Applied Geochemistry (v.39, #C)
Geochemical investigations of saltwater intrusion into the coastal carbonate aquifer of Mallorca, Spain by C. Garing; L. Luquot; P.A. Pezard; P. Gouze (1-10).
Coastal aquifers often display seawater intrusion resulting in the formation of a salty water wedge progressing inland. This study investigates the mass transfers in the mixing zone at the freshwater–seawater interface where the water is out of equilibrium with the rock-forming carbonates. Investigations were conducted in two boreholes, separated by 5 m, at the Ses Sitjoles test site (Mallorca Island, Spain) where repeated electrical conductivity logs of the formation and the saturating fluid, as well as regular pore-water sampling and permanent downhole multi-parameter monitoring of the water were performed over a period of 9 a. In the mixing zone, the significant acidification, the calcite saturation index profile and the Ca concentration profile cannot be explained by conservative mixing nor by dissolution–precipitation reactions only. Conversely, the analysis of organic C content and of the distinctly different time-resolved pH profiles measured in the two boreholes suggests the development of perennial biomass that triggers calcite dissolution. Moreover, the presence of biomass seems to be correlated with the permeability and vertical connectivity at the meter-scale. It is speculated that the mechanism could be self-activated because the microbiological activity induces calcite dissolution and tends to increase porosity and permeability that favors biomass development.
Mineralogy and environmental relevance of AMD-precipitates from the Tharsis mines, Iberian Pyrite Belt (SW, Spain) by T. Valente; J.A. Grande; M.L. de la Torre; M. Santisteban; J.C. Cerón (11-25).
This paper documents the solid phases associated to acid mine drainage (AMD) at the Tharsis mines (SW Spain). It provides an inventory of the AMD-precipitates, describing their main modes of occurrence and mineral assemblages. Results indicate that iron, aluminum and magnesium sulfates predominate in the assemblages. They occur as efflorescences composed of complex mixtures of metallic salts, and as ochres (jarosite combined with goethite). Also, their distributions illustrate two hydrochemical environments: the open pits, which reflect a proximal secondary paragenesis; and the downstream river banks (Meca River), which represent a more evolved paragenesis, resulting from the evolution of AMD produced throughout the system. These environments can be differentiated by composition and variety of minerals, which is considerably lower along the Meca River.The newly-formed minerals have monitoring significance and proved capable of participating in cycles of retention–liberation of hydrogen ions, sulfate, and metals. In a semi-arid climate, the importance of the AMD-precipitates as environmental indicators is stressed. They may help to understand the response of the system to the episodic rainfall events that occur after prolonged dry periods.
Concentration of metals in surface water and sediment of Luilu and Musonoie Rivers, Kolwezi-Katanga, Democratic Republic of Congo by Emmanuel K. Atibu; Naresh Devarajan; Florian Thevenon; Paola M. Mwanamoki; Joseph B. Tshibanda; Pius T. Mpiana; Kandasamy Prabakar; Josué I. Mubedi; Walter Wildi; John Poté (26-32).
The pollution and deterioration of most important vital rivers in the Katanga region, Democratic Republic of Congo (DRC) are mainly due to the discharge of untreated industrial effluents as well as to the mining and artisanal mineral exploitation activities. In this study, the concentrations of metals (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Mo, Ag, Sn, and Pb) and major elements (Na, Mg, and K) in mining effluents, water and sediment samples of two main rivers of the district of Kolwezi (Katanga, DRC) were subjected to analysis by Inductive Coupled Plasma-Mass Spectroscopy (ICP-MS). The results showed that, in general, the metal concentrations in the sampling sites from the mining effluent and river waters exceed largely the World Health Organization and the Aquatic Quality Guidelines for the Protection of Aquatic Life recommendation limits. The highest metal concentrations in water and sediment samples were detected surrounding the mining effluents discharge. In the surface sediments of Luilu River, the values of 47,468 and 13,199 mg kg−1 were observed for Cu and Co, respectively. For the sediment samples from Musonoie River, the maximum values of 370.8 and 240.6 mg kg−1 for Cu and Co, respectively were observed. The results of this study suggest that the mining effluents being discharged into the rivers and the accumulation of pollutants in sediments might represent a source of toxicity for aquatic living organisms and could pose significant human health risks. The measures to establish a monitoring program and the application of wastewater treatment techniques to the mining effluents prior to discharge are recommended to reduce the load of contaminants into the receiving systems.
Prediction of the concentration of chemical elements extracted by aqua regia in agricultural and grazing European soils using diffuse reflectance mid-infrared spectroscopy by J.M. Soriano-Disla; L. Janik; M.J. McLaughlin; S. Forrester; J. K. Kirby; C. Reimann (33-42).
The aim of this study was to develop partial least squares (PLS) models to predict the concentrations of 45 elements in soils extracted by the aqua regia (AR) method using diffuse reflectance Fourier Transform mid-infrared (MIR; 4000–500 cm−1) spectroscopy. A total of 4130 soils from the GEMAS European soil sampling program (geochemical mapping of agricultural soils and grazing land of Europe) were selected. From the full soil set, 1000 samples were randomly selected to develop PLS models. Cross-validation was used for model training and the remaining 3130 samples used for model testing. According to the ratio of standard deviation to root mean square error (RPD) of the predictions, the elements were allocated into two main groups; Group 1 (successful calibrations, 30 elements), including those elements with RPD ⩾ 1.5 (the coefficient of determination, R 2, also provided): Ca (3.3, 0.91), Mg (2.5, 0.84), Al (2.4, 0.83), Fe (2.2, 0.79), Ga (2.1, 0.78), Co (2.1, 0.77), Ni (2.0, 0.77), Sc (2.1, 0.76), Ti (2.0, 0.75), Li (1.9, 0.73), Sr (1.9, 0.72), K (1.8, 0.70), Cr (1.8, 0.70), Th (1.8, 0.69), Be (1.7, 0.66), S (1.7, 0.66), B (1.6, 0.63), Rb (1.6, 0.62), V (1.6, 0.62), Y (1.6, 0.61), Zn (1.6, 0.60), Zr (1.6, 0.59), Nb (1.5, 0.58), Ce (1.5, 0.58), Cs (1.5, 0.58), Na (1.5, 0.57), In (1.5, 0.57), Bi (1.5, 0.56), Cu (1.5, 0.55), and Mn (1.5, 0.54); and Group 2 for 15 elements with RPD values lower than 1.5: As (1.4, 0.52), Ba (1.4, 0.52), La (1.4, 0.52), Tl (1.4, 0.51), P (1.4, 0.46), U (1.4, 0.45), Sb (1.3, 0.46), Mo (1.3, 0.43), Pb (1.3, 0.42), Se (1.3, 0.40), Cd (1.3, 0.40), Sn (1.3, 0.38), Hg (1.2, 0.33), Ag (1.2, 0.32) and W (1.1, 0.19). The success of the PLS models was found to be dependent on their relationships (directly or indirectly) with MIR-active soil components.
Distribution of inorganic arsenic species in groundwater from Central-West Part of Santa Fe Province, Argentina by Mirna Sigrist; Antonela Albertengo; Lucila Brusa; Horacio Beldoménico; Mabel Tudino (43-48).
The distribution of inorganic arsenic species in groundwater used as drinking water supply by the peri-urban and rural population from central-western area of Santa Fe Province, Argentina, was studied. An analytical methodology based on an online system of atomic absorption spectrometry with hydride generation and flow injection (FI-HGAAS) was used for total inorganic arsenic determination. For speciation purposes, the distinction between As(V) and As(III) was performed through the on line coupling of FI-HGAAS to a solid phase system based on an anionic exchanger able to retain As(V) as oxyanion, allowing As(III) to be selectively determined. The concentration of As(V) was calculated as the difference between total arsenic and As(III) concentrations. Effects of matrix interference due to the nonselective behavior of the exchange resins were carefully laid. Results for 59 samples collected from 27 localities showed an almost exclusive predominance of pentavalent forms.
Diffusion–reaction studies in low permeability shale using X-ray radiography with cesium by Diana B. Loomer; Lisa Scott; Tom A. Al; K. Ulrich Mayer; Sergio Bea (49-58).
An X-ray radiography method for the determination of diffusion coefficients in rock has been modified to provide an estimation of diffusion–reaction parameters for the sorbing tracer, cesium (Cs+). Cesium tracer diffusion and sorption on cation exchange sites can be monitored in intact rock samples using data extracted from radiographs to plot time-series relative-concentration profiles of Cs+ as a function of transport distance. Cesium was found to be a very good tracer for these experiments because of the sensitivity of the X-ray attenuation measurements to Cs+ concentration.Reactive-transport modeling coupled with parameter estimation software was used to match experimental data and estimate pore diffusion coefficients for Cs+ (D p-Cs = 7.6 × 10−11 m2/s), single-site selectivity coefficients for Cs+ exchange (log K Cs+/Na+ = 1.5) and cation exchange capacity (CEC = 8.4 meq/100 g) for drill core samples of Queenston Formation shale, from the Michigan Basin in Ontario, Canada.
Characterisation of hydrous ferric oxides derived from iron-rich groundwaters and their contribution to the suspended sediment of streams by Stijn Baken; Carin Sjöstedt; Jon Petter Gustafsson; Piet Seuntjens; Nele Desmet; Jan De Schutter; Erik Smolders (59-68).
When Fe(II) bearing groundwaters surface in streams, particulate authigenic Fe-rich material is produced by oxidation. Such freshly precipitated Fe minerals may be transported as suspended sediment and have a profound impact on the fate of trace metals and nutrients in rivers. The objective of this study was to monitor changes in mineralogy and composition of authigenic material from its source to streams of increasing order. Groundwaters, surface waters, and suspended sediment in streams of different order were sampled in the Kleine Nete catchment (Belgium), a lowland with Fe-rich groundwaters (3.5–53.8 mg Fe/L; pH 6.3–6.9). Fresh authigenic material (>0.45 μm) was produced by oxidising filtered (<0.45 μm) groundwater and surface water. This material contained, on average, 44% Fe, and smaller concentrations of C, P, and Ca. Iron EXAFS (Extended X-ray Absorption Fine Structure) spectroscopy showed that the Fe was present as poorly crystalline hydrous ferric oxides with a structure similar to that of ferrihydrite. The Fe concentration in the suspended sediment samples decreased to 36–40% (stream order 2), and further to 18–26% (stream order 4 and 5). Conversely, the concentrations of organic C, Ca, Si, and trace metals increased with increasing stream order, suggesting mixing of authigenic material with suspended sediment from a different source. The Fe speciation in the suspended sediment was similar to that in fresh authigenic material, but more Fe–Fe interactions were observed, i.e. it was increasingly hydrolysed, suggesting ageing reactions. The suspended sediment in the streams of order 4 and 5 is estimated to contain between 31% and 59% of authigenic material, but more data are needed to refine this estimate. The authigenic material is an important sink for P in these streams which may alleviate the eutrophication risk in this catchment.
Do organic ligands affect forsterite dissolution rates? by Julien Declercq; Olivier Bosc; Eric H. Oelkers (69-77).
Far-from equilibrium, steady state forsterite dissolution rates were measured at pH ∼3 and 25 °C in aqueous solutions containing 0.1 m/kg NaCl and up to 0.1 mol/kg of 13 distinct dissolved organic ligands in mixed-flow reactors. The organic ligands considered in this study include those common in Earth surface environments and those considered as potential catalysts for use in CO2 sequestration efforts: acetate, oxalate, citrate, EDTA4−, glutamate, gluconate, malonate, aspartate, tartrate, malate, alginate, salycilate and humate. The presence of up to 0.1 mol/kg of each organic ligand altered forsterite dissolution rates less than 0.2 log units, which is the estimated uncertainty of the measured rates. Results obtained in this study, therefore, suggest that the presence of aqueous organic anions negligibly affects forsterite far-from equilibrium dissolution rates in most natural environments, and indicate that forsterite carbonation may not be appreciably accelerated by organic ligand catalysis.
Correlations between metals in tree-rings of Prosopis julifora as indicators of sources of heavy metal contamination by L.E. Beramendi-Orosco; M.L. Rodriguez-Estrada; O. Morton-Bermea; F.M. Romero; G. Gonzalez-Hernandez; E. Hernandez-Alvarez (78-84).
Heavy-metals (Cu, Pb and Zn) in tree-ring sequences of Prosopis juliflora, a tree species native to arid environments, were analyzed by ICP-MS. The tree-ring sequences were obtained from three specimens growing in an urban area previously reported as contaminated by the activity of a Cu smelter facility. The metal found in highest concentration in the wood was Zn, with concentrations up to 120 mg/kg and an enrichment factor up to 26; followed by Cu (up to 9.6 mg/kg, enrichment factor up to 8.6) and Pb (up to 1.4 mg/kg, enrichment factor up to 3). By assessing the correlation between different metal concentration trends, it was possible to infer two main pollution sources: vehicle traffic and Cu smelter emissions. Vehicle traffic is indicated by a correlation between Pb and Zn over time within individual trees, whereas contamination from the Cu-smelting facility is indicated by a correlation of Cu over time between trees. In tree A there was a significant within-tree correlation between Pb and Zn concentration trends (r = 0.856, P < 0.001), whereas Cu showed no correlation with the other metals. For tree B, there were no within-tree correlations between these metals, but when comparing the concentration–time trends between trees A and B, there was a significant correlation for Cu (r = 0.768, P < 0.01). The tree-ring sequence from tree C showed significant within-tree correlation for Cu:Zn (r = 0.430, P < 0.01) and for Pb:Zn (r = 0.753, P < 0.001). The highest enrichment values were found in tree A, located along the path of the growing-season dominant wind direction from the smelter facility, and not in the tree growing closer to the smelter (tree C), suggesting that the smelter’s emissions are dispersed to longer distances through the tall chimneys, attenuating the impact to the area directly closest to the smelter facility. It is concluded that Prosopis juliflora appears as a good bioindicator based on its metal accumulation capacity and lack of metal mobility among tree rings, thus providing information on the chronology and sources of heavy-metal pollution in urban and industrial areas.
Citrate adsorption can decrease soluble phosphate concentration in soil: Experimental and modeling evidence by Marek Duputel; Florence Van Hoye; Joële Toucet; Frédéric Gérard (85-92).
The adsorption/desorption of phosphate (PO4) on soil minerals is a major process regulating soluble phosphate concentrations (i.e. phosphorus availability) and ultimately PO4 bio-availability. Release of citrate by roots is widely recognized as an effective biological mechanism for increasing available phosphorus (P) in soil. However, interactions between citrate and PO4 are poorly understood and little investigated in soils. Using surface complexation modeling we recently predicted that citrate adsorption can decrease available P in soils depending mainly on soil type and on citrate, exchangeable calcium, and soil organic carbon concentrations. The most pronounced decrease was predicted in chromic cambisols. The intention of the present study was to verify the accuracy of our predictions against experimental data measured in a real chromic cambisol, and to improve our understanding of PO4-controlling processes through surface complexation modeling.The addition of a low to moderate citrate concentration (<50 μM) effectively decreased available P. In contrast, a concentration of 100 μM produced an increase of available P. The maximum decline of available P was observed at 20 μM of citrate. The agreement between simulated and measured values of P availability and total dissolved Ca concentrations was excellent at a citrate concentration ranging from 0 to 50 μM. Our model substantially underestimated the increase of available P measured at 100 μM of citrate. This discrepancy showed the presence of another PO4-controlling process, which revealed to be the dissolution of P-containing minerals, most probably illite.This study confirmed that the release of citrate in soils and its subsequent adsorption onto minerals can produce the decrease of P availability. Such citrate-induced decreases of available P can be observed after addition of a relatively low concentration. The threshold concentration was 20 μM in the chromic cambisol investigated here. This validation should have serious implications in the prospect of rhizosphere management for a better PO4 acquisition by plants through citrate release, as an adverse effect of citrate can be expected, depending on soil properties and citrate concentration. This study also further demonstrated the ability of surface complexation models to reproduce and predict dissolved concentrations of various ions in soil solutions as a function of environmental conditions.
On the potential for the Partial Triadic Analysis to grasp the spatio-temporal variability of groundwater hydrochemistry by Laurent Gourdol; Christophe Hissler; Lucien Hoffmann; Laurent Pfister (93-107).
Standard multivariate statistical techniques, such as principal components analysis and hierarchical cluster analysis, have been widely used as unbiased methods for extracting meaningful information from groundwater quality data. However, these classical multivariate methods deal with two-way matrices, usually parameters × sites or parameters × time, while often the dataset resulting from qualitative water monitoring programs should be seen as a datacube parameters × sites × time. Three-way matrices, such as the one proposed here, are difficult to handle and to analyse by classical multivariate statistical tools and thus should be treated with approaches dealing with three-way data structures. One possible analysis approach is the use of Partial Triadic Analysis (PTA). Applied to the dataset of the Luxembourg Sandstone aquifer, the PTA appears to be a new promising statistical instrument for hydrogeologists, for characterization of temporal or spatial hydrochemical variations induced by natural and anthropogenic factors. This new approach for groundwater management offers potential for (1) identifying a common multivariate spatial structure, (2) untapping the different hydrochemical patterns and explaining their controlling factors and (3) analyzing the temporal variability of this structure and grasping hydrochemical changes.
Can the long-term potential for carbonatization and safe long-term CO2 storage in sedimentary formations be predicted? by H. Hellevang; P. Aagaard (108-118).
A sedimentary formation perturbated by supercritical CO2 reacts by dissolving primary minerals and forming new secondary phases. In this process CO2 may be trapped in stable carbonate minerals and may thereby be immobilized for long time spans. The potential for mineral trapping can be estimated by solving kinetic expressions for the reservoir minerals and possible secondary phases. This is, however, not trivial as kinetic data are uncertain or even lacking for the minerals of interest. Here, the rate equations most commonly used for CO2 storage simulations have been solved, and the rate parameters varied, to obtain sensitivity on the total amount of CO2 stored as mineral carbonate. As various expressions are in use to estimate growth rates of secondary carbonates, three formulations were compared, including one taking into account mineral nucleation preceding growth. The sensitivity studies were done on two systems, the Utsira Sand being representative for a cold quartz-rich sand (37 °C, 100 bar CO2), and the Gulf Coast Sediment, being representative for a medium temperature quartz–plagioclase-rich system (75 °C, 300 bar CO2).The simulations showed that the total predicted CO2 mineral storage is especially sensitive to the choice of growth rate model and the reactive surface area. The largest sensitivity was found on α, fraction of total surface area available for reactions, with a reduction of one order of magnitude for all reacting phases leading to 3–4 times lower predicted CO2 mineral storage. Because the reactive surface area is highly uncertain for natural systems, the range in predicted results may be even larger. The short-term predictions (<100–1000 a), such as the onset of carbonate growth, were highly sensitive to nucleation and growth rates. Moreover, the type of carbonate minerals formed was shown to be model dependent, with the simplest model predicting an unlikely carbonate assemblage at low temperature (i.e., formation of dolomite at 37 °C). Therefore, to use kinetic models to upscale short-term (<months) laboratory experiments in time, to identify the past reactions and physical conditions of natural CO2 storage analogues, and finally to predict the potential for CO2 trapping in existing and future storage projects, more knowledge has to be collected, especially on the reactive surface area of CO2 storage reservoirs, and on the rate of secondary carbonate nucleation and growth.
Partitioning of Pb(II) during goethite and hematite crystallization: Implications for Pb transport in natural systems by Hong Phuc Vu; Samuel Shaw; Loredana Brinza; Liane G. Benning (119-128).
The interaction (e.g., adsorption and incorporation) of Pb with iron(III) (oxyhydr)oxide minerals has a significant influence on its partitioning and transport in many natural systems (e.g., rivers). The incorporation of Pb during ferrihydrite crystallization to hematite and goethite at neutral and alkaline pH, in the presence and absence of sulphate SO 4 2 - has been studied using X-ray Absorption Spectroscopy (XAS), X-ray Powder Diffraction (XRD), electron microscopic techniques and chemical extraction procedures. The XRD data showed that hematite and goethite were the end-products of crystallization at pH 5, whereas goethite was the sole product at pH 13. The Pb partitioning data revealed that upon crystallization at pH 5, ∼60% of the initially adsorbed Pb remained on the surface of the crystalline hematite/goethite, while ∼20% became incorporated with the remaining ∼20% released back into solution. Lead incorporation occurred primarily during the initial stage of ferrihydrite crystallization prior to hematite/goethite formation at pH 5. The presence of SO 4 2 - at pH 5 had little influence on the partitioning of Pb or mineral phases formed. At pH 13, 52% of the adsorbed Pb was incorporated during crystallization to goethite. Lead incorporation into this phase occurred over the entire crystallization process with adsorbed Pb incorporated during goethite crystal growth. X-ray Absorption Spectroscopy and unit cell size data demonstrated that Pb did not replace Fe within the structure of hematite or goethite, but was incorporated into defects or nanopores within the iron (oxyhydr)oxides.
Geochemical and palaeohydrological controls on the composition of shallow groundwater in the Netherlands by Jasper Griffioen; Sophie Vermooten; Gijs Janssen (129-149).
With the exception of the south of the country, the Netherlands has a strong bipartite hydrogeology: the Holocene part with a coastal dune belt and confining top layer of clay and peat further inland, and the Pleistocene, where thick phreatic aquifers dominate. This research aimed to ascertain the geochemical and palaeohydrological controls on the composition of shallow groundwater in 27 regions. Close to 6000 groundwater analyses were grouped and interpreted in terms of 1. salinity, 2. redox status, 3. acid/base and carbonate status and 4. natural nutrients NH4 and PO4. The a priori classification into geographical regions and geological formations revealed many statistically significant differences in medians, even for geologically or geographically related data groups. The compound-specific interpretation indicates that there are geogenically controlled, systematic differences in groundwater composition at the regional scale. The imprint of the geological sediments on the groundwater composition decreases in the order marine/estuarine via limnological, fluvial to aeolian. The imprints with respect to pH and carbonate status, natural nutrients and redox status are not necessarily interrelated. The vertical stratification in groundwater composition turns out to be often limited at the regional scale due to mutual occurrence of infiltrating and exfiltrating groundwater in regions and either the presence of a highly reactive Holocene, confining top layer or temporal changes in contamination. In the Holocene part, the salinity is controlled by the palaeoenvironmental conditions during the Holocene and by the recharge origin: the average Cl concentration decreases from estuarine via lagoonal to the former Zuider Sea (which was a bay). The most reduced states and also the highest nutrient concentrations and highest CO2 pressure are related to the presence of Holocene marine sediments in the confining top layer. Degradation of marine-derived organic matter as a nutrient source, is likely more intense in the Holocene deposits than that of peat and sedimentary remnants of terrestrial plants. A broad range in pH, carbonate status and redox status is encountered in the Pleistocene part. Here, the palaeohydrological evolution in terms of carbonate leaching together with the geological controls on the calcareous nature of the shallow sedimentary deposits cause regional differences in pH, calcite saturation and silicate weathering. One region with Late Pleistocene limnological deposits has deviating groundwater characteristics and appears more similar to the Holocene part of the Netherlands. Furthermore, reactive Fe is not abundant in all Pleistocene fluvial sediments nor is it maximally mobilised, as not all anoxic groundwater in these sediments is siderite-saturated. This leads to considerable intra- and inter-region variability.
Modern carbon burial in Lake Qinghai, China by Hai Xu; Jianghu Lan; Bin Liu; Enguo Sheng; Kevin M. Yeager (150-155).
The quantification of carbon burial in lake sediments, and carbon fluxes derived from different origins are crucial to understand modern lacustrine carbon budgets, and to assess the role of lakes in the global carbon cycle. In this study, we estimated carbon burial in the sediment of Lake Qinghai, the largest inland lake in China, and the carbon fluxes derived from different origins. We find that: (1) The organic carbon burial rate in lake sediment is approximately 7.23 g m−2 a−1, which is comparable to rates documented in many large lakes worldwide. We determined that the flux of riverine particulate organic carbon (POC) is approximately 10 times higher than that of dissolved organic carbon (DOC). Organic matter in lake sediments is primarily derived from POC in lake water, of which approximately 80% is of terrestrial origin. (2) The inorganic carbon burial rate in lake sediment is slightly higher than that of organic carbon. The flux of riverine dissolved inorganic carbon (DIC) is approximately 20 times that of DOC, and more than 70% of the riverine DIC is drawn directly and/or indirectly from atmospheric CO2. (3) Both DIC and DOC are concentrated in lake water, suggesting that the lake serves as a sink for both organic and inorganic carbon over long term timescales. (4) Our analysis suggests that the carbon burial rates in Lake Qinghai would be much higher in warmer climatic periods than in cold ones, implying a growing role in the global carbon cycle under a continued global warming scenario.
Flow dependent water quality impacts of historic coal and oil shale mining in the Almond River catchment, Scotland by Simon Haunch; Alan M. MacDonald; Neil Brown; Christopher I. McDermott (156-168).
The Almond River catchment in Central Scotland has experienced extensive coal mining during the last 300 years and also provides an example of enduring pollution associated with historic unconventional hydrocarbon exploitation from oil shale. Detailed spatial analysis of the catchment has identified over 300 abandoned mine and mine waste sites, comprising a significant potential source of mine related contamination. River water quality data, collected over a 15 year period from 1994 to 2008, indicates that both the coal and oil shale mining areas detrimentally impact surface water quality long after mine abandonment, due to the continued release of Fe and SO 4 2 - associated with pyrite oxidation at abandoned mine sites. Once in the surface water environment Fe and SO 4 2 - display significant concentration-flow dependence: Fe increases at high flows due to the re-suspension of river bed Fe precipitates (Fe(OH)3); SO 4 2 - concentrations decrease with higher flow as a result of dilution. Further examination of Fe and SO4 loading at low flows indicates a close correlation of Fe and SO 4 2 - with mined areas; cumulative low flow load calculations indicate that coal and oil shale mining regions contribute 0.21 and 0.31 g/s of Fe, respectively, to the main Almond tributary. Decreases in Fe loading along some river sections demonstrate the deposition and storage of Fe within the river channel. This river bed Fe is re-suspended with increased flow resulting in significant transport of Fe downstream with load values of up to 50 g/s Fe. Interpretation of major ion chemistry data for 2005–2006 indicates significant increases in Ca2+, Mg2+ and HCO 3 - in coal mined areas probably as a result of the buffering of proton acidity in mine waters; in the oil shale areas Na− and Cl− become increasing dominant possibly associated with increased urbanisation and saline pore water discharge from unprocessed oil shale waste. The study demonstrates the importance of considering the cumulative impact of point and diffuse contamination sourced from numerous small coal and oil shale mine sites on surface water quality.
Geogenic signatures detectable in topsoils of urban and rural domains in the London region, UK, using parent material classified data by J.D. Appleton; C.C. Johnson; E.L. Ander; D.M.A. Flight (169-180).
Systematic mapping of the chemical environment of urban areas from around the world have shown varying degrees of control of element distributions by the underlying parent material (PM). The purpose of the study reported here is to assess whether geogenic signatures that dominate soil chemistry in rural domains of Eastern England and which are not strongly impacted by human activities, can also be detected in the London urban region. A PM soil chemistry mapping method is used to determine the spatial variation of topsoil chemistry data in London and the surrounding rural areas. Analysis of variance (ANOVA) of the soil data for the London region indicates that 26–33% of the variance of Al, Ce, Cs, Ga, K, La, Mg, Mn, Nb, Nd, Rb, Ti, V and Y is explained by soil PM (surface geology), and a slightly lesser proportion (19–25%) of the variance for Ca, Co, Fe, I, Ni, Sc, Sr and Th. In comparison, soil PM explains only 5% of the variance of Cd. The variance of some other elements appears to be influenced by a mixture of geogenic and anthropogenic controls, including As, Ba, Cr, Cu, Mo, P, Pb, Sb, Se, Sn and Zn for which PM controls 12–16% of the variance. Geogenic soil chemistry patterns observed for the elements strongly influenced by PM in the rural areas surrounding London can be quite clearly followed into and through the London urban area. Spatial patterns of a range of elements primarily controlled by PM have not been destroyed even in a major urban centre with a recorded history dating back over 2000 years and which has been subjected to extensive urban development, destruction and redevelopment especially during the last 200–300 years.
Arsenic in a fractured slate aquifer system, New England, USA: Influence of bedrock geochemistry, groundwater flow paths, redox and ion exchange by Peter C. Ryan; Jonathan J. Kim; Helen Mango; Keiko Hattori; Ali Thompson (181-192).
Elevated As levels have been reported by the Vermont Geological Survey in groundwater from public and domestic bedrock wells in northwestern New England (USA). The study area in southwestern Vermont is underlain by pyrite-rich, organic-rich slates that were thrusted over carbonate and clastic sedimentary rocks of the continental shelf during the Ordovician Taconian Orogeny, and the distribution of wells with elevated As shows that they were completed in slates. Hydrochemical and bedrock geochemical analysis indicates that elevated As in the aquifer system is controlled by the following: (1) the presence of black slates that are rich in arsenian pyrite (200–2000 ppm As); (2) release of As via the dissolution of As-rich pyrite; (3) geochemically-reducing and slightly alkaline conditions, where high As values occur at Eh < 200 mV and pH > 7; and (4) physical hydrogeological parameters that foster low Eh and high pH, particularly long groundwater flow paths and low well yields (i.e. high residence time) which provides high rock to water ratios. Where all four factors affect As contents in groundwater, 72% of wells in a zone of distal groundwater flow/low-relief topography exceed 10 μg/L (ppb) and 60% of wells in this zone exceed 25 ppb As. Where flow paths are shorter in slates and groundwater has higher Eh and lower pH (i.e. in regions of higher-relief topography closer to recharge zones), only 3% of wells contain >10 ppb As and none contain >25 ppb.Overall, 28% (50/176) of low-elevation wells (<245 meters above sea level [masl]) exceed 10 ppb As; only 3% (2/60) of higher-elevation wells (245–600 masl) exceed 10 ppb As. Over the entire aquifer system, 22% of bedrock wells (52/236) exceed 10 ppb and the mean As concentration is 12.4 ppb. Strong positive correlations among Fe, SO4 and As in groundwater confirm that dissolution of pyrite is the dominant As source. Positive correlations among SO4, Na and As indicate that, in reducing (Eh < 200 mV) groundwater, Fe(II) is exchanged for Na on mineral surfaces following pyrite dissolution and As remains in solution; conversely, in oxidizing groundwater (recharge zones), Fe(II) is oxidized to Fe(III) and the subsequent formation of ferrihydrite removes As (V) from solution.