Aliso Canyon Air Quality Criteria

This page was last reviewed on July 28, 2016

July 26th, 2016. After several months of typical ambient methane levels in the Porter Ranch community the Air Resources Board removed their 5 methane monitors (Sites 1, 2, 5, 7, 8) in late July 2016. The South Coast Air Quality Management District will continue to operate 3 real time monitors, and will operate one of the two benzene monitors previously run by ARB. The Air Quality Criteria will now be updated weekly based on real time data from the three SCAQMD sites and the canister samples that will be collected once every six days. ARB continues to make methane measurements in the Los Angeles basin along with its collaborators, and also intends to make additional downwind flights to measure total methane from the Aliso Canyon Natural Gas Storage Facility. The website will be updated whenever new data is available.

Background

The South Coast Air Quality Management District (SCAQMD) and California Air Resources Board (ARB) staff have jointly developed a set of criteria to determine when air quality in the surrounding communities of the Aliso Canyon facility has returned to typical conditions – a checklist to determine when the air quality is no longer being impacted by the leak. Both agencies will continue to conduct air quality monitoring in and around the communities neighboring the facility for the foreseeable future.


*Results will be posted in 2-4 days after samples have been collected for laboratory analysis and quality assurance review. Laboratory samples are collected on a routine basis except for mercaptans. Laboratory samples are also collected for verified nuisance complaints or collected when the real time monitor data exceeds certain concentration thresholds, including samples for mercaptans. If a laboratory sample for mercaptan analysis was not collected, it is determined the criteria was met based upon the real-time monitoring.

The Air Quality Criteria are based mainly on measurements of the chemicals in natural gas of concern: methane, hydrogen sulfide, mercaptans, and benzene. The above table shows whether the criteria have been met on any individual day. The table differentiates between real-time monitoring at the 8 monitoring sites operated by SCAQMD and ARB staff, and samples that are collected in the field and returned to the laboratory for analysis. These samples may take 2-4 days before results are finalized, and are listed as pending until they are available. Lab samples considered in the evaluation include those collected by ARB, SCAQMD as well as SoCalGas.

Description of Determination Criteria for ARB and SCAQMD Monitoring of Aliso Canyon Well Leak Site (PDF, February 16, 2016)

Why Do Some Lab Samples Not Meet the Criteria?

The Laboratory Samples Criterion is being marked ‘Not Met’ due to 12 hour samples collected by SoCalGas that have methane greater than the threshold of 3ppm. All other Laboratory Samples met their perspective criterion, specifically for mercaptans, benzene and hydrogen sulfide. The map below shows where in the community those samples were located and the chart below shows the measured methane for all the 12 hr samples collected by SoCalGas. It is possible that the continued off gassing of residual methane is causing these exceedances. ARB and SCAQMD are investigating to determine the cause of these results.

map of 12 hour samples with methane concentrations greater than 3ppm
Map of 12 hour samples with methane concentrations >3ppm

Maximum methane 12-hour concentration chart
Click here to see larger image of 12-hour methane concentration chart. The above chart shows the 12 hour methane concentrations from SoCalGas sampling compared to the Criteria. The Night samples are collected from 6:00 pm to 6:00 am the following day. A Night sample >3ppm therefore require the designation of two days as ‘Not Meeting Criteria’. Each dot represents a sample collected at one of the sites shown on the map above.

More Information

Provided below are the data and rationale for the daily evaluation of the air quality criteria. The webpage will be expanded as more information becomes available in a format that can be posted.

Methane

Criteria from the joint SCAQMD/ARB document:

No “grab” (an instantaneous sample collected for subsequent laboratory analysis) measurements of methane concentrations conducted by SCAQMD, ARB, or SoCal Gas higher than 5 parts per million (ppm), nor hourly averages higher than 4 ppm, nor 12-hour or 24-hour averages higher than 3 ppm at the Aliso Canyon facility fenceline or in the Porter Ranch or surrounding communities, unless it is demonstrated that a source other than the Aliso Canyon facility is causing the higher measurements. Such other pollution sources will be fully investigated and appropriately addressed.

The following graphs show the real time data and the criterion:








The methane analyzer at Site #8 has been temporarily removed for maintenance and calibration
.

Hydrogen Sulfide

Criteria from the joint SCAQMD/ARB document:

No measurements of hydrogen sulfide (H2S) conducted by SCAQMD, ARB, or SoCal Gas higher than 10 parts per billion (ppb) (substantially below the California 1-hour standard of 30ppb for H2S) at the Aliso Canyon facility fence line or in the Porter Ranch or surrounding communities, unless it is demonstrated that a source other than the Aliso Canyon facility is causing the higher measurements. Such other pollution sources will be fully investigated and appropriately addressed.

The following graphs show the real time data and the criterion:



Mercaptans

Criteria from the joint SCAQMD/ARB document:

No measurements of mercaptans conducted by SCAQMD, ARB, or SoCalGas higher than 5 ppb at the Aliso Canyon facility fence line or in the Porter Ranch or surrounding communities, unless it is demonstrated that a source other than the Aliso Canyon facility is causing the higher measurements. Such other pollution sources will be fully investigated and appropriately addressed.

Mercaptans are added to natural gas to make it detectable through smell. If methane is below its criteria set above, the critieria for mercaptans will therefore be considered met. It should be noted that the threshold of 5ppb, the analytical detection limit, has not been exceeded since the leak started on October 23, 2015.

Instrumentation for real time measurements of mercaptans does not currently exist, and to determine levels of mercaptans a sample needs to be collected and brought to an analytical laboratory for analysis. All samples collected by SoCalGas (through 2/15/2016) have been below the analytical detection limit for mercaptans. The analytical detection limit, typically 5ppb, is much higher than the level at which mercaptans can be smelled. Additionally SCAQMD is analyzing for mercaptans in samples that are collected automatically at sites 3 and 4 whenever methane exceeds a threshold. This threshold is being lowered to 5ppm to be consistent with the AQ Criteria for methane. SQAQMD will also continue to collect samples whenever they get an odor complaint. Finally, ARB and SCAQMD collected a large number of samples on 2/16/2016 that were subsequently analyzed for mercaptans. These samples also had mercaptans below the laboratory detection limit.



Benzene

Criteria from the joint SCAQMD/ARB document:

No grab or 1-hour average measurements of benzene concentrations conducted by SCAQMD, ARB, or SoCal Gas higher than 2ppb (substantially below the California acute exposure standard of 8 ppb), nor 12-hour or 24-hour averages higher than 1ppb (equivalent to the California chronic exposure standard of 1 ppb) at the Aliso Canyon facility fence line or in the Porter Ranch or surrounding communities, unless the methane thresholds are not exceeded, which demonstrates that a source other than the Aliso Canyon facility is causing the higher benzene measurements. Such other pollution sources will be fully investigated and appropriately addressed.

The following graphs show the real time data and the criterion:

The benzene analyzer at Site #7 has been temporarily removed for maintenance and calibration. 


Infrared Camera

Criteria from the joint SCAQMD/ARB document:

Infrared cameras used at the Aliso Canyon facility by SCAQMD, ARB, or SoCalGas detect no natural gas leaks at the well SS-25 and within a radius of 1000 feet surrounding well SS-25.

ARB has a webpage dedicated to the efforts to collect infrared camera videos and images. Individual movies can be seen of the SS-25 well and the surrounding areas. Periodic small puffs of methane can be seen during the days after February 11, 2016 that represent controlled release of natural gas to relieve pressure in the well during the final sealing process of the well. These discrete event are not being treated as leaks.


Aircraft Measurements

Criteria from the joint SCAQMD/ARB document:

Methane measurements of the downwind plume of the Aliso Canyon facility conducted by Scientific Aviation using a specially instrumented aircraft show a large and sustained reduction compared to the release rate of 20,000 kilograms per hour measured prior to the establishment of control of the well on February 11, and indicate only limited, residual, off-gassing from the soil not anticipated to affect air quality in the Porter Ranch or surrounding communities.

Results of the Scientific Aviation are posted on the Aliso Canyon Main Webpage.


For more information, contact David Clegern by email or by phone at (916) 322-8286.

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