Soil heavy metal environmental quality assessment is a systematic process of sampling, laboratory analysis, and risk-based evaluation used to determine the concentration and ecological impact of toxic elements such as cadmium, lead, mercury, arsenic, and chromium in soil. Guided by national and international standards including GB 15618-2018, GB 36600-2018, and EPA methods, this assessment supports agricultural land management, contaminated site redevelopment, and remediation verification. This article provides a comprehensive technical overview of sampling procedures, target analytes, instrumental methods, pollution indices, and practical applications.
Technical Overview
Soil heavy metal environmental quality assessment refers to the integrated evaluation of soil contamination by potentially toxic elements through field sampling, chemical analysis, and comparison against risk-based regulatory criteria. Heavy metals such as cadmium (Cd), lead (Pb), mercury (Hg), arsenic (As), chromium (Cr), copper (Cu), nickel (Ni), and zinc (Zn) are persistent in the environment, non-biodegradable, and capable of accumulating through the food chain, posing long-term risks to human health and ecosystems.
The assessment framework in China is primarily governed by two national standards: GB 15618-2018, the risk control standard for soil contamination of agricultural land, and GB 36600-2018, the risk control standard for soil contamination of development land. These standards define risk screening values (RSV) and risk intervention values (RIV) for inorganic contaminants, differentiated by soil pH and land use type. Internationally, reference frameworks include ISO 17924 for bioaccessibility, EPA 6010D and EPA 6020B for elemental analysis, and various ASTM and EN methods for sampling and extraction.
Pollution and Risk Evaluation Indices
Beyond direct comparison with screening values, quantitative indices are widely applied to characterize contamination levels:
Single factor pollution index (Pi): evaluates the contamination degree of one individual element relative to its evaluation criterion.
Nemerow comprehensive pollution index (PN): integrates the mean and maximum single-factor indices to reflect overall soil quality.
Geo-accumulation index (Igeo): compares measured concentrations with geological background values to identify anthropogenic enrichment.
Potential ecological risk index (RI, Hakanson method): weighs metal concentrations by their biological toxicity response factors to estimate ecological hazard.
Health risk assessment model: estimates carcinogenic and non-carcinogenic risks via ingestion, inhalation, and dermal contact pathways, consistent with GB 36600-2018.
Testing Samples
Agricultural Land Soil
Surface soil samples (0-20 cm) collected from cultivated fields, orchards, vegetable plots, and paddy fields. These samples are assessed against the pH-dependent screening values of GB 15618-2018 to classify land into priority protection, safe utilization, or strict control categories.
Development and Construction Land Soil
Soil from residential, commercial, industrial, and public infrastructure sites, typically sampled at 0-0.5 m and 0.5-1.5 m depth intervals depending on the suspected contamination profile. Evaluation follows the first-class or second-class screening values of GB 36600-2018 according to the sensitive or non-sensitive land use scenario.
Industrial Site and Brownfield Soil
Soil from former electroplating, smelting, chemical, tannery, battery manufacturing, and pesticide production facilities. Sampling strategies are risk-based and often employ grid or judgmental (targeted) layouts around potential contamination sources, storage areas, pipelines, and waste disposal zones.
Mining and Smelting Area Soil
Soil and tailings-affected media from the surroundings of mines, smelters, and slag heaps, where multi-element enrichment of Cd, Pb, Zn, As, and Cu is common. Depth profile sampling is frequently used to evaluate vertical migration.
Background and Reference Soil
Undisturbed or minimally disturbed soil collected from areas remote from pollution sources, used to establish local geochemical baselines and support Igeo and enrichment factor calculations.
Testing Items
The core testing program comprises the eight regulated inorganic contaminants plus supplementary elements and soil parameters:
Basic eight heavy metals: arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), mercury (Hg), nickel (Ni), zinc (Zn).
Speciation analysis: hexavalent chromium (Cr(VI)), a highly toxic and mobile species requiring specific alkaline extraction.
Extended inorganic elements: antimony (Sb), beryllium (Be), cobalt (Co), molybdenum (Mo), selenium (Se), thallium (Tl), vanadium (V), manganese (Mn), and tin (Sn).
Supporting soil parameters: pH, organic matter content, cation exchange capacity (CEC), and soil texture, which influence metal mobility and the applicability of screening values.
Optional leaching tests: TCLP-style extraction (HJ/T 300 or EPA 1311) and horizontal vibration extraction (HJ 557) to evaluate metal leachability and groundwater risk.
Testing Methods
Sample Preparation
Field samples are air-dried at room temperature or freeze-dried for mercury-sensitive projects, disaggregated with non-metallic tools, and sieved through a 2 mm nylon sieve. A portion is further ground to pass a 0.149 mm (100 mesh) sieve for elemental digestion. All preparation steps avoid contact with metals to prevent cross-contamination.
Digestion and Extraction
Aqua regia extraction (HJ 803, ISO 12914): the routine approach for the eight heavy metals under GB standards, dissolving the environmentally reactive metal fraction.
Four-acid total digestion (HCl-HNO3-HF-HClO4, HJ 832): provides near-total elemental content including silicate-bound fractions.
Microwave-assisted digestion (EPA 3052, HJ 832): a closed-vessel method improving recovery and minimizing volatile losses of Hg and As.
Alkaline digestion for Cr(VI) (EPA 3060A): preserves the original valence state of chromium during extraction.
Instrumental Analysis
Inductively Coupled Plasma Mass Spectrometry (ICP-MS, HJ 803, EPA 6020B): ultra-trace simultaneous determination of multiple elements with detection limits at the ug/kg level, ideal for Cd, Pb, Tl, and Be.
Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, HJ 781, HJ 702): rapid multi-element determination for medium-to-high concentration samples such as Cu, Zn, Cr, and Ni.
Graphite Furnace Atomic Absorption Spectrometry (GFAAS, GB/T 17141): highly sensitive determination of trace Pb and Cd.
Flame Atomic Absorption Spectrometry (FAAS, GB/T 17138): routine measurement of Cu, Zn, and Ni.
Atomic Fluorescence Spectrometry (AFS, GB/T 22105): the preferred technique for Hg, As, Se, and Sb due to superior sensitivity for hydride-forming and cold vapor elements.
Cold Vapor Atomic Absorption Spectrometry (CV-AAS, GB/T 17136): dedicated mercury determination.
Portable and benchtop X-Ray Fluorescence (XRF, HJ 780): rapid on-site screening of Pb, Cr, Cu, Zn, As, and Ni to guide sampling density and delineate hotspots.
Quality Assurance and Quality Control
Each analytical batch incorporates method blanks, laboratory duplicates, matrix spikes, and certified reference materials (CRMs) such as GBW series soil standards. Calibration verification, spike recoveries of 80-120 percent, and relative deviation limits for duplicates ensure data reliability. All analyses are conducted by accredited third-party laboratories holding CMA and CNAS accreditations under ISO/IEC 17025.
Applications
Agricultural land classification and safe utilization zoning for grain production safety and soil-crop system management.
Contaminated site investigation and risk assessment prior to urban redevelopment, industrial land transfers, and brownfield regeneration.
Environmental impact assessment (EIA) and post-construction verification for industrial, mining, and infrastructure projects.
Remediation effectiveness evaluation, verifying that soil washing, solidification/stabilization, phytoremediation, or thermal desorption has achieved target cleanup levels.
Compliance monitoring around smelters, landfills, wastewater irrigation districts, and chemical industrial parks.
Geochemical baseline surveys and regional soil pollution censuses supporting land use planning and environmental regulation.
Due diligence audits for mergers, acquisitions, and real estate transactions involving potentially contaminated properties.
FAQ
What is the difference between GB 15618-2018 and GB 36600-2018?
GB 15618-2018 applies to agricultural land (farmland, orchards, pastures) and uses pH-dependent risk screening and intervention values focused on food safety and crop production. GB 36600-2018 applies to development land and classifies screening values by land use sensitivity, such as residential versus industrial, with emphasis on direct human health exposure pathways.
What happens if a soil concentration exceeds the risk screening value?
An exceedance does not automatically confirm unacceptable risk. It triggers further detailed investigation, including denser sampling, speciation or bioaccessibility analysis, and a site-specific health and ecological risk assessment. If the risk intervention value is exceeded, risk control or remediation measures are generally required.
Why are aqua regia and total digestion results different?
Aqua regia dissolves the reactive, environmentally relevant fraction of metals but cannot fully attack silicate minerals, so results for Cr and Ni in particular may be lower than total digestion with hydrofluoric acid. Regulatory GB screening values are based on aqua regia extraction, so the digestion method must match the evaluation criterion.
How is sampling density determined?
For preliminary investigations, grid-based sampling is common, with typical spacings of 40 m to 160 m for site investigations and one composite sample per 20 to 100 hectares for agricultural surveys, adjusted according to land use, contamination probability, and regulatory requirements.
How long does a complete assessment take?
Field sampling typically requires several days depending on site size and accessibility, while laboratory analysis of the eight heavy metals generally takes 10 to 15 working days. Projects involving speciation analysis, ecological risk modeling, or multiple sampling rounds require additional time for data validation and reporting.