Determination of Site Soil Environmental Background Values: Sampling Strategies, Analytical Methods,

Site soil environmental background value determination establishes the natural, baseline concentration levels of chemical substances—particularly heavy metals and metalloids—in soil at a specific location, providing a critical scientific reference for distinguishing geogenic enrichment from anthropogenic contamination. This technical article explains the sampling strategies, testing parameters, analytical procedures, and statistical methods used by accredited third-party laboratories to derive defensible soil background values in accordance with standards such as HJ 25.1, HJ/T 166, GB 36600, and the ISO 18400 series.

Technical Overview

A soil environmental background value is defined as the concentration range of a substance in soil that reflects natural geochemical and pedogenic conditions, essentially unaffected by identifiable anthropogenic pollution sources. In practical site investigation, background values serve as the scientific baseline against which measured contaminant concentrations are interpreted. Without a reliable background reference, elevated concentrations of elements such as arsenic, cadmium, or nickel cannot be correctly attributed to either natural geological enrichment or industrial contamination.

Background value determination is particularly important because the generic risk screening values in GB 36600-2018, the risk control standard for soil contamination of development land, are based on nationwide assumptions. In regions with naturally high geochemical backgrounds—for example, areas underlain by carbonate rocks or black shales that are naturally enriched in cadmium, arsenic, or thallium—soil concentrations may exceed generic screening values without any human contribution. A site-specific or regional background study provides the technical evidence needed to support refined risk assessment under HJ 25.3, regulatory communication, and rational remediation target setting.

A complete determination program integrates three pillars: representative sampling of undisturbed soils sharing the same parent material and soil type as the study site, validated laboratory analysis following recognized standards such as HJ/T 166, HJ 803, ISO 11466, and ISO 17294-2, and rigorous statistical treatment of the dataset including outlier screening, distribution fitting, and percentile-based derivation of the background value.

Testing Samples

Surface Soil Samples

Surface soil samples are typically collected from the 0–20 cm or 0–50 cm depth interval, where most environmental interaction occurs. Because surface horizons are the most exposed to atmospheric deposition and human activity, sample locations must be carefully selected in undisturbed areas away from roads, industrial stacks, fertilizer application zones, and fill material. Surface samples reflect the combined influence of parent material and long-term natural pedogenic processes.

Subsurface Soil Samples

Subsurface samples are recovered from depths of roughly 0.5 m to several meters using drill rigs or trial pits, depending on the depth of concern identified in the site conceptual model. These samples help verify vertical uniformity of element concentrations and detect any downward migration of anthropogenic contaminants that would disqualify a location from the background dataset.

Parent Material (C Horizon) Samples

The C horizon, or soil parent material, lies below the depth of significant biological activity and anthropogenic disturbance. For heavy metals and metalloids, C horizon samples often provide the closest approximation of true natural geochemical baseline conditions and are frequently used to corroborate background values derived from surface statistics.

Reference Area Samples

When the study site itself is suspected to be contaminated, background samples are collected from reference areas exhibiting the same soil type, parent material, topography, and hydrogeological setting but demonstrably free of industrial influence. Reference area sampling is designed in accordance with HJ 25.1-2019 and the ISO 18400 series, typically on a grid or stratified random layout, to capture natural spatial variability.

Testing Items

The parameter list for a background study is driven by the contaminants of concern identified during preliminary investigation and by the screening items regulated in GB 36600-2018. Typical programs include the following categories.

Regulated Heavy Metals and Metalloids

• Arsenic (As) — hydride generation atomic fluorescence spectrometry or ICP-MS
• Cadmium (Cd) — graphite furnace AAS or ICP-MS
• Total chromium (Cr) and hexavalent chromium (Cr(VI))
• Copper (Cu) — flame AAS, ICP-OES, or ICP-MS
• Mercury (Hg) — direct thermal decomposition or cold vapor techniques
• Nickel (Ni), lead (Pb), and zinc (Zn)

Extended Inorganic Parameters

Depending on the geological setting and site history, extended lists may include beryllium, antimony, thallium, vanadium, cobalt, manganese, selenium, fluorine, and barium. These elements are frequently elevated in specific lithologies and are often the very reason a background study is initiated.

Auxiliary Physicochemical Parameters

• Soil pH (potentiometric measurement in a soil–water suspension, HJ 962)
• Organic matter and organic carbon content
• Particle size distribution and soil texture classification
• Cation exchange capacity and soil moisture content

Auxiliary parameters support the interpretation of metal concentrations, because pH, organic matter, and clay content strongly influence the natural retention and mobility of trace elements in soil.

Testing Methods

Sampling Design and Sample Collection

Sampling locations are laid out using systematic grid, stratified random, or judgmental designs consistent with HJ 25.1-2019, HJ 25.2-2019, HJ/T 166-2004, and ISO 18400-102. Collection tools are made of stainless steel, wood, bamboo, or PTFE-coated materials; galvanized or brass implements are strictly avoided to prevent cross-contamination with zinc and copper. Samples are stored in polyethylene or glass containers, chilled during transport, and accompanied by chain-of-custody documentation. Field quality assurance measures include duplicate samples at a rate of at least 10 percent and equipment blank rinsates where applicable. A statistically robust background dataset generally requires a minimum of 30 valid sample results, with 50 or more preferred to characterize spatial variability adequately.

Sample Preparation and Pretreatment

Upon receipt at the accredited third-party laboratory, samples are air-dried at room temperature or freeze-dried, gently disaggregated, homogenized, and sieved through 2 mm and 0.149 mm nylon sieves in accordance with ISO 11464 and HJ/T 166. Sub-samples for mercury and volatile-sensitive parameters are typically kept refrigerated and analyzed promptly to avoid analyte loss.

Instrumental Analysis

• Aqua regia extraction (ISO 11466, HJ 803) followed by ICP-MS (ISO 17294-2), ICP-OES (ISO 11885), or atomic absorption spectrometry for the regulated metals, consistent with the extraction basis of GB 36600 screening values
• Total digestion with hydrofluoric acid based acid mixtures (ISO 14869-1) where total elemental content is required for geochemical comparison
• Microwave-assisted digestion (USEPA 3051A or equivalent) for improved throughput and reproducibility
• Mercury by direct thermal decomposition gold amalgamation (USEPA 7473) or cold vapor AAS
• Arsenic and selenium by hydride generation atomic fluorescence spectrometry (GB/T 22105 series)
• Hexavalent chromium by alkaline digestion (USEPA 3060A) followed by colorimetric determination (USEPA 7196A)

Every analytical batch incorporates method blanks, certified reference materials, laboratory duplicates, and matrix spikes to verify accuracy, precision, and recovery within the acceptance criteria of the cited standards.

Statistical Derivation of Background Values

The validated dataset is processed through an established statistical workflow. First, data are reviewed for representativeness and censored values are handled using appropriate substitution or distribution-based methods. Second, outliers are screened using the Grubbs test, Dixon test, or Tukey interquartile range method, and flagged results are investigated for physical explanations before exclusion. Third, the distribution form is tested with the Shapiro–Wilk or Kolmogorov–Smirnov test, applying logarithmic transformation where the data are log-normally distributed, which is common for trace elements. Finally, the background value is expressed as the 95th percentile of the screened dataset, or in some frameworks as the mean plus or minus two standard deviations, representing the upper limit of natural variation. The derived values are then compared with regional geochemical baselines and with the screening values of GB 36600-2018 and GB 15618 to confirm plausibility and support regulatory decision-making.

Applications

• Construction land safety assessment and brownfield redevelopment, where naturally elevated metals must be distinguished from actionable contamination
• Refined human health and ecological risk assessment under HJ 25.3, enabling site-specific remediation targets instead of generic defaults
• Environmental impact assessment, providing defensible baseline conditions before project construction
• Mining and industrial site closure, decommissioning, and soil remediation acceptance
• Regional geochemical basemapping, land use planning, and agricultural soil quality management
• Environmental liability disputes and litigation support, where source attribution of soil contamination is contested

FAQ

What is a soil environmental background value?

It is the concentration range of a substance in soil attributable to natural geological and soil-forming processes, minimally influenced by human activity. It functions as the reference baseline for judging whether observed concentrations at a site represent contamination or natural conditions.

When is background value determination necessary?

It is typically required when site investigation results exceed the screening values of GB 36600-2018 but the exceedances are suspected to originate from natural geochemical enrichment, or when regional regulators request site-specific reference values to support risk assessment and remediation planning.

How many samples are required for a valid background dataset?

Most technical guidance recommends a minimum of 30 valid data points after outlier screening, while 50 to 100 samples provide greater statistical confidence. The exact number depends on the heterogeneity of the parent material and the size of the study area.

Which sampling depth best represents natural background conditions?

For heavy metals, samples from the C horizon or other horizons below the depth of human disturbance usually best reflect the natural geochemical baseline. Surface and shallow samples remain valuable for comparing against exposure-relevant screening values and for detecting atmospheric deposition.

How is the final background value calculated?

After outlier elimination and distribution testing, the background value is commonly reported as the 95th percentile of the dataset, representing the upper bound of natural variability. Mean-based expressions such as mean plus two standard deviations are also used depending on the regulatory framework.

How long does a background determination project take?

Field sampling typically requires several days depending on site access and drilling requirements, laboratory analysis of inorganic parameters generally takes 10 to 15 working days, and statistical evaluation and reporting add further time. A complete program is usually completed within four to eight weeks.