Noise Engineers has been assisting in monitoring and evaluating the noise radiating to greater sage grouse leks near a new mining operation in Nevada for two seasons. The noise thresholds were set based on the pre-project noise levels measured in 2013. We are conducting continuous noise measurements during mating season to prevent impacts from the mine.
The greater sage grouse males make noises to attract a mate and environmental noise can interfere with that process. The greater sage grouse is not endangered but it is protected. We are working with the BLM, who has jurisdiction on this land and this project.
There is not a clear noise limit or guideline that can be used so we are all relying on the latest science. In this case, the latest science comes from a University of California Berkley biology professor, Dr Patricelli. She has done many studies related to gas pipelines near greater sage grouse breeding areas (leks).
Dr Patricelli states in her published studies that one should measure the existing L90 noise level before the operation (in this example a mine) is in place and operating. Then measure the L50 noise level once the mine is operating and it cannot be more than 10 dBA above the L90. We are only concerned about the noise level during breeding season (March 1 to May 15) and only during the morning hours when the birds are active (1hr before to 3 after sunrise).
The hourly L50 noise level is the median noise level, the noise level exceeded 50% of the time period (hour). The L90 is the noise level exceeded 90% of the time period (hour).
This is a key point: know what you are agreeing to. An agreement was made between the BLM and the mine before we became involved in the project during the 2014 season. In this case, the BLM said that the mine had to measure every morning during the entire mating season for as many years as it took to show that there was no longer a potential noise issue. If the mine could not identify the noise source, it was assumed that it came from the mine. If the mine had 7 hourly exceedances during any season then the mine could not operate during the morning hours.
The noise limits were set based on the noise measurements made during one week in 2013, before the mine was in operation. This one week measurement is not necessarily representative of the non-mine noise conditions during the breeding season during future years.
IDENTIFYING NOISE SOURCES
During the pre-mining noise measurements, in 2013, there were exceedances (the L50 was more than 10 dBA higher than the L90) during 4 of the 7 days. Hence, we expected many exceedances even without any mining activity. During each breeding season there were many exceedances that we needed to identify.
Ideally, there would be a person sitting out at the lek observing the noise sources or visiting the site occasionally. This could not be done because the BLM was concerned that it would disturb the greater sage grouse at the leks.
Since this was not an option, we installed weather stations at each lek and additional stations near the most significant noise sources in the vicinity. One close to the mine and one close to the major road in the vicinity.
We used sound level meters have the ability to measure hourly and minute noise data and the frequency spectrum. They also save an audio recording of each measurement. Because the site is remote, it was less expensive gather the data from a cell signal than to have someone download the data each day.
Most of the exceedances came from wind and from birds. After listening to several recordings and looking at the sound spectrum, we could quickly identify these sources and disregard these exceedances. It became more difficult when there was an equipment failure and we did not have weather data, a recording or noise data from one of the locations.
On several occasions, we did measure noise from heavy equipment noise. We correlated the noise measurements with the recordings at the mine to determine if it was coming from that direction. We also used the mining schedule to identify potential sources. We did identify a watering truck caused two exceedances and a nearby farm caused several others. My understanding is that the farm, while closer to the leks, did not need to conduct a noise monitoring program.
IMPACT OF NOISE MONITORING
One problem with this was that there is not an easy way to determine the impact of noise on the greater sage grouse breeding. You cannot see population changes from a single season to evaluate the effectiveness of the program and even if you did, you would need to evaluate control leks to minimize the influence of other variables.
A frustration with this project is that our extensive work may not have had very much impact on the greater sage grouse population in this region. There may have been other motivations behind requiring such an extensive and rigid noise monitoring program.
The mine hired a biologist to track what is happening to the greater sage grouse at the leks and found that ravens were eating many of the young. Ravens were the single largest contributor to the reducing greater sage grouse populations at these leks. Ravens, however, are protected, so reducing the raven population was not an option.
Another oddity is that one can buy hunting license and hunt greater sage grouse. Greater sage grouse populations are going down and the bird is protected but you can still hunt them.
Noise Engineers provides information and resources to help people address acoustical issues. In these episodes my goal is to provide resources, inexpensive tools, rules of thumb when dealing with acoustical issues. I would like to explain basic acoustic principles and answer any questions. I will describe actual projects to make this as practical as possible.