Environmental capacity of heavy metals in intensive agricultural soils: Insights from geochemical baselines and source apportionment

Soil environmental capacity (EC) of heavy metals (HMs) can be used as an index to evaluate the pollution status of HMs and to provide basic data for HM remediation. However, the commonly used soil EC for HMs usually are prone to bias due to the lack of local background values (BVs) and the consideration of the contribution from various HM sources. Here, a modified method was proposed to estimate the soil EC by integrating the establishment of local BVs and the quantitative evaluation of contributions from HM sources in an intensive agricultural area of Shouguang city, China. The local BVs of HMs were established using the relative cumulative frequency distribution method. The source-specific EC was quantified based on the local BVs and the contributions of HM sources identified by receptor model and variable importance analysis. Results showed that the average BV values of As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn were 7.67, 0.10, 62.84, 21.17, 0.031, 28.38, 19.25, and 59.60 mg kg-1, respectively, in the study area. The source-specific EC of Cd, Cu, Hg, and Zn were higher than their current EC, indicating an underestimation of soil capacity of HMs by the traditional method. The EC of HMs in these soils was generally medium indicated by their comprehensive EC index (PI) (PI >0.7), suggesting a low risk level of the targeted HMs. According to indexes such as the individual metal index (Pi) and enrichment factor (EF), special attention should be paid to Cd and Zn due to their low capacity (Pi <0.7) and high accumulation (EF > 2) in some points across this area. Altogether, our findings suggested that the modified method had a better capability for evaluating and predicting the enrichment status of soil HMs, which can be helpful for formulating the targeted measures to control HM pollution in such intensive agricultural areas.