Soil environmental background value investigation and determination establishes the natural baseline concentrations of metals, metalloids, and other substances in soil that has experienced minimal anthropogenic disturbance. These baseline values are fundamental for soil pollution assessment, environmental impact assessment, land-use planning, and the derivation of site-specific remediation targets. This article explains the complete technical workflow—from sampling design and laboratory analysis to statistical determination—in accordance with recognized standards such as HJ/T 166, GB/T 36197, GB 15618, and the ISO 18400 series.
Technical Overview
A soil environmental background value represents the concentration of a chemical substance in soil that reflects natural geological and pedogenic conditions, essentially unaffected by industrial, agricultural, or urban human activities. Background values are not universal constants; they vary with parent material, soil type, topography, climate, and biogeochemical cycling. Consequently, a rigorous background investigation must be designed around statistically valid sampling units that capture this natural variability.
The determination of background values serves as the scientific reference point against which contaminated or potentially contaminated sites are judged. Without a defensible local baseline, it is impossible to distinguish geogenic enrichment from anthropogenic pollution, to set realistic remediation goals, or to defend conclusions in environmental litigation or regulatory review.
A complete background value investigation typically follows five sequential stages:
- Desk study and sampling plan design based on geology, soil maps, land-use history, and remote-sensing interpretation
- Field sampling of surface soil, deep soil, soil profiles, and parent material following standards such as HJ/T 166, GB/T 36197, and ISO 18400
- Laboratory analysis of inorganic and organic parameters using validated instrumental methods
- Data validation, quality control review, and outlier screening
- Statistical determination of background values and background ranges, with reporting of statistical units and confidence measures
The resulting values are commonly compared against, and used to contextualize, risk control standards such as GB 15618 for agricultural land and GB 36600 for development land, as well as international references including ISO guidelines and ASTM soil characterization methods.
Testing Samples
Surface Soil Samples
Surface samples are generally collected from a depth of 0–20 cm for agricultural soils or 0–20 cm within the defined sampling unit. They reflect the soil horizon most exposed to atmospheric deposition, agronomic inputs, and biological activity. Multiple subsamples (typically 5–9) are collected in a zigzag, chessboard, or diagonal pattern within the sampling unit and composited to reduce micro-scale variability.
Deep Soil and Profile Samples
Deep samples, typically collected from below 1.5 m or from the C horizon, are critical because deeper strata are less influenced by surface human activity. Complete soil profiles are excavated or drilled to expose genetic horizons (A, B, C), and each horizon is sampled separately. The C horizon or underlying parent material often provides the closest approximation of true geogenic background.
Parent Material and Rock Samples
Where feasible, weathered bedrock or unconsolidated parent material is sampled to characterize the geochemical source term of the soil. This allows the investigation team to interpret whether elevated element concentrations in soil are inherited from mineralized or naturally enriched geology rather than caused by contamination.
Sample Handling and Preservation
All samples are placed in inert containers, uniquely labeled, and transported under chain-of-custody procedures. Samples destined for volatile or organic analyses are stored at low temperature in pre-cleaned amber glass jars, while samples for metals are collected in polyethylene or cloth bags. Field duplicates, transportation blanks, and equipment rinsate blanks are incorporated at prescribed frequencies, typically 5–10% of the sample population.
Testing Items
The analytical program is normally tiered into a mandatory core parameter set and an extended parameter set selected according to the geological setting and the intended application of the background values.
Core Inorganic Parameters
- Cadmium (Cd), Mercury (Hg), Arsenic (As), Lead (Pb), Chromium (Cr)
- Copper (Cu), Zinc (Zn), Nickel (Ni)
- Soil pH
- Organic matter content
Extended Inorganic Parameters
- Manganese (Mn), Cobalt (Co), Vanadium (V), Molybdenum (Mo)
- Selenium (Se), Thallium (Tl), Antimony (Sb), Beryllium (Be)
- Fluoride (F), Boron (B), Tin (Sn)
- Aluminum, iron, calcium, magnesium, sodium, and potassium as matrix and geochemical indicator elements
- Rare earth elements and other trace elements where regional geochemical mapping is required
Supporting Physicochemical Parameters
- Cation exchange capacity (CEC)
- Particle size distribution (clay, silt, sand fractions)
- Soil water content and bulk density
- Redox potential, where conditions warrant
Optional Organic Parameters
- Persistent organic pollutants such as organochlorine pesticides and polycyclic aromatic hydrocarbons, where historical land use suggests potential atmospheric or agronomic influence
Testing Methods
Sampling Design
Sampling units are delineated by integrating soil type, parent material, geomorphology, and land use so that each unit is geochemically homogeneous. Grid-based systematic sampling, stratified random sampling, or judgmental sampling of undisturbed reference areas may be employed. The number of samples per statistical unit must satisfy statistical adequacy, generally no fewer than 20–30 valid data points, and preferably 40 or more, to support distribution fitting and percentile estimation.
Sample Pretreatment
Samples for metal analysis are air-dried at room temperature or oven-dried at low temperature, disaggregated, and sieved through a 2 mm nylon sieve. A portion is further ground in an agate mill to pass a 0.149 mm (100 mesh) sieve for total-content digestion. Contamination from metal tools is strictly avoided throughout the pretreatment chain.
Digestion and Extraction
- Total digestion with hydrofluoric, nitric, and perchloric acids (or closed-vessel microwave digestion) for true total element contents
- Aqua regia extraction for environmentally relevant pseudo-total contents, consistent with HJ 803 and international practice
- Water-soluble and ion-exchangeable fraction extraction where speciation or mobility assessment is required
Instrumental Analysis
- Inductively coupled plasma mass spectrometry (ICP-MS) for multi-element determination at trace levels, with methods aligned to HJ 803 and HJ 680
- Inductively coupled plasma optical emission spectrometry (ICP-OES) for major and selected trace elements
- Graphite furnace atomic absorption spectrometry (GFAAS) for lead and cadmium per GB/T 17141
- Flame atomic absorption spectrometry (FAAS) for copper, zinc, and nickel per GB/T 17138 and GB/T 17139
- Atomic fluorescence spectrometry (AFS) for mercury and arsenic per GB/T 22105, with cold vapor techniques for mercury
- Ion-selective electrode for fluoride, and potentiometric determination of pH
- Gas chromatography–mass spectrometry (GC-MS) for optional organic parameters
Quality Assurance and Quality Control
Each analytical batch includes method blanks, laboratory duplicates, calibration verification standards, and certified reference materials (CRMs) traceable to recognized producers. Acceptance criteria typically require CRM recovery within the certified uncertainty range and relative deviation of duplicates within 10–20% depending on concentration level. Method detection limits must be well below expected background concentrations, ideally below one-tenth of the anticipated mean.
Statistical Determination of Background Values
- Outlier screening using box-plot analysis, Grubbs test, Thompson test, or the mean ± 2–3 standard deviation criterion, applied iteratively
- Distribution testing using the Shapiro–Wilk or Kolmogorov–Smirnov test; log-transformation where data follow a lognormal distribution, which is common for trace elements
- Calculation of central tendency (arithmetic or geometric mean) and dispersion (standard deviation or geometric standard deviation)
- Expression of the background value together with its range, commonly mean ± 2 standard deviations or the 95% confidence interval, with upper limits often anchored to the 90th–95th percentile
- Separate reporting for distinct statistical units, soil types, and depth intervals, accompanied by sample counts and statistical parameters
Applications
Determined soil environmental background values support a wide range of regulatory, industrial, and scientific purposes:
- Environmental impact assessment, where the local background serves as the pre-construction baseline for new industrial, mining, or infrastructure projects
- Contamination screening and site investigation, enabling regulators and consultants to distinguish anthropogenic enrichment from natural geochemical anomalies
- Derivation of site-specific remediation targets and risk-based screening levels for contaminated land remediation
- Agricultural land management, including the evaluation of farmland against GB 15618 risk control values for safe food production
- Regional and national soil environmental standard development, providing the empirical foundation for guideline values
- Geochemical mapping and mineral exploration, where multi-element background baselines guide anomaly identification
- Environmental forensics and litigation support, offering objective baseline evidence of historical impact
- Long-term soil monitoring programs, allowing trends in soil quality to be tracked against a stable reference
FAQ
What is the difference between a background value and a screening value?
A background value describes the natural, site-specific concentration of a substance in undisturbed soil, whereas a screening value, such as those in GB 36600 or GB 15618, is a risk-based threshold used to decide whether further assessment or intervention is required. A concentration can exceed background but remain below the screening value, indicating enrichment without unacceptable risk.
How many samples are required for a defensible background determination?
As a general rule, each statistical unit should contain at least 20–30 valid samples, and 40 or more is preferred so that the data distribution can be characterized and percentiles estimated with acceptable confidence. Fewer samples may be justified for small, clearly homogeneous units, but the statistical basis must always be documented.
Why are deep soil or parent material samples included?
Deep samples from the C horizon or from depths greater than approximately 1.5 m are minimally affected by surface human activity and therefore provide the best estimate of geogenic concentrations. Comparing surface and deep results helps identify whether elevated surface concentrations originate from natural sources or from deposition and contamination.
How are outliers in the dataset handled?
Outliers are screened using statistical tests such as the Grubbs or Thompson test, box-plot criteria, or the mean ± 2–3 standard deviation rule. Suspect points are reviewed against field records and geological maps before exclusion; genuinely mineralized areas are usually delineated as separate statistical units rather than being discarded, so that naturally enriched geology is not misclassified as pollution.
Can background values from one region be applied to another?
No. Background values are inherently local because they depend on parent material, soil-forming processes, and regional geochemistry. Transferring values between regions with different geology introduces significant error; a location-specific investigation by accredited third-party laboratories using standardized methods is always the reliable approach.
How long does a background value investigation take?
Typical project durations range from several weeks to a few months, depending on the size and accessibility of the study area, the number of statistical units, the parameter list, and the extent of profile drilling. Laboratory analysis of the full inorganic suite generally requires two to four weeks after sampling is completed.