Soil Pollution Key Special Program Risk Assessment: Technical Framework, Standards, and Testing Meth

Soil pollution key special program risk assessment is a systematic technical process for evaluating the human health and environmental risks of contaminated land under China's national soil pollution prevention framework. Conducted in accordance with standards such as GB 36600-2018, GB 15618-2018, and the HJ 25.3-2019 technical guideline, this assessment integrates site investigation, targeted contaminant analysis, and quantitative risk modeling to derive soil screening values, intervention thresholds, and site-specific remediation goals for land redevelopment and regulatory compliance.

Technical Overview

Soil pollution key special program risk assessment refers to the structured evaluation of contaminated soil and groundwater conducted under China's national soil pollution prevention and control framework, which was established through the Soil Pollution Prevention and Control Action Plan and reinforced by the Soil Pollution Prevention and Control Law of the People's Republic of China. The assessment supports the National Key R&D Program objectives by providing scientifically defensible data on contaminant distribution, exposure pathways, and quantified health risks for decision-making on land use, risk control, and remediation.

The assessment follows a tiered investigation approach. A preliminary site investigation establishes historical land use, identifies potential pollution sources, and screens for indicator contaminants. If contaminant concentrations exceed the risk screening values specified in GB 36600-2018 (Risk Control Standard for Soil Contamination of Development Land) or GB 15618-2018 (Risk Control Standard for Soil Contamination of Agricultural Land), a detailed investigation and site-specific human health risk assessment is required in accordance with HJ 25.3-2019.

Quantitative risk assessment is built on the classical four-step paradigm: hazard identification, exposure assessment, toxicity assessment, and risk characterization. Exposure pathways typically evaluated for development land include oral ingestion of soil, dermal contact, inhalation of soil particulates, and inhalation of volatile vapors from soil and groundwater. Risk characterization outputs include the hazard quotient (HQ) and hazard index (HI) for non-carcinogenic effects, and incremental lifetime cancer risk (CR) for carcinogenic effects, with commonly applied thresholds of HQ less than 1 and CR within the range of 10 to the minus 6 to 10 to the minus 4. Where calculated risks exceed acceptable levels, the assessment derives site-specific risk control values and remediation target concentrations.

Testing Samples

Surface Soil

Surface soil samples are generally collected from 0 to 0.5 meters below ground level, where direct human contact and inhalation exposure are most significant. Sampling density follows HJ 25.1-2019, typically requiring systematic grid-based sampling with at least one sampling point per 1,600 square meters for preliminary investigations of development land.

Subsurface and Deep Soil

Deep soil samples are collected from 0.5 meters to the saturated zone or bedrock to characterize vertical contaminant migration. Boreholes are advanced using direct-push, hollow-stem auger, or rotary drilling methods, with continuous lithological logging and samples recovered at each stratum change or at fixed intervals.

Groundwater

Groundwater samples are collected from temporary or permanent monitoring wells installed across the site, including upgradient background wells and downgradient monitoring points. Groundwater data are essential for evaluating leaching-to-groundwater pathways, vapor intrusion risks, and off-site migration potential, with quality evaluated against GB/T 14848-2017.

Soil Gas and Sub-slab Vapor

Soil gas samples are collected from temporary probes or permanent soil gas wells, and sub-slab vapor samples may be collected beneath existing buildings where volatile organic compounds are suspected. These samples support the assessment of vapor intrusion pathways into indoor air.

Agricultural Soil

For agricultural land assessment, topsoil samples are typically collected from the cultivated layer at 0 to 20 centimeters, with subsoil samples from 20 to 40 centimeters where deep plowing or vertical migration is a concern. Grid densities are adjusted according to land use type and survey objectives.

Sediment and Solid Waste Residues

Riverbed or ditch sediments, tailings, slag, and other solid waste residues within or adjacent to the site may be sampled to identify secondary pollution sources and evaluate contaminant release potential.

Testing Items

The contaminant screening list is determined during hazard identification but is typically aligned with the 85 indicators specified in GB 36600-2018 and the 8 basic items plus selected additional items in GB 15618-2018. Common testing categories include:

• Heavy metals and metalloids: arsenic, cadmium, hexavalent chromium, copper, lead, mercury, nickel, zinc, beryllium, cobalt, vanadium, antimony, thallium, and manganese
• Volatile organic compounds (VOCs): benzene, toluene, ethylbenzene, xylenes, styrene, chloroform, carbon tetrachloride, trichloroethylene, tetrachloroethylene, 1,2-dichloroethane, vinyl chloride, and related chlorinated solvents
• Semi-volatile organic compounds (SVOCs): polycyclic aromatic hydrocarbons such as benzo[a]pyrene, naphthalene, and phenanthrene; phthalate esters; nitroaromatics; and anilines
• Polychlorinated biphenyls (PCBs) and organochlorine pesticides such as DDT, chlordane, and hexachlorocyclohexane
• Petroleum hydrocarbons: total petroleum hydrocarbon fractions C6-C9 and C10-C40
• Dioxins and furans (PCDD/Fs) for sites with combustion, smelting, or chlor-alkali histories
• Other inorganic parameters: total cyanide, fluoride, and pH; groundwater additionally tested for major ions, ammonia nitrogen, nitrate, nitrite, and volatile organics

Testing Methods

All analytical work is performed by accredited third-party laboratories holding CMA (China Metrology Accreditation) and CNAS (China National Accreditation Service for Conformity Assessment) qualifications. Standard methods are drawn from the national environmental protection standards (HJ series), national standards (GB and GB/T series), and equivalent international methods from ISO, ASTM, and EPA method frameworks where applicable:

• Heavy metals: microwave digestion or aqua regia digestion followed by inductively coupled plasma mass spectrometry (ICP-MS) or inductively coupled plasma optical emission spectrometry (ICP-OES) per HJ 803-2016 and HJ 680-2013; mercury by cold vapor atomic fluorescence spectrometry per GB/T 22105.1; arsenic and selenium by atomic fluorescence spectrometry per GB/T 22105.2
• Hexavalent chromium: alkaline digestion followed by spectrophotometric determination per HJ 1082-2019
• VOCs: purge-and-trap gas chromatography-mass spectrometry (GC-MS) per HJ 605-2011, or headspace GC-MS per HJ 642-2013
• SVOCs: ultrasonic, accelerated solvent, or Soxhlet extraction followed by GC-MS per HJ 834-2017
• Organochlorine pesticides and PCBs: extraction followed by GC-MS per HJ 835-2017 and HJ 1021-2019
• Petroleum hydrocarbons: headspace GC for C6-C9 per HJ 921-2017 and GC for C10-C40 per HJ 1020-2019
• Dioxins and furans: high-resolution gas chromatography coupled with high-resolution mass spectrometry (HRGC-HRMS) per HJ 77.4
• Quality assurance: field duplicates, trip blanks, equipment rinsate blanks, matrix spikes, certified reference materials, and chain-of-custody documentation per HJ 25.2-2019 and HJ 164-2020

Risk modeling is performed using the exposure and fate-transport equations specified in HJ 25.3-2019, which are consistent with internationally recognized risk-based corrective action (RBCA) methodologies. Sensitivity analysis is conducted to identify the dominant exposure pathways and parameters driving the calculated risk, and site-specific parameters such as soil organic carbon content, hydraulic conductivity, and building ventilation rates are measured or conservatively assumed where data are unavailable.

Applications

Soil pollution key special program risk assessment results directly support a wide range of regulatory, transactional, and engineering decisions:

• Change of land use from industrial or commercial purposes to sensitive development land such as residential housing, schools, kindergartens, hospitals, and elderly care facilities, where risk assessment reports are a mandatory component of the development approval file
• Brownfield redevelopment and urban renewal projects requiring confirmation that residual contamination is compatible with the proposed land use scenario
• Classification of agricultural land into priority protection, safe utilization, and strict control categories, and the design of safe utilization strategies for moderately and severely polluted cultivated land
• Site closure and decommissioning of key soil-polluting enterprises, including chemical plants, pesticides manufacturers, coking and gasworks sites, electroplating facilities, smelters, and mining areas
• Accountability investigations to determine whether existing contamination originated from historical operations or third-party activities
• Derivation of site-specific remediation target values and technical remediation plans under HJ 25.4-2019, followed by remediation effect evaluation and post-remediation acceptance testing
• Periodic self-monitoring and hidden pollution investigation obligations for units included in the soil pollution key supervisory enterprise list

FAQ

What triggers the requirement for a formal risk assessment?

A detailed risk assessment is triggered when soil or groundwater contaminant concentrations from the preliminary or detailed investigation exceed the risk screening values in GB 36600-2018 for development land, or the risk screening values in GB 15618-2018 for agricultural land. It is also mandatory before converting former industrial land to sensitive uses such as residential or educational facilities.

What is the difference between screening values and intervention values?

Screening values are conservative, health-based thresholds below which contamination generally poses an acceptable risk and no further action is required. Intervention values are higher thresholds above which remediation or risk control measures are generally required. Concentrations between the two values require a site-specific quantitative risk assessment to determine whether action is necessary.

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

GB 36600-2018 applies to development land, including residential, commercial, and industrial uses. GB 15618-2018 applies to agricultural land such as cultivated land, orchards, grassland, and woodland. For sites undergoing conversion between these categories, the assessment is based on the intended future land use.

How many soil samples are typically required?

Under HJ 25.1-2019, preliminary investigations of development land generally require one surface soil sample per 1,600 square meter grid cell and one deep soil sample per 3,200 square meter grid, with a minimum number of sampling points per site. Detailed investigations increase density around confirmed contamination zones. Agricultural surveys apply different grid densities based on land type and survey tier.

How long does a complete assessment take?

Typical project durations range from several weeks for simple screening-level assessments to several months for complex industrial sites requiring multiple rounds of investigation, groundwater monitoring, vapor intrusion evaluation, and regulatory review of the risk assessment report.

What happens if the calculated risk exceeds acceptable levels?

The risk assessment derives site-specific risk control values and remediation target concentrations for each contaminant and exposure pathway. These values become the basis for the remediation and risk control plan, which may include source removal, soil replacement, solidification or stabilization, chemical oxidation, bioremediation, barriers, or land-use restrictions, followed by verification sampling and regulatory acceptance.