Radioactivity can be expressed in several different ways. A curie is a measurement of the intensity of radioactivity, or the rate of decay. A picocurie is one trillionth of a curie.  When found in soil, the amount is expressed in picocuries per gram; in water, it is picocuries per liter. Limits for what is considered safe or harmful can be based on these units of measurement. 

Agencies also have a hypothetical measure for how much radiation a person absorbs into his or her body after an exposure, the pathway into the body and the corresponding medical effects that radiation might cause. That hypothetical dose is expressed in millirem. Limits for what is safe can be based on this as well. 

The variations have to do with what an agency is measuring. In an email to Searchlight, Phil Rutherford, an independent consultant and health physicist specializing in radiation safety, described it this way: the DOE and NRC “regulate radiation and radioactive materials by limiting dose,” while the “EPA regulates radionuclides by limiting risk.” 

For added context, New Mexico has no statutory limit for plutonium in soil. As for water, the state’s Water Quality Control Commission adopted “monitoring and disclosure criteria” for the Rio Grande in 2010, New Mexico Environment Department spokesperson Jorge Estrada wrote in an email. That criteria is 1.5 picocuries of plutonium per liter of water and it only applies to LANL contaminants. It’s only been exceeded twice since it was adopted, he added, “one related to a stormwater sample and one shortly after the Los Conchas fire.”