Horizontal and Vertical Distributions of Chromium in a Chromate Production District of South Central China

To study the horizontal and vertical distribution of chromium (Cr) in the soil of a chromate production site (CPS) and its nearby area (NA-CPS) in south central China, 61 profiles (depth: 14 m) in the CPS and 69 samples (topsoil) were excavated following a grid-sampling method. The geographic coordinates, elevation, and types of soil layers were recorded, and the total Cr in the soil and the total Cr and Cr(VI) in the leachate of the soil and in the groundwater were determined. Migration of Cr in surface soils may be represented in terms of a multiple linear regression equation (R²_adj = 0.632). Distance, elevation, and pH are the primary factors that influence the horizontal distribution of Cr content in the surface soils, while the Cr concentration in different soil profiles mostly obeys the positive or negative binomial distributions. For a positive distribution, the Cr concentration decreases with increasing depth in the 0.0-8.0 m soil layer, under the fixing effect of soil. However, it shows an upward trend with a depth in the 8-14 m soil layer under the influence of Cr-polluted phreatic water. Under a negative distribution, Cr content is stable in the 0-6 m layer because of the influence of chromite ore processing residue mixed with miscellaneous fills, but it decreases obviously in the 6-14 m layer under the fixing effect of soil. Similar vertical distributions were observed for pH, L, L⁶⁺, and P⁶⁺. The decreasing amplitude of the Cr concentration for binomial distributions is mainly affected by the Cr concentration, pH, and LR of the soil. Moreover, P of soil increases with pH, and the type of soil layer is the primary factor influencing LR in the soil profiles. Our results of the horizontal and vertical distributions of Cr could be used to guide investigations that are focused on reducing the number of samples in the horizontal and vertical directions at CPSs, and to improve risk assessments of CPSs and nearby areas.