Applied Geochemistry (v.22, #2)

Sorption isotherms: A review on physical bases, modeling and measurement by G. Limousin; J.-P. Gaudet; L. Charlet; S. Szenknect; V. Barthès; M. Krimissa (249-275).
The retention (or release) of a liquid compound on a solid controls the mobility of many substances in the environment and has been quantified in terms of the “sorption isotherm”. This paper does not review the different sorption mechanisms. It presents the physical bases underlying the definition of a sorption isotherm, different empirical or mechanistic models, and details several experimental methods to acquire a sorption isotherm. For appropriate measurements and interpretations of isotherm data, this review emphasizes 4 main points: (i) the adsorption (or desorption) isotherm does not provide automatically any information about the reactions involved in the sorption phenomenon. So, mechanistic interpretations must be carefully verified. (ii) Among studies, the range of reaction times is extremely wide and this can lead to misinterpretations regarding the irreversibility of the reaction: a pseudo-hysteresis of the release compared with the retention is often observed. The comparison between the mean characteristic time of the reaction and the mean residence time of the mobile phase in the natural system allows knowing if the studied retention/release phenomenon should be considered as an instantaneous reversible, almost irreversible phenomenon, or if reaction kinetics must be taken into account. (iii) When the concentration of the retained substance is low enough, the composition of the bulk solution remains constant and a single-species isotherm is often sufficient, although it remains strongly dependent on the background medium. At higher concentrations, sorption may be driven by the competition between several species that affect the composition of the bulk solution. (iv) The measurement method has a great influence. Particularly, the background ionic medium, the solid/solution ratio and the use of flow-through or closed reactor are of major importance. The chosen method should balance easy-to-use features and representativity of the studied natural conditions.

In this paper a case study of hydraulic connectivity in a 300–1600 m deep, low temperature, sedimentary geothermal system in Kaifeng area, Henan province, China is presented. Based on lithologic data from 52 geothermal wells and chemical data on geothermal water (GW) from six depth-specific and representative wells, the system was chemically grouped into two main hot reservoirs (300–1300 m and 1300–1600 m deep), which were in turn, divided into six sub-reservoirs (SRs). Data on stable isotope (2H and 18O) ratios, radioactive isotope (14C) radiation in conjunction with computation of mineral–fluid chemical equilibria were used to establish the recharge source (a mountainous region in the southwestern part of Zhengzhou, 60 km away); evaluate groundwater age which varied with well depth from 15630 ± 310 a to 24970 ± 330 a; and assess the chemical equilibrium state within the system. The results of different analysis did not suggest an obvious hydraulic connection between the two main hot reservoirs. The location of the recharge zone and the geohydrologic characteristics of the study area demonstrate that the GW utilized from the system is mainly derived from confined waters of meteoric origin.

Reaction transport modelling of a freshening aquifer (Tertiary Ledo-Paniselian Aquifer, Flanders-Belgium) by Kristine Walraevens; Javier Cardenal-Escarcena; Marc Van Camp (289-305).
The semi-confined Ledo-Paniselian (Eocene) Aquifer in Flanders, Belgium is recharged in the areas with higher topography, where it is covered by the Bartonian Clay. Initial conditions in these sediments were marine. Hydrogeochemical modelling of the groundwater type distribution in the aquifer with a reaction transport model considers recharge of fresh CaHCO3 water by downward flow through the overlying clay. In the aquifer, in an upstream direction, progressively more freshened water types are found. A typical NaHCO3 type occurs upstream of the brackish-fresh water interface. The different stages of cation exchange produce a chromatographic sequence of groundwater types, which agrees well with observations. Cation exchange processes occurring mainly in the percolated clay, result in a Na+ increase, and peaks of K+, NH 4 + and Mg2+ in the aquifer, which are spatially separated as a result of groundwater flow. Calcite equilibrium, gypsum dissolution in the clay and sulphate reduction in the aquifer have also been included in the modelling.

Geochemical mapping in Aichi prefecture, Japan: Its significance as a useful dataset for geological mapping by Koshi Yamamoto; Tsuyoshi Tanaka; Masayo Minami; Koichi Mimura; Yoshihiro Asahara; Hidekazu Yoshida; Setsuo Yogo; Makoto Takeuchi; Masami Inayoshi (306-319).
Since 1994 a geochemical mapping project has been undertaken in Aichi prefecture, central Japan, with the aim of establishing a database to contribute to environmental assessment in the region. To date more than 1200 stream sediments have been collected and analyzed. The samples were analyzed for 10 major elements by X-ray fluorescence spectrometry and loss-on-ignition.The analyses show that stream sediments from areas with sedimentary bedrock are enriched in SiO2 and mafic elements such as TiO2 and Fe2O3 compared to areas with granitic bedrock. The SiO2 enrichment is mainly due to the amount of quartz and the enrichment in mafic elements is due to biotite, both of which are derived from the sedimentary bedrock. In contrast, sediments derived from granitic bedrock are characterized by high abundances of elements contained in feldspars such as Na2O and CaO. This contrast in elemental distribution (e.g., Na2O/TiO2 ratio) can be useful for geological mapping in the areas covered by vegetation and with poor bedrock exposure. The relative abundances of Ca, Na and K in the stream sediments suggest that the Inagawa granite, one of the main granite bodies in the study area, can be classified into four distinct types. This classification is distinct from previous classifications based on geological and petrochemical studies. It is confirmed that analysis of geochemical compositions of stream sediments has considerable potential as a tool in geological studies including lithological mapping.

Soils from many industrial sites in southeastern USA are contaminated with As because of the application of herbicide containing As2O3. Among those contaminated sites, two industrial sites, FW and BH, which are currently active and of most serious environmental concerns, were selected to characterize the occurrence of As in the contaminated soils and to evaluate its environmental leachability. The soils are both sandy loams with varying mineralogical and organic matter contents. Microwave-assisted acid digestion (EPA method 3051) of the contaminated soils indicated As levels of up to 325 mg/kg and 900 mg/kg (dry weight basis) for FW and BH soils, respectively. However, bulk X-ray powder diffraction (XRD) analysis failed to find any detectable As-bearing phases in either of the studied soil samples. Most of the soil As was observed by scanning electron microscopy, coupled with energy dispersive X-ray spectroscopy (SEM/EDX), to be disseminated on the surfaces of fine-grained soil particles in close association with Al and Fe. A few As-bearing particles were detected in BH soil using electron microprobe analysis (EMPA). Synchrotron micro-XRD and X-ray absorption near-edge structure (XANES) analyses indicated that these As-rich particles were possibly phaunouxite, a mineral similar to calcium arsenate, which could have been formed by natural weathering after the application of As2O3. However, the scarcity of those particles eliminated them from playing any important role in As sequestration.Synthetic acid rain sequential batch leaching experiments showed distinct As leaching behaviors of the two studied soil samples: BH soil, which has the higher As content, showed a slow, steady release of As, while FW soil, with a lower As content, showed a much quicker release and lower overall retention of As upon leaching. Sequential chemical extraction experiments were carried out using a simplified 4-step sequential chemical extraction procedure (SCEP) previously developed to characterize the fractionation of As and better understand the different leaching behaviors of the two studied soils. It was shown that only about 50% of the total extractable As was removed by the first two extraction steps, which represented the most weakly bonded and readily available As for environmental leaching. Compared with the sequential leaching experiments, it was further indicated that only half of the As associated with phases extracted by the second SCEP step was mobilized by SPLP leaching. Although microwave-assisted acid digestion results showed similar Al and Fe contents in both soils, the sequential chemical extraction experiments indicated that BH soil has a much higher content of amorphous Al and Fe phases and that a comparably higher portion of soil As was associated with those materials. The experimental results suggest that remediation efforts for the contaminated sites can be directed towards enhancing the formation of more stable As-bearing compounds in the soils to reduce the environmental leachability of As.

Differential settling and geochemical evolution of tailings’ surface water at the Central Manitoba Gold Mine by Barbara L. Sherriff; Nikolay V. Sidenko; Kristin A. Salzsauler (342-356).
Differential settling of sulfides and carbonates in the mill discharge, 70 a ago, has resulted in lateral zoning across the tailings at the Central Manitoba Gold Mine site. Potentially acid-generating minerals within the tailings with a negative net neutralizing potential (NNP) were deposited close to the discharge point, while remote areas accumulated less-dense carbonate-rich sediments. Subsequent erosion and redeposition of the tailings may have modified the original segregation, but have not destroyed the original zonation. In the regions more than 300 m from the discharge site, there is still sufficient calcite to neutralize acid generated from the oxidation of sulphides and the oxidized tailings have pH values of 7–8. In the areas close to the discharge point, the tailings generate more acid than can be neutralized by the dissolution of carbonates and the pH of the oxidized tailings drops to 3. The tailings deposit contains two ponds, the barren Blue Pond located near the discharge point and the Green Pond (which contains horsetail plants) 100 m north of the mill site. The Blue Pond is impacted by acidic drainage from carbonate-poor, sulphide-rich tailings, and characterized by a pH of 4.4 ± 0.1. Further from the water source, the Green Pond is surrounded by tailings with higher carbonate than sulphide content, giving a positive NNP and a pH of 7.4 ± 0.4. Aluminum and Cu are dissolved from the acidic oxidized tailings, precipitated in slimes in streams and then washed into the Blue Pond, where dissolution of Al(OH)3(s), jurbanite and basaluminite maintain the constant pH.

Geochemistry of an acidic chromium sulfate plume by L. Edmond Eary; Andy Davis (357-369).
The historical disposal of acidic chromium sulfate solutions into unlined lagoons between 1953 and 1970 at an industrial site resulted in formation of a dense aqueous phase liquid (DAPL) plume [specific gravity 1.11 g/cm3, pH 3, up to 4700 mg/L Cr(III), and up to 90,000 mg/L SO4]. The DAPL sank through the shallow glacial till aquifer to an underlying impermeable gneissic bedrock from where it migrated downgradient along buried channels incised in the bedrock. Because of its high density, the plume chemistry is sharply stratified vertically. Chromium(III) predominates in the DAPL because excess Cr(VI) not reduced in the original process has been reduced by Fe(II) derived from silicates, while Cr(OH)3(am) occurs as surface coatings on silicate minerals and as discrete particles mixed with Fe(OH)3(am) and Al(OH)3(am). The solubility of Cr(OH)3(am) accurately describes Cr(III) concentrations in the plume and nearby groundwater, while Al and Fe in solution are also consistent with solubility-controlling oxyhydroxides. Because of these solubility controls, metal cations are attenuated relative to more mobile Cl and SO4, resulting in a chromatographic separation of solutes downgradient from the plume origin. The good agreement between predicted and observed solution concentrations illustrates the utility of equilibrium modeling when interpreting metal transport characteristics and in determining the efficacy of natural attenuation in subsurface systems.

Hydro-geochemical controls on removal of Cr(VI) from contaminated groundwater by anion exchange by Biswajit Mukhopadhyay; Jon Sundquist; Eric White (370-387).
Column experiments using strongly basic anion exchange resin under acidic conditions of pH (3.6–5) and conducted with groundwater sources having strong acidic anions (Cl and SO 4 2 - ) in excess of weak acidic anions ( HCO 3 - and CO 3 2 - ) but with markedly different Cr(VI) concentrations demonstrate that the anion chemistry of groundwater controls the efficacy of the ion exchange process for removing dissolved oxyanions of Cr(VI). Although the proportions of CrO 4 2 - relative to HCrO 4 - in the influent waters are reduced due to acidification, Cr 2 O 7 2 - also exists when total Cr(VI) in the solution is high. Breakthrough of Cr occurs in the ion exchange column gradually and earlier than the expected breakthrough time calculated on the basis of flow characteristics and exchange capacity of the resin. The influent with higher Cr concentration (e.g., 95 mg/L) shows a square wave shaped breakthrough curve resembling an ideal type of breakthrough. The diffused nature in the early part of breakthrough is more pronounced with the influent with lower concentration of Cr (e.g., 3.3 mg/L). Thus, the breakthrough pattern with high concentration of Cr in the influent does not follow the prediction made in some previous studies describing an advection–reaction model of chromatographic flow, coupled with empirical reaction parameters obtained from chromate–chloride and chromate–sulfate isotherms experimentally determined in the range of aqueous Cr(VI) < 20 mg/L. One-dimensional advective–reactive–dispersive modeling with a full account of all the major anion exchanges, namely, Cl - ⇔ HCrO 4 - , Cl - ⇔ CrO 4 2 - , Cl - ⇔ Cr 2 O 7 2 - , Cl - ⇔ SO 4 2 - , Cl - ⇔ HCO 3 - , and Cl - ⇔ NO 3 - explains the effects of interfering anions on the Cr breakthrough. Available separation factors are used as first-order approximations of the equilibrium constants of the exchange reactions. If multi-component exchange is not considered the model shows that the Cr breakthrough is nearly ideal and the exchange capacity of the exchanger can be utilized to its full potential. When multi-component exchanges in the column are taken into consideration, then the model shows breakthrough patterns that closely match those observed in the experiments. Thus, the early and gradual breakthrough of Cr(VI) oxyanions during anion exchange cannot be wholly attributed to the unique characteristics of the chromate–chloride isotherms. The overall hydro-geochemistry of a site must be considered when evaluating the applicability of ion exchange to remediation of Cr-contaminated groundwater.

Evaluating main factors controlling aluminum solubility in acid forest soils, southern and southwestern China by Jingheng Guo; Xiaoshan Zhang; Rolf David Vogt; Jinsong Xiao; Dawei Zhao; Renjun Xiang; Jiahai Luo (388-396).
The mechanism controlling Al solubility in Chinese acidic forest soils is not clearly understood. This is the main limitation to the ability to generate adequate dose-response prediction models of the ecological effect of acid rain. To evaluate the relative significance of possible processes, soils and soil solutions from five forest catchments, located in southern and southwestern China, were collected and analyzed for chemical parameters. Monitoring showed that inorganic Al (Ali) was the dominant fraction in most soil solutions; organic Al (Alo) was usually less than 10% of total monomeric Al (Ala). Aluminum fractions varied significantly between and within the different sites, though appearing to follow a similar pattern. Over the entire pH range of 3.6–5.6, the pAl (i.e. −log of the Al3+ activity) closely correlated with solution pH, following regression slopes of 1.28 and 2.00 for upper and lower soil horizons, respectively. The variations in Al3+ activity could not be explained satisfactorily using mineral dissolution equilibria. Partial least square (PLS) regression showed that soil acidity (quality) and ionic strength (intensity) of the solution were the main explanatory variables for the variation in the concentration of Al fractions. Aluminum in upper horizons originated from both organic and inorganic solid Al pools, while aqueous Al in lower horizons was dominantly of inorganic origin. Aluminum solubility was strongly influenced by cation exchange, especially in the upper horizon. In the upper horizon, ionic strength (I) had a greater influence on Al solubility due to cation exchange reaction. In the lower horizon, dissolution of inorganic Al pools by the elevated H+ concentrations was the main Al release mechanism. So Al activity was more dependent on H+ (or pH) in the lower horizon.

Contents and δ 34S values of several S compounds, enumerations of S-reducing bacteria (SRB) and Fe-reducing bacteria (IRB), and Fe, Pb and In concentrations were determined for 210Pb-dated sediment cores from two lakes in Quebec, Canada. Both lakes are located approximately 70 km downwind of the Horne smelter and refinery in Rouyn-Noranda. Increases in Fe, Pb and In concentrations and a decrease in the δ 34S values of total S in both lake sediment cores coincide with the start-up of the smelter in 1927. The shift towards more negative δ 34S values was primarily caused by an increase in the extent of S isotope fractionation during bacterial (dissimilatory) SO4 reduction due to SO4 loading of the lakes after smelting began. Consequently, an enhanced accumulation of 32S-enriched reduced inorganic S compounds is evident in the sediments. δ 34S values of organic S in the sediments decreased only slightly due to the smelter emissions between 1930 and 1980. Hence, due to the sulfide depositing mechanisms, S isotope ratios constitute a useful tracer recording the onset of S pollution in sediments of the two previously SO4-limited lakes investigated. In contrast, total S concentrations alone are not reliable indicators for anthropogenic S loading in lake sediment records.

The Myra mine, now inactive, produced Zn and Cu concentrates from a Zn-rich, Kuroko-type, volcanogenic massive sulfide deposit located in the mountainous interior of Vancouver Island. The climate at the site is classified as “Marine West Coast”, with annual precipitation exceeding 2200 mm. Water from a losing stream on the mountainside above the mine follows preferential, fracture-controlled pathways to the upper workings before draining through the 10-Level portal. With a view toward mine decommissioning, portal discharge rate was monitored continuously over a 17-month period during which 46 water samples were collected. Effluent chemistry, dominated by Ca, HCO3 and SO4, shows moderate to high total base metal concentrations and near-neutral pH. Carbonatization, mainly of mafic rocks in the hangingwall, provides significant acid neutralizing potential. Metal concentrations vary seasonally, with smaller spikes associated with summer storm events, and a main peak associated with flushing of the workings during the first heavy autumn rains. Aqueous speciation modeling suggests that Fe and Al concentrations are controlled by the solubilities of hydrous ferric oxides and microcrystalline gibbsite, respectively. Concentrations of Zn, Cu and Cd appear controlled by sorption rather than by the solubilities of mineral phases. A comparison of precipitate concentrations observed in portal effluent with predictions from mass balance (inverse) modeling results suggests that less than 5% of the precipitated Fe and Al hydroxides are transported from the mine. However, amounts of sorbed Cu, Zn and Cd measured in the effluent are only slightly lower than modeled values. This suggests that the small fraction of (probably finer) Fe precipitates in portal effluent sorbs most of the Zn, Cu and Cd predicted by modeling. Based on mass balance calculations, metal loadings are explained by the oxidation of 3830 kg of pyrite, 600 kg of sphalerite and 190 kg of chalcopyrite, annually. Circum-neutral drainage conditions are maintained by the reaction of almost 19,800 kg of calcite, annually.

The sorption of Th(IV) onto γ-Al2O3 in the absence and presence of soil humic acid (HA)/fulvic acid (FA) was studied by batch technique. The influence of pH from 2 to 12, ionic strength from 0.01 M to 0.1 M KNO3, soil HA/FA concentrations from 2.5 mg/L to 17.5 mg/L, and foreign cations (Li+, Na+, K+) and anions ( NO 3 - , Cl) on the sorption of Th(IV) onto γ-Al2O3 was also tested. The sorption isotherms of Th(IV) were determined at pH 3.50 (±0.02) and analyzed with the linear, Freundlich, and Langmuir sorption models, respectively. The results demonstrated that the sorption of Th(IV) onto γ-Al2O3 increases steeply with increasing pH from 2 to 4. HA/FA was shown to enhance Th(IV) sorption at low pH, but to reduce Th(IV) sorption at high pH. It was assumed that the significantly positive influence of HA/FA on Th(IV) sorption onto γ-Al2O3 at pH 2–4 is attributable to the strong surface binding of HA/FA on γ-Al2O3 and subsequently to the formation of ternary surface complexes such as SO–O–HA–Th or SO–O–FA–Th. The results also demonstrated that the sorption is strongly dependent on the concentration of HA/FA, and slightly dependent on ionic strength. The sorption of Th(IV) onto γ-Al2O3 was also dependent on foreign ions in solution under the experimental conditions.

Lead contamination of fluvial sediments in an eroding blanket peat catchment by J.J. Rothwell; M.G. Evans; T.E.H. Allott (446-459).
Over the last few years there has been growing concern over the mobilisation of anthropogenically derived, atmospherically deposited Pb from upland blanket peat soils to receiving surface waters. The near-surface layer of blanket peat soils of the Peak District, southern Pennines, UK, is severely contaminated with high concentrations of Pb. Erosion of peat soils in this upland area may be releasing large quantities of previously deposited Pb into the fluvial system. Samples of fluvial sediments (suspended, floodplain, streamside fan, trash-line and channel bed) were collected from a severely eroding blanket peat catchment in the Peak District in order to investigate Pb contamination of fluvial sediments, to determine the mechanism for fluvial Pb transport and to determine if erosion of contaminated peat soils in the catchment is releasing Pb into the fluvial system. Concentrations of Pb associated with fluvial sediments are considerably higher than those in the catchment geology, but not as high as those in peat soils in the catchment. Intra- and inter-storm variability in the Pb content of suspended sediments can be explained by differences in organic matter content of these sediments and differences in erosion processes operating within the catchment. High Pb concentrations are associated with suspended sediments that have a high organic matter content. The results of this study suggest that organic matter is the principle vector for sediment-associated Pb in the fluvial system. Erosion of contaminated peat soils in the Peak District is releasing Pb into the fluvial system. The extent to which this is a problem in other peatland environments is an area requiring further research.

Factors responsible for high arsenic concentrations in two groundwater catchments in Taiwan by Sheng-Wei Wang; Chen-Wuing Liu; Cheng-Shin Jang (460-476).
High As contents in groundwater were found in two neighboring catchments – the Chianan plain and the southern Choushui river alluvial fan in Taiwan. The groundwater quality, the redox potential and the As distribution of the Chianan plain were characterized using factor analysis, redox zoning and a geochemical program. The results were compared with those of the southern Choushui river alluvial fan. Possible As release mechanisms are also elucidated. Factors 1 and 2 of the groundwater in the Chianan plain – the salinization factor and the As enrichment factor – are similar to those in the southern Choushui river alluvial fan. However, the spatial distribution of reductive tendency in the Chianan plain is different to that in the Choushui river alluvial fan, and yields spatially distinct hydrogeochemical environments in these two neighboring areas. The reduction potential in the Chianan plain is stronger than that in the Choushui river alluvial fan. The difference between the reduction potentials in these two vicinal areas influences the concentrations of As in groundwater. The reductive dissolution of As-rich Fe oxyhydroxide is suggested to be the major mechanism for release of As to the groundwater in the Chianan plain and the Choushui river alluvial fan of Taiwan.

Importance of geochemical transformations in determining submarine groundwater discharge-derived trace metal and nutrient fluxes by Aaron J. Beck; Yoko Tsukamoto; Antonio Tovar-Sanchez; Miguel Huerta-Diaz; Henry J. Bokuniewicz; Sergio A. Sañudo-Wilhelmy (477-490).
Seasonal (Spring and Summer 2002) concentrations of dissolved (<0.22 μm) trace metals (Ag, Al, Co, Cu, Mn, Ni, Pb), inorganic nutrients (NO3, PO4, Si), and DOC were determined in groundwater samples from 5 wells aligned along a 30 m shore-normal transect in West Neck Bay, Long Island, NY. Results show that significant, systematic changes in groundwater trace metal and nutrient composition occur along the flowpath from land to sea. While conservative mixing between West Neck Bay water and the groundwaters explains the behavior of Si and DOC, non-conservative inputs for Co and Ni were observed (concentration increases of 10- and 2-fold, respectively) and removal of PO4 and NO3 (decreases to about half) along the transport pathway. Groundwater-associated chemical fluxes from the aquifer to the embayment calculated for constituents not exhibiting conservative behavior can vary by orders of magnitude depending on sampling location and season (e.g. Co, 3.4 × 102– 8.2 × 103  μmol d−1). Using measured values from different wells as being representative of the true groundwater endmember chemical composition also results in calculation of very different fluxes (e.g., Cu, 6.3 × 103  μmol d−1 (inland, freshwater well) vs. 2.1 × 105  μmol d−1(seaward well, S = 17 ppt)). This study suggests that seasonal variability and chemical changes occurring within the subterranean estuary must be taken into account when determining the groundwater flux of dissolved trace metals and nutrients to the coastal ocean.