Does hydrocarbon contamination induce water repellency and changes in hydraulic properties in inherently wettable tropical sandy soils?

•Water repellency and hydraulic properties studied on hydrocarbon-contaminated soils•Laboratory simulated hydrocarbon contamination induced soil water repellency.•Hydrocarbon contamination increased (p < 0.05) saturated hydraulic conductivity.•Field soils from both the control and hydrocarbon-contaminated sites were wettable.•On tropical soils, hydrocarbon-induced water repellency appears to be non-persistent.

Hydrophobicity influences soil hydrological and ecological functions. Compared to naturally-occurring and fire-induced hydrophobicity, limited information is available on the impacts of hydrocarbon contamination on water repellency and hydraulic properties. Water repellency and hydraulic properties were measured on laboratory simulated, and field contaminated soils, 1 and 5 years after an accidental petroleum hydrocarbon spill. The objectives were; (1) to compare the water droplet penetration test (WDPT) to the molarity of ethanol droplet (MED) test, (2) to investigate the effect of hydrocarbon contamination on water repellency and hydraulic properties, and (3) to evaluate the performance of pedotransfer functions for hydraulic properties. The WDPT and MED tests gave qualitatively similar water repellency results as evidenced by a significant positive correlation (p < 0.05, r2 = 0.95) between the mean time for the two methods. Laboratory simulated hydrocarbon contamination induced soil water repellency. Saturated hydraulic conductivity (Ks) increased linearly with level of contamination (p < 0.05; r2 ≈ 0.8), indicating that rapid flow of water attributed to a reduction of the dielectric constant, and hence water–soil matrix interactions. No water repellency was observed in contaminated field soils (WDPT < 3 s), but the residual signature of hydrocarbon contamination was evident in other soil properties particularly electrical conductivity. This indicates that natural soils were inherently wettable and that hydrocarbon-induced hydrophobicity could be transient. This non-persistence was attributed to high decomposition rates stimulated by tropical conditions and nutrients added to promote revegetation. Predictions of pedotransfer functions were comparable to measured hydraulic data (p < 0.05, r2 > 0.8), confirming their general validity for water and solute transport modeling even on contaminated soils. The study confirmed the hypothesis that hydrocarbon contamination induces water repellency and reduces soil moisture retention at low suction (< 100 kPa) for laboratory contaminated soils, but effects were inconsistent for field samples. However, the increased saturated hydraulic conductivity associated with laboratory contaminated soils contradicted the original hypothesis. The findings imply that storms falling on initially dry recently contaminated soils may trigger contaminant transport and erosion via enhanced surface runoff, and rapid spreading of contaminants once they reach the groundwater systems. These hydrological impacts are critical for remediation of contaminated sites. Future research could use a contamination chronosequence/gradient to provide comprehensive information on the temporal evolution of water repellency and hydraulic properties under field conditions.