2020-06-24_REVISION - C1981035 (2)

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ENVIRONMENTAL ASSESSMENT Once the construction of the low-cover crossing is completed,the subsurface entries would be buried by at least 4 ft of compacted alluvium in the center of the Gulch where the dry streambed is incised into the Gulch. To the southeast of the intermittent streambed, the compacted alluvium cover would thicken progressively but unevenly and at the point of intersection with the coal seam outcrop on the southeast flank of the Gulch,would be approximately 25 ft deep. Moving west from the intermittent streambed,the compacted alluvium cover would thicken more quickly and evenly than on the southeast side and reach approximately 50 ft thick on the northwest flank of the Gulch at the intersection of the entries with the coal outcrop.Once the crossing is being utilized,noise from the passage of mining equipment and vehicles, and operation of the conveyor system through the buried crossing, would be attenuated by the compacted cover material. Direct noise effects would be negligible to minor and limited to directly above the buried crossing entries. The principal noise sources related to the continued mining operation of the surface facilities include the mine ventilation fan, conveyors, and warning alarm signals of load-out equipment. Noise from mining operations were determined (CDS 2014) to result in no impact. Mitigation of mine-site noise is not required. Indirect noise impacts are associated with the truck-traffic on CR 120 from coal transport. There are no published data measuring the noise or vibration associated with coal transport, but residents along CR 120 have reported noise and vibration levels that adversely affect their quality of life. State statute set noise limits for residential properties of 55 dBA daytime (7:00 a.m.to 7:00 p.m.) and 50 dBA nighttime at 25 ft from the property line. All haul trucks measured had noise levels below 86 dBA, a cited limit established by the State of Colorado.All noise measurements collected and/or reviewed by the applicant and LPC noise consultants agreed with the outcome of the noise monitoring results. Overall, it was determined that following GCCE's implementation of noise mitigations (i.e., installation of noise barriers), noise impacts would be negligible to minor, lasting for the duration of mining activities. Vibration Vibration studies did not find vibration to be above detectable levels at locations directly above the underground mining equipment and the surface facilities (CDS 2013).The peak vibration recorded at area residences was approximately eight times lower than the threshold for human perception (0.03 inches/second) and approximately 14 times lower than vibrations which could cause hairline cracking in historic plaster walls (minimal). These same levels of impacts would be reasonably expected to occur during mining of the Project Area, operation of the conveyor system and passage of mining equipment through the low-cover crossing. In response to continued resident comment, GCCE took measurements on the ground surface directly above the continuous miner equipment—approximately 300 ft above the continuous miner. No induced vibrations were recorded. Vibrations drop below the level detectable by human perception within 25 ft of this type of equipment(Aimone-Martin Associates 2015). Based on this information, it was determined that mining operation vibrations do not pose a significant impact from the Project, and therefore do not require further mitigation. Dunn Ranch Area Coal Lease by Application COC-78825 and Mine Plan Modification EA 3-24