American Soil Geochemical Landscapes Project Research Articles

Estimating the silica content and loss-on-ignition in the North American Soil Geochemical Landscapes datasets: a recursive inversion approach

A novel method of estimating the silica (SiO 2 ) and loss-on-ignition (LOI) concentrations for the North American Soil Geochemical Landscapes (NASGL) project datasets is proposed. Combining the precision of the geochemical determinations with the completeness of the mineralogical NASGL data, we suggest a ‘reverse normative’ or inversion approach to first calculate the minimum SiO 2 , water (H 2 O) and carbon dioxide (CO 2 ) concentrations in weight percent (wt%) in these samples. These can be used in a first step to compute minimum and maximum estimates for SiO 2 . In a recursive step, a ‘consensus’ SiO 2 is then established as the average between the two aforementioned SiO 2 estimates, trimmed as necessary to yield a total composition (major oxides converted from reported Al, Ca, Fe, K, Mg, Mn, Na, P, S and Ti elemental concentrations + ‘consensus’ SiO 2 + reported trace element concentrations converted to wt% + ‘normative’ H 2 O + ‘normative’ CO 2 ) of no more than 100 wt%. Any remaining compositional gap between 100 wt% and this sum is considered ‘other’ LOI and likely includes H 2 O and CO 2 from the reported ‘amorphous’ phase (of unknown geochemical or mineralogical composition) as well as other volatile components present in soil. We validate the technique against a separate dataset from Australia where geochemical (including all major oxides) and mineralogical data exist on the same samples. The correlation between predicted and observed SiO 2 is linear, strong ( R 2 = 0.91) and homoscedastic. We also compare the estimated NASGL SiO 2 concentrations with a sparser, publicly available continental-scale survey over the conterminous USA, the ‘Shacklette and Boerngen’ dataset. This comparison shows the new data to be a reasonable representation of SiO 2 values measured on the ground over the conterminous USA. We recommend the approach of combining geochemical and mineralogical information to estimate missing SiO 2 and LOI by the recursive inversion approach in datasets elsewhere, with the caveat to always validate results.

Exploratory Analysis of Geochemical Data and Inference of Soil Minerals at Sites Across Canada

Soil plays a crucial role in ecosystem functioning, e.g., soil minerals provide important provisioning and regulating ecosystem services. The determination of soil mineral composition can help to link geochemical processes to underlying bedrock and surficial geology, however analysing quantitative soil mineralogy by X-ray diffraction can be expensive. This study used data from the North American Soil Geochemical Landscapes Project sampled at sites (n = 560) across Canada; exploratory analysis of major elements from the C-horizon, < 2 mm size fraction, was carried out to determine whether geochemical data can infer site-specific qualitative soil minerals. Results for the raw geochemical data indicated relative variability of major elements across Canadian provinces with noticeable differences for silica and calcium. Geochemical data are compositional, and as such their statistical assessment is subject to the problem of closure. In the current study, all raw geochemical data were centred log-ratio-transformed prior to statistical analysis to overcome closure. Graphical measures indicated skewed element data prior to centred log-ratio transformation, which produced a more symmetric distribution. Correlations between elements suggested tentative soil mineral composition, such as silica and aluminum from aluminosilicates minerals. Principal component analysis of transformed geochemical data revealed three distinct groups of calcium, magnesium; iron, titanium, manganese; and aluminum, potassium, silica, sodium, while phosphorus had smaller relative variability independent of these groups. The interpretation of these groups was based on soil minerals and identified as carbonates, silicates, and weathered secondary oxides. These minerals corresponded geospatially to the regional bedrock geology of the sites across Canada, such as the sedimentary rock types from Western Canada to more variable minerals from igneous, metamorphic, or sedimentary rocks in Eastern Canada.

Estimation of soil gas permeability for assessing radon risk using Rosetta pedotransfer function based on soil texture and water content

Radon is a natural source of radioactivity and it can be found in all soils and rocks in the Earth. The presence of radon gas in indoor environments implies a serious risk for human health, already listed as carcinogenic by the World Health Organization. The most relevant methods to infer the risk for radon exposure are based on soil radon concentration and gas permeability that describe the effective radon movement in the soil. However, they neglect crucial soil properties and water content in soil, which can affect greatly soil permeability to gases. Additionally, soil permeability measurement remains expensive, difficult and time-consuming. In this paper we show a new and simple methodology to infer radon risk based on Rosetta3 pedotransfer function as well as soil texture and water content. We also determine the influence of soil texture both on the gas permeability variation in dependence on water content and on the parameter n of the van Genuchten –Mualem model, which establishes the shape of the relative permeability curves. We show that radon risk exposure may change importantly for the same soil with different soil water contents. We finally apply and validate the proposed method using radon permeability data from the Canadian component of the North American Soil Geochemical Landscapes Project (NASGLP). Results highlight that the proposed methodology provides reliable estimations of the gas permeability and reveal that the presence of water content may cross the boundary between two radon risk categories, and consequently, may change the radon risk category to safer situations.

The distribution of selected elements and minerals in soil of the conterminous United States

In 2007, the U.S. Geological Survey initiated a low-density (1 site per 1600km2, 4857 sites) geochemical and mineralogical survey of soil of the conterminous United States as part of the North American Soil Geochemical Landscapes Project. Three soil samples were collected, if possible, from each site; (1) a sample from a depth of 0 to 5cm, (2) a composite of the soil A-horizon, and (3) a deeper sample from the soil C-horizon or, if the top of the C-horizon was at a depth greater than 100cm, from a depth of approximately 80–100cm. The <2mm fraction of each sample was analysed for a suite of 45 major and trace elements following near-total multi-acid digestion. The major mineralogical components in samples from the soil A- and C-horizons were determined by a quantitative X-ray diffraction method using Rietveld refinement. Sampling ended in 2010 and chemical and mineralogical analyses were completed in May 2013. Maps of the conterminous United States showing predicted element and mineral concentrations were interpolated from actual soil data for each soil sample type by an inverse distance weighted (IDW) technique using ArcGIS software. Regional- and national-scale map patterns for selected elements and minerals apparent in interpolated maps are described here in the context of soil-forming factors and possible human inputs. These patterns can be related to (1) soil parent materials, for example, in the distribution of quartz, (2) climate impacts, for example, in the distribution of feldspar and kaolinite, (3) soil age, for example, in the distribution of carbonate in young glacial deposits, and (4) possible anthropogenic loading of phosphorus (P) and lead (Pb) to surface soil. This new geochemical and mineralogical data set for the conterminous United States represents a major step forward from prior national-scale soil geochemistry data and provides a robust soil data framework for the United States now and into the future.

Overview of the Canadian component of the North American Soil Geochemical Landscapes Project with recommendations for acquiring soil geochemical data for environmental and human health risk assessments

The involvement of the Geological Survey of Canada in the North American Soil Geochemical Landscapes Project from 2004 to 2009 led to the development of protocols to support environmental and human health risk assessments. Recommendations for acquiring soil samples, analysing them and the resulting geochemical data to support risk assessments, as well as outlines of a procedure for estimating geochemical background are presented. The following practices are proposed: (1) collection of soil samples from pedological horizons (C-horizon in particular) rather than depth-based intervals; (2) use of a spatially random sample design; (3) analysis of the unmilled &lt;2-mm fraction; (4) dissolution using the US EPA (US Environmental Protection Agency) (US EPA) Method 3050B aqua regia variant; (5) archiving of sample splits; and (6) evaluation of chemical data through the insertion, analysis and monitoring of QA/QC samples. Part of the Canadian project was also the in-situ measurement of soil gas radon and the radiometric estimates of soil K, U and Th concentrations, the protocols for which are discussed.

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.

Geochemistry of soils along a transect from Central Mexico to the Pacific Coast: A pilot study for continental-scale geochemical mapping

Abstract The Mexican Geological Survey (SGM), the National Institute of Statistics, Geography and Informatics (INEGI) and the Autonomous University of San Luis Potosi (UASLP) have established a multidisciplinary team with the objective of creating a national program of geochemical mapping of soils in Mexico. This is being done as part of the North American Soil Geochemical Landscapes Project in partnership with the US Geological Survey and the Geological Survey of Canada. As the first step, a pilot study was conducted over a transect that extends from the Mexico–US border near Ciudad Juarez in the north to the Pacific Ocean in the south. This pilot transect was conducted in two phases, and this paper presents results from the first phase, which sampled soils at about a 40-km spacing along a 730-km transect beginning in Central Mexico and ending at the Pacific Coast. Samples were collected from the A and C horizons at each site and 60 elements were analyzed. This pilot study demonstrates that geochemical mapping based on a 40-km spacing is adequate to identify broad-scale geochemical patterns. Geologic influence (i.e., soil parent material) was the most important factor influencing the distribution of elements along the transect, followed by the influence of regional mineralization. The study also showed that influence by human activities over the transect is minimal except possibly in large mining districts. A comparison of element abundance in the A horizon with the environmental soil guidelines in Mexico showed that the natural concentrations of the studied soils were lower than the established threshold for soil restoration with the exception of V and As. The former had a median value (75 mg/kg) approximately equal to the value established in Mexico for soil restoration in agricultural and residential lands (78 mg/kg), and the latter had three values higher than the 22 mg/kg threshold for soil restoration in agricultural and residential lands. These cases demonstrate the importance of knowing the national- and regional-scale geochemistry of Mexican soils as a support for the decision-making process, particularly for the proper formulation and application of soil guidelines designed to protect human and ecosystem health.

Pilot studies for the North American Soil Geochemical Landscapes Project – Site selection, sampling protocols, analytical methods, and quality control protocols

In 2004, the US Geological Survey (USGS) and the Geological Survey of Canada sampled and chemically analyzed soils along two transects across Canada and the USA in preparation for a planned soil geochemical survey of North America. This effort was a pilot study to test and refine sampling protocols, analytical methods, quality control protocols, and field logistics for the continental survey. A total of 220 sample sites were selected at approximately 40-km intervals along the two transects. The ideal sampling protocol at each site called for a sample from a depth of 0–5 cm and a composite of each of the O, A, and C horizons. The <2-mm fraction of each sample was analyzed for Al, Ca, Fe, K, Mg, Na, S, Ti, Ag, As, 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, HNO 3 , HClO 4 , and HF. Separate methods were used for Hg, Se, total C, and carbonate-C on this same size fraction. Only Ag, In, and Te had a large percentage of concentrations below the detection limit. Quality control (QC) of the analyses was monitored at three levels: the laboratory performing the analysis, the USGS QC officer, and the principal investigator for the study. This level of review resulted in an average of one QC sample for every 20 field samples, which proved to be minimally adequate for such a large-scale survey. Additional QC samples should be added to monitor within-batch quality to the extent that no more than 10 samples are analyzed between a QC sample. Only Cr (77%), Y (82%), and Sb (80%) fell outside the acceptable limits of accuracy (% recovery between 85 and 115%) because of likely residence in mineral phases resistant to the acid digestion. A separate sample of 0–5-cm material was collected at each site for determination of organic compounds. A subset of 73 of these samples was analyzed for a suite of 19 organochlorine pesticides by gas chromatography. Only three of these samples had detectable pesticide concentrations. A separate sample of A-horizon soil was collected for microbial characterization by phospholipid fatty acid analysis (PLFA), soil enzyme assays, and determination of selected human and agricultural pathogens. Collection, preservation and analysis of samples for both organic compounds and microbial characterization add a great degree of complication to the sampling and preservation protocols and a significant increase to the cost for a continental-scale survey. Both these issues must be considered carefully prior to adopting these parameters as part of the soil geochemical survey of North America.

A water-leach procedure for estimating bioaccessibility of elements in soils from transects across the United States and Canada

Abstract An objective of the North American Soil Geochemical Landscapes Project is to provide relevant data concerning bioaccessible concentrations of elements in soil to government and other institutions undertaking environmental studies. A protocol was developed that employs a 1-g soil sample agitated overnight with 40 mL of reverse-osmosis de-ionized water for 20 h, and determination of 63 elements following three steps of centrifugation by inductively coupled plasma–atomic emission spectrometry and inductively coupled plasma–mass spectrometry the following day. Statistical summaries are presented for those 48 elements (Ag, Al, As, B, Ba, Be, Br, Ca, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Fe, Ga, Gd, Ge, Hf, Ho, I, K, La, Li, Lu, Mg, Mn, Mo, Na, Nb, Nd, Ni, P, Pb, Pr, Rb, Re, S, Sb, Si, Sm, Sn, Sr, Tb, Ti, Tl, Tm, U, V, W, Y, Yb, Zn, Zr, and pH) for which

Soil chemistry in lithologically diverse datasets: The quartz dilution effect

Abstract National- and continental-scale soil geochemical datasets are likely to move our understanding of broad soil geochemistry patterns forward significantly. Patterns of chemistry and mineralogy delineated from these datasets are strongly influenced by the composition of the soil parent material, which itself is largely a function of lithology and particle size sorting. Such controls present a challenge by obscuring subtler patterns arising from subsequent pedogenic processes. Here the effect of quartz concentration is examined in moist-climate soils from a pilot dataset of the North American Soil Geochemical Landscapes Project. Due to variable and high quartz contents (6.2–81.7 wt.%), and its residual and inert nature in soil, quartz is demonstrated to influence broad patterns in soil chemistry. A dilution effect is observed whereby concentrations of various elements are significantly and strongly negatively correlated with quartz. Quartz content drives artificial positive correlations between concentrations of some elements and obscures negative correlations between others. Unadjusted soil data show the highly mobile base cations Ca, Mg, and Na to be often strongly positively correlated with intermediately mobile Al or Fe, and generally uncorrelated with the relatively immobile high-field-strength elements (HFS) Ti and Nb. Both patterns are contrary to broad expectations for soils being weathered and leached. After transforming bulk soil chemistry to a quartz-free basis, the base cations are generally uncorrelated with Al and Fe, and negative correlations generally emerge with the HFS elements. Quartz-free element data may be a useful tool for elucidating patterns of weathering or parent-material chemistry in large soil datasets.

The potential of mid- and near-infrared diffuse reflectance spectroscopy for determining major- and trace-element concentrations in soils from a geochemical survey of North America

Abstract In 2004, soils were collected at 220 sites along two transects across the USA and Canada as a pilot study for a planned soil geochemical survey of North America (North American Soil Geochemical Landscapes Project). The objective of the current study was to examine the potential of diffuse reflectance (DR) Fourier Transform (FT) mid-infrared (mid-IR) and near-infrared (NIRS) spectroscopy to reduce the need for conventional analysis for the determination of major and trace elements in such continental-scale surveys. Soil samples ( n = 720) were collected from two transects (east–west across the USA, and north–south from Manitoba, Canada to El Paso, Texas (USA), n = 453 and 267, respectively). The samples came from 19 USA states and the province of Manitoba in Canada. They represented 31 types of land use (e.g., national forest, rangeland, etc.), and 123 different land covers (e.g., soybeans, oak forest, etc.). The samples represented a combination of depth-based sampling (0–5 cm) and horizon-based sampling (O, A and C horizons) with 123 different depths identified. The set was very diverse with few samples similar in land use, land cover, etc. All samples were analyzed by conventional means for the near-total concentration of 49 analytes (C total , C carbonate and C organic , and 46 major and trace elements). Spectra were obtained using dried, ground samples using a Digilab FTS-7000 FT spectrometer in the mid- (4000–400 cm −1 ) and near-infrared (10,000–4000 cm −1 ) at 4 cm −1 resolution (64 co-added scans per spectrum) using a Pike AutoDIFF DR autosampler. Partial least squares calibrations were develop using: (1) all samples as a calibration set; (2) samples evenly divided into calibration and validation sets based on spectral diversity; and (3) samples divided to have matching analyte concentrations in calibration and validation sets. In general, results supported the conclusion that neither mid-IR nor NIRS would be particularly useful in reducing the need for conventional analysis of soils from this continental-scale geochemical survey. The extreme sample diversity, likely caused by the widely varied parent material, land use at the site of collection (e.g., grazing, recreation, agriculture, etc.), and climate resulted in poor calibrations even for C total , C organic and C carbonate . The results indicated potential for mid-IR and NIRS to differentiate soils containing high concentrations (>100 mg/kg) of some metals (e.g., Co, Cr, Ni) from low-level samples (

Geochemical studies of North American soils: Results from the pilot study phase of the North American Soil Geochemical Landscapes Project

Geochemical studies of North American soils: Results from the pilot study phase of the North American Soil Geochemical Landscapes Project