Air resources

Federal: USEPA established the Federal Clean Air Act which defines the Ambient Air Quality Standards (AAQS).

State: California Air Resources Board established the California Clean Air Act with their own AAQS.

Local: Great Basin Unified Air Pollution Control District (GBUAPCD) enforces regulations and administers permits when needed. There are specific rules and regulations applying to geothermal well drilling. The county and the Town of Mammoth Lakes both have an Air Quality Management plan.

Hydrogen sulfide (H2S), diesel particulate matter (DPM), oxides of nitrogen (NOx), and ozone are pollutants of concern released during construction and operation.

Regulatory O3 air quality levels will be exceeded in the short term with the production of NOx during construction and in the long term with the release of n-pentane due to plant operation. Even if the best technology available is used for this project, these residual impacts remain. However, this project is situated in an area where O3 and PM10 AAQS exceedance is tolerated.

Since the closest sensitive receptor from the plant is a campground 0.5 miles (0.80 km) away, and the closest residences are 0.8 miles (1.29 km) from the project area, only a qualitative health risk assessment is needed. Moreover, air pollutants will be carried away from areas at risk due to favourable dominant wind direction and power plant location.

See Tables 1 and 2 below.

Cumulative effects

Exact timing of nearby projects is unknown. Thus, no estimate of cumulative emissions can be made.

Mitigation

More efficient equipment will be used to reduce emissions and prevent fugitive dust release. A plan for n-pentane management will be submitted for approval by the Great Basin Unified Air Pollution Control District (GBUAPCD) Air Pollution Control Officer. Instruments will be acquired to monitor H2S concentrations and Volatile Organic Compound (VOC) leaks. Additional mitigation will be put in place if these emissions reach unacceptable levels.

Residual impacts

An unavoidable and significant amount of NOx and reactive organic gas (ROG) will be emitted during construction and operation respectively.

There is no monitoring station other than for PM10 in the county. As the proponent did not plan to monitor PM 2.5 either, we can only rely on estimates.

The definition of immediate vicinity around the project is unclear.

Based on current literature, the EIA’s assessed risk distance is reasonable for the major pollutants of concern (H2S, DPM, and n-pentane), except for NOx. In fact, modelling and observations suggest that this pollutant is persistent and can be carried over more than 5 km, which is six times the distance to the closest sensitive receptor. The fact that the maximum day emissions of NOx is three times the Imperial County Air Pollution Control District’s (ICAPCD) significance threshold is also concerning.

More transparency about the predicted air pollution dispersal path could be provided with modelling, which would allow us to assess when the plant's discharges are more likely to impact sensitive receptors. Dominant wind is from the west, away from the Mammoth Lakes community, but easterly wind occur 10-20% of the time year round. The public should be notified of this risk.

The cumulative effects section seems perfunctory. As some projects overshoot ROG and NOx air standards with their own emissions alone, considering the exact timing of each project is unnecessary to know that the study area will receive significant air pollution. Moreover, some pollutants linger, so an activity end date would be unrepresentative of the pollutant presence onsite.