In the late 2000s, questions emerged about whether hydrogen cyanide (HCN) emissions from gold mining cyanide leach operations could pose a risk to nearby communities. Rather than relying solely on conservative assumptions or modeling, the U.S. Environmental Protection Agency (EPA) invoked its authority under Clean Air Act Section 114 to request real-world data. In response, Air Sciences, working closely with the Nevada Mining Association (NvMA) and its member companies, spearheaded a comprehensive ambient monitoring program designed to answer a simple but critical question: What are the actual ambient concentrations of HCN at the fence line of operating gold mines?
Why HCN and Why Monitoring?
HCN is a hazardous air pollutant associated with cyanide-based gold extraction processes. While cyanide leaching is conducted in controlled, engineered systems, EPA sought to verify whether fugitive or process-related emissions could lead to off-site exposures of concern. To inform its regulatory decision-making, EPA requested direct ambient monitoring data rather than relying solely on emissions estimates or dispersion modeling.
This request set the stage for a collaborative, science-driven effort between regulators, industry, and technical experts.
Designing an EPA-Approved Monitoring Program
Air Sciences led the technical design and implementation of the monitoring effort, coordinating with NvMA, individual mine operators, and EPA staff. Key elements of the program included:
- Eight ambient monitoring sites strategically located at the fence line of selected Nevada gold mining operations.
- EPA-approved monitoring plans, ensuring consistency, transparency, and regulatory credibility.
- Integrated sampling and meteorological measurements, allowing observed concentrations to be interpreted in the context of site-specific wind patterns and operating conditions.
A representative example of this work is the Round Mountain Gold Corporation – Smoky Valley Common Operation HCN monitoring program conducted in Nevada and documented in the April 2010 final report. Ambient measurements collected at the Round Mountain mine demonstrated that HCN concentrations at and near the facility boundary were very low, often below analytical detection limits, and well within applicable public health benchmarks.
What the Data Showed
Across all monitored sites, the results told a consistent story:
- Ambient HCN concentrations at mine fence lines were below all applicable health-based screening levels.
- Data recovery and quality assurance metrics met or exceeded EPA expectations.
- Observed concentrations reflected normal operations, not atypical or short-term testing conditions.
These findings were reinforced by extensive source testing conducted at multiple Nevada gold mines, which showed very low HCN emission rates from cyanide leaching processes.
Regulatory Outcome: Data-Driven Decisions
The fence line monitoring program played an important role in EPA’s broader evaluation of hazardous air pollutants from gold mining. When EPA finalized the Gold Mine Ore Processing and Production Area Source NESHAP in 2011, the rule focused on mercury emissions and did not include HCN as a regulated pollutant under the standard.
This outcome reflected EPA’s conclusion that, based on real-world monitoring data, HCN emissions from gold mining cyanide leach operations did not present an unacceptable risk to public health.
Beyond HCN: A Blueprint for Other Non-Criteria Pollutants
One of the lasting legacies of this effort is the monitoring framework itself. The same fence line monitoring approach used for HCN can be readily adapted to other non-criteria pollutants where data gaps complicate risk evaluations. For example:
- Hydrogen fluoride (HF) near mineral processing or metallurgical operations.
- Ammonia or reduced sulfur compounds near industrial or mining sources.
- Site-specific hazardous air pollutants where modeling uncertainty is high.
By combining targeted ambient monitoring with meteorological data and robust QA/QC protocols, regulators and industry alike gain confidence that decisions are grounded in measured reality rather than worst-case assumptions.
Conclusion
The HCN fence line monitoring program stands as a clear example of how collaborative, science-based approaches can lead to better regulatory outcomes. Through leadership by Air Sciences, partnership with the NvMA, and constructive engagement with EPA, the gold mining industry demonstrated that transparent data collection can both protect public health and avoid unnecessary regulation.
In an era where environmental decisions are increasingly complex, this project remains a model for how monitoring, not speculation, can guide smart, defensible policy.
Contact us to learn more about our work and how Air Sciences’ expertise can support your environmental goals and regulatory negotiations regarding meaningful policy.
