Application of CSIA Technique in Differentiating Sources of Oil Contamination in Groundwater and Evidence of Natural Attenuation Potential in the Aquifer

Background and Objectives: Aromatic hydrocarbons such as benzene, toluene, ethylbenzene, and xylene (BTEX) are significant components of oil products. They can infiltrate groundwater due to leaks from underground gasoline tanks, storage tanks, and surface and subsurface oil products and crude oil storage facilities. Compound-Specific Isotope Analysis (CSIA) is recognized as a method for assessing natural attenuation processes of degradable pollutants, such as hydrocarbons, and for identifying various pollutant sources. This study aims to identify the sources of oil contamination in groundwater and evaluate natural attenuation processes in a contaminated aquifer, focusing on BTEX compounds, using CSIA, oil fingerprinting techniques, and hydrogeochemical data.
Materials and Methods: The hydraulic conductivities, ascertained through slug tests conducted on 24 monitoring wells, exhibit a wide range spanning from 7.9 × 10−4 to 1.4 × 10−6 m/s. The geometric mean of these values is calculated to be 7.5 × 10−5 m/s. Among the aquifers present, the uppermost one exhibits the highest level of oil pollution. Due to the presence of over 50 above-ground storage tanks and pipeline networks containing oil products such as gasoline, super gasoline, diesel, Euro diesel, kerosene, jet fuel (JP4) and aviation fuel(ATK), there is a potential risk of groundwater contamination by one or more of these products. Groundwater samples, floating oil substances from monitoring wells, and standard oil products were collected for analysis. BTEX compounds were extracted from water samples. These samples were then analyzed using GC-MS, GC-IRMS, and ICP-OES. The results were utilized for chemical and isotopic fingerprinting.
Results and Discussion: Monthly measurements of water and oil levels in monitoring wells indicated active leaks in both the northern and southern areas. CSIA revealed that in the northern area, toluene originated from oil products such as kerosene, ATK or jet fuel, regular gasoline, and diesel. Additionally, stable carbon and hydrogen isotope ratios for ethylbenzene, ortho-xylene, and para-xylene in Area A indicated that regular gasoline was the source of contamination in the northern area. δ13C and δ2H values in floating oil substances and groundwater samples suggested that in the southern section, toluene originated from high-octane gasoline. While GC-MS analysis confirmed the presence of ATK, JP4, gasoline, and diesel, identifying specific types of gasoline and diesel was challenging due to similar GC-MS chromatographs. To overcome this limitation, two-dimensional compound-specific isotope analysis was used, which could distinguish differences in isotopic signatures between similar chemical products like regular gasoline, high-octane gasoline, diesel, and Euro diesel, particularly through the analysis of carbon and hydrogen isotope ratios in toluene compounds.
Conclusion: The findings of this study indicate that effectively addressing the challenges of identifying sources of oil pollutants requires the simultaneous use of CSIA and fingerprinting techniques along with detailed hydrogeological analyses. Relying solely on one method independently is insufficient. Combining hydrogeological data with stable isotope analysis significantly enhances accuracy and reliability in identifying the sources of BTEX compounds. Examination of stable carbon isotopes in toluene, ortho-xylene, and para-xylene in groundwater samples showed a 4‰ depletion of carbon isotopes in ortho-xylene and para-xylene from east to west over a distance of approximately 1.5 kilometers in the flow direction, with a correlation coefficient above 80%. Depletion of other compounds showed similar trends but with lower correlations. Thus, geochemical and isotopic results indicated that natural attenuation processes are occurring in the aquifer. However, these processes are not sufficient to remediate the extent of contamination, and additional cleanup methods are necessary for effective pollution removal.