Soil Petroleum Hydrocarbon Analysis: Standards, Methods, and Applications

Soil petroleum hydrocarbon analysis is the core analytical process used to quantify total petroleum hydrocarbons (TPH) and related petroleum fractions in soil and sediment, supporting contaminated site assessment, remediation verification, and regulatory compliance under standards such as ISO 16703, EN 14039, and GB 36600. Accredited third-party laboratories combine structured field sampling protocols with gas chromatography techniques (GC-FID and GC-MS) to deliver carbon-range-resolved results for gasoline, diesel, and heavy oil contamination. This article explains the technical scope, sample types, testing items, analytical methods, quality control measures, and typical applications of petroleum hydrocarbon analysis in soil.

Technical Overview

Soil petroleum hydrocarbon analysis refers to the systematic sampling, extraction, and instrumental determination of petroleum-derived hydrocarbons in soil and sediment. Petroleum hydrocarbons are complex mixtures consisting of hundreds to thousands of individual aliphatic and aromatic compounds originating from crude oil and refined products such as gasoline, kerosene, diesel, lubricating oils, and heavy fuel oils. Because these products differ widely in volatility, water solubility, toxicity, and environmental persistence, modern laboratory practice quantifies both the total petroleum hydrocarbon (TPH) concentration and its distribution across defined carbon-number ranges and compound classes.

Common sources of soil contamination include leaking underground storage tanks (USTs) at fuel stations, pipeline ruptures, refinery and petrochemical plant operations, oilfield spills, improper disposal of waste oils, and historical industrial land use. Petroleum hydrocarbons in soil pose multiple risks: soluble fractions such as BTEX (benzene, toluene, ethylbenzene, and xylenes) can migrate into groundwater, heavier aromatic fractions such as polycyclic aromatic hydrocarbons (PAHs) persist in soil for decades and present carcinogenic risk, and volatile fractions can migrate upward as soil gas, creating vapor intrusion risks for buildings.

For analytical and regulatory purposes, petroleum hydrocarbons are conventionally divided into carbon ranges: the volatile or gasoline range (approximately C6 to C10, sometimes C5 to C12), the diesel range (approximately C10 to C28), and the oil or residual range (approximately C28 to C40). Most international soil methods, including ISO 16703:2011, EN 14039:2004, and the Chinese standard HJ 1051-2019, target the C10 to C40 range by gas chromatography with flame ionization detection (GC-FID), while the volatile C6 to C9 range is determined by headspace or purge-and-trap GC coupled with mass spectrometry or FID.

Assessment criteria vary by jurisdiction. In China, GB 36600-2018 establishes screening values for petroleum hydrocarbons (C10 to C40) of 826 mg/kg dry weight for first-class land uses such as residential areas and 4500 mg/kg for second-class land uses such as industrial and commercial sites, with corresponding control values of 5000 mg/kg and 9000 mg/kg respectively. Exceedance of a screening value typically triggers a detailed site investigation and risk assessment, while exceedance of a control value requires risk control or remediation measures.

Testing Samples

Because petroleum contamination in soil is highly heterogeneous and vertically stratified, sample representativeness is the single most decisive factor in data quality. Sampling programs should be designed according to the conceptual site model, historical land use, and suspected contaminant type, following the principles of the ISO 18400 series on soil sampling or equivalent national technical specifications.

Surface Soil Samples

Surface samples, typically collected from 0 to 20 cm or 0 to 50 cm depth, are used for initial screening, land-use compliance checks, and agricultural soil surveys. They are collected with stainless steel trowels or hand augers that are decontaminated between sampling points, transferred to pre-cleaned amber glass jars with polytetrafluoroethylene-lined caps, filled to minimize headspace, and transported under chain of custody at 0 to 6 degrees Celsius in the dark.

Subsurface Soil Samples

Subsurface investigation employs direct-push probing, hollow-stem augering, or rotary drilling with split-spoon samplers to retrieve continuous or interval cores. Samples are typically recovered every 0.5 to 1.5 meters, at lithological changes, and in targeted zones such as the smear zone near the water table where light non-aqueous phase liquids accumulate. Field screening with a photoionization detector, visual logging of staining, and odor observations guide the selection of intervals submitted for laboratory analysis.

Volatile Hydrocarbon Subsamples

For the determination of volatile petroleum hydrocarbons (C6 to C9, BTEX, and fuel additives), dedicated subsamples must be collected without homogenization. Following the closed-system approach of US EPA Method 5035A, undisturbed soil cores are transferred rapidly into small sealed vials pre-loaded with sodium bisulfate preservative or methanol, or captured in disposable syringe-type coring devices. These subsamples must never be composited or air-dried, and they are analyzed within 14 days of collection.

Sediment, Stockpiled Soil, and Drill Cuttings

Dredged sediments, remediation stockpiles, and drill cuttings from oil and gas operations are sampled using grid- or volume-based designs to support waste classification, disposal routing, and remediation mass balance calculations. For stockpiles, multiple increments are combined into composite samples when only extractable (non-volatile) hydrocarbons are targeted.

Associated Environmental Samples

Soil investigations are frequently paired with groundwater samples from monitoring wells and soil gas samples from vapor probes, allowing delineation of the dissolved plume and evaluation of vapor intrusion pathways as part of a multi-line-of-evidence site assessment.

Testing Items

Testing portfolios are tailored to the suspected petroleum product, the site conceptual model, and the applicable regulatory framework. Typical testing items include the following.

Total Petroleum Hydrocarbons (TPH)

TPH reports the aggregate concentration of extractable petroleum hydrocarbons within a defined carbon range, expressed in mg/kg dry weight. It is the primary screening parameter in virtually all soil quality frameworks and provides the basis for comparison with regulatory limits such as the GB 36600-2018 screening values.

TPH Carbon-Range Fractions

• C6 to C9 volatile petroleum hydrocarbons, indicative of gasoline, solvents, and lightly weathered light fuels.
• C10 to C40 extractable petroleum hydrocarbons, covering diesel, kerosene, lubricating oils, and heavy fuel oils.
• Aliphatic and aromatic sub-fractions obtained by silica gel column fractionation, required by fraction-based risk frameworks that assign different toxicity values to each fraction group.
• Individual n-alkane distributions (for example n-C10 through n-C40), useful for product identification and weathering assessment.

BTEX and Fuel Additives

Benzene, toluene, ethylbenzene, and o-, m-, p-xylenes are the most mobile and toxic water-soluble components of light fuels and are regulated individually in most soil standards. Fuel oxygenates such as MTBE, ETBE, and TAME are frequently included because of their high mobility and persistence in groundwater.

Polycyclic Aromatic Hydrocarbons (PAHs)

The sixteen priority PAHs from naphthalene to indeno(1,2,3-cd)pyrene and benzo(g,h,i)perylene are determined by GC-MS. Several of them, notably benzo(a)pyrene, are carcinogenic and are listed as controlled items in GB 36600-2018 and equivalent international frameworks.

Forensic and Weathering Indicators

Petroleum forensics relies on biomarker compounds such as hopanes and steranes, isoprenoid ratios such as pristane to phytane, and n-alkane depletion patterns to identify product type, distinguish multiple sources, and estimate the age and degree of weathering of a release.

Supporting Parameters

Soil moisture content, determined gravimetrically according to ASTM D2216 or ISO 11465, is measured on every sample to convert analytical results to a dry-weight basis. Depending on project objectives, supporting measurements may include pH, total organic carbon, and grain size distribution to assist data interpretation and remediation design.

Testing Methods

The analytical sequence for petroleum hydrocarbons generally consists of sample preparation, extraction, cleanup, instrumental determination, and quality-controlled data reporting. Method selection depends on the target carbon range and the regulatory context.

Sample Preparation and Preservation

Extractable-hydrocarbon samples are stored at 0 to 6 degrees Celsius and homogenized, and visible debris is removed before sub-sampling. Volatile-hydrocarbon vials are analyzed as received with their chemical preservative. All results are corrected to dry weight using the measured moisture content.

Extraction Techniques

• Solvent shaking or vortex extraction with acetone, n-hexane, or n-heptane, as specified in ISO 16703:2011 and HJ 1051-2019 for C10 to C40 hydrocarbons.
• Ultrasonic extraction, widely used for high-throughput soil screening.
• Accelerated solvent extraction at elevated temperature and pressure, consistent with US EPA Method 3545, offering low solvent consumption and high reproducibility.
• Microwave-assisted extraction according to ASTM D5765 or US EPA Method 3546 for heavily contaminated or matrix-rich samples.
• Soxhlet extraction according to US EPA Method 3540 for reference-level determination of semi-volatile hydrocarbons.
• Solvent extraction of methanol-preserved volatile subsamples followed by headspace or purge-and-trap introduction.

Cleanup and Fractionation

Raw extracts are cleaned on Florisil, silica gel, or alumina adsorption columns to remove polar biogenic organic matter such as humic and fatty acids that would otherwise inflate results. Where fraction-based criteria apply, a two-stage silica gel column separates the cleaned extract into aliphatic and aromatic fractions prior to separate GC determination.

Instrumental Determination

• TPH C10 to C40 by GC-FID according to ISO 16703:2011, EN 14039:2004, HJ 1051-2019, or US EPA Method 8015D. Quantification is performed against mineral oil or n-alkane calibration standards, integrating both the resolved peaks and the unresolved complex mixture hump between the n-C10 and n-C40 retention markers.
• Volatile petroleum hydrocarbons C6 to C9 by static headspace or purge-and-trap GC-MS or GC-FID, following HJ 1021-2019, US EPA Methods 5035A, 8260D, and 8015D, or ISO 22155 for volatile hydrocarbons in soil.
• PAHs by GC-MS in selected-ion monitoring mode according to ISO 18287 or US EPA Method 8270E.
• Historical infrared spectrophotometric methods, which measure total extractable oil at three wavelength bands, have largely been superseded by gas chromatographic methods because GC provides carbon-range resolution and better specificity against biogenic interferences.
• Advanced techniques such as comprehensive two-dimensional gas chromatography (GCxGC) and compound-specific isotope ratio mass spectrometry are applied in forensic source apportionment and litigation support.

Quality Assurance and Quality Control

Accredited third-party laboratories operate under ISO/IEC 17025 management systems and include method blanks, laboratory control samples, matrix spikes and matrix spike duplicates, surrogate recoveries (for example o-terphenyl and 5-alpha-androstane in GC-FID methods), and calibration verification standards in every batch. Field duplicates and field blanks assess sampling precision and cross-contamination. Typical laboratory quantification limits are on the order of 10 to 100 mg/kg dry weight for C10 to C40 and 0.01 to 0.1 mg/kg for volatile fractions, depending on the method and sample mass. Holding times are 14 days for volatile hydrocarbon subsamples, and extractable hydrocarbon samples are generally extracted within 14 days with extract analysis completed within 40 days.

Applications

Soil petroleum hydrocarbon analysis underpins decision-making across the entire lifecycle of contaminated land, from initial discovery through remediation closure. Representative applications include:

• Phase II environmental site assessments and brownfield redevelopment due diligence prior to land transactions or changes in land use.
• Investigation of leaking underground storage tanks at fuel stations, depots, and transportation hubs.
• Soil and groundwater quality surveys at refineries, petrochemical plants, and bulk liquid terminals.
• Emergency response and damage assessment following pipeline ruptures, tanker accidents, and oilfield spills.
• Baseline characterization and performance verification for remediation technologies such as soil vapor extraction, biopiles, thermal desorption, soil washing, chemical oxidation, and monitored natural attenuation.
• Waste classification and disposal routing for drill cuttings, contaminated spoil, and dredged sediments under hazardous waste identification regulations.
• Compliance demonstration against soil quality standards such as GB 36600-2018 during regulatory audits and site closure reviews.
• Forensic source apportionment, insurance claims, and litigation support where multiple responsible parties or releases are disputed.
• Ecological and agricultural risk assessment for farmland and sensitive receptors located near oil production and processing facilities.

FAQ

What is the difference between TPH and PAH testing?

TPH measures the aggregate concentration of all petroleum hydrocarbons within a defined carbon range and answers the question of how much petroleum is present. PAH testing quantifies sixteen specific individual aromatic compounds, several of which are carcinogenic, and answers the question of how toxic the contamination is. Most site investigations perform both, because PAHs occur in heavy fuels, tar, and creosote but represent only a small mass fraction of total petroleum.

Which carbon range should be tested, C6 to C9 or C10 to C40?

The choice follows the suspected product. Gasoline and solvent releases warrant C6 to C9 and BTEX analysis; diesel, kerosene, lubricating oil, and heavy fuel oil releases warrant C10 to C40 analysis. Where the product type is unknown, for example at mixed-use industrial sites, both ranges are recommended, and the combined result approximates total petroleum hydrocarbons from C6 to C40.

How should soil samples for volatile petroleum hydrocarbons be collected?

They must be collected as undisturbed, unpreserved-by-mixing subsamples using sealed vials containing sodium bisulfate or methanol preservative, following the closed-system technique of US EPA Method 5035A. The soil should never be homogenized, composited, or exposed to air, because volatilization losses of more than half the true concentration can occur within minutes of sample disturbance.

Can natural organic matter cause false positives in TPH results?

Yes. Biogenic compounds such as plant waxes, humic substances, and pine resins can be co-extracted and contribute to the total signal. Adsorption column cleanup on silica gel or Florisil, combined with chromatographic pattern interpretation by GC-FID, distinguishes petroleum signatures, which show characteristic n-alkane series and unresolved complex mixtures, from natural background organic matter.

How are results compared with the GB 36600-2018 limits?

Petroleum hydrocarbon (C10 to C40) results reported on a dry-weight basis are compared with the screening value applicable to the land-use class: 826 mg/kg for first-class land such as residential and public uses, and 4500 mg/kg for second-class land such as industrial and commercial uses. Results between the screening and control values trigger detailed investigation and risk assessment, while results above the control values require risk control or remediation.

How long does soil petroleum hydrocarbon analysis take?

For routine TPH and BTEX determinations, accredited third-party laboratories typically report results within five to ten working days after sample receipt. Rush services, multi-fraction characterization, and forensic fingerprinting programs require additional time due to the complexity of cleanup, fractionation, and data interpretation.

What should be considered when selecting a laboratory?

Choose an accredited third-party laboratory holding ISO/IEC 17025 accreditation covering the specific petroleum hydrocarbon methods and matrices involved, with documented method detection limits below the applicable regulatory screening values, a robust quality control program, and experience with the sampling and preservation requirements for volatile hydrocarbons.