Soil Environmental Quality Category Classification and Assessment

Soil environmental quality category classification and assessment is a systematic evaluation process that assigns agricultural and development land to risk-based management categories by comparing measured contaminant concentrations against national risk screening and control values. Performed in accordance with GB 15618-2018 and GB 36600-2018 under China's Soil Pollution Prevention and Control Law, this assessment provides the scientific basis for land use planning, safe utilization of farmland, and contaminated site management. Accredited third-party laboratories deliver the sampling, chemical analysis, and data interpretation required to support accurate category delineation and regulatory compliance.

Technical Overview

Soil environmental quality category classification and assessment is a core component of China's soil pollution management framework, established under the Soil Pollution Prevention and Control Action Plan and the Soil Pollution Prevention and Control Law of the People's Republic of China. The process evaluates contaminant concentrations in soil against national risk screening values and risk control values, and delineates land parcels into distinct management categories that determine permissible agricultural practices and land use decisions.

For agricultural land, GB 15618-2018 (Soil Environmental Quality — Risk Control Standard for Soil Contamination of Agricultural Land) defines the technical basis for a three-category management system:

• Priority Protection Category (Class I): soil is essentially clean, with contaminant concentrations at or below risk screening values; strict protection measures are applied to preserve soil quality and prevent degradation.
• Safe Utilization Category (Class II): contaminant concentrations exceed screening values but remain below risk control values; targeted agronomic measures, crop substitution, and soil conditioning are implemented to ensure safe food production.
• Strict Control Category (Class III): contaminant concentrations exceed risk control values, indicating severe pollution; planting of edible crops is restricted, and land use adjustment, risk control, or remediation is required.

For development land, GB 36600-2018 establishes risk screening and control values that distinguish between Class I land uses (residential, schools, medical and elderly care facilities) and Class II land uses (industrial and commercial sites), with more stringent criteria applied to sensitive receptor scenarios. Screening values in GB 15618-2018 are pH-dependent, with separate thresholds for four pH bands (below 5.5, 5.5 to 6.5, 6.5 to 7.5, and above 7.5) and differentiated criteria for paddy fields and other agricultural land, reflecting the influence of soil acidity on metal mobility and bioavailability.

A typical assessment workflow comprises five stages: collection and review of historical land use and pollution source data; grid-based field sampling and sample preservation; laboratory analysis of target parameters; comparison of measured concentrations against applicable screening and control values; and delineation of category boundaries supported by spatial analysis, exceedance statistics, and crop sensitivity evaluation. The final deliverable is a technical report containing category maps, unit-level classification results, and tailored risk management recommendations.

Testing Samples

Agricultural Surface Soil

Composite surface soil samples (0 to 20 cm depth) are collected from cultivated land, including paddy fields, dry farmland, vegetable production bases, and grain-producing areas. Samples are typically composited from multiple subsampling points within a defined grid cell to represent the average condition of each evaluation unit.

Orchard and Plantation Soil

Topsoil samples from orchards, tea plantations, and perennial economic crop land are collected to verify whether metal and pesticide residues comply with the screening values applicable to non-grain agricultural use, with sampling depth adjusted to the effective root zone.

Development Land Soil

Surface and sub-surface soil samples from former industrial sites, brownfields, and land earmarked for redevelopment are collected in accordance with stratified investigation principles. Soil gas and groundwater samples may be incorporated where volatile contaminants are suspected.

Background and Reference Samples

Regional background samples collected from areas free of obvious pollution sources are analyzed to establish local baseline concentrations, enabling the identification of anthropogenic enrichment and supporting the interpretation of exceedance results.

Testing Items

For agricultural land classification under GB 15618-2018, the following parameters are analyzed:

• Basic inorganic items (8 metals): cadmium (Cd), mercury (Hg), arsenic (As), lead (Pb), chromium (Cr), copper (Cu), nickel (Ni), and zinc (Zn).
• Selected organic items: total hexachlorocyclohexane (HCH), total DDT, and benzo[a]pyrene, with additional organochlorine pesticides and petroleum hydrocarbons added where land use history indicates potential sources.
• Supporting parameters: soil pH, organic matter content, cation exchange capacity, and soil texture, which are essential for applying the correct pH-dependent screening thresholds and evaluating contaminant bioavailability.

For development land risk assessment under GB 36600-2018, testing covers the 45 basic items, including heavy metals (arsenic, cadmium, chromium(VI), copper, lead, mercury, nickel), 27 volatile organic compounds, and 11 semi-volatile organic compounds, together with up to 40 additional items such as organochlorine pesticides, polychlorinated biphenyls, and petroleum hydrocarbons (C10 to C40) depending on site conditions.

Testing Methods

Sample preparation involves air-drying at room temperature, grinding, homogenization, and sieving to standardized fractions, followed by method-specific digestion or extraction. Analyses are performed using validated national and industry-standard methods in accredited laboratories, with routine quality assurance measures including method blanks, duplicate samples, matrix spikes, and certified reference materials.

• pH: potentiometric measurement in accordance with HJ 962-2018.
• Cadmium and lead: graphite furnace atomic absorption spectrophotometry per GB/T 17141-1997, or inductively coupled plasma mass spectrometry (ICP-MS) per HJ 680-2013.
• Copper, zinc, nickel, and chromium: flame atomic absorption spectrometry per GB/T 17138, GB/T 17139, and GB/T 17137, or inductively coupled plasma atomic emission spectrometry (ICP-AES) per HJ 803-2016 following aqua regia extraction.
• Mercury and arsenic: atomic fluorescence spectrometry (AFS) per GB/T 22105.1 and GB/T 22105.2, or HJ 702-2014 for soil and sediment.
• HCH and DDT: gas chromatography with electron capture detection per GB/T 14550.
• Benzo[a]pyrene and PAHs: high performance liquid chromatography per HJ 784-2016 or GC-MS per HJ 805-2016.
• VOCs and SVOCs: purge-and-trap or headspace GC-MS per HJ 605-2011 and HJ 642-2013, and GC-MS per HJ 834-2017.
• Petroleum hydrocarbons (C10–C40): gas chromatography per HJ 1021-2019.

Field sampling follows the technical specifications of HJ/T 166-2004 for soil environmental monitoring and NY/T 395 for farmland soil sampling, with positional records, chain-of-custody documentation, and coded sample transport ensuring full traceability from field to laboratory.

Applications

Category classification and assessment results serve multiple regulatory and practical purposes:

• Delineation of priority protection, safe utilization, and strict control zones in county-level agricultural land management plans.
• Design of safe utilization programs, including lime application, alternative crop varieties, agronomic regulation, and planting structure adjustment on Class II land.
• Implementation of strict control measures on Class III land, such as conversion to forestry or biomass crops, fencing, and risk isolation.
• Risk screening and detailed investigation of development land prior to urban renewal, land transfer, or change of land use.
• Post-remediation effectiveness verification, confirming that treated soil achieves the target category or land use standard.
• Compliance demonstration for enterprises listed as key soil pollution sources under statutory self-monitoring obligations.
• Support for food safety management by identifying cadmium and arsenic risks in rice and vegetable production areas.

FAQ

What are the three soil environmental quality categories for agricultural land?

Agricultural land in China is divided into the Priority Protection Category (unpolluted or slightly polluted, compliant with screening values), the Safe Utilization Category (lightly to moderately polluted, exceeding screening values but below control values), and the Strict Control Category (severely polluted, exceeding control values). Each category carries distinct management requirements under the Soil Pollution Prevention and Control Law.

What is the difference between risk screening values and risk control values?

Risk screening values in GB 15618-2018 and GB 36600-2018 define the concentration threshold below which soil contamination risk is generally considered acceptable. Risk control values define the concentration above which soil poses a high risk to human health or agricultural production, triggering strict control, risk mitigation, or remediation. Concentrations between the two thresholds indicate the need for detailed evaluation and safe utilization measures.

Why is soil pH measured during classification?

Screening values for metals such as cadmium, mercury, and lead in GB 15618-2018 vary across four pH bands, because lower soil acidity increases the solubility and crop uptake of many heavy metals. Accurate pH measurement is therefore essential for selecting the correct comparison threshold and avoiding misclassification.

How are categories finally determined?

Categories are assigned at the evaluation-unit level by combining laboratory results with screening value comparisons, point-level exceedance ratios, spatial interpolation, land use type, crop sensitivity, and local pollution source information. Professional judgment and regional technical guidelines govern cases where data are heterogeneous or borderline.

Can a land category change over time?

Yes. Category classification is dynamic. Following remediation, soil improvement measures, or new monitoring data, local authorities may update the category of a parcel through re-assessment, and soil achieving compliance can be upgraded, while newly polluted land may be downgraded to a stricter management class.

How many samples are required for a classification project?

Sampling density depends on the evaluation objective, land area, and heterogeneity of suspected pollution. Detailed investigation surveys typically use grid-based layouts with denser spacing around confirmed or suspected pollution sources, following HJ/T 166-2004 and applicable national technical guidelines, to ensure each delineated unit is adequately represented.