Distribution of Hydrocarbons in Soil Through Contamination from a Leaking Natural Gas Pipeline

Abstract

A plume of hydrocarbon contamination in soil from a leaking natural gas pipeline located at a depth of 80 cm was defined on the basis of discoloration of soil at 3 to 7 cm depth. Eleven sites were selected randomly on a grid superimposed on a map of the 240 m² plume and 48 soil samples at depths from 7 cm to 150 cm were collected. Samples were individually extracted using cyclohexane in a Soxhlet extraction apparatus. Condensed extracts were analyzed using capillary GC and GC/MS techniques to determine quantitatively the dinstribution of C₁₀ to C₃₅ hydrocarbons in soil. In solvent extracts of the soils, over 150 organic compounds were resolved and detected at total concentrations from 0.1 to 2700 ppm. The vertical distribution of hydrocarbons was consistent throughout the plume with higher concentrations of all components with increased proximity to the surface. The hydrocarbons moved vertically from the leak and diffused horizontally along an interface created at 15 to 30 cm by the addition 20 years earlier of a dense clay soil to a naturally high-gypsum base. Ratios of soil concentrations in three size ranges for the hydrocarbons were used to evaluate the physical mechanism for gaseous migration and environmental fate of the hydrocarbons. These ratios were not uniform at every site and depth throughout the plume. Results were consistent with differences in mobility and fate of the hydrocarbons in the soil based on volatility and adsorption. Polycyclic aromatic hydrocarbons present in the natural gas and in the pipeline residue were also found in some but not all soil samples under the conditions of extraction and analyses which were not optimized in the soil-extraction of the aromatic compounds.