Evaluation of Mercury Emissions, Occupational Exposure, and Air Dispersion Modeling at a Mineral Processing Facility
Project Overview
RoseyFields conducted a comprehensive environmental forensic investigation to evaluate potential mercury emissions generated during mineral processing and gold purification operations at an aggregate processing facility. The project integrated industrial hygiene, analytical chemistry, stack testing, environmental sampling, mass balance calculations, air dispersion modeling, and occupational exposure assessment to determine whether mercury released during thermal processing operations posed a risk to workers or nearby receptors. The investigation was performed in support of environmental compliance and litigation and evaluated both worker exposures within the processing facility and ambient mercury concentrations surrounding the site.
The evaluation focused on two primary emission sources associated with mineral processing: a dehydration oven used to dry concentrated mineral products and a smelting operation used to recover gold from mineral concentrates. Because naturally occurring mercury is present within gold-bearing mineral deposits, the investigation sought to quantify the amount of mercury released during thermal processing and determine whether resulting airborne concentrations complied with applicable California occupational exposure limits and environmental health criteria. The project incorporated laboratory analyses, stack testing, field measurements, engineering calculations, and atmospheric dispersion modeling to reconstruct mercury emissions from source through potential worker and community exposure.
A principal objective of the investigation was to develop a scientifically defensible evaluation of mercury emissions by integrating measured stack concentrations, analytical measurements of mercury contained within processed materials, emission mass balance calculations, and AERMOD atmospheric dispersion modeling. The resulting analysis provided a comprehensive assessment of both occupational exposure within the facility and potential impacts to surrounding receptors.
Chemicals of Concern
The investigation focused on elemental mercury and inorganic mercury compounds released during thermal treatment of naturally occurring mercury-bearing mineral concentrates. Mercury was present within black sand concentrates transported to the processing laboratory for gold recovery and was evaluated throughout multiple stages of mineral processing, including dehydration, magnetic separation, smelting, and final concentrate production. Analytical testing quantified mercury concentrations within incoming mineral concentrates, processed materials, and finished products while stack testing evaluated mercury vapor emissions generated during heating operations.
Because elemental mercury readily volatilizes when heated, the investigation emphasized airborne mercury vapor as the principal exposure pathway. Additional evaluation considered mercury deposition onto surrounding soils and facility surfaces, potential accumulation within process water and sediments, and long-term management of mercury-bearing residual materials generated during mineral processing.
Methods
RoseyFields conducted an integrated environmental and occupational exposure assessment combining industrial hygiene monitoring, laboratory analytical testing, stack emissions testing, contaminant mass balance calculations, and atmospheric dispersion modeling. Site investigations included direct observation of mineral processing activities, evaluation of processing equipment, documentation of emission sources, and identification of worker locations and potential receptor points throughout the facility. Facility-specific emission sources, building dimensions, ventilation systems, stack characteristics, and surrounding terrain were incorporated into the evaluation to develop representative emission scenarios.
Industrial hygiene investigations included both real-time mercury vapor monitoring and personal breathing zone air sampling conducted during routine mineral processing operations. Mercury vapor measurements were collected using calibrated direct-reading instrumentation capable of detecting low-level airborne mercury concentrations throughout work areas, including the minerals laboratory, drying room, outdoor processing areas, oven exhaust locations, employee workstations, and adjacent walkways. Personal breathing zone samples were collected using established occupational hygiene methodologies and compared with applicable California Occupational Safety and Health Administration (Cal/OSHA) permissible exposure limits and ceiling limits for elemental mercury. Statistical analyses, including upper tolerance limit calculations, were used to evaluate long-term worker exposure potential.
Representative mineral concentrate samples collected before and after processing were submitted to accredited analytical laboratories for determination of mercury concentrations using EPA analytical methods. Laboratory results were incorporated into detailed mass balance calculations to estimate the quantity of mercury volatilized during dehydration and smelting operations. By comparing mercury concentrations in raw materials with processed concentrates and finished products, RoseyFields estimated mercury losses attributable to thermal treatment and quantified emission rates suitable for atmospheric dispersion modeling.
Atmospheric transport was evaluated using the U.S. EPA AERMOD air dispersion model. Multiple model iterations were developed to evaluate varying emission rates, particle characteristics, stack configurations, meteorological conditions, and operational scenarios. Building Profile Input Program (BPIP) analyses were performed to account for building downwash effects, while receptor grids included employee work locations, facility boundaries, nearby structures, ventilation intakes, and the closest residential receptor. Model calibration incorporated measured stack testing results together with laboratory-derived mercury mass balance estimates to improve agreement between predicted and measured emissions. Sensitivity analyses were completed to evaluate uncertainty associated with alternative emission assumptions.
Results
The investigation demonstrated that occupational exposure to airborne mercury during routine mineral processing remained well below applicable Cal/OSHA occupational exposure limits. Statistical evaluation of personal breathing zone samples produced an upper tolerance limit corresponding to approximately one-third of the applicable eight-hour permissible exposure limit, indicating that routine worker exposures remained within acceptable regulatory criteria. Real-time monitoring identified generally non-detectable mercury concentrations throughout most work areas, with short-duration increases observed primarily during mineral drying and immediately following removal of heated materials from the dehydration oven.
Laboratory analyses confirmed that naturally occurring mercury was present within incoming black sand concentrates at concentrations ranging from hundreds to several thousand milligrams per kilogram, with measurable reductions following thermal processing consistent with volatilization of elemental mercury during dehydration and smelting. Mass balance calculations demonstrated that mercury losses varied according to the mercury content of individual mineral batches and processing conditions, providing quantitative estimates of emission rates for subsequent air dispersion modeling.
Stack testing and atmospheric dispersion modeling demonstrated rapid dilution of mercury emissions following release from the dehydration oven and smelter exhaust systems. Although elevated mercury vapor concentrations were measured immediately adjacent to emission points and directly within oven exhaust streams, modeled ambient concentrations decreased substantially with distance from the source. Predicted concentrations at employee work areas, facility boundaries, and nearby receptors remained well below applicable occupational exposure criteria under representative operating conditions. Air dispersion modeling further demonstrated that predicted mercury concentrations at the nearest residential receptor were effectively negligible.
The integrated evaluation also identified opportunities to further reduce potential mercury emissions through operational improvements, ventilation enhancements, and ongoing environmental monitoring. The project developed a conceptual mercury management framework addressing source characterization, emissions monitoring, periodic soil and sediment evaluation, process optimization, and continued occupational exposure surveillance to ensure long-term environmental compliance and worker protection.
Conclusions
This investigation demonstrates the value of integrating industrial hygiene, analytical chemistry, stack emissions testing, contaminant mass balance calculations, and atmospheric dispersion modeling to evaluate airborne mercury emissions from mineral processing operations. Rather than relying on a single line of evidence, RoseyFields combined laboratory analyses, direct field measurements, engineering calculations, and atmospheric modeling to reconstruct mercury transport from processing equipment through potential occupational and environmental exposure pathways.
The project further demonstrates that comprehensive environmental investigations require evaluation of contaminant generation, emission rates, atmospheric transport, worker exposure, and off-site impacts using scientifically defensible methodologies. Calibration of air dispersion models with measured stack emissions and analytical mass balance calculations provided increased confidence in predicted ambient mercury concentrations and supported evaluation of both occupational and community exposure scenarios.
This investigation was conducted under the direction of Dr. Paul Rosenfeld, an environmental scientist and environmental forensic expert with extensive experience in heavy metals investigations, air dispersion modeling, industrial emissions, occupational exposure assessment, contaminant fate and transport, and environmental risk assessment. Through RoseyFields LLC, Dr. Rosenfeld provides technical evaluation of industrial emission sources, atmospheric dispersion modeling, stack emission analysis, occupational exposure reconstruction, environmental forensic investigations, regulatory review, and expert witness services involving mercury, metals, industrial processes, and complex environmental contamination.
References
California Department of Industrial Relations, California Occupational Safety and Health Administration (Cal/OSHA). Mercury in the Workplace. May 2016.
U.S. Environmental Protection Agency. AERMOD: Description of Model Formulation.
U.S. Environmental Protection Agency. Building Profile Input Program (BPIP) User’s Guide.
Peak Environmental Health & Safety Engineering LLC. Exposure Assessment – Airborne Mercury, Perkins Facility Minerals Processing.
Soil Water Air Protection Enterprise (SWAPE). Evaluation of Potential Mercury Emissions from Gold Purification and Processes at the Perkins Facility.
EMSL Analytical, Inc. Stack Testing Analytical Results.
Sunland Analytical. Analytical Mercury Mass Measurements for Mineral Concentrates.