What is the legal threshold limit on the amount of a substance that is allowed in a public water system?

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The legal threshold limit for a substance in a public water system is the Maximum Contaminant Level. This is an enforceable standard set by regulators that specifies the highest allowable concentration of a contaminant in drinking water delivered to consumers. It’s designed to protect public health while reflecting what treatment and monitoring systems can reliably achieve. By contrast, the Maximum Contaminant Level Goal is a non-enforceable health-based target, while the Action Level is a trigger for corrective action when certain sampling results are exceeded (often for lead or copper), and a Treatment Technique is used when a numerical MCL isn’t feasible to enforce. So the enforceable limit you’re looking for is the Maximum Contaminant Level.