What are the permissible levels of trace elements (such as arsenic, copper, iron, lead, and zinc) in water for the water to still be considered safe?
Several of these trace elements are regulated by the EPA and are on their list of primary drinking water standards. These include arsenic, copper, and lead, as well as cadmium, chromium, mercury, and selenium. Iron is not a regulated contaminant because it is not known to cause health problems, but there is a secondary drinking water standard based on its tendency to stain laundry and plumbing fixtures. Manganese, copper (again), silver, and zinc are also included in the secondary standards. The primary and secondary standards are available on the EPA Web page Current Drinking Water Standards.
The above standards are national drinking water standards. Other water-quality standards are set by states to protect aquatic life. You can check with your state environmental or natural resources agency to see what aquatic life water-quality standards are in effect in your state. EPA is currently reviewing its recommendations for aquatic life criteria. Information on the current review of standards is available on Aquatic Life Water Quality Criteria.
The USGS National Analysis of Trace Elements also has current information on trace elements across the United States, including links to specific studies and current items in the news.