Soil Pollution Detailed Investigation Testing: Standards, Sampling, and Analytical Methods

Soil pollution detailed investigation testing is the second-phase site assessment mandated under China's Soil Pollution Prevention and Control Law whenever a preliminary survey indicates that contaminant concentrations may exceed the screening values of GB 36600-2018 or GB 15618-2018. Performed by accredited third-party laboratories in accordance with HJ 25.1-2019 and related technical specifications, the detailed survey precisely delineates the type, concentration, and three-dimensional extent of contamination to support risk assessment and remediation design. This article explains the sampling design, target analytes, analytical methods, regulatory applications, and frequently asked questions for detailed soil investigation programs.

Technical Overview

A detailed soil pollution investigation (detailed survey) is conducted when a preliminary site investigation identifies potential exceedances of soil screening values or significant data gaps remain. Its purpose is not simply to detect contamination, but to quantify it with sufficient spatial resolution to delimit pollution boundaries both horizontally and vertically. The results feed directly into human health and ecological risk assessment under HJ 25.3-2019 and into remediation feasibility design.

The legal framework derives from the Soil Pollution Prevention and Control Law of the People's Republic of China (effective January 1, 2019), the Measures for the Soil Environmental Management of Contaminated Land, and the technical guideline series HJ 25.1 through HJ 25.5. A typical detailed investigation project follows a defined workflow:

• Review of preliminary investigation data and refinement of the conceptual site model, including historical process areas, storage zones, drainage pathways, and underground utilities.
• Preparation of a sampling and analysis work plan, with grid-based point locations and stratified vertical sampling design.
• Field execution, including borehole drilling, lithological logging, on-site screening with portable X-ray fluorescence (XRF) and photoionization detectors (PID), and sample collection under chain-of-custody procedures.
• Laboratory analysis of soil, groundwater, and soil gas samples by CMA-accredited and ISO/IEC 17025-accredited laboratories.
• Statistical evaluation against GB 36600-2018 or GB 15618-2018 criteria, delineation of contamination plumes, and preparation of the detailed investigation report for upload to the national soil environment information platform.

Testing Samples

Surface Soil Samples

Surface samples on construction land are generally collected from the 0–0.5 m interval, where wind-blown deposition, spills, and historical surface handling leave the highest contaminant loads. For agricultural land surveys under GB 15618-2018, the cultivated layer (0–20 cm) and subsoil layer (20–40 cm) are sampled separately. In the detailed phase, discrete grab samples rather than multi-point composites are required, because averaging can mask localized hotspots that determine remediation boundaries.

Subsurface and Deep Soil Samples

Boreholes are advanced through suspect source zones, with discrete samples collected from each distinct lithological stratum and at any visual or olfactory evidence of contamination, sheen, or PID anomaly. Sampling density is typically highest in the vadose zone (intervals of roughly 0.5–1.0 m) and near suspected source depths, decreasing with depth. Drilling continues until uncontaminated strata are confirmed—commonly two consecutive clean horizons—or until bedrock or the saturated zone is reached, ensuring the full vertical extent of impact is captured.

Groundwater Samples

Detailed investigations install temporary or permanent monitoring wells, generally including at least one up-gradient background well and multiple down-gradient wells positioned through suspected source areas. Well screens straddle the water table where light non-aqueous phase behavior is a concern. Low-flow purging with field measurement of pH, conductivity, oxidation-reduction potential, dissolved oxygen, and turbidity is standard practice following HJ 164-2020, and samples are preserved and shipped under cold chain at approximately 4 °C.

Soil Gas and Sub-Slab Vapor Samples

Where volatile organic compounds are present, active soil gas probes and sub-slab vapor points are installed to evaluate the vapor intrusion pathway, which frequently governs human health risk for future residential or commercial occupants. Flow-controlled sampling into certified summa canisters or sorbent tubes, with leak tracers, ensures representative vapor data.

Sediment and Ancillary Samples

For sites bordering watercourses, drainage ditches, or former lagoons, sediment samples are collected to track off-site migration. Residual sludges, tank bottoms, and industrial solid wastes encountered during drilling may also be characterized to classify and manage them as hazardous or non-hazardous waste.

Testing Items

Analyte selection combines the mandatory indicator lists of the national standards with site-specific parameters derived from historical processes, raw materials, and waste streams. Typical categories include:

• Construction land basic items (45 indicators) per GB 36600-2018 Table 1: heavy metals and metalloids including arsenic, cadmium, hexavalent chromium, copper, lead, mercury, and nickel, together with volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs) such as benzene series, chlorinated solvents, nitrobenzene, aniline, chlorophenols, and benzo[a]pyrene.
• Construction land other items (40 indicators) per GB 36600-2018 Table 2: organochlorine and organophosphorus pesticides, polychlorinated biphenyls (PCBs), petroleum hydrocarbons (C10–C40), and dioxins/furans.
• Agricultural land items per GB 15618-2018: the eight basic indicators of cadmium, mercury, arsenic, lead, chromium, copper, nickel, and zinc, plus optional items such as total hexachlorocyclohexane (BHC), total DDT, and pH.
• Site-specific contaminants: total petroleum hydrocarbon fractions, polycyclic aromatic hydrocarbons (PAHs), cyanide, fluoride, sulfide, phenols, and process-related solvents identified from the site history.
• Groundwater parameters mirroring the soil analyte list, supplemented by routine hydrochemical indicators relevant to fate and transport assessment.

Testing Methods

Sampling and Field Screening

Field work follows HJ 25.1-2019 for investigation procedures, HJ 25.2-2019 for monitoring program design, HJ/T 166-2004 for soil sampling technique, and ISO 10381 series principles for sampling quality. In the detailed phase, soil sampling points within confirmed or strongly suspected contamination areas are typically arranged on a 20 m × 20 m grid, compared with 40 m × 40 m commonly applied in the preliminary phase, with infill points added to close concentration boundaries. Real-time XRF screening for metals and PID screening for volatiles guide depth-interval selection and sample prioritization.

Laboratory Analytical Methods

Nationally standardized methods are applied as follows:

• pH: HJ 962-2018, potentiometric determination.
• Mercury, arsenic, selenium, bismuth, antimony: HJ 680-2013, microwave digestion followed by atomic fluorescence spectrometry.
• Multi-element metals: HJ 803-2016, aqua regia extraction with inductively coupled plasma mass spectrometry (ICP-MS); lead and cadmium alternatively by GB/T 17141-1997 graphite furnace atomic absorption spectrometry.
• Hexavalent chromium: HJ 1082-2019, alkaline extraction followed by flame atomic absorption spectrophotometry.
• VOCs in soil: HJ 605-2011, purge-and-trap gas chromatography–mass spectrometry (GC-MS).
• SVOCs in soil: HJ 834-2017, GC-MS; PAHs by GC-MS or high performance liquid chromatography.
• Organochlorine pesticides: HJ 921-2017, GC-MS; PCBs: HJ 743-2015, GC-MS.
• Petroleum hydrocarbons (C10–C40): HJ 1021-2019, gas chromatography with flame ionization detection.
• Dioxins and furans: HJ 77.4-2008, isotope dilution high-resolution GC/HRMS.
• Groundwater VOCs: HJ 639-2012; groundwater metals: HJ 700-2014, ICP-MS.

Quality Assurance and Quality Control

All analyses are performed in CMA-accredited and ISO/IEC 17025-accredited laboratories. Quality systems incorporate trip, field, and method blanks; field duplicate samples at typically no less than 5–10% frequency; laboratory parallel determinations; matrix spikes and surrogate recoveries; certified reference materials; and continuing calibration verification. Reporting limits must be set well below the applicable GB 36600-2018 or GB 15618-2018 screening values so that non-detects are statistically meaningful. Holding times are strictly enforced—for example, unpreserved VOC vials are analyzed within short defined windows, and methanol preservation is applied for high-concentration samples under HJ 605-2011.

Applications

Detailed investigation testing serves a wide range of regulatory and commercial scenarios:

• Change of land use to sensitive functions such as residential development, schools, kindergartens, medical facilities, or elderly care, where a soil pollution investigation is legally required before planning approval.
• Transfer, rezoning, or cessation of production at land used by key supervised industries, including non-ferrous metal smelting, petroleum processing, chemical manufacturing, coking, electroplating, and tanning.
• Confirmation and delineation of contamination where preliminary survey results exceed GB 36600-2018 screening values, providing the data foundation for risk assessment under HJ 25.3-2019 and remediation design.
• Support for the national soil pollution detailed survey program, including categorization of agricultural plots into priority protection, safe utilization, and strict control classes under GB 15618-2018.
• Brownfield redevelopment, mergers and acquisitions due diligence, environmental compliance audits, and lender-required contamination liability assessments.
• Post-incident assessment following spills, leaks, or unauthorized dumping events requiring rapid plume delineation.

FAQ

What is the difference between a preliminary investigation and a detailed investigation?

The preliminary investigation is a first-pass assessment using coarser grids to determine whether contamination may exist and which analytes are relevant. The detailed investigation is triggered when preliminary soil or groundwater results exceed screening values, and it uses denser sampling (typically 20 m × 20 m grids) and stratified vertical profiling to precisely define the horizontal and vertical boundaries of each contaminant plume.

How deep must sampling extend?

Boreholes advance until uncontaminated strata are demonstrated—commonly two consecutive clean sampling intervals—or until bedrock or groundwater is encountered. Depth requirements are site-specific and driven by the preliminary findings, geology, and the depth of the water table, since contaminants such as chlorinated solvents can migrate well below surface sources.

Which criteria are used to judge exceedance?

For construction land, results are compared against the screening values and control values in GB 36600-2018, applying the first-class (sensitive use) or second-class (industrial use) thresholds according to the planned land function. Agricultural land is evaluated against GB 15618-2018 risk screening and intervention values. Exceedance of control or intervention values obligates entry into formal risk assessment and possible remediation.

What qualifications should the testing provider hold?

Analyses must be carried out by accredited third-party laboratories holding CMA accreditation for the specific soil and groundwater methods, ideally with ISO/IEC 17025 laboratory accreditation. The investigation report must be prepared by a professional institution and filed with the national soil environment information platform to be valid for subsequent regulatory processes.

How long does a detailed investigation take?

Field work typically requires one to three weeks depending on the number of points, borehole depths, and access constraints. Laboratory analysis generally requires 15 to 30 working days, with dioxin and full SVOC suites at the longer end. Including work plan approval, data evaluation, and report review, a complete detailed investigation commonly spans six to ten weeks.

What happens if contamination is confirmed?

The site is entered into the national contaminated land registry, and a risk assessment under HJ 25.3-2019 must be completed to derive site-specific remediation targets. Depending on the outcome, the responsible party implements risk control measures or remediation under HJ 25.4-2019, followed by effect evaluation under HJ 25.5-2018 before the land can be redeveloped.