Many metals generally occur in small concentrations in surface waters, and a number of them are nutrients required by aquatic biota in small amounts, but can be toxic at higher concentrations. CDPHE sets regulatory standards for most metals based on the uses identified for each stream segment (e.g., water supply, agriculture, recreation, aquatic life protection); if water quality samples frequently exceed these standards, the stream segment is placed on the State’s 303(d) or M&E (monitoring and evaluation) list for that particular constituent.