API Publ 4637-1996: Methods for the Selective Analysis of Petroleum Hydrocarbons in Groundwater and Soils

Scope and Application

API Publication 4637-1996, titled Selective Analysis of Petroleum Hydrocarbons in Groundwater and Soils: A Literature Review and Case Study, provides a comprehensive framework for the targeted characterization of petroleum-derived hydrocarbons in environmental media. Developed by the American Petroleum Institute (API), this publication addresses the critical need to distinguish between biogenic and anthropogenic hydrocarbon sources while enabling the quantitative measurement of specific hydrocarbon fractions such as gasoline-range organics (GRO), diesel-range organics (DRO), and heavier residual fractions.

The document is intended for environmental consultants, regulatory agencies, petroleum facility operators, and analytical laboratories engaged in site assessment, remediation monitoring, and natural resource damage evaluations. Its primary scope includes:

Literature review of existing analytical approaches for petroleum hydrocarbon fractionation and source identification.
A case study demonstrating fraction-specific analysis at a petroleum release site.
Recommendations for method selection, sample handling, and data interpretation to improve consistency across investigations.

Key Benefit: The selective analysis approach described in API Publ 4637-1996 reduces false positives from naturally occurring organic matter, enabling more accurate risk assessments and cost-effective remediation decisions.

Importantly, this publication does not prescribe a single mandatory method but instead provides a rigorous framework for adapting existing analytical techniques (e.g., EPA Methods 8015, 8260, 8270) to achieve hydrocarbon-class-specific data. It emphasizes the need to align analytical objectives with the physical-chemical properties of the contaminant mixture—especially boiling point ranges, carbon number distributions, and solubility characteristics.

Technical Requirements and Analytical Methods

2.1 Fractionation Approach

The core of API Publ 4637-1996 is the selective fractionation of petroleum hydrocarbons into three operational ranges:

Gasoline Range Organics (GRO): Typically defined as hydrocarbons with boiling points between C₅ and C₁₀ (approximately 35–210 °C). GRO includes alkylbenzenes and light aliphatic compounds.
Diesel Range Organics (DRO): Hydrocarbons boiling in the C₁₀ to C₂₈ range (about 210–430 °C), representing diesel fuel, fuel oils, and light lubricating oils.
Residual Range Organics (RRO): Higher boiling fractions (C₂₈+), encompassing heavy oils, greases, and weathered residues.

Caution: API Publ 4637-1996 emphasizes that operational definitions (e.g., carbon range cutoffs) must be explicitly stated in project documentation to avoid misinterpretation when comparing data across studies or laboratories.

2.2 Recommended Analytical Techniques

The publication evaluates several laboratory methods and provides guidance on their application for selective hydrocarbon analysis. The following table summarizes the principal methods discussed:

**Table 1 – Methods Evaluated in API Publ 4637-1996 for Selective Hydrocarbon Analysis**
Method	Target Fraction	Principle	Typical Detection Limit (μg/L water)	Notes
EPA 8015 (GC-FID)	GRO, DRO	Gas chromatography with flame ionization detection; direct injection for water, purge-and-trap for GRO	50–200	Most widely used for non-halogenated hydrocarbons
EPA 8260 (GC-MS)	GRO (individual VOCs)	Purge-and-trap gas chromatography/mass spectrometry	0.5–5	Provides speciation for BTEX and other volatile organic compounds
EPA 8270 (GC-MS)	DRO, RRO (semi-volatiles)	Extraction with CH₂Cl₂ followed by GC-MS	10–50	Suitable for PAHs and heavier aliphatics
Silica Gel Cleanup (EPA 3630)	All fractions	Removal of polar non-petroleum organics to minimize interference	N/A (preparation step)	Essential for selective analysis in soils with high natural organic matter
Carbon Range Analysis (CRA)	GRO, DRO, RRO	GC-FID with calibration based on carbon number windows	100–500 (depending on window)	Recommended by API Publ 4637 for refined product typing

2.3 Quality Control and Acceptance Criteria

Rigorous quality control is central to achieving reliable selective analysis. API Publ 4637-1996 specifies:

Surrogate recovery: Use of at least two non-interfering surrogate compounds (e.g., o-terphenyl for semi-volatiles, 4-bromofluorobenzene for volatiles) with acceptance windows of 70–130%.
Matrix spike/matrix spike duplicates (MS/MSD): Required at a frequency of 5% or one per 20 samples, whichever is greater. Relative percent difference (RPD) should be ≤30%.
Laboratory control samples (LCS): Analyzed with each batch to verify method performance. Criteria are method-specific but typically 80–120% recovery.
Method blank: Must contain no target analyte above one-half the reporting limit.
Silica gel treatment: Mandatory for all soil and water extracts suspected of containing biogenic interference. Post-treatment reduction of total hydrocarbon concentration by more than 50% indicates significant false-positive potential and should be documented.

Practical Tip: When using silica gel cleanup for water samples, adjust the elution solvent composition (typically pentane or hexane) to ensure complete recovery of aliphatic hydrocarbons while retaining polar interferences. Document all procedural modifications in the case narrative.

Implementation and Quality Control

3.1 Sampling Considerations

API Publ 4637-1996 stresses that selective analysis begins in the field. Hydrocarbon fractions are volatile and susceptible to loss during collection, transport, and storage. Key recommendations include:

Groundwater: Use dedicated, low-flow sampling techniques to minimize volatilization. Fill sample vials completely with zero headspace. Preserve with HCl (pH < 2) for volatile analysis and cool to 4 °C.
Soils: Collect samples in airtight containers, leaving minimal headspace. Freeze if analysis cannot be performed within 48 hours. Avoid composite sampling across layers with different hydrocarbon profiles.
Field blanks and trip blanks: One per day per sampling team. Analyze for all target fractions to identify cross-contamination.

3.2 Data Interpretation and Reporting

The publication provides guidance on interpreting selective analysis results, particularly for distinguishing petroleum hydrocarbons from natural organic matter (NOM). Two diagnostic indicators are highlighted:

Unresolved complex mixture (UCM) profile: A characteristic hump in the GC chromatogram (usually in the DRO range) indicates weathered petroleum. NOM typically produces a flat or structured baseline.
Carbon preference index (CPI): For n-alkanes, CPI values near unity suggest petroleum; values significantly greater than 1 (especially in C₂₅–C₃₃ range) indicate biogenic input.

Common Pitfall: Reporting total petroleum hydrocarbons (TPH) without fractionation can lead to overestimation of risk, especially in environments rich in organic detritus (e.g., wetlands, agricultural soils). Always apply selective analysis and silica gel cleanup when TPH exceeds background.

Compliance and Data Reporting

Although API Publ 4637-1996 is not a mandatory regulatory standard, its recommendations have been adopted by several state environmental agencies and referenced in ASTM practice E 1739 (Risk-Based Corrective Action for Petroleum Release Sites). Users should consider the following when applying this publication in a compliance context:

Regulatory acceptance: Many regulatory programs require or accept selective analysis for site closure decisions. Confirm with the overseeing agency which fraction definitions and methods are recognized.
Method modification policy: Any deviation from the analytical methods described must be documented and validated. A tiered approach (screening vs. definitive) is acceptable, but fraction-specific criteria must be met for risk-based comparisons.
Reporting level: Report all results at the laboratory’s method detection limit (MDL), not the project action limit. For fractions, report the carbon range (e.g., C₁₀–C₂₈) and the method used (GC-FID with or without silica gel).
Data usability: Apply the three-tier validation system recommended in the publication: Tier I (review QC surrogates and blanks), Tier II (assess MS/MSD and LCS), Tier III (evaluate chromatographic patterns and field duplicate precision).

Best Practice: When submitting data to a regulatory agency, include a narrative that explicitly references API Publ 4637-1996 as the guiding protocol, and attach chromatograms for at least 10% of samples to demonstrate fractionation quality.

The lasting value of API Publ 4637-1996 lies in its systematic approach to isolating petroleum-derived hydrocarbons from complex environmental matrices. By advocating for selective fractionation, silica gel cleanup, and rigorous quality control, the publication continues to serve as a foundational reference for environmental professionals working on petroleum site investigations and remediation monitoring.

Q: Is API Publ 4637-1996 a mandatory standard?
A: No. API Publ 4637-1996 is a technical publication (guidance document) rather than a consensus standard. However, its recommendations have been incorporated into several state cleanup regulations and ASTM risk-based corrective action standards. Many regulatory agencies accept data generated following this publication as defensible for site closure and risk assessment purposes.

Q: What is the main advantage of using selective analysis over total petroleum hydrocarbon (TPH) testing?
A: Selective analysis distinguishes between petroleum hydrocarbons and naturally occurring organic matter (NOM), reducing false positives that can lead to unnecessary remediation costs. It also enables fraction-specific risk assessment (e.g., GRO vs. DRO) and provides chromatographic patterns useful for source identification and weathering assessment.

Q: How does silica gel cleanup improve data quality?
A: Silica gel adsorbs polar organic compounds (e.g., humic acids, plant-derived waxes, detergents) that are not petroleum-derived. By removing these interferences before analysis, the technique ensures that the measured hydrocarbon concentration more accurately reflects the petroleum-derived fraction, which is critical for sites with high background NOM concentrations.

Q: Which carbon ranges are typically used for GRO, DRO, and RRO?
A: In API Publ 4637-1996, GRO is defined as hydrocarbons in the C₅ to C₁₀ boiling range, DRO as C₁₀ to C₂₈, and RRO as >C₂₈. These operational definitions can vary slightly by regulatory program; always confirm the expected carbon windows with the project-specific oversight agency.

Reference: API Publ 4637-1996, Selective Analysis of Petroleum Hydrocarbons in Groundwater and Soils: A Literature Review and Case Study, American Petroleum Institute, Washington, D.C., 1996.

This article is prepared for informational purposes and reflects the technical content of the original publication as interpreted by the author. Users should consult the full document and applicable regulations for specific application requirements.

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