Arsenic in agricultural and grazing land soils of Europe

Arsenic concentrations are reported for the <2 mm fraction of ca. 2200 soil samples each from agricultural (Ap horizon, 0–20 cm) and grazing land (Gr, 0–10 cm), covering western Europe at a sample density of 1 site/2500 km². Median As concentrations in an aqua regia extraction determined by inductively coupled plasma emission mass spectrometer (ICP-MS) were 5.7 mg/kg for the Ap samples and 5.8 mg/kg for the Gr samples. The median for the total As concentration as determined by X-ray fluorescence spectrometry (XRF) was 7 mg/kg in both soil materials. Maps of the As distribution for both land-use types (Ap and Gr) show a very similar geographical distribution. The dominant feature in both maps is the southern margin of the former glacial cover seen in the form of a sharp boundary between northern and southern European As concentrations. In fact, the median As concentration in the agricultural soils of southern Europe was found to be more than 3-fold higher than in those of northern Europe (Ap: aqua regia: 2.5 vs. 8.0 mg/kg; total: 3 vs. 10 mg/kg). Most of the As anomalies on the maps can be directly linked to geology (ore occurrences, As-rich rock types). However, some features have an anthropogenic origin. The new data define the geochemical background of As in agricultural soils at the European scale.     

History and Progress of the North American Soil Geochemical Landscapes Project,2001-2010

In 2007,the U.S.Geological Survey,the Geological Survey of Canada,and the Mexican Geological Survey initiated a low-density(1 site per 1600 km2,13323 sites) geochemical and mineralogical survey of North American soils(North American Soil Geochemical Landscapes Project).Sampling and analytical protocols were developed at a series of workshops in 2003-2004 and pilot studies were conducted from 2004-2007.The ideal sampling protocol at each site includes a sample from 0-5 cm depth,a composite of the soil A horizon,and a sample from the soil C horizon.The <2-mm fraction of each sample is analyzed for Al,Ca,Fe,K,Mg,Na,S,Ti,Ag,Ba,Be,Bi,Cd,Ce,Co,Cr,Cs,Cu,Ga,In,La,Li,Mn,Mo,Nb,Ni,P,Pb,Rb,Sb,Sc,Sn,Sr,Te,Th,Tl,U,V,W,Y,and Zn by inductively coupled plasma-mass spectrometry and inductively coupled plasma-atomic emission spectrometry following a near-total digestion in a mixture of HCl,HNO3,HClO4,and HF.Separate methods are used for As,Hg,Se,and total C on this same size fraction.The major mineralogical components are determined by a quantitative X-ray diffraction method.Sampling in the conterminous U.S.was completed in 2010(c.4800 sites) with chemical and mineralogical analysis currently underway.In Mexico,approximately 66% of the sampling(871 sites) had been done by the end of 2010 with completion expected in 2012.After completing sampling in the Maritime provinces and portions of other provinces(472 sites,7.6% of the total),Canada withdrew from the project in 2010.Preliminary results for a swath from the central U.S.to Florida clearly show the effects of soil parent material and climate on the chemical and mineralogical composition of soils.A sample archive will be established and made available for future investigations.     

Modern,Sangamon and Yarmouth soil development in loess of unglaciated southwestern Illinois

The Thebes Section in unglaciated southwestern Illinois contains a well preserved ∼500 kyr loess–paleosol sequence with four loesses and three interglacial soils. Various magnetic, mineralogical, and elemental properties were analyzed and compared over the thickness of soil sola. These proxies for soil development intensity have the following trend: Yarmouth Geosol>Sangamon Geosol>modern soil. Quartz/plagioclase, Zr/Sr, and TiO₂/Na₂O ratios were most sensitive to weathering. Frequency dependent magnetic susceptibility and anhysteretic remanent magnetization, greatest in A horizons, also correspond well with soil development intensity. Neoformed mixed-layered kaolinite/expandables, suggestive of a warm/humid climate, were detected in the Sangamon and Yarmouth soil sola. Clay illuviation in soils was among the least sensitive indicators of soil development. Differences in properties among interglacial soils are interpreted to primarily reflect soil development duration, with climatic effects being secondary. Assuming logarithmic decreases in weathering rates, the observed weathering in the Sangamon Geosol is consistent with 50 kyr of interglacial weathering (Oxygen Isotope Stage 5) compared to 10 kyr for the modern soil (Oxygen Isotope Stage 1). We propose that the Yarmouth Geosol in the central Midwest formed over 180 kyr of interglacial weathering (including oxygen isotope stages 7, 9, and 11).     

Chemical and K–Ar systematics of a mylonitic mica after natural and experimental interactions with varied fluids

Flow-through and batch-leaching experiments combining mineralogical, chemical and K–Ar isotopic analyses of mica separates from a mylonitic sample of the Tyndrum faulting system in Scotland (U.K.) were conducted to evaluate and simulate the natural interaction with H₂SO₄-loaded river water at pHs of 3–4. The flow-through experiments with H₂SO₄-loaded and pure deionized water completed by a batch-leaching experiment with 1 M HCl at room temperature had varied effects on soluble mineral phases, such as Fe-sulfates and Ca/Mg-carbonates, that were present in the varied mica size fractions, but none caused the mineralogical and K–Ar characteristics of the mica to differ from separates of the natural environment.Despite the limited number of K–Ar ages, size fractionation of the mylonitic mica identified two generations of mica with different REE patterns. The coarser (2–8 μm) fraction yielded a high La/Yb ratio of 4.3 and crystallized at 359 ± 6 Ma at a temperature of 250–300 °C, probably during a major tectonic-thermal activity. The smaller (< 0.5 μm) fraction yielded a La/Yb ratio of 2.1, a younger K–Ar age at 315 ± 5 Ma and a lower crystallization temperature of about 200 °C.     

Breaking the glass ceiling for high resolution Nd isotope records in early Cenozoic paleoceanography

We report an optimized method for extracting neodymium (Nd) from fossil fish teeth with a single-stage column (125 µl stem volume; LN Resin, Eichrom Industries, Darien Illinois) for isotopic analysis by multi-collector inductively coupled mass spectrometry (MC-ICMPS). Three reference materials (basalt: BCR-2, BHVO-2; phosphate: fossil bone composite) and splits of fossil fish teeth samples previously processed with existing two-stage column methods were processed using the single-stage column method. ¹⁴³Nd/¹⁴⁴Nd values of reference materials agree within error with published values, and the values for fish teeth correspond with sample splits processed with two-stage columns. Precision to ± ～ 0.23 ε_Nd was achieved for 30 ng Nd samples of reference materials, and Nd isotope measurements of fossil fish tooth sample replicates as small as 7 ng Nd were reproducible within long term instrumental uncertainty. We demonstrate the utility of the new method with the first high resolution Nd isotope record spanning the ～ 40.0 Ma middle Eocene Climatic Optimum, which shows an excursion of 0.65 ε_Nd during the peak warming at the study site (Ocean Drilling Program Leg 119, Site 738; 30 kyr sample spacing from 40.3 to 39.6 Ma). LN Resin is already used in standard methods for separating Nd, and Nd isotopes are routinely measured by MC-ICPMS with high efficiency inlet systems. Our innovation is a single, small volume LN Resin column for Nd separation. The streamlined approach results in a 10× increase in sample throughput.     

NEXAFS and XPS characterisation of carbon functional groups of fresh and aged biochars

The oxidation of surface functional groups on biochar increases its reactivity and may contribute to the cation exchange capacity of soil. In this study, two Eucalyptus wood biochars, produced at 450 °C (B450) and 550 °C (B550), were incubated separately in each of the four contrasting soils for up to 2 years at 20 °C, 40 °C and 60 °C. Carbon functional groups of the light fraction (< 1.8 g/cm³) of the control and biochar amended soils (fresh and aged for 1 and 2 years at 20 °C, 40 °C and 60 °C) were investigated using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). The spectra of biochar and light fractions of the control and biochar amended soils showed two distinct peaks at ∼285.1 eV and 288.5 eV, which were attributed to the C1s-π¹_CC transitions of aromatic C and C1s-π¹_CO transitions of carboxylic C, carboxyamide C and carbonyl C. The proportion of aromatic C was substantially greater in the light fraction of the biochar amended soils than the corresponding light fraction of the control soils. Also, the proportion of aromatic C was much higher in the light fraction of the B550 amended soils than in the corresponding B450 amended soils. Neither NEXAFS nor XPS results show any consistent change in the proportion of aromatic C of biochar amended soils after 1 year ageing. However, XPS analysis of hand-picked biochar samples showed an increase in the proportion of carboxyl groups after ageing for 2 years, with an average value of 8.9% in the 2 year aged samples compared with 3.0% in the original biochar and 6.4% in the control soil. Our data suggest that much longer ageing time will be needed for the development of a significant amount of carboxyl groups on biochar surfaces.     

Is silt the most influential soil grain size fraction?

The contribution of individual grain size fractions (2000–500, 500–250, 250–63, 63–2 and < 2 μm) to bulk soil surface area and reactivity is discussed with reference to mineralogical and oxalate and dithionite extractions data. The 63–2 μm fraction contributed up to 56% and 67% of bulk soil volume and BET surface area, respectively. Consideration of these observations and the mineralogy of this fraction suggest that the 63–2 μm fraction may be the most influential for the release of elements via mineral dissolution in the bulk soil.     

Carbon–mineral interactions along an earthworm invasion gradient at a Sugar Maple Forest in Northern Minnesota

The interactions of organic matter and minerals contribute to the capacity of soils to store C. Such interactions may be controlled by the processes that determine the availability of organic matter and minerals, and their physical contacts. One of these processes is bioturbation, and earthworms are the best known organisms that physically mix soils. Earthworms are not native species to areas previously glaciated, and the introduction of earthworms to these regions has been associated with often dramatic changes in soil structure and geochemical cycles. The authors are studying C mineral interaction along an approximately 200 m long earthworm invasion transect in a hardwood forest in northern Minnesota. This transect extends from the soils where earthworms are absent to soils that have been invaded by earthworms for nearly 30–40 years. Pre-invaded soils have an approximately 5 cm thick litter layer, thin (～5 cm) A horizon, silt rich E horizon, and clay-rich Bt horizons. The A and E horizons formed from aeolian deposits, while the clay-rich Bt horizons probably developed from underlying glacial till. With the advent of earthworm invasion, the litter layer disappears and the A horizons thicken at the expense of the O and E horizons. In addition, organic C contents in the A horizons significantly increase with the arrival of earthworms. Simultaneously, measured mineral specific surface areas suggest that minerals’ capacities to complex the organic matter appear to be greater in soils with active earthworm populations. Based on the data from two end member soils along the transect, mineral specific surface areas in the A and E horizons are larger in the earthworm invaded soil than in the pre-invasion soil. Additionally, within < 5 a of earthworm invasions, A horizon materials are turned from single grain to a strong medium granular structure. While A horizon organic matter content and organic C-mineral complexation increase after earthworm invasion, they are also more vigorously mixed. This growing data set, when ultimately combined with ongoing measurements of (1) the population dynamics of earthworms along the invasion transect, (2) C-mineral association (via surface adsorption and physical collusion in mineral aggregates) and (3) dissolved organic C will show how and how much soil capacity to store C is affected by burrowing organisms, which are often the keystone species of given ecosystems.     

Metal and arsenic distribution in soil particle sizes relevant to soil ingestion by children

Ingestion of soil is a common behaviour in young children as a means of exploring their surroundings. Much attention has been given to remediation of point-source polluted sites with regard to potential health risks for children. However, because of diffuse pollution and long-range atmospheric deposition, soil contaminant levels are generally increased in urban areas compared to their rural counterparts, even in areas located away from any point sources of pollution. Intake of urban soil can thereby result in significant amounts of the child’s daily metal intake. In the present study, soil samples were collected from 25 playgrounds around urban Uppsala, Sweden and analysed for contents of Al, As, Fe, Cr, Cu, Cd, Hg, Mn, Ni, Pb, W and Zn. Prior to aqua regia digestion, the samples were wet-sieved in order to separate soil particle fractions representing deliberate (<4 mm) and involuntary (<50 μm) soil ingestion by children, as well as a third size fraction of 50–100 μm representing soil that is easily transported by suspension. While the metal and As contents in the 50–100 μm fraction were similar to those of the <4 mm fraction, the <50 μm fraction had metal and As contents on average one and a half times higher than those of the <4 mm fraction. The metal and As contents correlated negatively with the sand content in both particle size fractions <4 mm and 50–100 μm, suggesting a general decrease in metal and As content with increasing sand content. However, a positive correlation was found between sand content and the metal and As contents of the finest fraction (<50 μm), suggesting that when the sand content is high, the bulk of the sorbed elements are on the finest particles. The difference between metal and As contents in the different size fractions was greater in the soil sample with the highest sand content than in the sample with the lowest sand content. This implies that texture is a significant factor in metal and As distribution in soils with moderate metal and As contents, when the number of binding sites associated with small particles is low. Tolerable daily intake (TDI) values for Pb and As were exceeded at all sites, and at two sites for Cd, for children with pica behaviour. A high ingestion rate of mainly small particles could also result in the TDI value for Pb being exceeded at 10 sites and that for As at one site. This study also found that soil analysis by the procedure recommended by Swedish authorities accurately represents the metal intake from deliberate soil ingestion, whereas involuntary soil ingestion of mainly small particles could result in metal intakes which are up to twice as high.     

Biogeochemical weathering of serpentinites: An examination of incipient dissolution affecting serpentine soil formation

Serpentinite rocks, high in Mg and trace elements including Ni, Cr, Cd, Co, Cu, and Mn and low in nutrients such as Ca, K, and P, form serpentine soils with similar chemical properties resulting in chemically extreme environments for the biota that grow upon them. The impact of parent material on soil characteristics is most important in young soils, and therefore the incipient weathering of serpentinite rock likely has a strong effect on the development of serpentine soils and ecosystems. Additionally, porosity generation is a crucial process in converting rock into a soil that can support vegetation. Here, the important factors affecting the incipient weathering of serpentinite rock are examined at two sites in the Klamath Mountains, California. Serpentinite-derived soils and serpentinite rock cores were collected in depth profiles from each sampling location. Mineral dissolution in weathered serpentinite samples, determined by scanning electron microscopy, energy dispersive spectrometry, electron microprobe analyses, and synchrotron microXRD, is consistent with the order, from most weathered to least weathered: Fe-rich pyroxene > antigorite > Mg-rich lizardite > Al-rich lizardite. These results suggest that the initial porosity formation within serpentinite rock, impacting the formation of serpentine soil on which vegetation can exist, is strongly affected both by the presence of non-serpentine primary minerals as well as the composition of the serpentine minerals. In particular, the presence of ferrous Fe appears to contribute to greater dissolution, whereas the presence of Al within the parent rock appears to contribute to greater stability. Iron-oxidizing bacteria present at the soil–rock interface have been shown in previous studies to contribute to the transition from rock to soil, and soils and rock cores in this study were therefore tested for iron-oxidizing bacteria. The detection of biological iron oxidation in this study indicates that the early alteration of these Fe-rich minerals may be mediated by iron-oxidizing bacteria. These findings help provide insight into the incipient processes affecting serpentinite rock weathering, important to the development of extreme serpentine soils and the biota that grow on them.     

Radon emanation of heterogeneous basin deposits in Kathmandu Valley,Nepal

Effective radium-226 concentration (EC_Ra) has been measured in soil samples from seven horizontal and vertical profiles of terrace scarps in the northern part of Kathmandu Valley, Nepal. The samples belong to the Thimi, Gokarna, and Tokha Formations, dated from 50 to 14 ky BP, and represent a diverse fluvio-deltaic sedimentary facies mainly consisting of gravelly to coarse sands, black, orange and brown clays. EC_Ra was measured in the laboratory by radon-222 emanation. The samples (n = 177) are placed in air-tight glass containers, from which, after an accumulation time varying from 3 to 18 days, the concentration of radon-222, radioactive decay product of radium-226 and radioactive gas with a half-life of 3.8 days, is measured using scintillation flasks. The EC_Ra values from the seven different profiles of the terrace deposits vary from 0.4 to 43 Bq kg^?1, with profile averages ranging from 12 ± 1 to 27 ± 2 Bq kg^?1. The values have a remarkable consistency along a particular horizon of sediment layers, clearly demonstrating that these values can be used for long distance correlations of the sediment horizons. Widely separated sediment profiles, representing similar stratigraphic positions, exhibit consistent EC_Ra values in corresponding stratigraphic sediment layers. EC_Ra measurements therefore appear particularly useful for lithologic and stratigraphic discriminations. For comparison, EC_Ra values of soils from different localities having various sources of origin were also obtained: 9.2 ± 0.4 Bq kg^?1 in soils of Syabru–Bensi (Central Nepal), 23 ± 1 Bq kg^?1 in red residual soils of the Bhattar-Trisuli Bazar terrace (North of Kathmandu), 17.1 ± 0.3 Bq kg^?1 in red residual soils of terrace of Kalikasthan (North of Trisuli Bazar) and 10 ± 1 Bq kg^?1 in red residual soils of a site near Nagarkot (East of Kathmandu). The knowledge of EC_Ra values for these various soils is important for modelling radon exhalation at the ground surface, in particular in the vicinity of active faults. Importantly, the study also reveals that, above numerous sediments of Kathmandu Valley, radon concentration in dwellings can potentially exceed the level of 300 Bq m^?3 for residential areas; a fact that should be seriously taken into account by the governmental and non-governmental agencies as well as building authorities.     

Sicilian bottled natural waters: Major and trace inorganic components

Sixteen bottled waters of various Sicilian brands, 11 natural mineral waters and five normal drinking waters, were analyzed for major and trace inorganic components by ion chromatography (IC) and inductively coupled plasma-mass spectrometry (ICP-MS), respectively. The bottled waters represent a variety of water types with significantly different compositions in terms of salinity, major components and trace elements. Chemically, they range from Ca–HCO₃ and Ca–SO₄ to Na–HCO₃ types. Total dissolved solids ranges from 54 to 433 mg/L, total hardness from 25 to 238 mg/L CaCO₃, and measured Na content from 5.7 to 57 mg/L. According to total dissolved ions, all the bottled waters were classified as oligomineral (50 < TDS < 500 mg/L). All the bottled waters analyzed here had elemental concentrations which did not exceed the guideline and directive values, although a high concentration of Al was noted for one bottled water (O7, central Sicily) and high Rb and V in a bottled water (O1) from the Etna volcanic area. With regard to trace elements, the chemical quality of bottled waters was assessed by a metal index (MI). Chemical characteristics were compared with 10 tap water samples from private houses or public places, representative of the public water supply in Palermo, the largest and most densely populated city in Sicily. The municipal waters analyzed, belonging to HCO₃-alkaline earth and Cl–SO₄-alkaline earth waters, showed concentrations of chemical inorganic components well within drinking water limits. The data also indicated that the water supplied by the municipal authority is of fair quality, although fairly hard and high in Na concentration. Several considerations indicate that there is no sufficient reason to prefer bottled waters to tap waters.     

Weathering versus atmospheric contributions to mercury concentrations in French Guiana soils

This work focuses on two possible sources of Hg in tropical soils, (i) lithogenic Hg from in situ weathering of soil parental material, and (ii) exogenic Hg from natural long-term atmospheric inputs and anthropogenic input from past and present industrial activities. The concentration of lithogenic Hg [Hg]_lithogenic was based on comparison of measured Hg concentration with those of elements resistant to weathering such as Nb, U, Zn, Fe. Exogenic Hg was quantified by subtracting [Hg]_lithogenic from total Hg concentrations. This calculation was applied to 4 French Guiana soil profiles, 3 profiles on the same toposequence (ferralsol, acrisol, hydromorphic soil) and one acrisol close to a Au mine, where elemental Hg is used. In all profiles, [Hg]_lithogenic varied slightly and was always below 40 μg kg⁻¹, whereas [Hg]_exogenic varied considerably and reached 500 μg kg⁻¹. The highest [Hg]_exogenic was calculated for the upper horizon of the acrisol close to Au mining activity, but also in the ferralsol. Concentrations of Hg were insignificant in the compact alterite in acrisols. It was concluded that pedogenesis processes that affect the natural Hg supply, combined with anthropogenic sources, explain the Hg concentrations in these tropical soils.     

A rapid and precise procedure for Pb isotopes in whole blood by Fe co-precipitation and MC-ICPMS analysis

Elevated Pb levels in humans through environmental exposure are a significant health concern requiring scientific study of the sources of, and physiological response to this toxin. This requires a simple and precise method for measuring radiogenic Pb isotopes and Pb levels in blood. Presented here is a combination of methods for separation and analysis of Pb previously used predominantly for geologic samples. This includes separation of Pb from the complex matrix of blood samples using an Fe co-precipitation method, followed by isotopic analysis by multi-collector inductively coupled plasma mass spectrometry. Evaluation of the efficacy of this procedure shows that the precision of sample preparations as measured by % difference between the ²⁰⁷Pb/²⁰⁶Pb of duplicate analyses averages 0.064% (n = 48). Using the same preparation and analysis techniques to measure Pb concentrations by isotope dilution resulted in a reproducibility of better than 6%. The method was successfully used to measure uptake of ingested soil Pb in a study of the bioavailability of Pb in contaminated soils.     

A regional-scale geochemical survey of soil O and C horizon samples in Nord-Trøndelag,Central Norway: Geology and mineral potential

A highly efficient, low-density sampling strategy was employed to study the geochemical expression of geological bodies and the mineral potential on the county scale in Central Norway. Soil O and C horizon samples (N = 752) were collected in Nord-Trøndelag and parts of Sør-Trøndelag, and analysed for 53 chemical elements (Ag, Al, As, Au, B, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Fe, Ga, Ge, Hf, Hg, In, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, P, Pb, Pd, Pt, Rb, Re, S, Sb, Sc, Se, Sn, Sr, Ta, Te, Th, Ti, Tl, U, V, W, Y, Zn, and Zr) and Pb isotopes in an aqua regia extraction. At the sample density of one site/36 km² the four metal deposits, which have been mined in the area within the last 50 years were all detected as geochemical anomalies. In addition, a number of new anomalies that may warrant follow-up surveys were found. In terms of geology the Grong–Olden Culmination is marked by a distinct ²⁰⁶Pb/²⁰⁷Pb isotope anomaly. Geochemical differences distinguish the most important belts of mafic metavolcanic lithologies in the area. Though the Fosdalen iron ore deposit is only marked in the soil O horizon, the C horizon outlines the more prominent anomalies of possibly economic interest. Climatic factors like the input of marine aerosols along the coast are clearly visible in the soil O horizon. Low-density geochemical mapping of two sample materials provides important complementing information for the interpretation of the geochemical variation in Nord-Trøndelag county.     

Bacteriohopanepolyol signatures of bacterial populations in Western Canadian soils

Bacteriohopanepolyols (BHPs) are naturally occurring compounds derived from bacteria. Their quantity and diversity in five Western Canadian soils, which vary mostly in vegetative cover, were examined using high performance liquid chromatography – atmospheric pressure chemical ionization – mass spectrometry (HPLC–APCI–MS). Eighteen BHP compounds including tetra-, penta- and hexa-functionalised components, as well as composite components, were identified. Concentrations were highest in the forest-grassland transition soil [515 μg/g organic carbon (OC)], followed by the forest soil (431 μg/g OC) and the grassland soils (333–306 μg/g OC). The distribution trends measured using HPLC–ACPI–MS agree with hopanoid measurements using gas chromatography–mass spectrometry (GC–MS) but intact BHPs were detected at a concentration that was an order of magnitude higher using HPLC–APCI–MS. Adenosylhopane was the most abundant BHP in all the samples and comprised on average 27% of total BHPs, supporting earlier work indicating that adenosylhopane is an excellent soil-specific biomarker. The soil samples vary in vegetative cover and this is likely one of the main reasons for observing variation in BHP composition, suggesting that BHP biomarkers may be a valuable tool for assessing bacterial community structure in soil when used in cooperation with other molecular microbial ecology methods (such as DNA genotyping).     

Total concentration,speciation and mobility of potentially toxic elements in soils around a mining area in central Iran

The current study was designed to investigate the extent and severity of contamination as well as the fractionation of potentially toxic elements (As, Cd, Cr, Cu, Pb, Zn, Ni) in minesoils and agricultural soils around a Pb–Zn mine in central Iran. For this purpose, 20 agricultural soils and eight minesoils were geochemically characterized. Results showed that minesoils contained elevated concentrations of As (12.9–254 mg kg⁻¹), Cd (1.2–55.1 mg kg⁻¹), Pb (137–6239 mg kg⁻¹) and Zn (516–48,889 mg kg⁻¹). The agricultural soils were also polluted by As (5.5–57.1 mg kg⁻¹), Cd (0.2–8.5 mg kg⁻¹), Pb (22–3451 mg kg⁻¹) and Zn (94–9907 mg kg⁻¹). The highest recorded concentrations for these elements were in soils influenced directly by tailing ponds. Chromium, Cu and Ni content in agricultural soils (with average value of 74.1, 34.6 and 50.7 mg kg⁻¹, respectively) were slightly higher than the minesoils (with average value of 54.5, 33.1 and 43.4 mg kg⁻¹, respectively). Sequential extraction data indicated that there were some differences between the speciation of PTEs in soil samples. In the agricultural soils, Zn and Cd were mainly associated with carbonate bound fraction, As and Pb with reducible fraction, Cu with oxidisable fraction and Cr and Ni with residual phase. With respect to mobility factor values, Zn and Cd in the agricultural soils have been found to be the most mobile while As mobility is negligible. Also, the mobility factor of As, Cd and Pb in agricultural soils adjoining tailing ponds was high. In minesoil sample Cd was most abundant in the carbonate form, whereas other studied elements were mainly present in the reducible and residual fractions; therefore, despite the high total concentrations of As, Pb and Zn in the minesoils, the environmental risk of these elements was low. Based on the obtained data, a portion of Cu, Cr and Ni input was from agricultural activities.     

Calcium–phosphate treatment of contaminated soil for arsenic immobilization

The application of As-based herbicides at several industrial sites has resulted in numerous localized areas of As-contaminated soil. In this study, an As-contaminated soil (As = 278 mg/kg) collected from an industrial site located in the southeastern USA was subjected to inorganic phosphate (P_i) treatments. Although P_i treatments have been previously used for flushing As from contaminated soils, in this study, contaminated soil was amended with P_i to study the possible immobilization of As through a co-precipitation mechanism. Specifically, the P_i amendment was aimed at simultaneous flushing of As from the soil with orthophosphoric acid and co-precipitating it as Ca–phosphate–arsenate phases. Bench-scale P_i treatment experiments were performed at different pH conditions, with and without the addition of Ca. Sorption of P_i on BH soil in the presence or absence of additional Ca was determined, along with the associated mobilization of As from the soil. A significant amount of the HNO₃-digestible As (up to 55% at pH 4, 10–15% at pH 8, and ∼30% at pH 11) was released from the contaminated soil during the P_i sorption experiments. This increased mobility of As after the addition of P_i resulted from the competitive desorption of As from the soil. Although P_i sorption at high pH (>8) was largely controlled by precipitation, As did not co-precipitate with P_i. Aqueous geochemical modeling indicated that the lack of As co-precipitation during P_i-only treatment primarily resulted from the deficiency of Ca in the system. When additional Ca (16.9 mmol) was supplied along with P_i (3.38 mmol), the mobility of As decreased significantly at circum-neutral to high solution pH. Geochemical modeling suggested that the leachable As in the soil was potentially precipitated as As-bearing Ca–P_i phases. X-ray diffraction analysis of precipitates separated from the treated soil and from the synthetic leachate confirmed that the formation of a poorly crystalline carbonate apatite phase occurred as a consequence of the treatment. The results of this study support the potential application of Ca–P_i treatment for remediation of As-contaminated soil at environmentally relevant pH conditions.     

High cadmium concentration in soil in the Three Gorges region: Geogenic source and potential bioavailability

This study investigated the distribution and sources of Cd in soils from a Cd-rich area in the Three Gorges region, China. The results showed that in the study area arable soils contain 0.42–42 mg kg⁻¹ Cd with 0.12–8.5 mg kg⁻¹ in the natural soils, corresponding to high amounts of Cd (0.22–42 mg kg⁻¹) in outcropping sedimentary rocks in the area. Both lognormal distribution and enrichment factor (EF) plots were applied in an attempt to distinguish between geogenic and anthropogenic origins of Cd in the local soils. The lognormal distribution plots illustrated that geogenic sources dominated in soils with low and moderate Cd concentrations (<8.5 mg kg⁻¹), whereas anthropogenic sources (agricultural activities, coal mining) significantly elevated Cd contents in some arable soils (>8.5 mg kg⁻¹). The enrichment factor plots illustrated that the majority of the soil samples had EF values of <5, pointing to a geogenic origin of Cd in the soils, whereas some arable soils had EF values >5, pointing to an additional anthropogenic input of Cd to the soils. Sequential extraction results showed that Cd soluble in water and weak acid (water-soluble, exchangeable and carbonate fraction of the soil) accounts for an average of 31% of the total soil Cd, which indicates high potential for Cd mobility and bioavailability. The findings point to a potential health risk from Cd in areas with high geogenic background concentrations of this metal.     

Pb-concentrations and Pb-isotope ratios in soils collected along an east–west transect across the United States

Analytical results for Pb-concentrations and isotopic ratios from ca. 150 samples of soil A horizon and ca. 145 samples of soil C horizon collected along a 4000-km east–west transect across the USA are presented. Lead concentrations along the transect show: (1) generally higher values in the soil A-horizon than the C-horizon (median 21 vs. 16.5 mg/kg), (2) an increase in the median value of the soil A-horizon for central to eastern USA (Missouri to Maryland) when compared to the western USA (California to Kansas) (median 26 vs. 20 mg/kg) and (3) a higher A/C ratio for the central to eastern USA (1.35 vs. 1.14). Lead isotopes show a distinct trend across the USA, with the highest ²⁰⁶Pb/²⁰⁷Pb ratios occurring in the centre (Missouri, median A-horizon: 1.245; C-horizon: 1.251) and the lowest at both coasts (e.g., California, median A-horizon: 1.195; C-horizon: 1.216). The soil C-horizon samples show generally higher ²⁰⁶Pb/²⁰⁷Pb ratios than the A-horizon (median C-horizon: 1.224; A-horizon: 1.219). The ²⁰⁶Pb/²⁰⁷Pb-isotope ratios in the soil A horizon show a correlation with the total feldspar content for the same 2500-km portion of the transect from east-central Colorado to the Atlantic coast that shows steadily increasing precipitation. No such correlation exists in the soil C horizon. The data demonstrate the importance of climate and weathering on both Pb-concentration and ²⁰⁶Pb/²⁰⁷Pb-isotope ratios in soil samples and natural shifts thereof in the soil profile during soil-forming processes.