Soil Heavy Metal Potential Risk Assessment: Testing Methods, Standards, and Risk Evaluation Framewor

Soil heavy metal potential risk assessment is a systematic technical process used to identify, quantify, and evaluate the risks that heavy metal contamination in soil may pose to human health, agricultural product safety, and ecosystems. This article explains the regulatory basis, sampling schemes, analytical methods, ecological and health risk indices, and practical applications of soil heavy metal potential risk assessment, providing reference for farmland management, land redevelopment, and environmental compliance.

Technical Overview

Soil heavy metal potential risk assessment is a scientific evaluation procedure that determines the concentration levels of heavy metals and metalloids in soil, compares them against regulatory screening values and geochemical background values, and quantifies the potential risks to human health and the receiving environment. Unlike organic pollutants, heavy metals such as cadmium (Cd), lead (Pb), mercury (Hg), and arsenic (As) are non-biodegradable; they persist in soil for decades, accumulate through the food chain, and can become toxic to crops, groundwater, and human populations even at low concentrations.

The main sources of heavy metal contamination in soil include mining and smelting activities, electroplating and metal processing, chemical manufacturing, wastewater irrigation, long-term application of phosphate fertilizers and pesticides containing metallic impurities, atmospheric deposition from fossil fuel combustion, vehicle emissions, and improper disposal of electronic and industrial waste.

A complete risk assessment workflow typically follows these stages: desk study and site reconnaissance, sampling plan design, field sampling and laboratory analysis, data validation, comparison with screening and intervention values, calculation of ecological risk indices, human health risk modeling, risk classification, and formulation of risk control or remediation recommendations. The evaluation is governed by frameworks such as GB 36600-2018 and GB 15618-2018 in China, the risk assessment guidance series published by US EPA, and ISO 18400-series soil sampling standards, and is routinely executed by accredited third-party laboratories with ISO/IEC 17025-accredited analytical capabilities.

Testing Samples

Agricultural and Farmland Soil

Surface soil samples (0–20 cm for cultivated land, 0–15 cm for paddy fields) collected from vegetable plots, grain fields, and orchards. The primary concern is the transfer of Cd, Pb, As, and Hg into food crops, so sampling is often supplemented by simultaneous collection of crop tissue samples to evaluate the soil–plant uptake pathway.

Industrial Site and Brownfield Soil

Soil from current or former industrial plants, including electroplating facilities, smelters, tanneries, chemical works, fuel stations, and battery manufacturers. Sampling follows a systematic grid combined with judgmental placement at suspected source areas, and typically targets both surface soil and subsurface soil at multiple depth intervals (for example 0–0.5 m, 0.5–1.5 m, 1.5–3 m).

Mining and Smelting Area Soil

Soil surrounding tailings ponds, waste rock piles, and ore processing zones, where multi-metal co-contamination (Pb, Zn, Cd, Cu, As) is common. Both horizontal distribution grids and vertical profile sampling are used to characterize the migration depth of contaminants.

Urban, Residential, and Recreational Soil

Soil from kindergartens, schools, parks, residential gardens, and roadside green belts, where children are the most sensitive receptors and direct ingestion and dermal contact are the dominant exposure pathways.

Landfill and Waste Disposal Site Soil

Soil around municipal and industrial landfills used to evaluate leakage and lateral migration of Hg, Pb, Cd, Cr, and other metals from waste masses into surrounding ground.

Testing Items

The core parameter set for soil heavy metal risk assessment comprises the eight priority metals and metalloids:

• Arsenic (As) — total and, where required, speciation
• Cadmium (Cd)
• Chromium (Cr) — total chromium and hexavalent chromium (Cr(VI))
• Copper (Cu)
• Mercury (Hg)
• Nickel (Ni)
• Lead (Pb)
• Zinc (Zn)

Depending on land use and site history, the scope is frequently extended to include antimony (Sb), beryllium (Be), cobalt (Co), manganese (Mn), molybdenum (Mo), selenium (Se), thallium (Tl), vanadium (V), and tin (Sn).

Supporting parameters that strongly influence metal mobility and risk interpretation are also measured:

• pH value (a key input, because agricultural screening values are pH-dependent)
• Soil organic matter content and cation exchange capacity (CEC)
• Soil texture (particle size distribution) and moisture content
• Chemical speciation of metals by sequential extraction (BCR scheme: acid-soluble, reducible, oxidizable, and residual fractions), which reveals the bioavailable fraction rather than only total content

Testing Methods

Sample Preparation and Pretreatment

Field samples are transported under chain-of-custody procedures, air-dried at room temperature or freeze-dried for Hg-sensitive projects, crushed, and passed through 2 mm and 0.149 mm (100 mesh) sieves. Composite samples are reduced by coning and quartering to obtain representative analytical portions. Sampling itself is performed in accordance with HJ/T 166-2004, the HJ 25.1-2019 site investigation procedure, and the ISO 18400 series.

Digestion and Extraction

• Aqua regia extraction (HCl–HNO3) — the routine pseudo-total digestion used for regulatory comparison, following ISO 11466, ISO 12914, US EPA Method 3051A, and related HJ procedures
• Four-acid total digestion (HNO3–HF–HClO4–HCl) — complete dissolution of the silicate matrix for true total metal content
• US EPA Method 3050B — partial digestion widely used for environmental monitoring comparability
• Alkaline digestion (US EPA Method 3060A) — mandatory pretreatment for hexavalent chromium
• BCR or Tessier sequential extraction — for metal speciation and bioavailability studies

Instrumental Determination

• Inductively coupled plasma mass spectrometry (ICP-MS) — ultra-trace multi-element determination of Cd, Pb, Tl, Ni, and other metals, following US EPA Method 6020B and ISO 17294-2, with detection limits down to the sub-microgram-per-kilogram level
• Inductively coupled plasma optical emission spectrometry (ICP-OES) — simultaneous determination of Cu, Zn, Cr, Ni, and Mn at higher concentrations, following US EPA Method 6010D and ISO 11885
• Graphite furnace atomic absorption spectrometry (GFAAS) — sensitive determination of Pb and Cd, following US EPA Method 7010 and GB/T 17141
• Flame atomic absorption spectrometry (FAAS) — routine determination of Cu and Zn
• Cold vapor atomic absorption spectrometry (CV-AAS, US EPA Method 7471B) and direct thermal decomposition (US EPA Method 7473) — determination of total mercury
• Atomic fluorescence spectrometry (AFS) — highly sensitive determination of Hg, As, Se, and Sb, following GB/T 22105 and HJ 680
• Portable and benchtop X-ray fluorescence (XRF) spectrometry — rapid in-situ screening following US EPA Method 6200 and ISO 13196, always followed by laboratory confirmation of positive findings
• UV-visible spectrophotometry (US EPA Method 7196A) or ion chromatography (US EPA Method 7199) — determination of hexavalent chromium after alkaline digestion

Quality Assurance and Quality Control

Reliable risk assessment depends on defensible data. An accredited third-party laboratory operating under ISO/IEC 17025 applies the following controls to every batch: method blanks, laboratory duplicates with relative deviation limits, matrix spikes with recovery acceptance windows (typically 75–125 percent), calibration verification standards, analysis of certified soil reference materials, documented limits of detection and quantification, and complete data validation before results are used for risk characterization.

Risk Assessment Methodology

Comparison with Regulatory Screening Values

The first-tier evaluation compares measured concentrations with the risk screening values in GB 15618-2018 (agricultural land, pH-dependent) and GB 36600-2018 (development land, differentiated between sensitive and non-sensitive land uses). For example, GB 36600-2018 screening values include cadmium 20/65 mg/kg, arsenic 20/60 mg/kg, mercury 8/38 mg/kg, lead 400/800 mg/kg, and hexavalent chromium 3.0/5.7 mg/kg for the two land-use classes. Concentrations below screening values generally indicate acceptable risk; concentrations between screening and intervention values trigger detailed site-specific risk assessment; concentrations above intervention values indicate unacceptable risk requiring risk control or remediation.

Geochemical Accumulation Index (Igeo)

The Igeo quantifies enrichment relative to local geochemical background: Igeo = log2 [ Cn / (1.5 × Bn) ], where Cn is the measured concentration and Bn is the background value. Results are graded from practically uncontaminated (Igeo ≤ 0) to extremely contaminated (Igeo > 5).

Potential Ecological Risk Index (Hakanson Method)

This is the most widely applied ecological risk model. The single-element risk factor is Eir = Tir × Cf, where Cf is the contamination factor (measured concentration divided by background) and Tir is the toxic-response factor (Hg = 40, Cd = 30, As = 10, Cu = Ni = Pb = 5, Cr = 2, Zn = 1). The comprehensive risk index RI is the sum of all Eir values and is classified from low risk to extremely high risk.

Contamination Factor, Pollution Load Index, and Nemerow Index

The pollution load index (PLI) is the n-th root of the product of individual contamination factors and expresses overall site pollution. The Nemerow comprehensive pollution index (PN) combines the mean and maximum single pollution indices and is routinely used in farmland soil grading in China, classifying soil as clean, still clean, slightly polluted, moderately polluted, or heavily polluted.

Human Health Risk Assessment

Site-specific health risk assessment, performed according to HJ 25.3-2019 or the US EPA risk assessment framework, calculates the average daily intake of each metal through incidental soil ingestion, dermal contact, and inhalation of soil dust, separately for children and adult receptors. The results are expressed as a hazard quotient (HQ) and hazard index (HI) for non-carcinogenic effects (acceptable when HI < 1) and as an incremental lifetime cancer risk (CR) for carcinogenic metals such as As, Cd, and Ni (generally considered acceptable within 10-6 to 10-4). Where risks are unacceptable, the back-calculation yields risk-based remediation goals that define the cleanup targets for the site.

Applications

• Farmland classification and safe utilization — supporting the zoning of agricultural land into priority protection, safe utilization, and strictly controlled categories, and guiding crop variety adjustment and soil amendment
• Brownfield redevelopment and merger due diligence — quantifying environmental liabilities before land transactions and confirming that post-remediation soil meets the intended land-use standard
• Environmental impact assessment — providing the baseline soil quality data required for construction project approval
• Remediation design and verification — establishing risk-based remediation targets and verifying cleanup effectiveness through before-and-after comparison
• Mining area ecological restoration — delineating contamination footprints around tailings and smelters and prioritizing areas for intervention
• Food safety and traceability — evaluating the soil–crop transfer of Cd, Pb, and As in production bases for grain and vegetables
• Environmental disputes and litigation — providing legally defensible analytical evidence from accredited third-party laboratories

FAQ

What is the difference between screening values and intervention values?

Screening values are conservative concentration thresholds below which health and ecological risks are generally acceptable without further evaluation. Intervention values are higher thresholds above which risks are considered unacceptable and risk control or remediation is mandatory. Concentrations between the two require a detailed, site-specific risk assessment to decide whether action is needed.

Which standard applies to my project: GB 15618-2018 or GB 36600-2018?

GB 15618-2018 applies to agricultural land (farmland, orchards, pasture) and its screening values vary with soil pH. GB 36600-2018 applies to development land such as residential, commercial, and industrial sites and distinguishes between sensitive and non-sensitive land uses. If a site will change from industrial to residential use, it must be assessed against GB 36600-2018 for the future, more sensitive use.

Does aqua regia digestion recover the true total metal content?

No. Aqua regia provides a pseudo-total extraction that dissolves most environmentally relevant metal fractions but does not fully attack silicate lattices, so results can be lower than four-acid or HF-based total digestion. However, aqua regia data are directly comparable with most regulatory screening values and are the standard basis for compliance assessment.

Can portable XRF replace laboratory analysis?

Portable XRF is an excellent in-situ screening tool for Pb, Cr, Cu, Zn, and Ni because it delivers results in seconds and allows dense grid coverage. However, it has insufficient sensitivity for Cd and cannot measure Hg or hexavalent chromium, so positive or borderline findings must always be confirmed by ICP-MS, AAS, or AFS analysis in an accredited laboratory.

How many soil samples are required?

For contaminated site investigations under HJ 25.1-2019, systematic grids of approximately 40 m × 40 m are commonly used in the preliminary stage, tightened to about 20 m × 20 m in detailed investigation where exceedances are found. Farmland assessments typically use grids scaled to the assessment objective, from 100 m to several hundred meters, with at least one background point outside the influence area.

How long does the full assessment take?

Field sampling for a typical site requires three to ten working days depending on grid density and depth. Laboratory analysis generally requires five to ten working days after sample receipt, and the complete report — including index calculation, health risk modeling, and conclusions — is typically delivered within four to six weeks of project initiation.