Hydrogen Sulfide Monitoring on the Gulf Coastline

Hydrogen sulfide (H₂S) is associated with some oil and natural gas extraction. The oil being spilled in the Gulf, however, is called "sweet," a term applied to oil with low sulfur and H₂S. Hydrogen sulfide can also come from marshes and sewage treatment plants, and decaying organic materials.  It is often described as a "rotten egg" odor. More information on H₂S can be found on ATSDR's Toxicological Profile for Hydrogen Sulfide

EPA has been monitoring for H₂S in a number of locations near Venice and Mobile, Alabama.  While concentrations in urban areas are generally below 1 part per billion (ppb), concentrations near specific natural or manmade sources can be much higher. In most areas where these monitors were deployed, we have seen H₂S values below detectable limits. However, in the Venice area we have seen multiple hourly values above 0.1 (the lowest detectable limit on the monitors that are in use) and some above 0.5 parts per million (ppm).  The lack of similar H₂S readings at nearby monitors indicates that the source is unlikely to be from the oil spill and more likely is related to a local situation unrelated to the spill. 

There are various types of H₂S monitors. The H₂S monitors being used in these locations are emergency response monitors, which are usually deployed in industrial emergencies.   While these monitors have advantages in emergencies, they have limitations in their resolution at lower concentrations. Additionally, the accuracy of these monitors can be affected by factors such as high humidity, and there is some concern that some H₂S values may be incorrect due to instrument error. 

A more sophisticated H₂S monitor is in use in Chalmette, LA, (approximately 50 miles from Venice)  This monitor is operated by the Louisiana Department of Environmental Quality (LDEQ), and is reporting H₂S levels significantly lower than what has been measured by the emergency response monitors.  Typical values from the LDEQ monitor ranged from 0.002 to 0.004 ppm during this same time period.  

To detect H₂S at levels below the emergency response monitor’s limits, EPA has set up high-resolution H₂S monitoring equipment near Venice; co-locating it with an existing emergency monitor.  This will help determine if the values being recorded by the emergency response monitors are indeed correct or if they are biased by the instrument and the weather effects. Co-location of the two instruments will also allow us to determine if the elevated readings are a result of instrument problems with the areaRAE or are indeed real values that will warrant further investigating.  If we do identify these elevated values to be accurate, EPA will work to determine the source of these values.

The high-resolution data are presented on the AirNow page

*Please note that from 6:00 a.m. (CST) July 23, 2010, through 2:00 p.m. (CST) July 27, 2010, the monitor was relocated and taken offline during Tropical Storm Bonnie. No data were reported during this time.

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