Soil Contamination Site Investigation and Assessment

Soil contamination site investigation and assessment is the systematic process of identifying, quantifying, and evaluating pollutant risks at potentially contaminated land before redevelopment, transaction, or remediation. Guided by internationally recognized frameworks such as ASTM E1527, ASTM E1903, the ISO 18400 series, and China's national standards GB 36600-2018 and HJ 25.1-2019, accredited third-party laboratories deliver phased site investigations, comprehensive chemical testing, and human health and ecological risk assessment. This article explains the technical workflow, sample types, analytical parameters, testing methods, and typical applications of professional site investigation and assessment services.

Technical Overview

Soil contamination site investigation and assessment is a phased, science-based process used to determine whether hazardous substances are present at a site, to delineate their vertical and horizontal extent in soil, groundwater, and soil gas, and to evaluate the risks they pose to human health and the environment. It provides the technical foundation for land transaction due diligence, brownfield redevelopment planning, risk control decisions, and remediation design.

The investigation generally follows a three-phase structure recognized in international practice and codified in standards such as ASTM E1527 (Phase I environmental site assessment), ASTM E1903 (Phase II environmental site assessment), the ISO 18400 series for soil sampling, and China's national technical guidelines HJ 25.1 (site environmental investigation), HJ 25.2 (site monitoring), and HJ 25.3 (risk assessment), together with the risk control standards GB 36600-2018 for development land and GB 15618-2018 for agricultural land.

• Phase I - Preliminary investigation: non-intrusive review of historical records, production processes, chemical inventories, and pollution incidents, combined with site reconnaissance and stakeholder interviews to identify potential contamination sources and develop a conceptual site model.
• Phase II - Detailed investigation: intrusive soil borings, monitoring well installation, and systematic sampling with laboratory analysis to confirm the presence, concentration, and spatial distribution of contaminants.
• Phase III - Risk assessment and remediation planning: exposure assessment, toxicity assessment, and risk characterization to derive site-specific remediation targets and support risk control strategies.

A typical project workflow proceeds from information collection and sampling plan design, through field sampling and field screening, to laboratory analysis, data validation, comparison against screening values, and finally risk assessment reporting. All analytical work should be performed by accredited third-party laboratories operating under ISO/IEC 17025 quality management systems to ensure legally defensible data.

Testing Samples

Surface Soil Samples

Collected from the upper soil horizon, typically 0 to 50 cm below ground surface, using stainless-steel trowels or hand augers. Sampling locations are arranged on systematic grids or placed judgmentally at suspected source areas identified in the conceptual site model. Surface samples are used to evaluate direct-contact exposure pathways and characterize surface contamination distribution.

Subsurface Soil Samples

Retrieved from drilled boreholes using direct-push technology, hollow-stem augers, or rotary drilling rigs. Samples are collected at regular depth intervals, for example every 1.5 to 3 meters, and at lithological boundaries or field screening anomalies. Field portable instruments such as photoionization detectors and X-ray fluorescence analyzers support real-time selection of sampling depths. Boreholes typically extend until uncontaminated soil is confirmed by two consecutive sampling intervals below applicable screening values.

Groundwater Samples

Obtained from temporary or permanent monitoring wells installed in shallow and deep aquifers. After well development, low-flow purging techniques minimize turbidity and sample disturbance. Groundwater data delineate dissolved-phase contaminant plumes and are evaluated against the GB/T 14848 groundwater quality standard or site-specific criteria derived from the risk assessment.

Soil Gas and Vapor Samples

Collected from soil gas probes or sub-slab installations to evaluate the vapor intrusion pathway for volatile organic compounds, in accordance with ASTM E2600. These samples are critical for sites with chlorinated solvents, petroleum hydrocarbons, or planned occupied buildings.

Surface Water, Sediment, and Leachate Samples

Surface water and sediment samples from adjacent ditches, ponds, or receiving water bodies characterize off-site migration pathways. Leachate samples, prepared using horizontal vibration extraction or toxicity characteristic leaching procedures, are used to assess whether excavated soil or waste exhibits hazardous characteristics under the GB 5085 series identification standards.

Testing Items

Parameter selection is driven by historical land use, the conceptual site model, and the screening values in GB 36600-2018, which specifies 85 basic items and 40 additional items for development land. Typical parameter groups include:

• Physical and chemical properties: pH, moisture content, organic matter, electrical conductivity, and particle size distribution.
• Heavy metals and metalloids: arsenic, cadmium, total chromium, hexavalent chromium, copper, lead, mercury, nickel, zinc, manganese, antimony, thallium, beryllium, vanadium, cobalt, and selenium.
• Volatile organic compounds (VOCs): benzene, toluene, ethylbenzene, and xylenes (BTEX); chlorinated ethenes and ethanes such as tetrachloroethylene, trichloroethylene, and vinyl chloride; chlorobenzenes; and petroleum-related volatiles.
• Semi-volatile organic compounds (SVOCs): phenols, nitroaromatics, anilines, chlorophenols, and phthalate esters.
• Polycyclic aromatic hydrocarbons (PAHs): 16 priority PAHs, including benzo[a]pyrene, naphthalene, and phenanthrene.
• Total petroleum hydrocarbons (TPH): fractionated as C6-C9 and C10-C40 for fuel and oil contamination characterization.
• Pesticides: organochlorine pesticides such as HCHs and DDTs, organophosphorus pesticides, and pyrethroids.
• Polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs).
• Inorganic contaminants: total and easily released cyanide, fluoride, ammonia nitrogen, and sulfide.
• Emerging contaminants: per- and polyfluoroalkyl substances (PFAS) for sites with firefighting foam, electroplating, or fluoropolymer histories.

Testing Methods

Field screening methods provide rapid, real-time decision support. Portable X-ray fluorescence analyzers screen metals in soil consistent with ISO 13196, photoionization and flame ionization detectors screen volatile organics, and membrane interface probe logging maps contaminant profiles continuously with depth.

Laboratory methods are selected to achieve detection limits well below applicable screening values:

• Heavy metals: acid digestion with aqua regia following ISO 11466, followed by quantification using ICP-MS (ISO 17294-2) or ICP-OES; mercury by direct thermal decomposition atomic absorption or atomic fluorescence spectrometry; arsenic and selenium by hydride generation atomic fluorescence; hexavalent chromium by alkaline digestion and spectrophotometry consistent with ISO 15192.
• VOCs: headspace or purge-and-trap extraction coupled with gas chromatography-mass spectrometry (GC-MS), following ISO 15009 and HJ 605.
• SVOCs: pressurized fluid or ultrasonic extraction with gel permeation clean-up and GC-MS determination, following ISO 22155 and HJ 834.
• PAHs: GC-MS or high-performance liquid chromatography with fluorescence detection, following ISO 18287.
• Total petroleum hydrocarbons: gas chromatography with flame ionization detection, quantified as the C10-C40 fraction per ISO 16703.
• Pesticides and PCBs: solvent extraction with dispersive solid-phase clean-up followed by GC-MS/MS or LC-MS/MS determination.
• Dioxins and furans: high-resolution gas chromatography coupled with high-resolution mass spectrometry (HRGC-HRMS).
• Cyanide, fluoride, and ammonia: spectrophotometry, ion-selective electrode, and ion chromatography techniques consistent with ISO 10359.

Quality assurance and quality control follow ISO/IEC 17025 requirements and the monitoring specifications of HJ 25.2, including chain-of-custody documentation, method blanks, field and laboratory duplicates, matrix spikes, surrogate recoveries, and independent data validation before results are used for risk-based decisions.

Applications

Site investigation and assessment services support a wide range of regulatory, commercial, and engineering objectives:

• Brownfield redevelopment and land use conversion from industrial to residential, educational, or other sensitive purposes.
• Closure, relocation, and change-of-use applications for industrial enterprises requiring soil and groundwater compliance confirmation.
• Environmental due diligence for mergers and acquisitions, real estate transactions, and green financing disclosures.
• Decommissioning assessments for gas stations, oil depots, and underground storage tank systems.
• Legacy contamination sites, including former chemical plants, electroplating facilities, smelters, pesticide manufacturers, and steel works.
• Mining areas, tailings storage facilities, and hazardous waste disposal or landfill sites.
• Agricultural land quality evaluation and safe utilization planning under GB 15618-2018.
• Remediation baseline characterization, remedial effectiveness verification, and post-remediation acceptance monitoring.
• Technical support for environmental disputes, insurance claims, and litigation involving soil and groundwater liability.

FAQ

What is the difference between Phase I and Phase II site investigations?

Phase I is a non-intrusive assessment that reviews historical records, site conditions, and operational information to identify recognized environmental conditions and potential contamination sources. Phase II is an intrusive investigation involving soil borings, monitoring wells, sampling, and laboratory analysis to confirm and delineate actual contamination where Phase I identifies potential risks.

How deep should soil borings and sampling extend?

Borings should penetrate through all potentially affected strata, typically to the water table or the base of the unsaturated zone. Drilling may be terminated only when contamination is absent, for example when two consecutive sampling intervals return results below screening values, or when competent bedrock is encountered, consistent with HJ 25.1 and HJ 25.2.

What are the screening and intervention values in GB 36600-2018?

GB 36600-2018 establishes risk screening values and risk intervention values for two land-use categories: Category I covers sensitive uses such as residential, educational, and medical facilities, while Category II covers industrial, commercial, and logistics uses. Exceeding a screening value triggers mandatory detailed investigation and risk assessment; exceeding an intervention value generally requires risk control or remediation.

When is a quantitative human health risk assessment required?

A risk assessment following HJ 25.3 is required when contaminant concentrations exceed screening values or when land use is converted to a more sensitive category. The assessment quantifies exposure through ingestion, dermal contact, and inhalation pathways, compares carcinogenic risk and hazard quotients against acceptable thresholds, and derives site-specific remediation target values.

How many sampling points are required for a site investigation?

Sampling density depends on site area, heterogeneity, and investigation phase. Preliminary investigations commonly use systematic grids, for example 40 m by 40 m to 100 m by 100 m depending on site size, supplemented by judgmental points at suspected source areas, while detailed investigations densify the grid around confirmed contamination to fully delineate its extent.

How long does a complete site investigation and assessment take?

A Phase I assessment typically requires two to four weeks; a Phase II investigation including field work and laboratory analysis generally requires one to three months; and a quantitative risk assessment adds another two to six weeks. The overall timeline depends on site complexity, data gaps, regulatory review cycles, and the extent of confirmed contamination.