Applied Geochemistry (v.15, #7)

Pb isotope composition of tree rings (Celtis Australis) and urban aerosols have been determined to assess whether arboreal species can be used as bio-geochemical tracers of the evolution of heavy metal pollution to the environment. Particular care was paid to setting up a high quality analytical technique to work with arboreal species with low Pb content. The Pb isotope composition of tree rings from 1950 to 1995 is within the range of European aerosols and is correlated with the temporal evolution of Pb isotopes measured in air particulates from Firenze. The entire data set (tree rings and air particulates) demonstrate that Pb isotope composition of tree rings can be used successfully as a proxy of the atmospheric Pb isotope composition of urban areas. This, in turn, suggests that tree rings are potentially a powerful bio-geochemical tracer for monitoring air pollution history due to human activities.

Hydrogeochemistry and transport of organic contaminants in an urban watershed of Chesapeake Bay (USA) by Gregory D. Foster; Eldon C. Roberts; Barry Gruessner; David J. Velinsky (901-915).
Stream water samples were collected in the two main free-flowing branches of the Anacostia River watershed above the head of tide over a one year time period. Both the Northeast and Northwest Branches drain large suburban and urban land areas that flow into the more urbanized tidal portion of the Anacostia River within Washington, DC. Large volume (40–75 l) water samples were filtered, and the suspended particulate matter and filtrate were analyzed for polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and organochlorine pesticides (OCPs) at sub-nanogram per liter concentrations using ultra-trace analytical methods. Higher amounts of PCBs, PAH, and OCPs in the tidal Anacostia River occurred primarily in the particulate phase during high flow events. Polycyclic aromatic hydrocarbons in the particulate phase within fluvial transport consisted primarily of pyrogenic homologues characteristic of weathered or combusted petroleum products. Fluxes were exceptionally high for PAHs which showed annual fluxes to the tidal Anacostia River comparable to those determined for the much larger mainstem Potomac River. Aromatic hydrocarbons in runoff from urban regions may serve as an important source of PAH fluxes to the tidal waters of Chesapeake Bay.

Fluid geochemistry of the Acqui Terme-Visone geothermal area (Piemonte, Italy) by Luigi Marini; Vittorio Bonaria; Massimo Guidi; Johannes C. Hunziker; Giulio Ottonello; Marino Vetuschi Zuccolini (917-935).
The main geothermal reservoir of Acqui Terme-Visone hosts Na–Cl waters, which are in chemical equilibrium at 120–130°C with typical hydrothermal minerals including quartz, albite, K-feldspar, illite, chlorite (or smectite), anhydrite, calcite and an unspecified Ca-Al-silicate. In the Acqui Terme-Visone area, these geothermal waters ascend along zones of high vertical permeability and discharge at the surface almost undiluted or mixed with cold, shallow waters. To the SW of Acqui Terme, other ascending geothermal waters, either undiluted or mixed with low-salinity waters, enter relatively shallow secondary reservoirs, where they reequilibrate at 65–70°C.Both chemical and isotopic data indicate that bacterial SO4 reduction affects all these waters, especially those discharged by the secondary reservoirs. Therefore, geothermal waters must get in contact with oil, acquiring the relatively oxidized organic substances needed by SO4-reducing bacteria. This oil–water interaction process deserves further investigations, for potential economic implications.

The B isotopic composition, in combination with O and H isotopes and hydrochemical tracers, is utilized to constrain the evolution of basement-hosted groundwaters via water–rock interactions and fluid infiltration from external (sedimentary) reservoirs. Two distinct groundwater types have been identified in the Central European crystalline basement (N Switzerland–SW Germany): fresh groundwaters characterized by low values of δ 11B (−3.5 to −0.6‰), δ 18O (−12.0 to −10.0‰), and δD (−86.8 to −71.9‰), and brackish groundwaters with distinctly heavier B, O, and H isotopic compositions (δ 11B=+6.4 to +17.6‰, δ 18O=−9.4 to −5.6‰, δD=−67.6 to −60.8‰). Fresh groundwaters show a systematic decrease in δ 11B, related to an increase in B concentrations (and degree of total mineralization), along the pathway of groundwater migration which can only be interpreted in terms of leaching of crystalline host rocks. A δ 11B value of −3.3‰ is inferred for the crustal B source (mainly Hercynian granites) involved in the leaching process, in agreement with the known δ 11B range of granitic rocks. The evolution of brackish groundwaters, derived from crystalline basement reservoirs with little water circulation, is more complex. As indicated by B–O–H stable isotope and hydrochemical (e.g. B/Cl, Na/Cl, and Br/Cl) constraints, brackish groundwaters from the study area are influenced by admixture of sediment-derived fluids which infiltrated from Late Paleozoic (Permo-Carboniferous) and Early Mesozoic (Lower Triassic) sedimentary strata. The data presented show that B isotopes are sensitive to mixing processes of fluids derived from different crustal reservoirs and, hence, may be utilized as a tracer for constraining the internal (autochthonous) vs external (allochthonous) origin of salinity in basement-hosted groundwaters.

The enhancement of mobility of radionuclides in the geosphere through complexation by humic substances is a source of uncertainty in performance assessment of radioactive waste repositories. Only very few data sets are available which are relevant for performance assessment of an underground repository for radioactive waste. Using the equilibrium dialysis-ligand exchange method developed at the Paul Scherrer Institut, conditional stability constants for the formation of complexes of Aldrich humic acid with Ca2+, NpO2 +, Co2+, Ni2+, UO2 2+ and Eu3+ and complexes of Laurentian soil- and Suwannee River fulvic acid with Co2+, UO2 2+ and Eu3+ were measured. pH was varied between 5 and 10 and ionic strength between 0.02 and 0.2 M. The data are presented as equilibrium coefficients that are free from any model assumptions. The equilibrium coefficients increased in the order Ca2+≅NpO2 +<Co2+< Ni2+<UO2 2+< Eu3+. The quality of the data is assessed in an extended discussion of statistical and systematical errors, and by a critical ‘rereview’ of the auxiliary stability constants used for the calculation of the equilibrium coefficients. An approximate overall uncertainty of 0.5 log-units is estimated for the stability data reported. The conditional stability constants were found to increase markedly with increasing pH in the case of Co2+, UO2 2+ and Eu3+. For Ni2+, Ca2+ and NpO2 + this effect was less pronounced. For all metal ions tested, the influence of ionic strength was of less importance, and the conditional stability constants did not show a significant dependence on the type of humic substances investigated.

The modelling of metal-humate interactions has been a field of active research for more than 3 decades but despite all efforts there is still far from a consensus concerning humic binding models. The authors demonstrate that a synopsis of large sets of reliable experimental data for Ca, Co, Ni, Eu, Am, Cm, Np(V) and U(VI) reveals a consistent picture of the influence of metal concentration, pH and ionic strength on metal-humate interactions. However, this consistent behaviour cannot be interpreted with high numerical accuracy by simple binding models, the need for more adjustable parameters increases proportional to the width of the experimental parameter range to be fitted. This experience triggered the proposal of a pragmatic approach for performance assessment purposes. The “conservative roof” approach does not aim to accurately model all experimental data but rather allows estimates to be made of the maximum effects on metal complexation to be expected from humic substances. A specific “conservative roof” model is applied to situations generally to be expected in deep groundwater and selected cases of interest for planned Swiss repositories of radioactive waste are discussed in detail.

Stream discharges and concentrations of dissolved and colloidal metals (Al, Ca, Cu, Fe, Mg, Mn, Pb, and Zn), SO4, and dissolved silica were measured to identify chemical transformations and determine mass transports through two mixing zones in the Animas River that receive the inflows from Cement and Mineral Creeks. The creeks were the dominant sources of Al, Cu, Fe, and Pb, whereas the upstream Animas River supplied about half of the Zn. With the exception of Fe, which was present in dissolved and colloidal forms, the metals were dissolved in the acidic, high-SO4 waters of Cement Creek (pH 3.8). Mixing of Cement Creek with the Animas River increased pH to near-neutral values and transformed Al and some additional Fe into colloids which also contained Cu and Pb. Aluminium and Fe colloids had already formed in the mildly acidic conditions in Mineral Creek (pH 6.6) upstream of the confluence with the Animas River. Colloidal Fe continued to form downstream of both mixing zones. The Fe- and Al-rich colloids were important for transport of Cu, Pb, and Zn, which appeared to have sorbed to them. Partitioning of Zn between dissolved and colloidal phases was dependent on pH and colloid concentration. Mass balances showed conservative transports for Ca, Mg, Mn, SO4, and dissolved silica through the two mixing zones and small losses (<10%) of colloidal Al, Fe and Zn from the water column.

Humic substances (HS) isolated from swamp water, surface soil, peat and brown coal were characterized using elemental composition, solid state 13C CP/MAS NMR and Py-GC/MS analysis. A substantial amount of information with regard to the source, maturity, depositional environment and degree of degradation of humic substances was obtained. The elemental composition, atomic ratio, 13C NMR spectra and Py-GC/MS results indicated that vascular plant matter is the main contributor to humic substances. Humic substances from swamp water and from soil in the surrounding area of the swamp were found to be identical, suggesting that the swamp humic substances are derived from surrounding soils. Humic substances from brown coal were characterized by a higher degree of humification, including loss of polysaccharides, degradation in lignin content and increase in aromaticity. Compared with humic substances from brown coal, the humic substances from peat showed a lower degree of humification and were considerably more aliphatic in nature.

Bacteriogenic Fe oxides (BIOS) and groundwater samples were collected 195 m underground at the Stråssa Mine in central Sweden. Ferrous iron oxidizing bacteria, including stalked Gallionella ferruginea and filamenous Leptothrix sp., were prominent in the BIOS samples. The BIOS samples were found to contain only poorly ordered (amorphous) hydrous ferric oxide, as determined by X-ray diffraction. Inductively coupled plasma mass spectroscopy revealed hydroxylamine-reducible Fe and Mn oxide contents that ranged from 55 to 85% on a dry weight basis. Concentrations of Sr, Cs, Pb and U in filtered groundwater ranged from 0.002 to 1.8 μM. Solid phase concentrations of these heavy metals in the BIOS spanned the 0.04–2.23 mmol/kg range. Distribution coefficients (K d values), calculated as the ratio between BIOS and dissolved heavy metal concentrations, revealed solid phase enrichments that, depending on the heavy metal and Fe oxide content of the sample, extended from 103.0 to 104.7. At the same time, however, a strong inverse linear relationship was found between log  K d values and the corresponding mass fraction of reducible oxide in the samples, implying that metal uptake was strongly influenced by the relative proportion of bacterial organic matter in the composite solids. Based on the metal accumulation properties of the BIOS, an important role can be inferred for intermixed Fe oxides and bacterial organic matter in the transport and fate of dissolved metals in groundwater systems.

Groundwater colloid properties: a global approach by Claude Degueldre; Iñes Triay; Jae-Il Kim; Peter Vilks; Marcus Laaksoharju; Norbert Miekeley (1043-1051).
This study presents and discusses groundwater colloid results from various geological formations ranging from crystalline to sedimentary, from organic rich to organic poor systems and from subsurface to very deep aquifers. Colloid presence and their potential mobility are justified on the basis of colloid stability properties in the investigated groundwaters. The colloid concentration is a function of pH, redox potential, concentrations of Na, K, Ca, Mg and organic carbon, as well as the status of the chemical and physical steady state of the hydrogeochemical system. The colloid properties are discussed with a non-site specific approach.

Numerical analysis of hydrogeochemical data: a case study (Alto Guadalentı́n, southeast Spain) by Juan Carlos Cerón; Rosario Jiménez-Espinosa; Antonio Pulido-Bosch (1053-1067).
This study examines the spatial variability of the factors obtained from the application of correspondence analysis to a hydrogeochemical data set. The goal was to synthesize the hydrogeochemical information using this multivariate statistical technique, by setting a series of factors which clarified the main properties of one aquifer. Then, a geostatistical framework to obtain a probabilistic assessment of groundwater quality was established. Experimental and theoretical semivariograms of the selected factors, considered as regionalized variables, were computed. These variographic information and factor values in the experimental sites were used in the ordinary kriging, which provides unbiased and linear estimates of the regionalized variables. These estimates were used to compile maps of the chosen factors, which explain their spatial distribution.The selected case study was the alluvial aquifer of Alto Guadalentı́n which is situated in southeast Spain, in the Internal Zones of Betic Cordilleras. These waters are chiefly SO4 and Cl types, but HCO3 facies are common in the central sector of the basin. High temperature, acid pH, problems of overexploitation and pollution by CO2-gas characterise these waters. Available groundwater quality monitoring data were used to calibrate the numerical model. The present study focused on setting the main physical and chemical attributes and establishing the spatial pattern of groundwater quality and the temporal changes in this pattern.