Assessing the quality of oil-contaminated saline soil using two composite indices

It is essential to ascertain the quality status of contaminated soils prior to implementing bioremediation. In this study we assessed the quality of oil-contaminated saline soils in the Yellow River Delta, China using two composite indices. Five sites were selected according to the distance to the Yellow River estuary shoreline, and soils in each site were sampled from different distances to oil well port along the four cardinal directions. Four enzymes including dehydrogenase (DHO), catalase (CAT), polyphenol oxidase (PPO) and lipase (LIP), and physicochemical properties of soils at depth of 0–20 cm were analyzed. Soil electrical conductivity (EC) and total organic carbon (TOC) decreased significantly with the increasing of distance to the shoreline or to the well port. Enzyme activities decreased with the increasing EC except for the PPO, while they oppositely responded to TOC. Considering inconsistencies in assessing soil quality using individual indicators, we introduced two composite indices (geometric mean of the assayed enzyme activities (GMea) and integrated quality index derived from principal components analysis). The GMea decreased with EC, while it increased with TOC and the integrated quality index. The threshold values of GMea and the integrated quality index for healthy soil were confirmed according to the EC and TOC of the uncontaminated soils. If the values of the two indices moved far away from the thresholds, the soil quality became worse. The soil quality was mainly affected by crude oil contamination when the values of the two indices were higher than the thresholds; oppositely, the soil quality was influenced by soil salinity. Our results suggest that the two composite indices could be used for evaluating the quality of crude-oil-contaminated saline soil.