theoretical percolate below fertilized lawns. Nitrogen isotopes alone could not be used to determine NO3 sources in several wells because denitrification blurred the original isotopic signatures. The range of δ 11B values in native ground water in the study area (−8.2‰ to +21.2‰) is large. The samples with the low δ 11B values have a geochemical signature characteristic of hydrothermal systems. Physical and chemical data suggest B is not being strongly fractionated by adsorption onto clays. δ 11B values from local STP effluent (−2.7‰) and wash water from a domestic washing machine (−5.7‰) were used to plot mixing lines between wastewater and native ground water. In general, wells with elevated NO3 concentrations fell along mixing lines between wastewater and background water on plots of δ 11B against 1/B and Cl/B. Combined use of δ 15N and δ 11B in the study area was generally successful in identifying contaminant sources and processes that are occurring, however, it is likely to be more successful in simpler settings with a well-characterized δ 11B value for background wells.

Marine sediments preserve useful information for reconstructing oceanographic conditions and environmental changes in the past. In this study, the authors have evaluated an efficient near-infrared reflectance spectroscopy (NIRS) method for simultaneous determination of total C (TC), total inorganic C (TIC), total organic C (TOC), magnetic susceptibility (MS), the ratio of di- and tri-unsaturated C37 alkenones ( U 37 k ′ ) , and δ 18O of foraminiferal shells in marine sediments. Near-infrared diffused reflectance spectra (1100–2500 nm) of 172 sediments recovered from the IMAGES Core MD972151, located in the southwestern South China Sea, were analyzed and correlated with the abovementioned parameters using partial least squares regression (PLSR) procedures. The leave-one-out cross-validation results showed that TC and TIC were successfully predicted (r 2  > 0.93) while MS, U 37 k ′ , and δ 18O were estimated with moderate accuracy (r 2  > 0.76). The concentrations of TOC, however, could not be estimated with high precision (r 2  = 0.34), which is possibly a result of the low and narrow range of organic C in this specific core. The results indicate that NIRS has the potential to be a rapid analytical tool for continuous marine core examination, although uncertainties associated with TOC need further investigation.

Municipal compost-based mixture for acid mine drainage bioremediation: Metal retention mechanisms by Oriol Gibert; Joan de Pablo; José Luis Cortina; Carlos Ayora (1648-1657).
An upflow packed column was operated to evaluate the potential of a mixture of municipal compost and calcite to promote sulphidogenesis in the remediation of a simulated mine water at high flows (>0.1 m d−1). Results showed that the pH was neutralised and metals (Fe, Al, Zn, Cu) were significantly removed. Metal removal was attributed to the combined result of precipitation as metal (oxy)hydroxides and carbonates, co-precipitation with these (oxy)hydroxides and sorption onto the compost surface rather than to precipitation as metal sulphides. The two last mechanisms are especially significant for Zn, whose hydroxide is not expected to precipitate at pH 6–7. Before the saturation of compost sorption sites, 60% of the influent Zn was estimated to have been removed by co-precipitation with Fe- and Al-(oxy)hydroxide and 40% by sorption onto the municipal compost.

Groundwater recharge, circulation and geochemical evolution in the source region of the Blue Nile River, Ethiopia by Seifu Kebede; Yves Travi; Tamiru Alemayehu; Tenalem Ayenew (1658-1676).
Geochemical and environmental isotope data were used to gain the first regional picture of groundwater recharge, circulation and its hydrochemical evolution in the upper Blue Nile River basin of Ethiopia. Q-mode statistical cluster analysis (HCA) was used to classify water into objective groups and to conduct inverse geochemical modeling among the groups. Two major structurally deformed regions with distinct groundwater circulation and evolution history were identified. These are the Lake Tana Graben (LTG) and the Yerer Tullu Wellel Volcanic Lineament Zone (YTVL). Silicate hydrolysis accompanied by CO2 influx from deeper sources plays a major role in groundwater chemical evolution of the high TDS Na–HCO 3 type thermal groundwaters of these two regions. In the basaltic plateau outside these two zones, groundwater recharge takes place rapidly through fractured basalts, groundwater flow paths are short and they are characterized by low TDS and are Ca–Mg–HCO 3 type waters. Despite the high altitude (mean altitude ∼2500 masl) and the relatively low mean annual air temperature (18 °C) of the region compared to Sahelian Africa, there is no commensurate depletion in δ 18O compositions of groundwaters of the Ethiopian Plateau. Generally the highland areas north and east of the basin are characterized by relatively depleted δ 18O groundwaters. Altitudinal depletion of δ 18O is 0.1‰/100 m. The meteoric waters of the Blue Nile River basin have higher d-excess compared to the meteoric waters of the Ethiopian Rift and that of its White Nile sister basin which emerges from the equatorial lakes region. The geochemically evolved groundwaters of the YTVL and LTG are relatively isotopically depleted when compared to the present day meteoric waters reflecting recharge under colder climate and their high altitude.

Effect of chemical reactions on the hydrologic properties of fractured and rubbelized glass media by K.P. Saripalli; P.D. Meyer; K.E. Parker; M.J. Lindberg (1677-1686).
Understanding the effect of chemical reactions on the hydrologic properties of geological media, such as porosity, permeability and dispersivity, is critical to many natural and engineered sub-surface systems. Influence of glass corrosion (precipitation and dissolution) reactions on fractured and rubbelized (crushed) forms HAN28 and LAWBP1, two candidate waste glass forms for a proposed immobilized low-activity waste (ILAW) disposal facility at the Hanford, WA site, was investigated. Flow and tracer transport experiments were conducted using fractured and rubbelized forms, before and after subjecting them to corrosion using vapor hydration testing (VHT) at 200 °C temperature and 200 psig pressure, causing the precipitation of alteration products. Data were analyzed using analytical expressions and CXTFIT, a transport parameter optimization code, for the estimation of the hydrologic characteristics before and after VHT. It was found that glass reactions significantly influence the hydrologic properties of ILAW glass media. Hydrologic properties of rubbelized glass decreased due to precipitation reactions, whereas those of fractured glass media increased due to reaction which led to unconfined expansion of fracture aperture. The results are unique and useful to better understand the effect of chemical reactions on the hydrologic properties of fractured and rubbelized stony media in general and glass media in particular.

Mineralogical examinations were performed to characterize the formation of secondary minerals and natural removal process of dissolved As and trace metals (Pb, Zn and Cu) from sulfide oxidation. Laboratory-based leaching tests were also conducted to determine whether the concentrations of As and trace metals in the leachates from waste-rock materials and contaminated soil could be affected by the presence acids such as acid rainwater or acid mine drainage. Waste-rock materials and contaminated soil were compared by 4-day leaching tests using HNO3 solutions of increasing acidity (0.00001–0.1 mole/L). Mineralogical studies of the waste rocks confirmed the presence of Fe-(oxy)hydroxides (e.g. goethite), jarosite, elemental S, Fe-sulfates, amorphous Fe–As phases, anglesite and covellite as secondary minerals. These secondary minerals act as mineralogical scavengers of dissolved trace metals, SO 4 2 - and acidity released by sulfide oxidation. Arsenic was attenuated by the adsorption on Fe-(oxy)hydroxides and/or the formation of an amorphous Fe–As phase, with a Fe/As ratio = 1 (maybe scorodite: FeAsO4  · 2H2O). Electron probe microanalyses data showed that the Fe-(oxy)hydroxides had high concentrations of Pb (up to 21 wt%), with appreciable amounts of As (up to 7.7 wt%), Zn (up to 4.6 wt%) and Cu (up to 2.5 wt%) indicating that dissolved metals were co-precipitated and adsorbed onto Fe-(oxy)hydroxides, Fe(Mn)-hydroxides and Fe-sulfates.The results of the leaching experiments within the pH-range 3.5–5.0 indicated that acidic rainstorms may leach minor amounts of Pb (ca. 1.7–4.0% of total), Zn (ca. 0.8–2.2% of total), Cu (ca. 0.0–0.2% of total) and As (ca. 0.02–0.1% of total) from waste rocks, including the dissolution of soluble secondary minerals previously formed during prolonged dry periods, while dissolution of these elements was negligible from the contaminated soil. In the pH-range 1.0–3.0, the leaching of Pb (ca. 2.4–31% of total) and As (ca. 0.1–5.8% of total) from the waste rocks was significant, which could influence the concentration of these metals in mine runoff. Strongly acidic solutions may also appreciably dissolve Zn (0.0–48% of total) and Cu (0.0–34% of total) in contaminated soil. Leach tests showed that the formation of less soluble secondary minerals had high retention of As, Pb, Zn and Cu, unless their solubilities were increased after the addition of strongly acidic solutions (pH of below 2.0). The precipitation of secondary minerals and the adsorption of trace metals are efficient mechanisms for decreasing the mobilities of As and other trace metals in the surface environment.Geochemical modeling indicated that the leach solutions were supersaturated with respect to goethite, hematite, and magnetite at pH greater than 2.9. Ferrihydrite, Fe(OH)3, was not in equilibrium with the solution during the entire experiment, suggesting that amorphous Fe-hydroxide can only form in small amounts. X-ray analyses of Seobo mine-waste samples indicated the presence of jarosite and goethite, which retain As, Pb, Zn and Cu during precipitation and co-precipitation.

Sustainability of humic acids in the presence of magnesium oxide by Nathalie A. Wall; Sara A. Mathews (1704-1713).
The Waste Isolation Pilot Plant (WIPP) is a U.S. Department of Energy (DOE) repository for the disposal of transuranic waste generated from the U.S. defense program. Humic acids (HA), known to strongly bind to actinides and which may be generated by the degradation of organic materials present in the waste, may influence the performance of the WIPP. This work presents experimental results on the effect of WIPP brines, ERDA-6 and GWB, and of the WIPP engineered barrier, Premier Chemicals magnesium oxide, on the solubility of HA. In the absence of MgO, a portion of HA initially precipitates, but a constant concentration of HA remains in solution: over a 60 day period of time, at least 290 ± 10 mg/L HA can remain soluble in de-ionized (DI) water, 30 ± 4 mg/L in 95% ERDA-6, and 31 ± 4 mg/L in 95% GWB. For solutions initially containing from 0 to 400 mg/L HA, the ratio of the initial HA concentration and the soluble HA concentration is 1.3 in DI water, 13 in 95% ERDA-6, and 11 in 95% GWB. In the presence of MgO, all HA precipitate within 60 days in systems with a high liquid-to-solid ratio (10.0 g/g) or a low ratio (2.4 g/g). This phenomenon is due to HA precipitation and/or sorption on the surface of MgO.

The characteristics of ribbed moraines, with an emphasis on till geochemical prospecting, were studied in the area of Peräpohjola, southern Finnish Lapland. Percussion drilling, test pits and trenches were used for till sampling. The samples were partially extracted and then analysed using ICP-AES or GFAAS methods in the geolaboratory of the Geological Survey of Finland. The distribution of Au and Cu in fine fraction (<0.06 mm) of till in both horizontal and vertical dimensions showed that the uppermost part of the moraine ridges contained the highest metal contents. In the coarser fractions of till (0.06–0.5 and >2 mm), the anomalies were located on the distal side of the ribbed moraine ridges, down-ice from mineralized bedrock. Geochemical patterns together with fresh pyrite grains in till heavy mineral concentrates indicate a short glacial transport distance of the mineralized debris. This conclusion is also supported by the presence of a large proportion of local rock fragments and boulders in the uppermost till unit and at the surface, which is a result of glacial quarrying during the ribbed moraine formation.

Chemical components in geological samples are often measured by more than one method to be able to measure high concentrations in resistant minerals as well as low concentrations in other mineral phases. For quality control, it is necessary to evaluate if the results from these methods are consistent or not. A total of 1884 sediment samples taken from the Leinster area of Ireland were analysed by both instrumental neutron activation analysis (INAA) and atomic absorption spectrometry (AAS) for 5 chemical elements (Co, Cr, Fe, Ni, and Zn). It is well known that INAA detects the total concentrations of chemical elements, but it has much higher detection limits than AAS. In this study, descriptive statistics, scatter plots, statistical tests and GIS mapping techniques were employed to compare the differences between the results from the two analytical methods, and to evaluate the impact of detection limits on data quality.Descriptive statistics and scatter plots were effective in showing the discrepancies between values of the two methods. Scatter plots showed outliers and the poor accuracy of INAA for concentrations close to or below the detection limits. It is suggested that errors may have happened during both INAA and AAS processes for some samples. Statistical results of both nonparametric sign and Wilcoxon signed rank tests showed that the differences between concentrations measured with the two methods were significant at the level of p  < 0.001, and the average differences were 20–30% for the 5 elements. Meanwhile, more than 25% of all samples showed differences greater than 40–60% for these elements. After removing values lower than 3 times detection limits of both methods, only a slight improvement was achieved for the consistency between the two methods. Besides detection limits, the incomplete digestion prior to AAS may have played an important role. The higher AAS values of Ni than its INAA values may have been caused by multiple detection limits in different batches of laboratory analyses. The high detection limits of INAA also caused spatial patterns on the GIS maps showing differences between the two methods.The current geochemical database may be sufficient for some aspects of mineral exploration, but not for other geochemical studies, such as baseline database construction and environmental investigations. Better results could have been achieved by initial error checking and correction, improvement on the detection limits of INAA, and improvement on the acid digestion for AAS. It is possible that other geochemical databases contain similar data quality issues demonstrated in this one. This study demonstrates how straightforward statistical methods can be applied to check/verify the quality of geochemical databases.

The Amazon. Bio-geochemistry applied to river basin management by Yves Tardy; Vincent Bustillo; Claude Roquin; Jefferson Mortatti; Reynaldo Victoria (1746-1829).
A hydrochemical model, using hydrograph separation, developed for the Niger basin, has been proposed as a strategic tool for studying the watershed dynamics at any time and space scales. The model is applied to the Amazon basin, including the main channel and its major tributaries. The database corresponds to a sampling and analytical program developed over 8 cruises at 9 stations (about 70 samples), collected in the framework of the CAMREX Project (1982–1984). The model, based on a hydrograph separation of 3 reservoirs, is successful in extrapolating and predicting the geochemical and environmental behaviour of such large basins, naturally submitted to large secular or annual, regular or even catastrophic climatic oscillations. Several topics have been considered. (1) Coherence among the physico-chemical analyses: dissolved species (pH, NH 4 + , Na+, K+, Ca2+, Mg2+, NO 3 - , HCO 3 - , Cl, DOC, SO 4 2 - , HPO 4 2 - , SiO2, O2 and CO2), and inorganic or organic suspended load (fine and coarse fractions FSS, CSS, POCF, POCC). (2) Hydrograph separation in 3 reservoir contributions: R S, the superficial or rapid runoff, R I, the hypodermic or intermediate runoff, including the flood plain contributions, and R B the ground water or base flow. (3) Estimation of the isotopic and physico-chemical features of each of the 3 flow components: R S, R I, and R B. (4) Determination of the 3 hydrological parameters (size of the reservoir, drying up coefficient, and residence time of water), characterizing each of the 3 flow components (R S, R I, and R B), in each of the 9 basins considered. (5) Hydrological and geochemical balances for all the parameters analysed either (a) cruise by cruise for all tributaries and the Amazon River at Obidos, or (b) among each of the 3 river flow components. (6) Isotopic data set of δ 18O in waters, tests of coherence of the hydrograph separation model. (7) Relationships between isotopic signatures and morphological or hydroclimatical parameters characterizing the river–soil–vegetation systems. The developed procedure presents a new tool in environmental predictions, emphasizing the potentiality of geochemical interpretation of complex hydrochemical data sets.