Significant quantities of water, primarily for processing and cooling, are needed to produce fuel. Various crude oils have different levels of metal contaminants. The metal of most concern in crude oil is mercury, the concentration of which can vary widely from one crude oil source to another. Mercury is important to refiners as a pollutant because it causes corrosion in process units and its discharge into local waterways surrounding an oil refinery is harmful to the environment.

Online monitors or offline analyzers can be used in oil refineries to obtain real-time data on trace metal contaminants in the process and discharge water; helping to ensure regulatory compliance and protect the environment.