Health risk assessment of contaminated site soil is a systematic, quantitative process that evaluates the probability and severity of adverse human health effects caused by exposure to chemical pollutants in soil. By integrating site investigation data, receptor-specific exposure scenarios, and toxicological evidence under frameworks such as ASTM E2081, ISO 15800, GB 36600-2018, and HJ 25.3-2019, the assessment determines whether a site is safe for its intended land use and derives risk-based remediation targets. This article provides a complete technical reference covering assessment principles, sampling strategies, analytical parameters, testing methods, and practical applications.
Technical Overview
Health risk assessment of site soil contamination is a multi-disciplinary, science-based evaluation that quantifies the potential adverse effects of chemical contaminants on human receptors at a given site. The assessment links contaminant concentrations measured during site investigation with land-use scenarios, exposure pathways, and dose–response relationships, producing numerical risk indices that support decisions on remediation, risk control, or no further action. It is the core decision-making tool for brownfield redevelopment, industrial facility closure, and land-use zoning under modern soil pollution prevention regulations. All chemical analyses supporting the assessment are performed by accredited third-party laboratories operating in compliance with ISO/IEC 17025.
The Four-Step Assessment Framework
- Hazard Identification: Review of site history, process records, and investigation data to identify contaminants of potential concern (COPCs), typically by comparing measured soil concentrations against screening values such as those in GB 36600-2018 or the USEPA Regional Screening Levels (RSLs).
- Exposure Assessment: Characterization of current and future land use, receptor populations (child and adult residents, commercial and industrial workers, construction workers), and complete exposure pathways, including incidental soil ingestion, dermal contact, inhalation of fugitive dust, and vapor intrusion of volatile contaminants into indoor air.
- Toxicity Assessment: Assignment of carcinogenic slope factors (oral and inhalation) and non-carcinogenic reference doses or reference concentrations, sourced from authoritative toxicological databases such as the USEPA IRIS, PPRTV, CalEPA, and the parameter tables annexed to HJ 25.3-2019.
- Risk Characterization: Calculation of lifetime excess carcinogenic risk (Risk = CDI × SF) and hazard quotients (HQ = CDI / RfD) for each pathway and receptor, summation into cumulative risk and hazard index, and comparison against jurisdiction-specific acceptable risk thresholds.
Receptors, Pathways, and Acceptable Risk Criteria
The most sensitive receptor is typically a child resident, for whom soil ingestion rates per unit body weight are highest. Complete pathways evaluated in a standard assessment include direct soil ingestion, dermal absorption of contaminants from soil adhered to skin, inhalation of resuspended soil particulates, inhalation of soil gas migrating into buildings, and, where relevant, migration to groundwater followed by potable or vapor pathways. An acceptable outcome generally requires a lifetime excess carcinogenic risk between 10^-6 and 10^-5 (with 10^-4 used as an upper bound in some jurisdictions) and a hazard quotient or hazard index not exceeding 1 for non-carcinogenic effects.
Primary Standards and Technical Guidelines
- ASTM E1739 — Risk-Based Corrective Action (RBCA) applied at petroleum release sites
- ASTM E2081 — Risk-Based Corrective Action applied at chemical release sites
- ISO 15800 series — Soil quality, characterization of soil related to human exposure assessment
- ISO 10381 series — Soil quality, sampling design and sampling techniques
- GB 36600-2018 — Soil environmental quality, risk control standard for soil contamination of development land
- HJ 25.1, HJ 25.2, and HJ 25.3-2019 — Technical guidelines for site investigation, sampling, and risk assessment of land for construction
- USEPA Risk Assessment Guidance for Superfund (RAGS) Parts A–F and the Regional Screening Levels framework
- ISO/IEC 17025 — General requirements for the competence of the testing laboratories performing supporting chemical analyses
Testing Samples
Surface Soil Samples
Surface soil (typically the 0–0.5 m layer, or 0–20 cm for certain exposure models) is collected using stainless steel trowels, hand augers, or cores, because this horizon governs direct-contact, ingestion, and dust inhalation pathways. Samples are transferred directly into pre-cleaned amber glass jars with minimal headspace for VOC analysis and into sealed containers for metals and SVOCs, then transported under chain of custody at 4 °C. Coordinates, depths, lithology, color, odor, and any staining are logged for every location.
Subsurface Soil Samples
Subsurface samples are obtained using direct-push technology, hollow-stem augers, or sonic drilling rigs, with continuous lithologic logging and field screening (photoionization detection for VOCs, X-ray fluorescence screening for metals). Samples are targeted at suspected source zones, such as former tank farms, sumps, wastewater lagoons, and spill areas, and are collected at systematic vertical intervals down to the maximum depth of impact, the capillary fringe, or the water table.
Soil Gas and Sub-Slab Vapor Samples
Where volatile contaminants are present, soil gas is collected from temporary drive-point probes or permanent vapor implants installed at depths of approximately 0.5–1.5 m below ground surface or building slabs. Samples are drawn at controlled, low flow rates into passivated evacuated canisters or onto thermal desorption tubes to minimize ambient air short-circuiting, and are used to evaluate the vapor intrusion pathway for current or future buildings.
Supporting Groundwater and Background Samples
Groundwater from monitoring wells is sampled to assess contaminant leaching and groundwater-borne vapor intrusion, using low-flow purging techniques with field measurement of pH, redox potential, conductivity, dissolved oxygen, and turbidity. Background soil samples are collected from unaffected, geologically similar off-site locations to distinguish anthropogenic contamination from naturally elevated geogenic concentrations, which is particularly important for arsenic and heavy metals.
Quality Assurance Samples
- Field duplicates at a frequency of at least 10% to quantify sampling precision
- Trip blanks accompanying VOC sample shipments to identify contamination during transport
- Equipment (rinsate) blanks to verify decontamination of reusable sampling tools
- Matrix spikes and surrogates to assess matrix effects and analytical recovery
- Complete chain-of-custody documentation from field to laboratory
Testing Items
The analyte list is tailored to site history and detected contaminants of concern, but typically covers the following parameter groups aligned with GB 36600-2018 and international risk screening frameworks.
Heavy Metals and Metalloids
Common metals include arsenic, cadmium, mercury, lead, hexavalent chromium, total chromium, copper, nickel, zinc, beryllium, antimony, thallium, vanadium, cobalt, manganese, selenium, and molybdenum. Speciation of chromium and mercury is frequently required because hexavalent chromium and methylmercury carry substantially higher toxicological weighting than their trivalent and inorganic forms.
Volatile Organic Compounds (VOCs)
Target VOCs include benzene, toluene, ethylbenzene, xylene isomers, styrene, chloroform, carbon tetrachloride, methylene chloride, 1,2-dichloroethane, 1,1,1-trichloroethane, tetrachloroethylene, trichloroethylene, vinyl chloride, cis- and trans-1,2-dichloroethylene, chlorobenzene, and fuel oxygenates such as MTBE. VOCs dominate both the carcinogenic risk and the vapor intrusion pathway at most solvent- and fuel-impacted sites.
Semi-Volatile Organic Compounds (SVOCs)
SVOCs comprise the sixteen priority polycyclic aromatic hydrocarbons (from naphthalene through benzo[a]pyrene, indeno[1,2,3-cd]pyrene, and dibenz[a,h]anthracene), phthalate esters such as bis(2-ethylhexyl) phthalate, chlorinated phenols, chlorobenzenes, nitrobenzene, aniline, and nitroaromatic compounds. PAHs are the principal risk drivers at gasworks, coking, and coal-processing sites.
Pesticides, PCBs, and Petroleum Hydrocarbons
Organochlorine pesticides include alpha-, beta-, and gamma-hexachlorocyclohexane, DDT with its DDE and DDD metabolites, aldrin, dieldrin, endrin, heptachlor, and chlordane. Polychlorinated biphenyls may be quantified as Aroclor mixtures or individual congeners. Total petroleum hydrocarbons are reported by carbon fraction, for example C6–C10 and C10–C40, to support fraction-based toxicity assessment. For sites such as former incinerators, chlor-alkali facilities, or herbicide plants, dioxins and furans (PCDD/Fs) are quantified as toxic equivalents.
Soil Parameters for Exposure Modeling
Site-specific values of pH, soil organic carbon content, dry bulk density, moisture content, porosity, texture, and hydraulic conductivity are measured because they directly control exposure model outputs such as volatilization factors, particulate emission factors, and contaminant leaching rates used in the risk equations.
Testing Methods
Sampling and Handling Protocols
Sampling follows ASTM D6169 and the ISO 10381 series, together with HJ 25.2-2019 and HJ/T 166-2004 for sites in China. Low-level VOC soils are collected with the EnCore-type syringe samplers or methanol-preserved vials described in EPA Method 5035A to minimize volatilization losses. Soil gas programs follow ASTM D5314 and D7758 (passive soil gas) with confirmatory sub-slab sampling beneath existing structures where building occupancy is a concern.
Instrumental Analytical Methods
- Metals: Aqua regia or total digestion followed by ICP-OES (EPA 6010D) or ICP-MS (EPA 6020B; HJ 803-2016); arsenic and mercury by atomic fluorescence spectrometry or cold vapor AAS (EPA 7471C); hexavalent chromium by alkaline digestion (EPA 3060A) with UV-visible colorimetric determination (EPA 7196A; HJ 1082-2019).
- VOCs: Purge-and-trap or headspace GC-MS (EPA 8260D; HJ 605-2011 and HJ 642-2013), with reporting limits set well below the governing screening values.
- SVOCs and PAHs: Solvent extraction by sonication, Soxhlet, or accelerated solvent extraction (EPA 3550C, 3540C, 3545A) followed by GC-MS (EPA 8270E; HJ 834-2017) or HPLC with fluorescence detection (EPA 8310).
- Pesticides and PCBs: GC-ECD or GC-MS (EPA 8081B and 8082A; HJ 743-2015 and HJ 921-2017).
- Petroleum hydrocarbons: GC-FID with silica gel cleanup (EPA 8015D; HJ 1021-2019).
- Dioxins and furans: High-resolution GC-MS (EPA 1613B; HJ 77.4-2008) reported as WHO toxic equivalents.
- Soil gas and indoor air: Canister-based GC-MS per EPA TO-15 or thermal desorption tubes per EPA TO-17.
Risk Calculation and Modeling
Point estimates of risk are computed using the standard intake equations in USEPA RAGS Part A or HJ 25.3-2019, with spreadsheet or commercial software tools implementing the ASTM E2081 tiered RBCA framework. Vapor intrusion is screened with the Johnson–Ettinger model and refined with measured soil gas, sub-slab, and indoor air data. In Tier 3 probabilistic assessments, Monte Carlo simulation propagates parameter uncertainty across full distributions rather than conservative single-point values, providing defensible risk bounds for complex or high-value sites.
Applications
Risk-based assessment outcomes directly determine land-use suitability, remediation scope, and regulatory closure, making the methodology applicable across the entire contaminated land management lifecycle.
- Brownfield redevelopment, verifying that residual contamination is compatible with proposed residential, educational, commercial, or recreational reuse
- Closure and decommissioning of industrial facilities, including chemical plants, gasworks and coking sites, refineries, electroplating shops, pesticide manufacturers, and smelters
- Derivation of site-specific remediation targets that may be higher than generic screening values, substantially reducing excavation and disposal volumes and project cost
- Environmental due diligence for property transactions, mergers, and acquisitions, quantifying residual liability
- Regulatory submissions and pollution risk control area delineation under soil pollution prevention legislation
- Vapor intrusion evaluation for existing occupied buildings overlying volatile contaminant plumes
- Post-remediation verification demonstrating that residual risks satisfy acceptable thresholds
- Technical support for environmental litigation and insurance claims
FAQ
What risk levels are considered acceptable?
A lifetime excess carcinogenic risk of 10^-6 to 10^-5 is generally regarded as acceptable, with 10^-4 serving as the regulatory upper bound in some jurisdictions. For non-carcinogenic effects, a hazard quotient or cumulative hazard index of 1 or below indicates no appreciable concern. Both criteria must be satisfied for every receptor and exposure pathway.
How many samples and what sampling depth are required?
Sample density is driven by site area, geology, and the heterogeneity of contamination, typically combining systematic grid sampling with judgmental placement at former source areas. Vertical profiling extends from the surface to the deepest identified impact or the water table, often 3 to 10 m or deeper at source zones, with intervals of roughly 1 to 2 m adjusted according to field screening results.
What is the difference between Tier 1, Tier 2, and Tier 3 assessments?
Tier 1 compares measured concentrations directly against generic screening values such as GB 36600-2018 or USEPA RSLs. Tier 2 recalculates risk using site-specific exposure and soil parameters under ASTM E2081 or HJ 25.3-2019. Tier 3 applies probabilistic techniques such as Monte Carlo simulation to characterize uncertainty and support refined, defensible remediation goals.
Can a site-specific risk assessment reduce the remediation scope?
Yes. Because generic screening values are intentionally conservative, site-specific exposure parameters, actual soil properties, and land-use restrictions frequently raise calculated remediation targets above default values, reducing excavation volumes, treatment costs, and project duration while maintaining equivalent health protection.
When is a vapor intrusion assessment necessary?
Vapor intrusion must be evaluated whenever volatile contaminants such as chlorinated solvents or petroleum hydrocarbons are present in soil or shallow groundwater, particularly beneath or near existing or planned occupied buildings. The evaluation typically progresses from Johnson–Ettinger screening to soil gas and sub-slab measurements, with indoor air sampling reserved for confirmation.
How long does a complete assessment take?
A typical program requires two to eight weeks for site investigation and sampling, two to four weeks for laboratory analysis by an accredited third-party laboratory, and two to four additional weeks for risk calculation, report preparation, and regulatory review, with complex sites requiring phased work over several months.