Applied Geochemistry (v.18, #11)

Areas showing anomalously high levels of heavy metals (eg. Cu, Pb, Sn, Zn) in both peri-urban and rural soils in parts of western Lancashire (UK) were indentified during a regional geochemical survey. The sites were identified as areas of former peat fenland that had been filled in with a variety of domestic and industrial wastes including furnace slag, as well as inert rubble. The very high levels of heavy metals present in the soil raises concerns about possible translocation into food crops, as this is an important market gardening area. There is no direct evidence of a health risk, and the high-pH soils will limit metal mobility and bioavailability, but these soils are worthy of further multidisciplinary scientific investigation.

The effect of SO4 on the ferrihydrite adsorption of Co, Pb and Cd: ternary complexes and site heterogeneity by Peter J. Swedlund; Jenny G. Webster; Gordon M. Miskelly (1671-1689).
Ferrihydrite adsorption of Co, Pb and Cd is enhanced in the presence of sulfate. This effect, which has been observed previously for Cu and Zn, is not predicted by the diffuse layer model using adsorption constants derived from single sorbate systems. However, by including ternary surface complexes with the stoichiometry ≡FeOHMeSO4, where ≡Fe is an Fe on the ferrihydrite surface and Me is Co, Pb, or Cd, the effect of SO4 on cation adsorption is accurately predicted for the range of cation, ferrihydrite and SO4 concentrations studied. Surface site heterogeneity for Co, Pb and Cd adsorption in the absence of SO4 also was considered. While Co adsorption could be described with a 2-site model (with site densities of 5×10−3 and 2×10−1 mol mol Fe−1) Pb adsorption required a third site with a higher affinity and a site density of 3.5×10−4 mol molFe−1. Cadmium adsorption was reasonably well described with a 2-site model but showed some evidence for a third higher affinity site. Recommended values of adsorption constants for use in SO4-rich environments, such as acid mine drainage, are presented.

Humidity changes in southern Tunisia during the Late Pleistocene inferred from U–Th dating of mollusc shells by Christiane Causse; Bassam Ghaleb; Najiba Chkir; Kamel Zouari; Hedi Ben Ouezdou; A Mamou (1691-1703).
Calcareous deposits, mainly consisting of mollusc shell accumulations, which have been dated by the U/Th disequilibrium method, mark the shorelines of paleolake highstands in the Great Chotts Area of Southern Tunisia. The 5 sites studied consist of discontinuous accumulations of fossils of marine-like organisms e.g.: Cerastoderma glaucum, Melania tuberculata, Melanopsis praemorsa, Cerithium rupestre. U/Th isochron plots and age frequency histograms for 39 shell samples are reported here. Limited variations for U content and 234U/238U activity ratios (AR) of shells support the hypothesis of closure of the geochemical system with respect to this element. It is remarkable that 234U/238U AR of shells collected in Chott Fejej or Chott Jerid are clustered around different values, reflecting probably different groundwater recharge from the Continental Intercalaire (CI) or Complexe Terminal (CT) aquifers. Furthermore waters collected near Wadi el Akarit show 234U/238U AR values comparable to those observed for shells. 14C determinations made on aliquots of some of these samples suggested an age distribution between 18 and 34 ka BP. The U/Th data of these 39 shell samples imply that 4 distinct flood episodes of these lakes occurred at about 30, 95–100, 130–150 and 180–200 ka. For the episode centred around 30 ka, the frequency histogram of ages shows a multimodal age group that could represent the existence of several humid pulses rather than a unique event. Moreover, the comparison of δ13C and δ18O with those of older humid Pleistocene phases, when very large palaeolakes have been recorded, suggests that these young carbonate shells are not related to a true highstand lake. It is suggested that they represent a period of less humid climatic conditions with carbonate accumulation in minor water ponds in which intensive biological activity could have taken place. It should be noted that this period was less arid than the present.

Armoring of limestone is a common cause of failure in limestone-based acid-mine drainage (AMD) treatment systems. Limestone is the least expensive material available for acid neutralization, but is not typically recommended for highly acidic, Fe-rich waters due to armoring with Fe(III) oxyhydroxide coatings. A new AMD treatment technology that uses CO2 in a pulsed limestone bed reactor minimizes armor formation and enhances limestone reaction with AMD. Limestone was characterized before and after treatment with constant flow and with the new pulsed limestone bed process using AMD from an inactive coal mine in Pennsylvania (pH=2.9, Fe =150 mg/l, acidity =1000 mg/l CaCO3). In constant flow experiments, limestone is completely armored with reddish-colored ochre within 48 h of contact in a fluidized bed reactor. Effluent pH initially increased from the inflow pH of 2.9 to over 7, but then decreased to <4 during the 48 h of contact. Limestone grains developed a rind of gypsum encapsulated by a 10- to 30-μm thick, Fe-Al hydroxysulfate coating. Armoring slowed the reaction and prevented the limestone from generating any additional alkalinity in the system. With the pulsed flow limestone bed process, armor formation is largely suppressed and most limestone grains completely dissolve resulting in an effluent pH of >6 during operation. Limestone removed from a pulsed bed pilot plant is a mixture of unarmored, rounded and etched limestone grains and partially armored limestone and refractory mineral grains (dolomite, pyrite). The ∼30% of the residual grains in the pulsed flow reactor that are armored have thicker (50- to 100-μm), more aluminous coatings and lack the gypsum rind that develops in the constant flow experiment. Aluminium-rich zones developed in the interior parts of armor rims in both the constant flow and pulsed limestone bed experiments in response to pH changes at the solid/solution interface.

Sorption and distribution of adsorbed metals in three soils of India by H. Veeresh; S. Tripathy; D. Chaudhuri; B.R. Hart; M.A. Powell (1723-1731).
The mobility and bioavailability of heavy metals depends on the metal retention capacity of soil and also on the geochemical phases with which metals are associated. Laboratory batch experiments were carried out to study the sorption and distribution of Cd, Ni and Pb in 3 soils differing in their physicochemical properties from India: Oxyaquic Haplustalf (SL1), Typic Haplustalf (SL2) and Typic Haplustert (SL3). The heavy metal adsorption was studied by isotherms and the distribution coefficient (K D) for each metal was obtained from the linear regressions of the concentration of metal remaining in equilibrium solution and the amount adsorbed. In general, the sorption capacity for all the metals decreased in the order: SL3>SL2>SL1. Among metals, the sorption capacity in all the soils decreased in the order: Pb>>Ni>Cd. Distribution of sorbed metals at various equilibrating concentrations was studied by sequential extraction. Results showed significant differences in the distribution of metals in these soils. At higher additions (such as 200 μM l−1) most of the metals were extracted in their more mobile fractions, exchangeable and/or inorganic in contrast to their original partitioning in soils, where they were preferentially associated with the less mobile residual fraction. Largest percentages of metals extracted in the exchangeable fraction corresponded to those soil–metal systems with smaller K D values, e.g. Cd, Ni and Pb in SL1 and Cd and Ni in SL2. In neutral and alkaline soils (SL2, pH=7.1, and SL3, pH=8.6) Pb was predominantly extracted from the inorganic fractions and this corresponded to higher K D values for Pb in these soils. The predominance of metals associated with the exchangeable fraction together with low K D values indicates higher mobility of metals retained in the acidic soil (SL1, pH=5.2) compared with the others.

The Rabbit Lake U mine in-pit tailings management facility (TMF) (425 m long×300 m wide×91 m deep) is located in northern Saskatchewan, Canada. The objectives of this study were to quantify the distribution of As phases in the tailings and evaluate the present-day geochemical controls on dissolved As. These objectives were met by analyzing pore fluid samples collected from the tailings body for dissolved constituents, measuring Eh, pH, and temperature of tailings core and pore fluid samples, conducting sequential extractions on solid samples, conducting geochemical modeling of pore fluid chemistry using available thermodynamic data, and by reviewing historical chemical mill process records. Dissolved As concentrations in 5 monitoring wells installed within the tailings body ranged from 9.6 to 71 mg/l. Pore fluid in the wells had a pH between 9.3 and 10.3 and Eh between +58 and +213 mV. Sequential extraction analyses of tailings samples showed that the composition of the solid phase As changed at a depth of 34 m. The As above 34 m was primarily associated with amorphous Fe and metal hydroxides while the As below 34 m was associated with Ca, likely as amorphous poorly ordered calcium arsenate precipitates. The change in the dominant As solid phases at this depth was attributed to the differences in the molar ratio of Fe to As in the mill tailings. Below 34 m it was <2 whereas above 34 m it was >4. The high Ca/As ratio during tailings neutralization would likely precipitate Ca4(OH)2(AsO4)2:4H2O type Ca arsenate minerals. Geochemical modeling suggested that if the pore fluids were brought to equilibrium with this Ca-arsenate, the long-term dissolved As concentrations would range between 13 and 126 mg/l.

Siderophores are low-molecular weight organic molecules secreted by plants and micro-organisms in response to Fe stress. With stability constants commonly exceeding 1030, siderophores are considered to have higher affinities for Fe(III) than for any other major or trace element dissolved in soil solution. However, several siderophores have affinities for trace metals that approach those for Fe(III), and certain actinides form siderophore complexes of surprisingly high stability. The purpose of this study was to examine the role of hydroxamate siderophores in controlling Pb sorption to an Fe(III) oxide adsorbent. Goethite [α-FeOOH], prepared by standard methods and identified by X-ray diffraction, gave a specific surface of 36 m2 g−1 as determined by N2 multipoint BET analysis. Adsorption experiments were performed aseptically using a batch method with a goethite concentration of 1.0 g l−1 and an ionic strength of 0.01 M NaClO4. Soluble Pb and Fe were measured between pH 3 and 8 by first adding Pb (10 μM) and then siderophore (10, 20, or 40 μM) to the goethite suspension. Three hydroxamate siderophores were employed: desferrioxamine B (DFB), ferrichrome (FC), and rhodotorulic acid (RA). Following 20 h reaction, Pb and Fe in solution were measured by ICP–MS and ICP–AES, respectively. The efficacy of siderophore-mediated Pb desorption varied with siderophore type and generally increased with pH and siderophore/Pb molar ratio. Desferrioxamine B, at pH 6.5 and a DFB/Pb molar ratio of 4, solubilised nearly 25% of the total sorbed Pb. In the presence of 10 μM FC, Pb adsorption largely mimicked that for the siderophore-free system, whereas significant amounts of Pb were desorbed with 20 μM FC at pH >5.5. The dihydroxamate siderophore, RA, was the least effective Pb chelator, requiring 20 μM to desorb detectable amounts of Pb.

Metal geochemistry in a mine-polluted estuarine system in Spain by Charlotte B Braungardt; Eric P Achterberg; Francoise Elbaz-Poulichet; Nicholas H Morley (1757-1771).
The Rio Tinto and Rio Odiel drain the Iberian Pyrite Belt, an important metal-rich sulphide deposit. The rivers are highly acidic (pH 2.2–3.6) and have milli-molar SO4 and Fe concentrations and micro-molar Co, Cu, Mn and Zn concentrations. Observed dissolved metal levels were at a maximum during autumn and early winter surveys (e.g. Rio Tinto: 460–856 μM Cu), and lower in late winter, spring and summer (121–175 μM Cu). This variability is attributed to the production of concentrated acid mine drainage (AMD) during periods of enhanced microbial activity at higher temperatures in summer, and a subsequent run-off of the AMD into the rivers with the first rain in the autumn. Lower temperatures and dilution by winter floods resulted in a reduction of river metal concentration towards the end of the wet season. Metal distributions in the estuarine mixing zones of the Tinto and Odiel were governed by the acidity. The lack of metal transfer from the dissolved to the particulate phase in the low salinity region is attributed to the electrostatic repulsion between the metal cations and positive charges on particle surfaces, and/or to the protonation of adsorption sites at low pH. Dissolved Pb was an exception and showed marked removal in the low salinity zone at low pH (pH 2.5), due to its particle reactive nature. The gross metal fluxes from the Rio Tinto and Rio Odiel are important on a global scale, for example amounting to 8.1 and 1.6% of the estimated global riverine dissolved Zn and Cu fluxes. The fluxes of metals from the estuary contribute to enhanced dissolved metal concentrations observed in the Gulf of Cadiz.

The geographic distribution of elevated groundwater As concentrations in a fractured silicate bedrock aquifer in central New Hampshire correlates with the presence of pegmatites which border late Devonian granites and intrude metasedimentary rocks. As concentrations in the pegmatites averaged 9.6 mg/kg, which is much higher than the associated granites (0.24 mg/kg) and metasedimentary rocks (0.8 mg/kg). As was concentrated in these pegmatites by partial melting of calcareous metapelites and subsequent recrystallization as granites with low As concentrations and pegmatites with high As concentrations. Arsenic behaves similarly to B, which was also concentrated into these late stage fluids. Arsenopyrite (FeAsS) with an oxidation reaction rim of scorodite (FeAsO4·2H2O) was observed in aquifer materials. Elevated As concentrations have been observed to occur in other New England locations in pelitic metasediments intruded by anatectic plutons. It is proposed that pegmatite formation from partial melting of pelitic metasediments may be an important mechanism for concentrating As in crystalline aquifer materials, which can then cause local As enrichment of groundwaters. Groundwater As concentrations ranged from 26 nmol/l to 5300 nmol/l with a median value (210 nmol/l) that is more than 30 times higher than the median for groundwaters from all of New Hampshire (6.5 nmol/l). High chloride concentrations (>1 mmol/l), resulting from road salt contamination of recharge waters, suggest that groundwaters are most likely young (<50 years). A systematic relationship was observed between pH and dissolved As and Fe concentrations. All waters with elevated As concentrations (>670 nmol/l or ∼50 μg/l) have very low Fe concentrations (<18 μmol /l) and high pH (>7); samples with low As (<340 nmol/l) have variable Fe concentrations, but all occur at low pH (<7). pH-dependent Fe oxyhydroxide adsorption of As oxyanions is consistent with the observations. At pH>∼7, Fe oxyhydroxides form rapidly and have a neutral or negative net surface charge that does not readily adsorb As. At pH <∼7, Fe oxyhydroxide formation is slow and depends on dissolved O2 availability, however the resultant Fe oxyhydroxides have a positive net surface charge, and appear to adsorb As readily.

Strontium isotopes and rare earth elements as tracers of groundwater–lake water interactions, Lake Naivasha, Kenya by S. Bwire Ojiambo; W. Berry Lyons; Kathy A. Welch; Robert J. Poreda; Karen H. Johannesson (1789-1805).
Strontium isotope compositions and rare earth element (REE) concentrations are presented for groundwater and surface water samples collected from the Lake Naivasha watershed in the East African Rift, Kenya. The chief objective of the study is to test the suitability of REEs, in conjunction with Sr isotopes, as tools for investigating groundwater–lake water interactions. In general, the REE concentrations and 87Sr/86Sr ratios support the authors’ earlier investigations where Cl mass balance, δ18O, δD, and He isotopes were employed to study groundwater–lake water interactions in the Naivasha watershed. The REE data suggest that a significant amount of the groundwater south of Lake Naivasha (i.e., 50–85%) consists of lake water recharge to the aquifer system. Specifically, mixing calculations conducted using REE data of Lake Naivasha water and groundwaters indicate that between 70 and 85% of groundwater directly south of the lake is likely lake water. These values are somewhat higher than the authors’ previous estimates determined with conservative stable H isotopes (δD, 50–70%). For both cases, however, the data demonstrate that water originating in Lake Naivasha contributes significantly to the underlying groundwater flow system, hence supporting earlier evidence that the lake's freshness reflects rapid loss of water and dissolved solutes to the local groundwater system. Overall, lake and groundwater Sr isotope compositions support seepage of lake water into the underlying aquifer along the lake's south shore. The 87Sr/86Sr data also provide additional insight into the geochemical evolution of waters of the Lake Naivasha watershed indicating that the initial source of Sr to these waters is likely chemical weathering reactions involving basaltic rocks within the recharge zones of the watershed along the Rift Valley flanks. Furthermore, with increasing residence time of groundwaters within the aquifer system and flow down and along the rift valley floor, Sr isotope compositions of groundwaters become more radiogenic, reflecting rock–water interactions with chemically differentiated and radiogenic peralkaline rhyolite volcanic rocks. The importance of the longer aquifer residence times and radiogenic source rocks is especially apparent for geothermal waters of the Olkaria Geothermal Field that have 87Sr/86Sr ratios (i.e., 0.70747) similar to local comendites.