Cleanup goals for petroleum-contaminated pastures according to oil density, concentration, and soil type

The common 1% oil cleanup criterion was tested for pasture production according to oil type and concentration, in soil types frequently contaminated in southeastern Mexico. Reductions in aerial biomass of <i>Brachiaria humidicola</i> were measured over six months in soils contaminated with crude oils of varying grades (light, medium, heavy, and extra-heavy). Dose-response curves for heavy crude-contaminated soils showed acceptable criteria (90% pasture) of 0.71, 0.56, 1.23, ~0.20 and < 0.10% oil for an Arenosol, Vertisol, Gleysol, Fluvisol and an Acrisol, respectively. Generally, for all crude oils, the 1% level resulted in pasture reductions of ~20–70, ~25–60, ~50–65, and ~35–65% in the Arenosol, Vertisol, Fluvisol, and Acrisol, respectively. Still, in the Gleysol it was variable (reduction of ~10% to an increase of ~15%). Thus, the 1% oil cleanup criterion may be suitable for some soils with large amounts of smectite clays and organic matter (such as Gleysols). Still, for most soils, it may not be strict enough to prevent soil fertility deterioration, and soils with large amounts of non-smectite fines may be particularly impacted. Therefore, lower cleanup levels need to be considered, as well as low-cost regenerative agricultural practices to recover soil fertility in contaminated soils, when these cleanup levels are not achievable.