Soil Pollution Source Testing: Standards, Analytical Methods, and Source Apportionment

Soil pollution source testing is a systematic process of field sampling, laboratory analysis, and source apportionment used to determine which contaminants are present in soil, at what concentrations, and where they originate. Accredited third-party laboratories conduct this testing in accordance with national and international standards such as GB 36600-2018, GB 15618-2018, ISO 10381, and ASTM environmental site assessment guidance. This article explains the sampling procedures, testing items, analytical methods, quality controls, and practical applications of soil contamination source identification.

Technical Overview

Soil pollution source testing, also known as soil contamination source identification or source apportionment, combines field investigation, regulated sampling, and instrumental analysis to characterize pollutants in soil and trace them back to their origin. Common pollution sources include industrial discharges, mining and smelting operations, agricultural inputs such as fertilizers, pesticides and sewage sludge, waste disposal sites, fuel spills, and atmospheric deposition from traffic and combustion processes.

The testing process is designed to answer three fundamental questions: which contaminants are present and at what concentrations; how the contamination is distributed horizontally and vertically across the site; and which source profile, whether industrial, agricultural, or natural geogenic background, best explains the observed contamination pattern. Results are benchmarked against risk screening and intervention values defined in standards such as GB 36600-2018 for development land, GB 15618-2018 for agricultural land, and internationally recognized ISO and ASTM soil quality guidance.

• Contaminant identification and quantification against regulatory screening values
• Vertical and horizontal delineation of contamination plumes
• Isotopic and statistical source apportionment for liability determination
• Background reference sampling to distinguish geogenic from anthropogenic inputs

Testing Samples

Representative sampling is the single most important determinant of data quality in source identification. Sampling plans are established according to ISO 10381 and ISO 18400 series guidance, HJ/T 166-2004 for soil environmental monitoring, and HJ 25.1-2019 and HJ 25.2-2019 for contaminated site investigation and monitoring. A complete sampling program typically includes the following sample categories.

Surface Soil Samples

Surface samples are generally collected from 0 to 20 cm for agricultural land or 0 to 50 cm for development land using grid-based, systematic random, or judgmental layouts. Surface soil captures contamination from atmospheric deposition, spills, runoff accumulation, and agricultural inputs, and it is the primary matrix for initial screening against GB 36600-2018 and GB 15618-2018 limit values.

Stratified Profile and Borehole Samples

Subsurface samples are collected at defined depth intervals, typically 0.5 m, 1 m, 2 m, and 3 to 6 m, using drill rigs or hand augers. Vertical profiling determines whether contaminants have migrated downward toward groundwater, defines the base of the contamination plume, and provides the three-dimensional geometry needed for source tracing and remediation design.

Suspected Source and Background Reference Samples

Source tracking requires parallel characterization of suspected inputs, including industrial wastewater, sludge, fly ash, waste residues, fuels, paints, and sediments from adjacent drainage channels. Undisturbed background soils located up-gradient and beyond the influence of the suspected source are collected concurrently to establish natural baseline concentrations for comparison.

Testing Items

The testing scope is selected based on land use, site history, and the suspected pollution source. For development land, the basic program under GB 36600-2018 covers 45 basic items comprising heavy metals, volatile organic compounds, and semi-volatile organic compounds, with an additional 40 optional items. For agricultural land, GB 15618-2018 specifies eight basic screening items.

Physical and Chemical Properties

• pH and electrical conductivity (ISO 10390)
• Organic matter content and total organic carbon
• Cation exchange capacity and soil texture classification
• Moisture content, redox potential, and soluble salts

Heavy Metals and Inorganic Pollutants

• Basic regulated metals for development land: arsenic, cadmium, hexavalent chromium, copper, mercury, nickel, and lead
• Basic regulated metals for agricultural land: cadmium, mercury, arsenic, lead, chromium, copper, nickel, and zinc
• Extended inorganic list: antimony, beryllium, cobalt, manganese, thallium, vanadium, total chromium, selenium, and iron
• Cyanide, fluoride, and sulfide where relevant to industrial sources

Volatile and Semi-Volatile Organic Compounds

• Volatile organic compounds including benzene, toluene, ethylbenzene, xylenes, chlorinated ethenes, chloroethanes, chloroform, and carbon tetrachloride
• Semi-volatile organic compounds including nitrobenzene, aniline, phenol derivatives, pentachlorophenol, phthalate esters, naphthalene, and benzo(a)pyrene

Pesticides, POPs, and Petroleum Hydrocarbons

• Organochlorine pesticides such as DDT and hexachlorocyclohexane isomers
• Organophosphorus and pyrethroid pesticide residues
• Polychlorinated biphenyls and dioxins and furans expressed as toxicity equivalents
• Total petroleum hydrocarbons in the C10 to C40 range, BTEX, and fuel oxygenates
• Emerging contaminants including per- and polyfluoroalkyl substances and pharmaceutical residues

Testing Methods

Sampling Protocols and Field Quality Assurance

Initial investigation grids commonly follow HJ 25.1-2019, with 40 m by 40 m grids for larger sites and 20 m by 20 m grids for smaller or highly suspect areas. Volatile organic compound samples are collected with minimal disturbance directly into pre-preserved vials, never homogenized, and transported at 4 °C in the dark. Field quality controls include duplicate samples, equipment rinsate blanks, transport blanks, portable X-ray fluorescence screening consistent with ASTM D8064, and documented chain of custody.

Sample Preparation

For inorganic analyses, samples are air-dried, crushed, and sieved in accordance with ISO 11464. For organic analyses, samples are freeze-dried and sieved as required by the specific method. Volatile organic compound subsamples remain sealed and unopened until analysis to prevent loss of target analytes.

Inorganic and Heavy Metal Analysis

• Aqua regia digestion per ISO 11466 followed by ICP-MS (HJ 803) or ICP-OES (HJ 781)
• Total element determination by hydrofluoric acid digestion per ISO 14869-1
• Mercury, arsenic, selenium, and antimony by atomic fluorescence spectrometry (HJ 680, GB/T 22105 series)
• Lead and cadmium by graphite furnace atomic absorption (GB/T 17141) and copper, zinc, and nickel by flame atomic absorption (GB/T 17138, GB/T 17139)
• Hexavalent chromium speciation by alkaline extraction (HJ 1082)
• Sequential extraction using harmonized BCR or Tessier schemes to assess metal mobility and bioavailability

Organic Compound Analysis

• VOCs by purge-and-trap or headspace GC-MS (HJ 605, HJ 742)
• SVOCs by accelerated solvent, Soxhlet, or ultrasonic extraction followed by GC-MS (HJ 834)
• Polycyclic aromatic hydrocarbons by GC-MS (HJ 805) or HPLC with fluorescence detection (HJ 784)
• Organochlorine pesticides by GC-MS or GC-ECD (GB/T 14550, HJ 835) and pyrethroids by GC-MS (HJ 960)
• Polychlorinated biphenyls by GC-MS (HJ 743) and dioxins and furans by high-resolution GC-MS (HJ 77.4)
• Total petroleum hydrocarbons C10-C40 by GC-FID (HJ 1021)

Source Apportionment Techniques

• Lead isotope ratio fingerprinting (206Pb/207Pb, 208Pb/206Pb) and cadmium isotope analysis to distinguish industrial, gasoline-derived, and geogenic lead
• Compound-specific isotope analysis to differentiate co-mingled fuel and solvent plumes
• Receptor modeling using Positive Matrix Factorization and chemical mass balance
• Multivariate statistics including principal component analysis, cluster analysis, and enrichment factor calculations
• Geostatistical kriging contour maps that relate plume geometry to candidate source locations
• Direct fingerprint matching against source materials such as sludge, ash, effluent, and background soils

Data Validation and Laboratory Quality Control

All analyses are performed under ISO/IEC 17025 management systems. Quality controls include method blanks, laboratory control samples, matrix spikes with recovery targets typically of 70 to 130 percent, calibration verification standards, certified reference materials, and duplicate samples with relative percent difference generally below 30 percent. Method detection limits are required to be well below the applicable regulatory screening values so that compliance decisions are statistically defensible.

Applications

• Phase I and Phase II environmental site assessments prior to brownfield redevelopment and land use change, consistent with ASTM E1527 and ASTM E1903 practice
• Risk screening and detailed risk assessment of development land under GB 36600-2018 and HJ 25.3-2019
• Agricultural soil quality classification and safe food production surveys under GB 15618-2018
• Mining, smelting, and metal processing district investigations with multi-element and isotopic fingerprinting
• Gas station, refinery, and fuel depot site assessments for petroleum hydrocarbons, BTEX, and oxygenates
• Landfill and waste disposal impact studies on surrounding soil and groundwater
• Pollution incident response, forensic investigation, and litigation support for apportioning liability among responsible parties
• Verification and validation of remediation effectiveness against cleanup target values

FAQ

Which standards govern soil pollution source testing?

The core framework consists of GB 36600-2018 for development land, GB 15618-2018 for agricultural land, the HJ 25.1, HJ 25.2, and HJ 25.3 technical series for site investigation, monitoring, and risk assessment, HJ/T 166-2004 for soil monitoring, and international sampling guidance from the ISO 10381 and ISO 18400 series. Individual analytes are quantified using the corresponding GB, HJ, ISO, or ASTM standard methods listed above.

How is the actual pollution source identified?

Source identification combines several independent lines of evidence: the geometry and depth profile of the contamination plume, comparison of contaminant fingerprints and concentration ratios against background values, isotope signatures of elements such as lead or cadmium, statistical receptor models such as Positive Matrix Factorization, and direct chemical matching between soil contamination and suspected source materials. Convergence of multiple evidence streams provides defensible source attribution.

What is the difference between screening values and intervention values?

Screening values define contaminant concentrations below which health risks are generally considered acceptable without further evaluation. Exceedance of a screening value does not automatically require remediation; instead, it triggers a detailed risk assessment under HJ 25.3-2019, which may conclude with site-specific intervention or cleanup target values.

Why is both total and speciated metal analysis required?

Regulatory limits such as those in GB 36600-2018 are set specifically for hexavalent chromium because of its high toxicity and mobility, whereas trivalent chromium is far less hazardous. Similarly, sequential extraction distinguishes metals bound in stable mineral phases from those in exchangeable and reducible fractions, which controls actual ecological risk and remediation strategy.

What are typical sample holding times and conditions?

Soil samples for volatile organic compounds must generally be analyzed within 7 to 14 days when preserved and stored at 4 °C in the dark. Mercury analyses are typically completed within 28 days, other metals within 180 days, and extractable organics are extracted within 7 to 14 days with analysis within 40 days of extraction.

How many sampling points are needed?

The required density depends on site size, heterogeneity, and investigation stage. Preliminary investigations commonly use 20 m by 20 m or 40 m by 40 m grids per HJ 25.1-2019, with denser judgmental spacing around suspected source areas, storage tanks, drain outlets, and spill locations. Detailed investigation and remediation verification stages require progressively denser grids to close data gaps.