Right now - arsenic is regulated in wastewaters statewide due to water quality standards that were conceived of over 20 years ago. Because arsenic can be quite detrimental to human health and for surface waters that are a source of drinking water, it is important to protect the drinking water supply by adopting protective standards.
The Safe Drinking Water Act regulates arsenic at 10 micrograms per liter (ug/L), or 10,000 nanograms per liter as a maximum contaminant level (or MCL). Also, the practical quantitation limit, or PQL for arsenic currently stands at about 1 micrograms per liter. Having such large discrepancies between the water quality standard, the drinking water MCL, and the PQL creates challenges in implementation of standards within the permitting framework.
The division worked with stakeholders and the commission throughout the 2010s and the early part of this decade to implement statewide temporary modifications because of uncertainty in what the standard should be, and the challenges presented by these discrepancies. While being an interim solution that is unique to Colorado, this solution is only temporary. Each time we extend it, we have to get it approved by our partners at EPA Region 8. Colorado has utilized the temporary modification tool since 2011 (Reg. No. 38.79) to recognize the underlying, use-based standard as a goal while using the level generally deemed to be technologically feasible to set permit limits (i.e., 0.02-3 ug/L). Currently, the arsenic temporary modification is scheduled to expire in 2029.
As part of the Water Quality Roadmap, the division is currently scoping out what implementation of the protective arsenic standards will look like in the permitting framework. Keep informed of our progress through our quarterly roadmap meetings, which will also offer regular stakeholder input opportunities. The next Roadmap meeting will be held on March 5, 2026.
