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Abbreviation for "National Pollutant Discharge Elimination System." The US Congress passed this Clean Water Act to control discharges of contaminants. Discharges are allowed in to US water only by NPDES permits. Drilling fluids, drill cuttings, produced water, drilling rig deck drainage and blowout preventor fluids are covered specifically. Barite must be monitored for heavy metals to allow mud discharges. Oils are banned from discharge.