Fertility impacts in crude oil-contaminated soil based on type and quantity of clay

Artificial soil mixtures were prepared with 10–40% of either kaolinite or bentonite clay, plus Sphagnum moss, and clean sand. Mixtures were placed in plastic containers in the field, and let mature for six months (in a tropical climate). They were then contaminated with 3% of either light, medium or heavy crude oil, and let weather for another six months. Afterward, compaction was measured in the field, and core samples were taken for laboratory determinations. No correlation was found between soil density and compaction, and it appeared that some of the density determinations were unreliable due to interferences from the petroleum. Compaction was low (< 0.5 MPa) for smectite-based soil, but much higher (up to 1.24 MPa) in kaolinite-based soil, especially with higher clay levels contaminated with heavy crude. Water repellency for kaolinite-based soil was low to null (especially for 30 and 40% clay), but repellency for smectite-based soil was three orders of magnitude higher, in the severe to extreme range. However, it is uncertain whether these laboratory determinations on smectite-based soils could be applied to the field. Regional, smectite-rich soils tend to be moist and low-lying in the landscape, unlikely to manifest water repellency, even during the dry season. Field capacity in smectite-based soil showed low impacts (reductions of 0–1% of moisture content with higher amounts of clay), but kaolinite-based soil showed considerable reduction, especially at 30% clay (being 15% less). The importance of these results for contamination, characterization and remediation of regional soils is discussed, as well as recommendations for future research.