Each claim states how it is known and cites its evidence.
EstablishedA concentration requires a named material and explicit units before it means anything.
General principle, shown here for context — not an observation about this record.
A concentration is meaningless without its material and its units. The USGS states both wherever it reports one: concentrations in soils and sediment in milligrams per kilogram, parts per million or parts per billion, and in water in milligrams or micrograms per litre — with the material named in every case.
- method: Units and materials as declared by the cited USGS publications.
- generality: GENERAL_PRINCIPLE
Evidence support: Supported by the cited evidence · 2 sources, multiple sources, independence unknown
Evidence: Gosen 2017 · Primary authority; Smith 2013 · Primary authority
EstablishedGRTS design, ~1 site per 1,600 km², conterminous United States; sampling completed 2010, analyses May 2013
A property of a dataset or model, shown here for context — not an observation about this record.
This is a systematic national survey, not an exploration programme. Sites across the conterminous United States were placed by generalized random tessellation stratified design at an average of about one site per 1,600 square kilometres, and the <2-millimetre fraction was analysed for 45 elements. Sites were chosen by the design and not by where anything was expected, so the density of samples carries no information about where mineralisation is, and a high value at a site is a property of that site’s soil rather than a signal about its surroundings.
- generality: DATASET_PROPERTY
- scope: Coverage is the conterminous United States. Nothing in this dataset speaks to Alaska, Hawaii or anywhere outside it, and absence from it is absence from a survey rather than absence from the ground.
Evidence support: Supported by the cited evidence · one source
Evidence: Smith 2013 · Primary authority
EstablishedPer-element method and preparation; inorganic and organic carbon computed, not measured
A property of a dataset or model, shown here for context — not an observation about this record.
A row of this dataset is not one analysis. Each element carries its own method and its own sample preparation: copper by ICP-AES after a four-acid near-total digestion, arsenic by hydride-generation AAS after a sodium peroxide fusion, mercury by cold-vapour AAS after an HNO3–HCl digestion. Two elements in the same row are therefore not comparable in provenance merely because both are reported in milligrams per kilogram. Two of the columns are not measurements at all: inorganic carbon is a stoichiometric calculation from XRD mineral abundances by a stated formula, and organic carbon is total carbon minus that calculation.
- generality: DATASET_PROPERTY
- method: Each method named here is the metadata’s own wording for that element.
- precision note: Near-total is not total. A four-acid digestion and an alkaline fusion liberate different proportions of an element from resistant minerals, so a concentration is a concentration under a preparation, not a property of the soil alone.
Evidence support: Supported by the cited evidence · one source
Evidence: Smith 2013 · Primary authority
Established< below limit; N.D. no detect, limit unstated; <= bounded difference; INS insufficient sample; N.S. no sample
A property of a dataset or model, shown here for context — not an observation about this record.
The USGS soil geochemistry tables report five different kinds of non-value, and they are five different facts. "<n" means the analysis ran and the element was below the lower limit of determination for that method, with n being the limit. "N.D." means no detect, with no limit stated anywhere. "<=" marks organic carbon bounded above because it is computed as a difference and one term was missing. "INS" means there was too little sample to analyse. "N.S." means no sample reached the laboratory at all. The first three are observations about the soil; the last two are observations about the sampling. None of them is zero, and none of them is the limit beside it.
- generality: DATASET_PROPERTY
- precision note: A value reported as "<1 mg/kg" is not 1 mg/kg and is not 0 mg/kg. The concentration lies somewhere below 1 and the analysis does not say where.
Evidence support: Supported by the cited evidence · one source
Evidence: Smith 2013 · Primary authority
EstablishedA defined national soil sampling frame with specified depths and horizons.
The USGS national soil survey defines its material precisely: one site per 1,600 square kilometres across 4,857 sites, with a sample from 0 to 5 centimetres, a composite of the A horizon, and a deeper sample from the C horizon or from 80 to 100 centimetres where the C horizon lies deeper than a metre.
- method: Protocols developed at a 2003 workshop and refined through pilot studies from 2004 to 2007.
Evidence support: Supported by the cited evidence · one source
Evidence: Smith 2013 · Primary authority