C-horizon Soils Research Articles

Spatiotemporal variations of chemical weathering intensity in large drainage basin and its potential climatic implications: A case study from the Yangtze River Valley

The knowledge on continental chemical weathering has a role in understanding carbon cycle and climate change. However, regional-scale spatiotemporal analysis of chemical weathering in large drainage basins is relatively seldom performed. In this study, based on 718 A- and C-horizon soil samples, we examined the spatiotemporal variations of chemical weathering intensity (i.e., the Chemical Index of Alteration (CIA)) in the Yangtze River Valley (YRV) by using trend surface analysis (TSA) and spatial correlation analysis (SCA). We found that: (1) the average CIA values of 68.4 and 68.5 for the A- and C-horizon soils reflect an overall intermediate chemical weathering in the YRV; (2) the weathering intensities follow a latitudinal distribution pattern that they increased from 50 to 70 in the north/northwest to 70–90 in the south/southeast of the YRV; (3) the general trend of chemical weathering is preferential leaching of Ca and Na over K; (4) TSA and SCA suggest a relatively dominant control of precipitation on the weathering intensity than temperature; (5) regions with frequent, long-duration and intense heavy precipitation (≥5 days/y and daily rainfall ≥50 mm) and heatwave (≥2 times/y, ≥15 days per time, and ≥ 40 °C) are featured with anomalous silicate weathering whose intensity was diminished from C- to A-horizon; and (6) the absolute differences of the mean CIA values between C- and A-horizon whether in the YRV or sub-regions were <1, indicating a stable silicate weathering. We conclude that significant latitude effect embodies the dominant control of monsoon climate on the chemical weathering in the YRV. Local climate extremes (e.g., rainstorms) are accompanied by anomalous silicate weathering. However, decreasing weathering intensity from bottom up does not necessarily indicate alleviated climate extremes, but probably the loss of terrestrial carbon stocks.

Suitability of surficial media for Ni–Cu–PGE exploration in an established mining camp: a case study from the South Range of the Sudbury Igneous Complex, Canada

A geochemical study over the southwestern part of the South Range of the Sudbury Igneous Complex was completed to assess the suitability of surficial media (humus, B-horizon soil and C-horizon soil) for delineating geochemical anomalies associated with Ni–Cu–PGE (platinum group element) mineralization. Another objective was to test whether Na pyrophosphate can eliminate the effects of anthropogenic contamination in humus. Results of this study suggest that the natural geochemical signature of humus is strongly overprinted by anthropogenic contamination. Despite no indication of underlying or nearby mineralization, metal concentrations in humus samples by aqua regia collected downwind from smelting operations are higher compared to background, including up to 13 times higher for Pt, 12 times higher for Cu and nine times higher for Ni. The high anthropogenic background masks the geogenic signal such that it is only apparent in humus samples collected in the vicinity of known Ni–Cu–PGE deposits. Results of this study also demonstrate that anthropogenically derived atmospheric fallout also influences the upper B-horizon soil; however, lower B-horizon soil (at &gt;20 cm depth) and C-horizon soil (both developed in till) are not affected. Glacial dispersal from Ni–Cu–PGE mineralization is apparent in C-horizon till samples analysed in this study. Compared to the background concentrations, the unaffected C-horizon till samples collected immediately down-ice of the low-sulfide, high precious metal Vermilion Cu–Ni–PGE deposit are enriched over 20 times in Pt (203 ppb), Au (81 ppm) and Cu (963 ppm), and over 30 times in Ni (1283 ppm). Supplementary material: supplementary tables and figures are available at https://doi.org/10.6084/m9.figshare.c.5691080

Exploration geochemistry of surficial media over the high-grade McArthur River uranium deposit, Saskatchewan, Canada

ABSTRACT An orientation survey using surficial media was performed over the high-grade McArthur River unconformity-related U deposit (Saskatchewan, Canada) to test whether or not secondary dispersion of elements related to the ore body or alteration zone can be detected at the surface more than 500 m above the deposit. Organic-rich Ah-horizon soils, Fe-rich B-horizon soils, C-horizon soils, tree cores of Jack pine (Pinus banksiana), and glacially dispersed boulders of Manitou Falls Formation sandstone that host the U deposit were collected in four sampling grids near the mine site. Two of the grids overlaid the trace of the P2 fault that hosts the deposit and extends nearly to the surface, one grid overlaid both the P2 fault and one of the high-grade ore bodies (Zone 4), and one grid was located 2.5 km away from the ore body surface trace in the barren hanging wall of the P2 fault. The grid overlying the Zone 4 ore body had the highest proportion of samples with elevated U and low 207Pb/206Pb ratios, the latter indicative of radiogenic Pb from a high-U source, measured in two size fractions of Ah-horizon soils using Na pyrophosphate leach, pine tree cores using total digestion, and sandstone boulders using 2% HNO3 leach. A handful of pathfinder elements, such as As, Co, Ni, and Pb, are variably associated with the U and radiogenic Pb. Sandstone boulders with an assemblage of dravite + kaolinite ± illite, determined using shortwave infrared (SWIR) spectroscopy and matching the alteration mineralogy in the Manitou Falls Formation above the U deposit, were prevalent in the grid above the Zone 4 ore body and in the adjacent grid in the direction of glacial dispersion. A coarse fraction of the B-horizon soils, leached with 5% HNO3, highlighted the grid above the Zone 4 ore body to a lesser extent, whereas HNO3 leaches and aqua regia digests of C-horizon soil separates did not highlight the P2 fault or ore body trace due to influence by parent till mineralogy. Results of environmental monitoring at the mine site, which was active at the time of sampling, suggest that dust containing U, Pb, and radionuclides from waste rock piles and a ventilation shaft could influence A-horizon soil geochemistry near the mine site, and that U and radiogenic Pb anomalies in B- and C-horizon soils near the water table are close to a treated mine effluent discharge point. However, older trees that record elevated U and radiogenic Pb in annual rings that pre-date mining activity, and alteration mineralogy and geochemistry of boulders that are less susceptible to the influences of mining activity, add confidence that the geochemical anomaly in diverse surficial media above the Zone 4 ore body represents secondary dispersion from the underlying U deposit.

Elemental concentrations and in vitro bioaccessibility in Canadian background soils.

Elemental concentrations and bioaccessibility were determined in background soils collected in Canada as part of the North American Geochemical Landscapes Project. The concentrations of As, Cr, Cu, Co, Ni and Zn were higher in the C-horizon (parent material) compared to 0-5cm (surface soil), and this observation along with the regional distribution suggested that most of the variability in concentrations of these elements were governed by the bedrock characteristics. Unlike the above-stated elements, Pb and Cd concentrations were higher in the surface layer reflecting the potential effects of anthropogenic deposition. Elemental bioaccessibility was variable decreasing in the order Cd>Pb>Cu>Zn>Ni>Co>As>Cr for the surface soils. With the exception of As, bioaccessibility was generally higher in the C-horizon soils compared to the 0-5cm soils. The differences in metal bioaccessibility between the 0-5cm and the C-horizon and among the provinces may reflect geological processes and speciation. The mean, median or 95th percentile bioaccessibility for As, Cr, Cu, Co, Ni and Pb were all below 100%, suggesting that the use of site-specific bioaccessibility results for these elements will yield more accurate estimation of the risk associated with oral bioavailability for sites where soil ingestion is the major contributor of human health risk.

Assessment of Metal Enrichment Factors in Waters Draining a Mixed-Land-Use Catchment

Pollution assessment in a given area depends mostly on the precise determination of the so-called background values. In this work, enrichment factors (EFs) for iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn) in suspended solids (SS) of a stream draining a rural catchment (NW Spain) were determined to assess whether the observed concentrations of these metals represent natural or contaminated levels. Aluminum was used as a normalizer element. Four approaches were used for defining background values used in the EF analyses: a local background value for metals concentrations in SS, a local background value for metals concentrations in C-horizon soils, mean crust values, and mean shale values. The most satisfactory result was obtained when using local background values for metals concentrations of SS, suggesting that on average the Fe, Mn, Cu, and Zn concentrations measured represent natural levels. However, some signs of enrichment were observed (16–28 percent of the samples with EF > 1.5).

The use of property-scale portable X-ray fluorescence data in gold exploration: advantages and limitations

This study examines field portable X-ray fluorescence (pXRF) data for unsieved B- and C-horizon soil samples from two contrasting Au exploration programs in northern Canada and compares them to laboratory results from sieved samples to determine if a minimalist approach to sample preparation, equipment calibration and analysis produces results that are adequate for exploration purposes. The collection of 14 651 residual soil samples on detailed grids at the Whiskey Au project in the Black Hills of the Yukon Territory, Canada during the 2011 field season allows a comparison of pXRF data from unsieved samples with inductively-coupled plasma mass spectrometry (ICP-MS) data from the &lt;100-µm grain size fraction digested in aqua regia. XRF data were generated on two devices: an Innov-X Delta handheld and a Niton FXL desk-top unit. Data for selected elements from the pXRF units show trends in gridded percentile plots comparable to the ICP-MS data, although the continuity of these trends reflects the degree to which the pathfinder element concentrations exceed the lower orders of detection. Acceptable merged gridded images for As and Cu from the two devices were generated without data levelling, whereas the Pb, Mo and Ni data require leveling prior to gridding in order to provide a reasonable fit to the laboratory data. Data for elements that occur in concentrations close to the lower limit of detection by pXRF, such as Sb, may provide some useful information but must be used with caution. Data for Fe also show a poor correlation, possibly due to heterogeneity of Fe in the samples, as well as due to incomplete digestion of all Fe-bearing minerals in some samples using an aqua regia digestion. A total of 680 till samples collected from regional and detailed sampling grids on the Kiyuk Lake Au property in southern Nunavut, Canada during the 2012 field season were analysed by pXRF and also by ICP-MS following an aqua regia digestion. pXRF data were generated on an Innov-X Delta using unsieved samples for comparison with ICP-MS data from the &lt;70-µm grain size fraction. The pathfinder element As shows good agreement with the laboratory data, whereas Ni, Mo and Sb data give poor matches due to proximity of the data to the lower orders of detection by pXRF. The use of pXRF data on soil samples with no sample preparation other than drying and no instrument calibration is a robust approach for Au exploration using particular elements, such As and Cu, provided the data are monitored for consistency. Even greater compatibility between pXRF and laboratory data would be expected where sample preparation and site-specific calibrations are implemented. Other pathfinder elements for Au, including W, Sb, Bi, Ag and Te, as well as Au itself, typically occur at levels in soil either close to or below the lower order of detection of the current generation of field-portable analytical devices. The use of pXRF in Au exploration allows for quick decision-making and provides near-real time sampling guidance in the field where ‘fit for purpose’ data for suitable pathfinder and lithologically-controlled elements can be obtained.

Partitioning of Zn, Cd, Pb, and Cu in organic-rich soil profiles in the vicinity of a zinc smelter

A five-step sequential extraction procedure was applied to organic-rich soil samples from five soil profiles situated 1–8 km from a zinc smelter. The partitioning of Zn, Cd, Pb, and Cu into five operationally defined fractions (exchangeable, “carbonate’’-bound, reducible, oxidizable, and residual) was studied at different soil depths down to 35cm. In the surface soil (0–1 cm) a major part of Pb and Cu was extracted in the oxidizable fraction, whereas for Zn and Cd slightly more was extracted in the ‘‘carbonate”-fraction than in the other four fractions. Extracted metal proportions in the oxidizable fraction were respectively of the order of 30%, 20%, 50%, and 80% for Zn, Cd, Pb, and Cu in the surface soil for all sites, but these proportions decreased with soil depth. In the surface soil less than 20% of all the elements were extracted in the residual fraction, but the proportions associated with this fraction generally increased with soil depth. In the C-horizon, differences in extracted proportions of Pb and Cu in the residual fraction were probably due to geochemical factors, whereas for Zn the low extracted proportion at a highly contaminated site (20%) may be due to Zn migration to the C-horizon at this site. For Cd the extracted proportions in the C-horizon were lower than for the other elements, generally below 20%, presumably because Cd is weaker in terms of its adsorption to the soil than the other elements studied. Total concentrations of the metals decreased strongly with increasing distance from the smelter, but less systematic differences were observed for their distributions among fractions. Potentially bioavailable metal proportions (exchangeable + “carbonate”-bound fraction) in the surface soil were about 50%, 60%, 20%, and 10% for Zn, Cd, Pb, and Cu, respectively. In C-horizon soil the mobility sequence Cd>Zn>Pb = Cu was generally observed. The present results indicate that the concentrations and chemical fractionation of Zn, Pb, and Cd in these soils represent a considerable risk to natural terrestrial food chains.

Degradation of the potato glycoalkaloid α-solanine in three agricultural soils

The toxic glycoalkaloids produced by the potato plant ( Solanum tuberosum L.) have previously been found in upper soil from a potato field during several months. Further insight into the fate of the glycoalkaloids is needed, as only little information about their degradation in soil is available. Degradation of the glycoalkaloid, α-solanine, has been followed for 42 d in three agricultural soils with common texture and carbon contents. A similar degradation pattern was found in all soils, and the kinetics was well described by a sum of two first-order equations. Overall, degradation rates for the initial first reaction were in the range 0.22–1.64 d −1. Estimated half-lives were in the range 1.8–4.1 d for the three top soils at 15 °C; the fastest degradation was observed in the sandy soil. The major proportion of α-solanine in the sandy soil was degraded by the fast process, while the proportion was lower for the two other soils. Fast degradation appeared to be related to the presence of low amount of sorbents. Additionally, degradation was followed at 5 °C in A- and C-horizon soil from the sandy location, and for both horizons the half-lives were of similar length (4.7–8.7 d). For the slow process, degradation rates were in the range 0.000–0.123 d −1, and residuals were still present in all soils and all temperatures at the end of the experiment (d 42). Overall, fast degradation was found in both top- and subsoil even at low temperatures, and the risk for α-solanine leaching to the groundwater appears to be low.

Relative spatial soil geochemical variability along two transects across the United States and Canada

Abstract To support the development of protocols for the proposed North American Soil Geochemical Landscapes project, whose objective is to establish baselines for the geochemistry of North American soils, two continental-scale transects across the United States and Canada were sampled in 2004. The sampling employed a spatially stratified random sampling design in order to estimate the variability between 40-km linear sampling units, within them, at sample sites, and due to sample preparation and analytical chemical procedures. The 40-km scale was chosen to be consistent with the density proposed for the continental-scale project. The two transects, north–south (N–S) from northern Manitoba to the USA–Mexico border near El Paso, Texas, and east–west (E–W) from the Virginia shore north of Washington, DC, to north of San Francisco, California, closely following the 38th parallel, have been studied individually. The purpose of this study was to determine if statistically significant systematic spatial variation occurred along the transects. Data for 38 major, minor and trace elements in A- and C-horizon soils where less than 5% of the data were below the detection limit were investigated by Analysis of Variance (ANOVA). A total of 15 elements (K, Na, As, Ba, Be, Ce, La, Mn, Nb, P, Rb, Sb, Th, Tl and W) demonstrated statistically significant (p The patterns of relative variability differ by transect and horizon. The N–S transect A-horizon soils show significant between-40-km scale variability for 29 elements, with only 4 elements (Ca, Mg, Pb and Sr) showing in excess of 50% of their variability at the within-40-km and ‘at-site’ scales. In contrast, the C-horizon data demonstrate significant between-40-km scale variability for 26 elements, with 21 having in excess of 50% of their variability at the within-40-km and ‘at-site’ scales. In 36 instances, the ‘at-site’ variability is statistically significant in terms of the sample preparation and analysis variability. It is postulated that this contrast between the A- and C- horizons along the N–S transect, that is dominated by agricultural land uses, is due to the local homogenization of Ap-horizon soils by tillage reducing the ‘at-site’ variability. The spatial variability is distributed similarly between scales for the A- and C-horizon soils of the E–W transect. For all elements, there is significant variability at the within-40-km scale. Notwithstanding this, there is significant between-40-km variability for 28 and 20 of the elements in the A- and C-horizon data, respectively. The differences between the two transects are attributed to (1) geology, the N–S transect runs generally parallel to regional strikes, whereas the E–W transect runs across regional structures and lithologies; and (2) land use, with agricultural tillage dominating along the N–S transect. The spatial analysis of the transect data indicates that continental-scale maps demonstrating statistically significant patterns of geochemical variability may be prepared for many elements from data on soil samples collected on a 40 × 40 km grid or similar sampling designs resulting in a sample density of 1 site per 1600 km2.

LOW-DENSITY REGIONAL GEOCHEMICAL MAPPING OF GOLD AND PALLADIUM HIGHLIGHTING THE EXPLORATION POTENTIAL OF NORTHERNMOST EUROPE

The <2-mm fraction of 605 samples of the C-horizon of podzols collected from an area of 188,000 km2 in the European Arctic was analyzed for Au and Pd by a method using reductive coprecipitation to enrich Au and platinum group elements (PGE) prior to analysis. Due to the low detection limits (0.1 ppb Au, 0.2 ppb Pd) only 2 percent of all samples returned values below the detection limit for Au, and 10 percent were below the detection limit for Pd. The resulting geochemical maps show clear regional distribution patterns, notwithstanding the very low sample density of 1 site per 300 km2. All known Au and PGE occurrences are associated with geochemical anomalies and several new targets for exploration were identified. Low-density geochemical mapping of Au and Pd with low detection limits in C-horizon soils can thus be used as a relatively inexpensive tool to define prospective areas within large regions.

Total fluoride in Guangdong soil profiles, China: Spatial distribution and vertical variation

A total of 260 soil profiles were reported to investigate the fluoride distribution and vertical variation in Guangdong province. The soil fluoride contents followed an approximately lognormal distribution. Although the soil fluoride geometric mean concentration of 407 mg/kg is lower than that of China, its content varied from 87 to 2860 mg/kg. An upper baseline concentration of 688 mg/kg was estimated for surface soils. In A-, B-, and C-horizon soil fluoride spatial distribution presented similar patterns that high fluoride concentration mainly located in limestone, purple shale, and sandshale areas, indicated that soil fluoride spatial distribution was primarily dependent on the regional bedrock properties rather than anthropogenic inputs. From A- to C- horizon soil fluoride geometric mean concentration had an increasing tendency of 407, 448, and 465 mg/kg. This vertical variation was the result of the intensive eluviation under the subtropical hydrothermal condition, and had closely related with soil properties, such as lower organic matters and clay content variations. Moreover, the soil degradation and erosion was also an important pathway of soil fluoride movement, as a result the soil fluoride exported into surface and groundwaters would reach about 4.1 × 10 4 t year − 1 in the study area.

Mercury in Soils, Lakes, and Fish in Voyageurs National Park (Minnesota): Importance of Atmospheric Deposition and Ecosystem Factors

Concentrations of methylmercury in game fish from many interior lakes in Voyageurs National Park (MN, U.S.A.) substantially exceed criteria for the protection of human health. We assessed the importance of atmospheric and geologic sources of mercuryto interior lakes and watersheds within the Park and identified ecosystem factors associated with variation in methylmercury contamination of lacustrine food webs. Geologic sources of mercury were small, based on analyses of underlying bedrock and C-horizon soils, and nearly all mercury in the 0- and A-horizon soils was derived from atmospheric deposition. Analyses of dated sediment cores from five lakes showed that most (63% +/- 13%) of the mercury accumulated in lake sediments during the 1900s was from anthropogenic sources. Contamination of food webs was assessed by analysis of whole, 1-year-old yellow perch (Perca flavescens), a regionally important prey fish. The concentrations of total mercury in yellow perch and of methylmercury in lake water varied substantially among lakes, reflecting the influence of ecosystem processes and variables that affect the microbial production and abundance of methylmercury. Models developed with the information-theoretic approach (Akaike Information Criteria) identified lake water pH, dissolved sulfate, and total organic carbon (an indicator of wetland influence) as factors influencing methylmercury concentrations in lake water and fish. We conclude that nearly all of the mercury in fish in this seemingly pristine

Microbial degradation of street dust polycyclic aromatic hydrocarbons in microcosms simulating diffuse pollution of urban soil

Diffuse pollution with polycyclic aromatic hydrocarbons (PAHs) of topsoil in urban regions has caused increasing concerns in recent years. We simulated diffuse pollution of soil in microcosms by spiking sandy topsoil (A-horizon) and coarse, mineral subsoil (C-horizon) with street dust (PM63) isolated from municipal street sweepings from central Copenhagen. The microbial communities adapted to PAH degradation in microcosms spiked with street dust in both A-horizon and C-horizon soils, in spite of low PAH-concentrations. The increased potential for PAH degradation was demonstrated on several levels: by slowly diminishing PAH-concentrations, increased mineralization of 14C-PAHs, increasing numbers of PAH degraders and increased prevalence of nah and pdo1 PAH degradation genes, i.e. the microbial communities quickly adapted to PAH degradation. Three- and 4-ring PAHs from the street dust were biodegraded to some extent (10-20%), but 5- and 6-ring PAHs were not biodegraded in spite of frequent soil mixing and high PAH degradation potentials. In addition to biodegradation, leaching of 2-, 3- and 4-ring PAHs from the A-horizon to the C-horizon seems to reduce PAH-levels in surface soil. Over time, levels of 2-, 3- and 4-ring PAHs in surface soil may reach equilibrium between input and the combination of biodegradation and leaching. However, levels of the environmentally critical 5- and 6-ring PAHs will probably continue to rise. We presume that sorption to black carbon particles is responsible for the persistence and low bioaccessibility of 5- and 6-ring PAHs in diffusely polluted soil.

Some statistical relationships between stream sediment and soil geochemistry in northwestern Wisconsin—can stream sediment compositions be used to predict compositions of soils in glaciated terranes?

Mean stream sediment chemical compositions from northwestern Wisconsin in the north central United States, based on more than 800 samples, differ significantly from mean A-horizon and C-horizon soil compositions, based on about 380 samples of each horizon. Differences by a factor greater than 1.5 exist for some elements (Ca, Mn, Mg, P, Ti, Ni, Pb, Se, Zn). A very large database of stream sediment geochemistry exists for the region (more than 2200 samples) and for the U.S. (roughly 400,000 samples), whereas data on the chemistry of soils is much sparser both regionally and nationally. Therefore, we have attempted to quantify trends in compositional differences between stream sediments and nearby soils to test whether the abundant stream sediment data can be used to predict soil compositions. A simple computational technique of adjusting the stream sediment compositions according to the ratio of means of soils and stream sediments was conducted. A variety of techniques of correction and interpolation of data were tested and indicate that repetitive testing of results allows an optimum correction to be achieved. Predicted soil compositions compared to analytically determined soil compositions show a range of results from relatively good correspondence for some elements to rather poor correspondence for others. In general, predictions are best at midranges of compositions. The technique does not predict well more extreme or anomalous values. Thus, this technique appears to be useful for estimating background soil compositions and delineating regional compositional trends in soils in situations where large amounts of stream sediment analyses and smaller amounts of soil analyses are available. The technique also provides probabilistic qualifications on the expected error between predicted and actual soil compositions so that individual users can judge if the technique provides data of sufficient accuracy for specific needs.

Secondary geochemical dispersion in the Precambrian auriferous Hutti-Maski schist belt, Raichur district, Karnataka, India. Part I: anomalies of As, Sb, Hg and Bi in soil and groundwater

The nature of secondary geochemical dispersion of As, Sb, Hg and Bi in soil and ground water of the semi-arid, tropical, Archaean, auriferous, Hutti-Maski greenstone belt has been investigated for identification of appropriate geochemical techniques for Au exploration in similar terrains. Results indicate that the <180 μm size-fraction of C-horizon soil is an appropriate sampling medium for delineating pedogeochemical anomalies of As, Sb, Hg and Bi related to gold mineralisation. These pedogeochemical anomalies along with anomalous values of alkalinity, chloride, sulphate, As and Sb in groundwater are controlled significantly by primary mineralisation located along shear zones in the greenstone belt. Arsenic anomalies in soil are broad, whereas, those of Sb and Bi are restricted to narrow zones directly over mineralised areas. In contrast, Hg anomalies around known mineralised areas are irregular and do not clearly demarcate the mineralised areas. The study indicates that anomalies of As, Sb and Hg in soil are principally hydromorphic, whereas those of Bi are clastic. The study recommends use of groundwater sampling at 2–3 km spacing with routine analysis of chloride, sulphate and alkalinity along with As and Sb in the first phase. This may be followed up with sampling of C-horizon of soils on a 1 km square grid for As-anomalies. Arsenic-anomalous areas may be sampled for As, Sb, Hg and Bi on a 500 m square grid for detailed exploration.

Efficiency of cyanidation in gold exploration using soils

Cyanidation allows gold to be extracted from larger samples than is practical by fire assay and may thus provide both a simpler analytical procedure and better sample representativity. Here we compare recovery of gold by cyanidation and fire assay for the −212 +106 μm and −106 +53 μm light mineral fractions and the −53 μm fraction of C-horizon soils from a variety of gold deposits and weathering regimes in Canada and the U.S. Results for cyanidation and fire assay are strongly correlated and for many samples, gold recovery by cyanidation is within ±20% of values reported by FA–AAS. Somewhat unexpectedly, there is good recovery of gold from unground light mineral fractions in which the gold is encapsulated rather than being present as free gold. Abundance and size distribution of the gold should be considered when deciding if cyanidation offers any real advantage over fire assay-based methods.

Comparison of elemental contents in O- and C-horizon soils from the surroundings of Nikel, Kola Peninsula, using different grain size fractions and extractions

Forty-five soil samples from the O- and C-horizons were taken in a 12,000 km 2 area on the borders of Finland, Norway and Russia. The nickel smelter at Nikel, the ore roasting plant at Zapoljarnij, both in Russia, and the iron ore mine and processing plant at Kirkenes in Norway, are all situated within this area. Element levels and variation in both soil horizons (two different extractions of the O-horizon and two different grain size fractions of the C-horizon) are presented and discussed. No good correlation exists between element concentrations measured in the O-horizon and the parent C-horizon. Of the 30 analysed elements, 7 are clearly enriched (>2×) in the O-horizon, while 11 show higher concentrations (>3×) in the C-horizon. In Russia, near industry, a number of heavy metals (Co, Cu, Ni) show extreme enrichments in the O-horizon soils. All the anthropogenic contaminants show a marked decrease in element concentrations from the Russian nickel industry towards the west in the O-horizon samples. Chemistry of the C-horizon is very little affected by the emissions. For the O-horizon samples, a close correlation was observed between 1 M ammonium acetate- and concentrated nitric-acid extractable element concentrations. Element availability in 1 M ammonium acetate, however, differs widely from 65% (K) to less than 1% (Si, Ti). Of the heavy metals examined, Cd and Zn are most readily extractable. Copper is more strongly bound in the O-horizon than Ni or S. Interestingly, Na, K, Mg and P are less available at sites affected by emissions from industry than in background areas. Element content and variation are generally higher in the <0.063 mm than the <2 mm fraction of the C-horizon samples by a factor of 1.6 to 2.8. In addition, correlation between O-horizon and C-horizon chemistry is higher for the fine fraction of the C-horizon. The fine fraction of mineral soils is thus considered more appropriate for the detection of element sources and for mapping regional differences than the <2 mm fraction. Comparison of the results of the two extractions used for the O-horizon samples and the two fractions used to analyse the C-horizon samples helps to distinguish anthropogenic from geogenic element sources.

Enrichment of metals in the organic surface layer of natural soil: identification of contributions from different sources

The humus layer of natural soils is frequently used as a sampling medium in geochemical exploration surveys. This procedure has been questioned because the concentration level in the humus layer does not necessarily reflect the geochemistry in the underlying mineral material. Four sources of metals to the organic surface layer are identified: (1) bioturbation; (2) the ‘vascular pump’; (3) atmospheric input of marine origin; and (4) airborne pollution. Two methods to define and discuss the contributions from these sources are presented for a survey of Norwegian soils. The first method is based on calculation of ratios of concentrations between humus and undisturbed subsoil within regions of similar topography, climate and air pollution. Concentration ratios consistently below 0.1 and low inter-region variation, such as for Al and Fe, characterize elements supplied to the humus by mixing-in of mineral soil, while concentration ratios appreciably higher than 0.1 identify some additional mechanism. The latter is evident for elements such as Pb, Zn and Cd (air pollution), Sr and Mg (atmospheric input of sea-salt), and Ca (the ‘vascular pump’). The second method involves subtraction of the contributions from mineral matter, assuming that the ash content reflects the mass, and C-horizon soil the composition of the mineral matter in the humus layer. As mineral grains mixed into the humus layer are often fairly strongly weathered compared with the undisturbed mineral soil, this method will result in a certain over-compensation; nevertheless it leads to some interesting conclusions as is shown for Ba (the ‘vascular pump’), and Zn and V (long-range transport and local air pollution).

Platinum distribution in soil profiles of the Tulameen ultramafic complex, southern British Columbia

Exploration for chromitite-associated Pt deposits is hampered by a poor understanding of the distribution and behaviour of Pt in the surficial environment. This study investigates Pt content, residence sites and PGE mineralogy of soils developed on till and colluvium associated with the Tulameen ultramafic complex in southern British Columbia. Seventy-six soil profiles, as well as sediments, bogs and waters were sampled above the dunite core of the Tulameen complex, within which Pt occurrences consist of massive-to-discontinuous segregations of platiniferous chromitite. Platinum content of the −70 mesh fraction of soils and sediments was determined by lead fire assay-inductively coupled plasma emission spectrometry. Samples from fourteen selected profiles were then examined in detail to determine Pt mineralogy and its distribution between different size, density and magnetic fractions. Platinum concentrations in the −70 mesh fraction of C horizon soils range from 2 to 885 ppb and are closely related to soil dunite content, as estimated from MgO content and verified by XRD mineralogy. Dunite colluvium (mean: 24.2% MgO), locally-derived dunitic till (mean: 16.5% MgO) and exotic non-dunitic till (mean: 5.7% MgO) have median Pt concentrations of 88 ppb, 36 ppb and 8 ppb, respectively. This trend is evident in all size and density fractions. Platinum content of heavy mineral (SG > 3.3) fractions is 10–20× greater than in light mineral fractions. Platinum is most abundant in the heavy magnetic fraction from non-dunitic tills and dunitic tills remote from known mineralization, but the proportion of Pt in the heavy non-magnetic fraction increases with increasing proximity to mineralization. Scanning electron microscopy and microprobe studies of heavy fractions from C-horizons identified Pt-Fe-Cu alloys as free grains, and as inclusions in Mg-silicates and chromites. Chromite occurs as Mg-Cr-rich anhedral fragments and as Fe-rich euhedral to subhedral crystals. The latter are relatively more abundant in the magnetic fraction and are interpreted as Pt-poor grains disseminated throughout the dunite. The Mg-Cr-rich chromite fragments are relatively more important in the non-magnetic fraction and are interpreted as remnants of Pt-bearing massive chromitite segregations. The abundance of chromite fragments in soils near chromitite segregations accounts for the high Pt content of the non-magnetic heavy fractions of these soils. The −270 mesh fraction or the magnetic heavy fraction of C-horizon soils would be the most suitable sample media for reconnaissance geochemical sampling. However, the greater contrast in Pt concentrations, more limited dispersal and Mg-Cr-rich chromite association of the non-magnetic heavy fraction make it a more suitable medium for detailed geochemical sampling.

Substrate‐ and nutrient‐limited toluene biotransformation in sandy soil

AbstractLab microcosm tests of the rate of toluene biodegradation were performed using soil from the A, B, and C horizons of the unsaturated zone of a sandy field site. Most of the soil samples had been previously exposed to toluene. Toluene biodegradation was rapid, occurring at a time scale comparable to the rate of sorption in many of the microcosms and demonstrating the potential for bioremediation of these contaminants in unsaturated soil. In the A horizon, with an initial toluene concentration in the solution phase of 4.5 mg/L, degradation was controlled by substrate‐limited growth on toluene as the primary substrate. Soil from the B and C horizons initially showed similar behavior with a lower toluene concentration of about 2.5 mg/L. The maximum utilization rate (μmax) for soil from all three depths was 2.0 d−1. With repeated exposure to moderate to high concentrations of toluene, transformation in the B‐ and C‐horizon soil appeared to be zero order, at a rate of 1.0 to 2.0 μg tol‐uene/g soil/d. In C‐horizon soil that had been taken directly from the field, the transformation rate was almost immeasurably slow. Addition of nitrogen as either ammonium or nitrate accelerated the degradation, showing that nitrogen was the most limiting nutrient.The apparent adaptation period observed before rapid toluene removal was fit by a substrate‐limited growth model. Greater numbers of toluene‐degrading microorganisms were found in soil exposed to toluene than in unexposed soil, supporting bio‐mass growth as the explanation for the adaptation period. The results of enumeration of heterotrophs compared to the numbers of toluene degraders suggested that a small proportion of the total viable microorganisms were responsible for degradation of toluene.