Soil pollution investigation and testing is a systematic process of sampling, chemical analysis, and risk evaluation used to identify and quantify contaminants in soil for environmental compliance, land redevelopment, and remediation planning. Accredited third-party laboratories perform these investigations in accordance with national and international standards such as GB 36600, HJ 25.1, ISO 18400, and ASTM D6221 to deliver defensible, high-quality data. This article explains the technical scope, testing parameters, analytical methods, and typical applications of professional soil contamination surveys.
Technical Overview
Soil pollution investigation and testing refers to the structured assessment of soil quality at a given site to determine whether hazardous substances are present at concentrations that may pose risks to human health or the environment. A typical investigation follows a phased approach: preliminary site assessment based on historical land-use records, site reconnaissance, intrusive sampling, laboratory analysis, and comparison of results against regulatory screening values.
In regulatory frameworks such as China's Soil Pollution Prevention and Control Law and comparable international programs, land designated for redevelopment, industrial closure, or agricultural conversion must undergo formal contamination investigation. The core technical workflow involves designing a sampling grid, collecting surface and sub-surface soil samples (often complemented by groundwater and soil gas samples), preserving samples under controlled conditions, and analyzing them for target contaminants using validated instrumental methods.
The investigation typically concludes with a site characterization report that maps contaminant distribution, identifies pollution sources and pathways, and supports a human health and ecological risk assessment. Results determine whether the site is suitable for its intended use or whether remediation is required.
Testing Samples
Surface Soil Samples
Surface soil (typically 0 to 20 cm or 0 to 50 cm depth) is collected using stainless steel trowels, hand augers, or core samplers. Surface samples reflect recent deposition, spill residues, atmospheric deposition, and agricultural inputs such as fertilizers and pesticides. At each sampling point, approximately 500 g to 1 kg of soil is collected, with sub-samples often combined to form a composite representative of the sampling unit.
Subsurface Soil Samples
Deeper soil layers are accessed through drilled boreholes, direct-push rigs, or excavated test pits. Subsurface profiling is essential for identifying vertical migration of dense non-aqueous phase liquids, leached metals, and legacy contamination buried beneath fill material. Samples are usually recovered at regular depth intervals (for example, every 1 to 2 meters) and at lithological transitions identified by field logging.
Soil Gas and Pore Air Samples
For sites with suspected volatile organic compound (VOC) contamination, soil gas is collected using temporary drive points or permanent vapor probes. Soil gas data support vapor intrusion assessments, particularly for residential and sensitive land-use scenarios.
Companion Groundwater Samples
Monitoring wells installed within investigation boreholes allow collection of groundwater samples. Because soil contamination frequently migrates to groundwater, combined soil and groundwater datasets provide a complete picture of contaminant fate and transport at a site.
Testing Items
The parameter list depends on land use, historical activities, and applicable screening standards. Under GB 36600-2018, the screening of construction land requires a mandatory base set of 45 parameters, with additional items triggered by site history. Common testing categories include:
- Heavy metals and metalloids: arsenic (As), cadmium (Cd), chromium (Cr, including hexavalent chromium Cr(VI)), copper (Cu), lead (Pb), mercury (Hg), nickel (Ni), zinc (Zn), antimony (Sb), beryllium (Be), cobalt (Co), selenium (Se), thallium (Tl), manganese (Mn)
- Volatile organic compounds (VOCs): benzene, toluene, ethylbenzene, xylenes (BTEX), chlorinated solvents such as trichloroethylene, tetrachloroethylene, and chloroform, as well as the full target list of EPA Method 8260-type analytes
- Semi-volatile organic compounds (SVOCs): phenols, nitroaromatics, anilines, phthalate esters, and polycyclic aromatic hydrocarbons (PAHs) including benzo(a)pyrene and naphthalene
- Persistent organic pollutants: polychlorinated biphenyls (PCBs), organochlorine pesticides such as DDT, HCH, aldrin, and dieldrin, as well as emerging contaminants like per- and polyfluoroalkyl substances (PFAS)
- Petroleum hydrocarbons: total petroleum hydrocarbon (TPH) fractions, mineral oil, and oil-range organics relevant to fuel stations, refineries, and chemical plants
- Inorganic and non-metal parameters: pH, cyanide, fluoride, ammonia nitrogen, nitrate, nitrite, sulfate, chloride
- Soil physical and chemical properties: organic matter content, cation exchange capacity (CEC), soil texture and particle size distribution, bulk density, moisture content, redox potential
- Ecological and agricultural indicators: available phosphorus, available potassium, total nitrogen, exchangeable bases, and microbial biomass where fertility assessment is required
Testing Methods
Sampling Design and Field Protocols
Sampling layouts follow systematic, stratified, or judgmental designs. Grid-based systematic sampling with a specified point density (for example, one point per 1,600 square meters for initial screening of large parcels) is standard for preliminary investigations. Sampling operations comply with HJ/T 166, HJ 25.1 and HJ 25.2, and the ISO 18400 series. QA/QC measures in the field include equipment decontamination, duplicate samples, trip blanks, and chain-of-custody documentation.
Sample Preparation and Preservation
Laboratory preparation involves air-drying or freeze-drying (for volatile-sensitive analytes), removal of debris, crushing, and sieving (commonly 2 mm or 0.25 mm fraction). VOC samples are collected with minimal disturbance into pre-weighed vials with preservatives and kept at 4 degrees Celsius. Holding times specified in the applicable standards are strictly observed.
Heavy Metal Analysis
- Arsenic and mercury: atomic fluorescence spectrometry (AFS) or ICP-MS following aqua regia or water bath digestion
- Multi-element metals (Cu, Pb, Ni, Cd, Cr, Zn): inductively coupled plasma mass spectrometry (ICP-MS) or inductively coupled plasma optical emission spectrometry (ICP-OES) after acid digestion per EPA 3050B or EPA 6010/6020-type procedures
- Hexavalent chromium: alkaline digestion followed by colorimetric determination with diphenylcarbazide (EPA 3060A / 7196A-type methods)
- Rapid on-site screening: portable X-ray fluorescence (XRF) spectrometry for real-time metal screening and sampling optimization
Organic Contaminant Analysis
- VOCs: purge-and-trap or headspace gas chromatography-mass spectrometry (GC-MS) following EPA 8260-type methodology
- SVOCs and PAHs: solvent extraction (ultrasonic or accelerated solvent extraction) followed by GC-MS or high-performance liquid chromatography (HPLC) with fluorescence detection
- Organochlorine pesticides and PCBs: solid-liquid extraction with florisil or gel permeation cleanup, followed by GC-MS or GC-ECD
- Petroleum hydrocarbons: gravimetric infrared methods or GC-based fractionated TPH analysis
Data Evaluation and Risk Screening
Analytical results are compared against soil screening values in GB 36600-2018 for construction land (with different criteria for first-class sensitive and second-class non-sensitive land uses) or GB 15618-2018 for agricultural land. Where exceedances occur, further detailed investigation, human health risk assessment models (such as the RBCA or Csoil-type frameworks adapted in local guidelines), and remediation target calculations are performed. All laboratory data must pass QC criteria including method blanks, laboratory control samples, matrix spikes, and calibration verification.
Applications
- Redevelopment of former industrial sites: chemical plants, steel mills, electroplating facilities, tanneries, pesticide manufacturers, and coking plants requiring soil investigation before land transfer or construction
- Fuel stations and oil depots: TPH and BTEX assessment for storage tank closure and site restoration
- Agricultural land quality surveys: heavy metal and pesticide residue monitoring for food safety and soil fertility management
- Mining and smelting areas: identification of arsenic, lead, cadmium, and thallium contamination footprints and ecological risk evaluation
- Landfill and waste disposal sites: leachate impact assessment for surrounding soils and groundwater
- Environmental liability and litigation support: baseline environmental assessments during mergers, acquisitions, and property transactions
- Remediation validation: post-remediation sampling to confirm that cleanup targets have been achieved and sites are fit for intended use
- Emergency response: rapid characterization of soils following spills, fires, or unauthorized dumping incidents
FAQ
What standards govern soil pollution investigation in China?
The principal technical documents include GB 36600-2018 for construction land screening, GB 15618-2018 for agricultural land, HJ 25.1-2019 for site investigation techniques, HJ 25.2-2019 for monitoring, HJ 25.3 for risk assessment, and HJ/T 166-2004 for general soil environmental monitoring specifications. International projects frequently reference the ISO 18400 series and ASTM environmental site assessment standards.
How deep should soil samples be collected?
Sampling depth depends on the contamination scenario and proposed land use. Typical investigations sample surface soil (0 to 0.5 m) and sub-surface intervals down to several meters, extending to the water table or to bedrock where dense contaminants or landfill impacts are suspected. Boreholes are advanced until clean soil is confirmed over a defined interval or the conceptual site model is satisfied.
How many sampling points are required?
Point density is defined by the investigation phase and site size. Preliminary grid screening commonly applies one point per 400 to 1,600 square meters depending on land-use sensitivity, while detailed investigation grids are intensified around identified exceedances. Regulatory guidelines specify minimum requirements for each phase.
How long does a soil investigation take?
A typical program spans two to six weeks: one to two weeks for desktop review and sampling plan preparation, one to two weeks for fieldwork, and one to two weeks for laboratory analysis and reporting, depending on site complexity, parameter list, and analytical workload.
What happens if contaminant levels exceed screening values?
Exceedances trigger a detailed investigation with denser sampling to delineate the lateral and vertical extent of contamination, followed by a human health and ecological risk assessment. Based on calculated site-specific remediation targets, a remediation plan is developed, implemented, and verified through confirmation sampling before the site is released for its intended use.
Why should testing be performed by an accredited laboratory?
Accredited laboratories operate under ISO/IEC 17025 quality management systems, use validated methods with documented detection limits, and participate in proficiency testing schemes. Regulatory authorities, lenders, and buyers typically accept only data reports bearing CMA or equivalent accreditation marks, ensuring legal defensibility of the investigation results.