Exposure to toxic contaminants in the environment harms human and animal health and disturbs the integrity and function of the impacted ecosystem. The impact could be local, regional, and global. The concentration of a toxic substance below or above detection limits or thresholds in environmental samples is frequently recorded as non-detect. We discuss inferences based on exact and modified likelihood methods for the location-scale family with values below the detection limit, and as a special case for the normal distribution with a comparison between the methods. We demonstrate the procedure using Niagara River monitoring data.
Keywords: EM algorithm; Likelihood; Modified likelihood; Toxic contaminants; Type I censoring; Water quality.