Determination of Mineral Oil Content in Groundwater: Analytical Methods, Standards, and Quality Cont

Mineral oil is one of the most closely monitored indicators in groundwater quality assessment, typically originating from leaking fuel storage tanks, industrial discharges, pipeline failures, and spill incidents. Accurate determination of mineral oil content in groundwater depends on standardized sampling procedures and validated instrumental techniques such as infrared spectrophotometry, gravimetry, and gas chromatography. This article summarizes the applicable standards, sample types, analytical methods, detection limits, and quality control measures commonly applied by accredited third-party laboratories for groundwater mineral oil testing.

Technical Overview

Mineral oil in groundwater refers to a complex mixture of petroleum-derived hydrocarbons, including aliphatic, alicyclic, and aromatic compounds, generally spanning the carbon range of approximately C10 to C40, together with lighter volatile fractions. Because petroleum products are ubiquitous in industrial, transportation, and energy activities, mineral oil serves as a key indicator of anthropogenic contamination in aquifers. Sources include leaking underground storage tanks at fuel stations, refinery and petrochemical facilities, oil depots, machinery workshops, landfills, and accidental spills.

In China, groundwater quality is governed by GB/T 14848-2017 (Standard for Groundwater Quality), which classifies groundwater into five categories and assigns Class III (multipurpose, human-health-based) limits for petroleum-related indicators, while GB 3838-2002 sets corresponding surface water criteria. International reference values include the hydrocarbon oil index defined in ISO 9377-1 and the historical mineral oil indicator parameter of 10 micrograms per liter in the EU Drinking Water Directive. Analytical determination is typically performed by infrared (IR) spectrophotometry, gravimetry, ultraviolet or fluorescence spectrophotometry, and gas chromatography, with the choice depending on regulatory requirements, expected concentration, and data objectives such as contamination source identification or remediation verification.

Testing Samples

Groundwater from Monitoring Wells

Samples are collected from dedicated or temporary monitoring wells installed and purged in accordance with HJ 164-2020 (Technical Specifications for Groundwater Environmental Monitoring). Low-flow purging with oil-free equipment is recommended until field parameters such as pH, electrical conductivity, dissolved oxygen, oxidation-reduction potential, and turbidity stabilize, minimizing suspended solids that can bias oil measurements.

Drinking Water and Source Well Samples

Water from production wells, municipal supply wells, and agricultural or domestic wells is tested to verify compliance with drinking water and irrigation quality requirements, particularly in areas adjacent to fuel stations, industrial parks, or known contamination plumes.

Site Investigation and Remediation Samples

Groundwater samples gathered during contaminated site investigation and risk assessment activities, following procedures such as HJ 25.1, HJ 25.2, and HJ 25.3, are analyzed to delineate plume extent, support risk characterization, and confirm the effectiveness of remediation.

Sample Collection and Preservation

• Collect samples directly into pre-cleaned amber glass bottles of at least 500 mL, preferably 1 L, with PTFE-lined caps; plastic containers must never be used due to adsorption and contamination risks.
• Fill bottles completely and record the exact volume, since the entire bottle contents, including any oil adhering to the walls, are typically extracted to ensure quantitative recovery.
• Acidify to pH below 2 with hydrochloric acid and store at approximately 4 degrees Celsius in the dark.
• Analyze within 7 days for IR-based methods (HJ 970-2018, HJ 637-2018); acidified samples for gravimetric oil and grease may be held up to 28 days under EPA Method 1664B.

Testing Items

Core and related testing items for groundwater mineral oil evaluation include:

• Mineral oil (petroleum) in water by infrared spectrophotometry.
• Oil and grease, including n-hexane extractable material (HEM) and silica-gel-treated non-polar material (SGT-HEM).
• Hydrocarbon oil index (HOI) in accordance with ISO 9377-1.
• Total petroleum hydrocarbons fractionated as C6 to C9 (volatile) and C10 to C40 (semi-volatile and heavier) fractions.
• Associated petroleum indicators such as benzene, toluene, ethylbenzene, and xylenes (BTEX) and polycyclic aromatic hydrocarbons (PAHs) for source identification and risk assessment.

Testing Methods

Infrared Spectrophotometry

Infrared spectrophotometry is the primary regulatory method for determining mineral oil in groundwater in China, specified in HJ 970-2018 and HJ 637-2018, which superseded GB/T 16488-1996. The sample is acidified, extracted with an infrared-transparent solvent, and the absorbance of the CH stretching vibrations is measured at three characteristic wavenumbers, approximately 2930, 2960, and 3030 cm-1, corresponding to methylene, methyl, and aromatic C-H groups. Calibration uses mixed standards such as n-hexadecane, iso-octane (or pristane), and toluene to correct for differing hydrocarbon compositions. Modern methods have replaced ozone-depleting carbon tetrachloride with tetrachloroethylene as the extraction solvent. Method detection limits are typically on the order of 0.01 mg/L, making the technique suitable for the low concentration ranges expected in groundwater.

Gravimetric Method

The gravimetric approach, described in EPA Method 1664B and ISO 9377-2, acidifies the sample to pH below 2, extracts it with n-hexane, dries the extract over sodium sulfate, evaporates the solvent, and weighs the residual oil. Treatment of the extract with silica gel adsorbs polar substances such as animal and vegetable oils and natural organic matter, isolating the non-polar mineral oil fraction. The method is robust and simple but has practical quantitation limits generally in the range of 1 to 5 mg/L, so it is better suited to grossly contaminated water than to routine trace-level groundwater monitoring.

Ultraviolet and Fluorescence Spectrophotometry

Ultraviolet methods measure the absorbance of petroleum ether extracts, commonly at wavelengths near 225 nm, and are sensitive to aromatic components. Fluorescence spectrophotometry, applied for example in GB 17378.4 for marine waters, excites the extract in the ultraviolet region and measures the emitted fluorescence, achieving detection limits down to the low microgram-per-liter range. Both techniques are sensitive and rapid but respond mainly to aromatic hydrocarbons, so calibration standards must closely match the oil type, and they are frequently supplemented or confirmed by infrared or chromatographic methods.

Gas Chromatography (GC-FID and GC-MS)

Gas chromatographic methods determine the hydrocarbon oil index per ISO 9377-1: the extract is cleaned on Florisil or silica gel and analyzed by GC-FID, integrating the total resolved and unresolved response between n-C10 and n-C40. This provides both a quantitative total and a distribution profile useful for distinguishing gasoline, diesel, kerosene, and lubricating oil sources and for evaluating weathering and biodegradation. GC-MS further enables PAH analysis, petroleum biomarker fingerprinting, and speciation of volatile petroleum hydrocarbons by purge-and-trap techniques, supporting forensic source apportionment and detailed risk assessment.

Quality Assurance and Interference Control

• Multi-point calibration with correlation coefficients typically required at 0.999 or better, plus continuing calibration verification standards.
• Method blanks, laboratory duplicates, matrix spikes, and surrogate recoveries within acceptance limits, commonly 70 to 130 percent.
• Silica gel cleanup to remove polar animal, vegetable, and humic interferences when reporting mineral oil specifically.
• Whole-bottle extraction and internal rinsing of the container to recover oil adsorbed on glass walls.
• Participation in interlaboratory proficiency testing schemes by accredited third-party laboratories to demonstrate ongoing competence.

Applications

Determination of mineral oil content in groundwater supports a wide range of environmental and regulatory applications:

• Routine groundwater quality monitoring within national and regional monitoring networks and industrial park surveillance programs.
• Leak detection and compliance verification at fuel stations, oil depots, refineries, and petrochemical facilities.
• Contaminated site investigation, risk assessment, and remediation effectiveness validation under site management frameworks.
• Environmental impact assessment and construction project acceptance for projects with potential oil contamination risks.
• Emergency response to spills and pipeline failures, including plume delineation, trend tracking, and forensic source identification.
• Safety evaluation of drinking water sources and irrigation wells in areas influenced by industrial or transportation activities.
• Due diligence assessments for land transactions and brownfield redevelopment.

FAQ

What is the difference between mineral oil, oil and grease, and total petroleum hydrocarbons?

Mineral oil refers specifically to non-polar petroleum hydrocarbons. Oil and grease is a broader extractable-matter measurement that may include animal and vegetable oils, which are separated by silica gel treatment when a mineral-only result is required. Total petroleum hydrocarbons is a chromatography-based measurement of hydrocarbons within defined carbon ranges, offering greater compositional detail than bulk optical or gravimetric methods.

Which method is most suitable for trace-level groundwater monitoring?

Infrared spectrophotometry per HJ 970-2018 is the standard regulatory choice in China, with detection limits around 0.01 mg/L. Fluorescence spectrophotometry and GC-based hydrocarbon oil index methods offer comparable or lower limits and are preferred when hydrocarbon profiling or international data comparability is required.

Why must glass bottles be used for sampling?

Oil readily adsorbs onto plastic surfaces and plasticizers can leach into the sample, causing both negative and positive bias. Pre-cleaned glass bottles with PTFE-lined caps prevent contamination and allow the entire sample, including wall-adsorbed oil, to be extracted quantitatively.

How are interferences from naturally occurring organic matter handled?

Humic substances and other polar organics can inflate optical and gravimetric results. Acidification, solvent extraction selectivity, and silica gel adsorption cleanup are applied to isolate the non-polar mineral oil fraction, and chromatographic confirmation is used when ambiguity remains.

What do elevated mineral oil results in groundwater indicate?

Concentrations exceeding regulatory thresholds generally indicate an active or historical petroleum source such as a leaking tank, pipeline, spill, or industrial discharge. Follow-up investigations typically combine vertical plume profiling, BTEX and PAH analysis, hydrocarbon fingerprinting, and repeated monitoring to establish extent, trend, and remediation requirements.