Soil total petroleum hydrocarbons (TPH) testing quantifies petroleum-derived contamination in soil and underpins environmental site investigation, risk assessment, and remediation verification. Using techniques such as GC-FID, headspace GC-MS, and infrared spectrophotometry, accredited third-party laboratories report TPH across defined carbon ranges (C6-C9, C10-C40) against regulatory benchmarks such as GB 36600-2018. This article explains the principles, sampling requirements, analytical workflow, quality control, and typical applications of TPH analysis in soil.
Technical Overview
Total petroleum hydrocarbons (TPH) is a collective term for the measurable mixture of hydrocarbons, mainly aliphatic alkanes, branched and cyclic paraffins, and aromatic compounds, derived from crude oil and refined petroleum products present in soil. Because petroleum contains hundreds to thousands of individual compounds, no single method can quantify every constituent. Instead, testing quantifies the aggregate mass of extractable hydrocarbons within defined carbon-number ranges, most commonly the volatile fraction (C6-C9) and the semi-volatile, extractable fraction (C10-C40).
Soil petroleum contamination originates from crude oil extraction and refining, leaking underground storage tanks and pipelines, fueling stations, accidental spills, industrial discharges, and historical land use. Lighter hydrocarbon fractions are highly mobile, migrate rapidly to groundwater, and drive vapor intrusion risks, while heavier fractions persist in soil, adsorb strongly to organic matter and clay particles, and frequently carry polycyclic aromatic hydrocarbons (PAHs) of significant toxicological concern.
Regulatory frameworks worldwide control TPH in soil. In China, GB 36600-2018 (Soil Environmental Quality - Risk Control Standard for Soil Contamination of Development Land) lists C10-C40 as a mandatory screening item, with soil screening values of 826 mg/kg for sensitive land use and 4,500 mg/kg for non-sensitive land use. International practice relies on ISO, EN, and ASTM standards together with US EPA reference methods, as well as risk-based tools such as TPH fractionation into aliphatic and aromatic sub-fractions grouped by equivalent carbon number.
Testing Samples
Surface and Subsurface Soil
Samples are collected from boreholes, test pits, or hand-auger locations at depths guided by the site conceptual model and contaminant distribution, typically from the ground surface down to several meters. Stratigraphic logs, field screening with photoionization detectors, and depth-discrete sampling help delineate the vertical and horizontal extent of petroleum impacts.
Industrial, Commercial, and Brownfield Soil
Soil from refineries, chemical plants, fuel depots, gas station sites, former manufactured gas plants, and derelict industrial land represents the largest share of TH testing demand, supporting due diligence, redevelopment, and regulatory closure.
Farmland and Agricultural Soil
Soil adjacent to oil production zones, irrigated with potentially affected water, or impacted by spills is tested to evaluate farmland quality, crop safety, and the need for soil restoration.
Sediment, Oily Sludge, and Solid Waste
River and lake sediments near petroleum facilities, tank-bottom sludge, drilling cuttings, and contaminated soil stockpiles are analyzed to classify waste, determine disposal or treatment routes, and verify treatment outcomes.
Testing Items
The core testing items for soil petroleum hydrocarbon evaluation include:
- Total petroleum hydrocarbons (C6-C9), the volatile gasoline-range fraction
- Total petroleum hydrocarbons (C10-C40), the diesel-to-heavy-oil range fraction regulated under GB 36600-2018
- Combined total TPH (C6-C40) reported as the sum of volatile and extractable fractions
- Fractionated TPH, splitting hydrocarbons into aliphatic and aromatic sub-fractions by equivalent carbon bands for detailed risk-based assessment
- Supporting organic analytes: BTEX (benzene, toluene, ethylbenzene, xylenes), MTBE, and the 16 priority PAHs
- Matrix parameters required for data interpretation: moisture content for dry-weight correction, pH, organic carbon content, and particle size distribution
Testing Methods
Sample Collection, Preservation, and Preparation
Sampling follows recognized procedures such as HJ/T 166-2004 for soil environmental monitoring and the closed-system approach of US EPA Method 5035 for volatile fractions. Volatile TPH samples are transferred quickly into airtight, zero-headspace vials (or methanol-preserved vials), chilled to 4 degrees C in the dark, and analyzed within 7 to 14 days. Samples for extractable TPH are held at 4 degrees C, extracted within 14 days, and extracts are analyzed within 40 days. Preparation includes homogenization, removal of debris, air- or freeze-drying for heavy hydrocarbons, sieving, and parallel moisture determination at 105 degrees C for dry-weight reporting.
Extraction and Cleanup
Extractable petroleum hydrocarbons are recovered from soil using solvent extraction with n-hexane/acetone or dichloromethane/acetone mixtures. Common extraction techniques include ultrasonic extraction, Soxhlet extraction (US EPA Method 3540C), and accelerated solvent extraction (US EPA Method 3545A) under elevated temperature and pressure. Extracts are dried with anhydrous sodium sulfate and cleaned through silica gel, Florisil, or alumina columns (US EPA Method 3630C) to remove polar and biogenic interferences; activated copper may be added to eliminate elemental sulfur.
GC-FID Determination for C10-C40
Gas chromatography with flame ionization detection is the internationally recognized reference technique for the extractable fraction, standardized in ISO 16703:2011, EN 14039:2004, and US EPA Method 8015D. Hydrocarbons are separated on a non-polar capillary column between the n-C10 and n-C40 retention-time markers, and the total integrated area, including the unresolved complex mixture hump, is quantified against external calibration standards. Results are reported in mg/kg dry weight, with typical limits of quantification in the range of 1 to 10 mg/kg.
Headspace GC-MS Determination for C6-C9
The volatile gasoline-range fraction is measured by static headspace equilibration followed by gas chromatography-mass spectrometry, following methods such as HJ 1021-2019. Soil is heated in sealed vials so that volatile hydrocarbons partition into the headspace, which is then injected onto the GC column. This approach minimizes volatilization losses during handling and achieves detection limits at the sub-0.1 mg/kg level.
Infrared Spectrophotometry
Infrared methods, including HJ 1051-2019 and approaches derived from US EPA Method 418.1, quantify total petroleum content by measuring carbon-hydrogen stretch absorbances near 2930, 2960, and 3030 cm-1 in solvent extracts. They provide rapid screening capability, although results can be affected by biogenic organics and the choice of extraction solvent, so cleanup and confirmation with chromatographic methods are recommended for critical decisions.
Quality Assurance and Quality Control
Reliable TPH data depend on a full quality system, including:
- Method blanks and solvent blanks to demonstrate freedom from contamination
- Laboratory control samples and matrix spikes with typical recovery acceptance limits of 70-130 percent
- Surrogate standards such as o-terphenyl or deuterated n-alkanes added before extraction to monitor method performance
- Field and laboratory duplicates with relative percent difference generally within 30 percent
- Continuing calibration verification, retention-time marker checks, and dry-weight correction of all results
Applications
Soil TPH testing supports decision-making throughout the entire contaminated land management cycle:
- Phase II environmental site investigations conducted in line with HJ 25.1-2019 and equivalent international practice
- Human health and ecological risk assessment under HJ 25.3-2019 and site-specific cleanup goal development
- Remediation design verification, treatment process monitoring, and site closure acceptance
- Brownfield redevelopment, land-use transfer, and environmental due diligence in mergers and acquisitions
- Operation, monitoring, and decommissioning of oil and gas facilities, tank farms, and pipelines
- Emergency spill response, extent delineation, and post-incident cleanup confirmation
- Agricultural soil safety evaluation and regulatory compliance reporting
FAQ
What is the difference between TPH and PAH testing?
TPH measures the aggregate mass of all petroleum hydrocarbons within a defined carbon range and indicates the overall level of oil contamination. PAH testing quantifies a specific subset of individual heavier aromatic compounds, many of which are carcinogenic. The two analyses are complementary: TPH defines the extent of petroleum impact, while PAH results drive the toxicological risk evaluation.
Why are C6-C9 and C10-C40 analyzed by different methods?
The volatile C6-C9 fraction evaporates readily and is easily lost during conventional drying and solvent extraction, so it must be measured from sealed vials using headspace GC-MS. The heavier C10-C40 fraction is not volatile and is best recovered by solvent extraction followed by GC-FID. Combining both results provides complete coverage of the TPH range.
What detection limits and reporting units apply?
GC-FID methods for C10-C40 typically achieve limits of quantification of about 1 to 10 mg/kg dry weight, headspace GC-MS for C6-C9 reaches sub-0.1 mg/kg levels, and infrared screening methods generally report at around 10 mg/kg. All regulatory comparisons are made on a dry-weight basis.
How should soil samples for TPH be preserved?
Samples should be collected with minimal disturbance, sealed in airtight containers with no headspace for volatile analysis, stored at 4 degrees C in the dark, and delivered to the laboratory quickly. Volatile TPH samples should be analyzed within 7 to 14 days, while extractable TPH samples should be extracted within 14 days of collection.
How are TPH results compared with regulatory limits?
Dry-weight results are compared against applicable screening values. Under GB 36600-2018, C10-C40 concentrations exceeding 826 mg/kg on sensitive-use land or 4,500 mg/kg on non-sensitive-use land trigger further detailed investigation and risk assessment, and potentially remediation under the site-specific risk control framework.
Can naturally occurring organic matter cause false positives?
Yes. Humic substances, plant oils, and other biogenic organics can co-extract and inflate apparent TPH values. Silica gel or Florisil cleanup, chromatographic separation, and review of the hydrocarbon distribution pattern by experienced analysts are used to distinguish petroleum sources from natural organic matter.