Distribution of Residual Diesel Components in Soil Pores After Bioremediation Assessed by the Measurement of Hydraulic Properties.

Distribution of residual diesel components in soil pores was assessed by measuring hydraulic properties of bioremediated aged diesel contaminated soils.  Aged diesel-contaminated subsurface soils (1.5~2 m) are incubated for bioremediation process over 80 d under the two different column treatment; I) no nutrient added and II) nutrient balanced.  Nutrient added loosen soil was also run (treatment III) for same bioremediation process.  Mass depletion of diesel was negligible in treatment I) while treatment III) exhibit the greatest extent of diesel biodegradation.  For all cases, greatest rates of diesel biodegradation occurred during the first 20 d, followed by a much retarded biodegradation rate in the remaining incubation period.  Reduction of the degradation rate due to entrained diesel within inaccessible soil pores was hypothesized and tested by measuring the hydraulic properties of two column soils.  At given hydraulic pressure heads of 0 ~ 15 cm, water flow (e. g., hydraulic conductivity) through column II (nutrient balanced) was consistently greater than that of column I soil (no nutrient added), indicating less hydrophobicity of column II soil.  In addition, the water retention of column II soil was also greater at pressure heads < 100 cm (equivalent to pore size of > 30 mm), suggesting that bioremediation of hydrocarbons results in enhanced wettability of larger soil pores.  However, the same effect was not apparent at pressure heads > 100 cm, where smaller size soil pores were responsible for the water retention, indicating that 1) water wettability was not improved in smaller size pore and 2) diesel components present in smaller soil pores (e. g., < 30 mm) remained after bioremediation.  The results of this study suggest that diesel components entrained in small pores with limited microbe accessibility may be recalcitrant to bioremediation.