Delineating the hazard zone of multiple soil pollutants by multivariate indicator kriging and conditioned Latin hypercube sampling

Soil pollution delineation is a difficult task because there may be multiple pollution sources and complex transport schemes. In this study, four heavy-metal concentrations, namely Cr, Cu, Ni, and Zn, were collected at 1082 sampling sites in central Taiwan. Conditioned Latin Hypercube Sampling (cLHS) is used to determine the most cost-effective sampling strategies for long-term monitoring of multiple heavy metals. Then, the study estimates the probability of multiple hazardous heavy metals using multiple-variable indicator kriging (MVIK) based on sufficient cLHS samples. The results suggest that heavy-metal sampling patterns, including the size and configuration, affect the spatial distribution of probable hazards, especially when the sample size is small. Unlikely random sampling, the cLHS method replicates the variability and distribution of variables. In this study, an area is defined as a hazard zone if the amount of any heavy metal exceeds the corresponding regulatory threshold. The heavy-metal delineations (e.g., where the MVIK hazard probability exceeds 0.85) cover over 20% of the study area and correlate highly with the locations of industrial plants and irrigation systems in the area. Hence, MVIK coupled with cLHS provides a way to assess the presence of multiple hazards efficiently and effectively in future monitoring and environmental management projects.