The Effect of Soil-Particle Size on Hydrocarbon Entrapment Near a Dynamic Water Table

The entrapment of residual hydrocarbon globules by water table fluctuations can produce a long-term contamination threat to groundwater supplies that is difficult to remove. The mobilization of entrapped hydrocarbon globules depends on the balance between capillary and gravitational forces represented by the Bond number. It is important to estimate the potential for hydrocarbon entrapment at a spill site due to its influence on the effectiveness of remediation efforts. The present work focuses on the influence of particle diameter on hydrocarbon entrapment for a typical LNAPL (light nonaqueous-phase liquid). Laboratory column tests have been conducted using a dual-beam gamma densitometer to measure saturations of the three phases (water, air, and hydrocarbon). Soltrol 170®, a solvent manufactured by Phillips 66 Co., is used as the hydrocarbon. Residual saturation of the Soltrol is measured after fluctuations in water table level to establish the distribution and consistency of hydrocarbon entrapment below the water table. Glass particles of nearly uniform size were used to represent a sandy soil. In the experiments, average particle sizes ranged from 210 to 6000 μm. Data were also taken using the synthetic soil matrix approved by the U.S. Environmental Protection Agency (EPA) for contamination studies. Results show that the distribution of trapped LNAPL is quite uniform and that the average residual saturation is about 13% up to a particle diameter of 710 μm. Above this diameter, residual saturation decreases with particle size. The corresponding critical Bond number, determined experimentally, agrees well with the predicted value of 1.6.