Variation in Petroleum Hydrocarbon Chain Lengths with Depth at a Former Crude Oil and Natural Gas ProductionFacility

An investigation was performed at a former crude oil and natural gas production facility to evaluate whether releases from the product flowlines, gathering lines or water injection lines had impacted soil beneath the site. Thirty-six trenches were initially excavated and sampled beneath the former piping runs to a maximum depth of 6?m. After the trenching investigation, nine soil boreholes were advanced and sampled to a depth of approximately 18?m to further delineate the lateral and vertical extent of impacted soil. Soil samples collected from the trenches and boreholes were analyzed for total petroleum hydrocarbons (TPH) in accordance with ASTM Method 2887. The results of the investigation indicated that TPH impacted soil was present within several areas of the 40-ha site. The petroleum hydrocarbons generally had chain lengths ranging from C₆ to C₃₅, characteristic of light crude oil. The impacted soil also contained condensate, the volatile portion of crude oil. Condensate consists of short-chain hydrocarbons (C₁ to C₁₂) and is characterized by low levels of aromatic volatile organic compounds (VOCs). The condensate typically was more prevalent at depths below 4.5?m than the less volatile, longer chain length hydrocarbons. Statistical analysis of TPH data collected during subsequent excavation activities showed that the mean percentage of condensate was significantly greater at depths below 4.5?m than in shallower samples. In contrast, the mean percentage of TPH compounds in the diesel range (C₁₄ to C₂₃) was significantly greater in samples collected at depths above 4.5?m. The difference in the mean percentage of heavier hydrocarbons (C₂₄ to C₄₄+) with depth was not statistically significant.