Applied Geochemistry (v.15, #6)

Ground and surface waters collected from two undisturbed Zn–Pb massive sulphide deposits (the Halfmile Lake and Restigouche deposits) and active mines in the Bathurst Mining Camp (BMC), NB, Canada were analysed for the rare earth elements (REE). REE contents are highly variable in waters of the BMC, with higher contents typical of waters with higher Fe and lower pH. There are significant differences between ground- and surface waters and between groundwaters from different deposits. The REE contents of surface waters are broadly similar within and between deposit areas, although there are spatial variations reflecting differences in pH and redox conditions. Surface waters are characterised by strong negative Ce anomalies ([Ce/Ce*]NASC as low as 0.08), produced by oxidation of Ce3+ to Ce4+ and preferential removal of Ce4+ from solution upon leaving the shallow groundwater environment. Groundwaters and seeps typically lack significant Ce anomalies reflecting generally more reducing conditions in the subsurface environment and indicating that Ce oxidation is a rapid process in the surface waters. Deeper groundwaters at the Halfmile Lake deposit are characterised by REE patterns that are similar to the host lithologies, whereas most groundwaters at the Restigouche deposit have LREE-depleted patterns compared to NASC. Halfmile Lake deposit groundwaters have generally lower pH values, whereas Restigouche deposit groundwaters show greater heavy REE-complexation by carbonate ions. Shallow waters at the Halfmile Lake and Stratmat Main Zone deposits have unusual patterns which reflect either the adsorption of light REE onto colloids and fracture-zone minerals and/or precipitation of REE–phosphate minerals. Middle REE-enrichment is typical for ground- and surface waters and is highest for neutral pH waters. The labile portion of stream sediments are generally more middle REE-enriched than total sediment and surface waters indicating that the REE are removed from solution by adsorption to Fe- and Mn-oxyhydroxides in the order middle REE≥light REE>heavy REE.

An investigation of the chemistry of surface precipitates on stream pebbles and Potamopyrgus (Hydrobia) jenkinsi (Smith) shells has been undertaken on samples collected from a contaminated river in SW England, UK. Samples were analysed by LA-ICP-AES to evaluate the technique as a suitable method to use on small environmental samples, and to determine if the Pb/Fe and Zn/Mn ratios of the coatings on the two substrates were comparable. The ratios in the coatings are similar in these samples types, but are generally higher on the pebble samples. Results have been compared to a previous investigation of pebble coating chemistry in the river, and are shown to have the same spatial distribution but very different absolute values due to changes in environmental conditions in the river over time.

The East Midlands Triassic (Sherwood Sandstone) aquifer which has been the subject of detailed radiometric age studies, is used to investigate both inert and reactive constituents of groundwater as indicators of residence time. Detailed resampling of the aquifer in 1992 has provided a considerable body of new inorganic geochemistry data, though without radiocarbon. Several inert indicators are defined including the isotopic ratios δ 18O, δ 2H, 36Cl, noble gas ratios, as well as the halogen elements (Cl, Br, F, I) and their element ratios. These form a group of essentially unreactive tracers primarily reflecting changing rainfall inputs and palaeoclimatic conditions, except at outcrop where human impacts are also seen clearly. The concentrations of Cl, mainly from atmospheric sources, remain below 25 mg l−1 Cl over a distance of some 30 km from outcrop. Reactive indicators, the result of time-dependent water–rock interactions, include δ 13C, Mg/Ca, Sr/Ca, Na/Cl and show diagnostic trends along the flow lines. However the concentrations of certain trace elements — Li, Rb, Cs, Mn and Mo — which are not limited by solubility constraints show linear trends along the present day flow gradient. This water–rock interaction is taking place in groundwaters with low total mineralisation and it can be demonstrated that reactions involving these elements and isotopes are occurring entirely within the aquifer since high salinity groundwaters are found below the Sherwood Sandstone.There is good correlation between some of the elements and 14C activities in the 1977 data set and this has been used to derive a concentration–age relationship for the 1992 set of data. A chemical timescale for the aquifer is then established using (a) Li and (b) a combination of five trace elements. The age of the fresh groundwater is thus shown to be up to 100 ka BP, indicating the likelihood of semi-continuous recharge during the Devensian glacial period preceding the glacial maximum, when no recharge occurred (10–20 ka BP). This approach may be of value in extending groundwater dating beyond the radiocarbon timescale as well as interpreting sites where no radiocarbon data are available. The scatter from the mean age line indicates those groundwaters which are derived from either rapid or slower than average flow zones within the aquifer, as well as age stratification. This stratification is borne out by a depth profile of groundwater from a new borehole near to outcrop which is shown, by a combination of chemical and isotopic tracers, to contain Holocene overlying late Pleistocene water.

The geochemistry, as defined by amounts of easily reducible Mn (ERMn; Mn oxides), reducible Fe (RFe; Fe oxides), organic matter (% loss on ignition), total metal (Cu, Pb and Zn) and metals associated with the ERMn, RFe and organic matter components of deposited sediments (DS) and suspended particulate matter (SPM) were contrasted over a 1-year period (two-way ANOVA with sediment type and month as the two factors) within the Fraser River Estuary, BC, Canada. The geochemistry of SPM as compared to DS was distinctly different. The geochemistry of SPM displayed a marked seasonality. By contrast, seasonal differences in the geochemistry of DS were much less pronounced over the 12-month sampling period. Concentrations of organic matter and RFe in SPM were significantly greater (two-way ANOVA; P<0.05) in winter months (maximums of 23% and 53 g kg−1, respectively) as compared to the rest of the year (maximums of 9.3% and 11 g kg−1, respectively). Concentrations of organic matter in DS did not change over the 12-month period; however, RFe in DS was significantly greater in winter months (7.3 g kg−1) as compared to summer months (2.3 g kg−1). Easily reducible Mn in both SPM and DS was highly variable throughout the year with no apparent seasonal dependence. Total concentrations of Cu, Pb and Zn and their partitioning among the 3 sediment components (i.e. ERMn, RFe and organic matter) were also month-dependent (two-way ANOVA, P<0.05); metal concentrations in SPM were up to 17 times greater than DS with a higher proportion of these metals associated with the easily reducible component (oxides of Mn and amorphous forms of Fe oxides) during winter as compared to summer months. Trace metal concentrations and partitioning in DS showed the same seasonal trends, although not to the same degree as occurred for SPM, throughout the 4 seasons of study. Seasonal changes in the partitioning of metals in addition to greater proportions of the metal occurring in an easily reducible form in SPM relative to DS has potentially important implications for sediment ingesting organisms capable of filter-feeding on both SPM and DS. Specifically, metal bioavailability to sediment ingesting organisms from SPM may be seasonally dependent with periods of greatest exposure occurring during winter months, as compared to DS where no seasonal dependence occurs. To identify main vectors of metal exposure to sediment ingesting organisms, both the type of sediment and when they are feeding on the particular type of sediment need to be determined.

Experimental modeling of platinum sorption on organic matter by Laura P Plyusnina; Tanya V Kyz'mina; George G Likhoidov; Gennadii A Narnov (777-784).
In connection with the discovery of a new type of Pt deposit in low-rank brown coals and black shales, the interaction of Pt-bearing aqueous solutions with fractionated organic matter (asphaltenes and asphaltenic acids) was studied at 200–400°C and 1 kbar total pressure. It was found that chemical sorption onto the organic matter lowers Pt content in the aqueous solutions by about two orders of magnitude relative to organic-free systems. Thermal maturation of the asphaltenes leads to its aromatization and concomitant sorption of Pt from n×10−4 mPt (mol per kg of dry matter) at 200°C to n×10−2 mPt at 400°C. Thus, the Pt chemisorption on activated carbonized organic matter may be an effective mechanism of Pt accumulation in C-bearing rocks.
Keywords: Aqueous medium; 200–400°C; P tot =1  kbar ; Platinum; Solubility; Kinetics; Organic matter; Thermal maturation; Extracting; Redox buffering; Black shales;

The ferrozine method revisited: Fe(II)/Fe(III) determination in natural waters by E Viollier; P.W Inglett; K Hunter; A.N Roychoudhury; P Van Cappellen (785-790).
The original ferrozine method has been modified to sequentially determine the Fe(II)/Fe(III) speciation in small volumes of fresh and marine water samples, at the submicromolar level. Spectrophotometric analyses of the Fe(II)–ferrozine complex are performed on a single aliquot before and after a reduction step with hydroxylamine. The procedure is calibrated using Fe(III) standards stable under normal conditions of analysis. It is shown also that the presence of high concentrations of dissolved NOM (natural organic matter) do not create any significant artifacts. The method was used to measure Fe(II) and Fe(III) depth distribution in salt marsh pore waters and in a stratified marine basin.
Keywords: Iron speciation; Fresh and marine water; Spectrophotometry; Ferrozine;

Hydrochemical and isotopic characterisation of the Bathonian and Bajocian coastal aquifer of the Caen area (northern France) by Florent Barbecot; Christelle Marlin; Elisabeth Gibert; Laurent Dever (791-805).
This paper describes the geochemical evolution of groundwater in the Bathonian and Bajocian aquifer along its flowpath. Since this aquifer represents one of the main sources of fresh water supply in the Caen area and has been subjected to a Holocene marine intrusion, its management requires a sound knowledge of (1) the primary conditions and (2) the potential influence of either natural or anthropogenic pressures. Groundwater vertical sampling validity is discussed with the contribution of high resolution temperature logging. The main processes of geochemical evolution along a groundwater flow line and the sea-water intrusion characteristics are discussed using ionic concentrations (Br, F and major elements) and isotopes (water δ 2H and δ 18O, TDIC δ 13C and A14C, sulphate δ 18O and δ 34S). As the 13C content of TDIC is used as a tracer of water-rock interaction, it shows evidence of specific chemical and isotopic evolutions of groundwater within the aquifer, both related to water-rock interaction and mineral equilibria in groundwater. All the above-mentioned tracers evolve downflow: cation concentrations are modified by exchange with clay minerals allowing a high F concentration in groundwater, whereas Br and SO2− 4 concentrations appear to be redox condition dependant. Superimposed on these geochemical patterns, δ 18O and δ 2H compositions indicate that aquifer recharge has varied significantly through time. The chemical evolution of groundwater is locally affected by a salty water intrusion that is characterised by mixing between Flandrian fresh water and sea-water which has interacted with peat as evidenced by a high Br/Cl ratio and SO2− 4 reduction.
Keywords: Geochemistry; Coastal aquifer; Radiocarbon; Holocene;

The concentrations of major and trace elements were determined (aqua regia leach and ICP-AES analyses) in stream, lake and dredged sediments downstream of the historical Antskog iron- and copperworks, S.Finland. The levels of Ag, Cd, Cu, Pb and Zn are highly elevated in all studied sediment types: roughly half of the studied lake-sediment samples contain >5 ppm Ag, >15 ppm Cd, >0.1% Cu, >0.1% Pb and >0.3% Zn. In the dredged sediment material located onshore, the concentrations of Ag, Cu and Pb are comparable to those in the polluted lake-sediment samples, while in stream sediments elevated metal concentrations are found especially in samples characterised by high concentrations of organic material. The source of the elevated metal concentrations is the historical metalworks at Antskog, mainly the copperworks of the 19th century. Compared to the limit values for contaminated soils in Finland, the concentrations of Cu, Pb and Zn are on average elevated by factors >10 in the polluted horizons of lake sediments, >5 in the dredged sediment located onshore and >2 at the most heavily contaminated site in the stream. Since the surface waters in the area are used for agricultural purposes and for various leisure activities, it is necessary to make further detailed investigations into the extent of the metal pollution and to determine species, mobility and bioavailability of the metals.

In this paper the groundwater in-situ generation of dissolved organic carbon (DOC) is discussed based on the origin of groundwaters, their physico-chemical and isotopic properties, chemical composition and the dissolved inorganic carbon (DIC) concentration and its 13C content. Three aquifer systems are investigated. Two of these have relatively well defined hydrological and geochemical conditions (Fuhrberg and Munich) and are used as reference systems. The third aquifer (Gorleben) is a complex system containing DOC concentrations up to 200 mg C/L in deep groundwaters. From this aquifer system 19 groundwaters from different hydrogeochemical conditions are analyzed. The in-situ generation of DOC is found to occur in conjunction with the microbiologically mediated mineralization of sedimentary organic carbon (SOC). Thereby, SO4 is reduced and phosphate is released into the groundwater. Where SO4 is depleted, the mineralization of SOC occurs via fermentation, resulting in CH4 generation.

Experiments have been carried out to study the uptake of radiocaesium by sediment cores from Esthwaite Water, Cumbria, UK in order to test models to describe the simultaneous uptake of radiocaesium from the water column and diffusion of activity within the sediment. A new, simplified method of simulating time dependent diffusion of tracers in sediments is developed and tested. Comparison of experimental with field measurements showed that rates of diffusion of activity which had been introduced by diffusion across the sediment–water interface (as in the present experiments) were approximately one order of magnitude greater than those determined from in situ Chernobyl and weapons test activity–depth profiles. Measured total solids–aqueous distribution coefficient (K d t )) values in the experiments were ca. 2000 l kg−1, more than one order of magnitude lower than those measured in situ (ca. 7×104 l kg−1). Much better agreement, however, was observed between the exchangeable distribution coefficients (K d e), measuring ‘mobile’ Cs, which were ca. 2000 l kg−1 in the experiments and ca. 4900 l kg−1 in the field. Modelling the removal of Chernobyl radiocaesium from Esthwaite Water showed that the majority of the activity was transported to the sediments by attachment to and settling of suspended particles. It is concluded that activity deposited on particulates (forming the majority of activity observed in situ) was more strongly bound to sediments than that introduced by direct diffusion across the sediment–water interface, leading to significantly different mobility of these two fractions of 137Cs in sediments. Model sensitivity analyses showed that removal of activity across the sediment–water interface is more strongly influenced by diffusion within the sediments than by the thickness of the benthic boundary layer.

An extensive saline plume (>250 km2) within the regionally important unconfined aquifer in the Neogene Ogallala Formation overlies the Panhandle oil and gas field in the Southern High Plains, Texas, USA. Relative to upgradient Ogallala water, the plume waters have δ 18O (−6.7 to −8.8‰) and δD (−42 to −88‰) values that tend to be depleted and have higher Cl (>150 mg/l) and SO4 (>75 mg/l) concentrations. Various end-member-mixing models suggest that the plume composition reflects the presence of paleowaters recharged during Middle to Late Wisconsinan time rather than salinization associated with petroleum production. Paleowaters probably mixed with salt-dissolution zone waters from the underlying Upper Permian formations before discharging upward into the Ogallala Formation. Cross-formational discharge is controlled primarily by the geometry of the underlying units, as influenced by the Amarillo uplift, pinch-out of the laterally adjoining confined aquifer in the Triassic Dockum Group, variations in the saturated thickness of the Ogallala aquifer and the presence of potential pathways related to salt dissolution.

Impact of nitrogen fertilizers on the natural weathering-erosion processes and fluvial transport in the Garonne basin by Khadija Semhi; Philippe Amiotte Suchet; Norbert Clauer; Jean-Luc Probst (865-878).
Knowledge of the impact of N-fertilizers on the weathering-erosion processes of soils in intensively cultivated regions is of prime importance. Nitrification of NH4 fertilizers produces HNO3 in the basin of the Garonne river, enhancing soil degradation. Their influence on the weathering rates was determined by calculating the consumption rate of atmospheric/soil CO2 by soil weathering and erosion, and its contribution to the total dissolved riverine HCO3 . This contribution was found to be less than 50% which corresponds normally to a complete carbonate dissolution by carbonic acid, suggesting that part of the alkalinity in the river waters is due to carbonate dissolution by an acid other than carbonic acid, probably HNO3.

A study of speciation of Sb in bisulfide solutions by X-ray absorption spectroscopy. by J.Frederick W. Mosselmans; George R. Helz; Richard A.D. Pattrick; John M. Charnock; David J. Vaughan (879-889).
Direct evidence of the structure of thioantimonide species in alkaline aqueous solutions is provided by X-ray absorption spectroscopy. Twenty solutions containing thioantimonide species were prepared by dissolving stibnite (Sb2S3) in deoxygenated aqueous NaHS solutions; the solution pH range was 8–14, the [Sbtot] 1–100 mM and the [HS] 0.009–2.5 M. The structural environment of the dissolved Sb was determined by EXAFS analysis of the Sb K-edge over the temperature range 80–473 K.Many of the solutions contain a species with Sb bonded to four S atoms at 2.34 Å, consistent with the presence of a [Sb(V)S4 3−] species, demonstrating that oxidation of Sb(III) to Sb(V) has occurred on dissolution. There is evidence that the complementary reduced phase is H2. In three solutions, the Sb has three nearest neighbor S atoms and two of these solutions have an additional S shell of two atoms at 2.9Å, with one showing evidence of an Sb shell at 4.15 Å. This provides evidence of the presence of multimeric Sb(V) thioantimonide species. Analysis of several solutions reveals the presence of a species with three Sb–S interactions of 2.41–2.42 Å, supporting the presence of a Sb(III) species such as Sb2S2(SH)2. Six solutions have S coordination numbers from 2.7–4 Å and Sb–S distances of 2.37–2.39 Å, and are likely to contain mixtures of at least two species in concentrations such that each make a significant contribution to the EXAFS. There was no clear relationship between either [Sbtot] or [HS] and the type of species present, but Sb(III) species were only present in the solutions with high pH. The effect of temperature was most significant in one solution, where at 423 K partial hydrolysis occurred and the presence of a species such as Sb2S2(OH)2, with an Sb–O distance of 1.91 Å, is indicated.The study provides new information on the coordination environment of thioantimonide species, complementary to previous studies and provides a basis for a better understanding of Sb speciation in aqueous solutions found in hydrothermal systems, anoxic basins and man-made, high pH environments. In particular it demonstrates the need for Sb(V) to be considered in theoretical and experimental studies of such systems. However, more definitive interpretation of some of the data is inhibited by the presence of mixtures of species and the lack of information on the outer coordination shells that would confirm the presence of multimeric species.