HomeMy WebLinkAbout1.06 Impact Analysis
North Hangs Mine
February 2023 27
Impact Analysis
Sections 7-1002 and 7-1003 of the Garfield County Land Use and Development Code (Code)
address mitigation efforts required for permitted gravel mining operations in Garfield County.
Garfield County requires adherence to Colorado State Statutes (CRS) §25-12, which
establishes statewide standards for noise level limits for various time periods and areas.
1. General Impact Mitigation
All impacts discussed in this section are primarily mitigated by two factors: 1) the inherent
topographical differences between the mining operations onsite and 2) the surrounding land and
groves of trees along the Colorado River that will be maintained. All mining activities will take
place in an area of the site that sits below the existing grade, with the exception of topsoil
stripping and initial stockpiling. Processing will occur several feet below ground level to reduce
noise propagation. Topsoil stripping will be conducted in short term periods of the year, typically
in the early spring or late fall. Trees will be planted to in-fill the existing vegetation along the
Colorado River and along the south side of processing and stockpiling. These trees will mitigate
the visual and noise impacts of the operation to the Town of Silt, Colorado and the I-70 corridor.
Note that any reference to ‘Exhibits’ or ‘Maps’ refers to Exhibits and Maps contained within the
attached Colorado Division of Reclamation, Mining, and Safety (CDRMS) application
components for the North Hangs Mine unless specifically stated otherwise.
2. Adjacent Land Use
The proposed Land Use Change would allow for the development of a gravel mine at the
location. This use is consistent with the industrial/commercial aspect of adjacent land including
an existing gravel mine 1000 feet southeast of the proposed North Hangs Mine and the I-70
corridor along the southern boundary of the property. To the west is agricultural land used as
irrigated pasture for cattle. To the north is the Colorado River, with Colorado Parks and Wildlife
open space across the river to the northeast. Across the river to the northwest is agricultural
land consisting of rangeland and a single residence.
3. Site Features
Map C-1 of the CDRMS permit application in Appendix 4 shows the site features including
floodplain area, wetlands, water pathways, ditches, topography, vegetative cover, and more.
4. Soil Characteristics
A soil report was generated using the United States Department of Agriculture’s NRCS Web
Soil Survey (WSS) and is included in the CDRMS permit application in Appendix 4 as part of
Exhibit I. The WSS provides soil data and information produced by the National Cooperative
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Soil Survey. The majority of the site (89.6%) is split between three soil types, Wann,
Halaquepts, and Torrifluvents (see Map C-1). Each of these primary soil types are described on
the following pages.
Suitability for Reclamation and Landscaping
All three soil types provide an average of at least 12 inches of suitable material for revegetation.
All three soils are listed as class VII without irrigation. Class VII are not suitable for cultivation
and are restricted to grazing by domestic animals or wildlife; all are listed as climatically limited
soils.
Soil Type Descriptions
Wann sandy loam, 1 to 3 percent slopes
Map Unit Setting
National map unit symbol: jnzc
Elevation: 5,000 to 6,500 feet
Farmland classification: Prime farmland if irrigated and reclaimed of excess salts and sodium
Map Unit Composition
Wann and similar soils: 85 percent
Minor components: 15 percent
Estimates are based on observations, descriptions, and transects of the mapunit.
Description of Wann
Setting
Landform:Terraces, valley floors
Landform position (three-dimensional):Tread
Down-slope shape: Linear, convex
Across-slope shape: Linear, convex
Parent material: Alluvium derived from sandstone and shale
Typical profile
H1 - 0 to 8 inches: sandy loam
H2 - 8 to 60 inches: fine sandy loam, sandy loam, coarse sandy loam
H2 - 8 to 60 inches:
H2 - 8 to 60 inches:
Properties and qualities
Slope:1 to 3 percent
Depth to restrictive feature: More than 80 inches
Drainage class: Poorly drained
Runoff class: Very low
Capacity of the most limiting layer to transmit water (Ksat): High (2.00 to 6.00 in/hr)
Depth to water table: About 0 inches
Frequency of flooding: OccasionalNone
Frequency of ponding: None
Calcium carbonate, maximum content:10 percent
Maximum salinity: Nonsaline to very slightly saline (0.0 to 2.0 mmhos/cm)
Available water capacity: Very high (about 26.3 inches)
Interpretive groups
Land capability classification (irrigated): 4w
Land capability classification (nonirrigated): 6w
Hydrologic Soil Group: A/D
Ecological site: R048AY265CO - Salt Meadow
Hydric soil rating: Yes
Custom Soil Resource Report 18
Minor Components
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Torrifluvents
Percent of map unit:5 percent
Hydric soil rating: No
Kim
Percent of map unit:5 percent
Hydric soil rating: No
Arvada
Percent of map unit:5 percent
Hydric soil rating: No
Halaquepts, nearly level
Map Unit Setting
National map unit symbol: jnxr
Elevation: 5,400 to 7,400 feet
Frost-free period: 101 to 135 days
Farmland classification: Not prime farmland
Map Unit Composition
Halaquepts, nearly level, and similar soils:85 percent
Estimates are based on observations, descriptions, and transects of the mapunit.
Description of Halaquepts, Nearly Level
Setting
Landform:Terraces, fans, valleys
Landform position (three-dimensional):Tread
Down-slope shape: Convex, linear
Across-slope shape: Convex, linear
Parent material: Alluvium
Typical profile
H1 - 0 to 8 inches: clay loam
H2 - 8 to 24 inches: loam
H3 - 24 to 60 inches: stratified very gravelly cobbly sand
Properties and qualities
Slope:0 to 6 percent
Depth to restrictive feature: More than 80 inches
Drainage class: Poorly drained
Runoff class: Medium
Capacity of the most limiting layer to transmit water (Ksat): Moderately high (0.20 to 0.60 in/hr)
Depth to water table: About 0 inches
Frequency of flooding: Occasional None
Frequency of ponding: None
Calcium carbonate, maximum content:10 percent
Gypsum, maximum content: 5 percent
Maximum salinity: Moderately saline to strongly saline (8.0 to 16.0 mmhos/cm)
Sodium adsorption ratio, maximum:30.0
Available water capacity: Low (about 3.9 inches)
Interpretive groups
Land capability classification (irrigated): None specified
Land capability classification (nonirrigated): 6w
Hydrologic Soil Group: C/D
Hydric soil rating: Yes
Torrifluvents, nearly level
Map Unit Setting
National map unit symbol: jnz3
Custom Soil Resource Report 16
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Elevation: 5,000 to 7,000 feet
Mean annual precipitation: 12 to 15 inches
Mean annual air temperature: 46 to 48 degrees F
Frost-free period: 90 to 120 days
Farmland classification: Not prime farmland
Map Unit Composition
Torrifluvents and similar soils:85 percent
Minor components:15 percent
Estimates are based on observations, descriptions, and transects of the mapunit.
Description of Torrifluvents
Setting
Landform: Distributaries, rivers, flood plains
Down-slope shape: Linear, convex
Across-slope shape: Linear, convex
Parent material: Alluvium
Typical profile
H1 - 0 to 36 inches: loam
H2 - 36 to 60 inches: sand
Properties and qualities
Slope:0 to 6 percent
Depth to restrictive feature: More than 80 inches
Drainage class: Moderately well drained
Runoff class: Low
Capacity of the most limiting layer to transmit water (Ksat):Moderately high to high (0.60 to 2.00
in/hr)
Depth to water table: About 12 to 36 inches
Frequency of flooding: Occasional None
Frequency of ponding: None
Calcium carbonate, maximum content:5 percent
Gypsum, maximum content:1 percent
Maximum salinity: Very slightly saline to moderately saline (2.0 to 8.0 mmhos/cm)
Sodium adsorption ratio, maximum:2.0
Available water capacity: Moderate (about 7.6 inches)
Interpretive groups
Land capability classification (irrigated): None specified
Land capability classification (nonirrigated): 7w
Hydrologic Soil Group: C
Hydric soil rating: No
Minor Components
Wann
Percent of map unit:10 percent
Landform: Terraces
Hydric soil rating: Yes
Fluvaquents
Percent of map unit:5 percent
Landform: Marshes
Hydric soil rating: Yes
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5. Geology and Hazard
A geologic hazards study by Kumar & Associates was conducted on the property. It can be
found in Appendix 3.
6. Groundwater and Aquifer Recharge Areas
Mining activity at the North Hangs Mine will be conducted below the groundwater table. This is
facilitated through the dewatering process during mining; dewatering pumps drain the active pit
of groundwater to allow for mining activity to take place below natural grade. The dewatering of
the pits will take place under an approve CDPHE discharge permit and CDWR gravel well
permit.
Groundwater is located approximately four feet below the surface of the ground onsite. No wells
are located within 600 feet of the proposed pit 2. Mining activity will expose groundwater from the
very beginning of operations. Dewatering of the pit will take place to bring the groundwater table
below the operating floor. This allows equipment to safely and effectively mine the alluvial
deposit. The pit pump will be active continuously during the mining season, which typically is
mid-April to early-November, depending on weather. The pit pump will not run during flood
events or when temperatures drop below freezing.
The only known aquifer in the area is the shallow alluvial aquifer of the Colorado River. Depth to
water table in the permit area varies depending on location and time of year but averages
roughly four feet below ground surface. According to the U.S. Geological Survey’s Ground
Water Atlas of the United States, the only groundwater aquifer system below the alluvial one in
this area is the Unita-Animas Aquifer System. This is located beneath the Mancos Shale that
underlays the site alluvium and will not be reach by mining or affected by mining
Wastewater and sewage will be stored in a portable vault toilet that will be service by a
contractor. This will prevent sewage or effluent from wastewater from entering the Colorado
River alluvial aquifer.
The 100-YR floodplain of the Colorado River encompasses the entire site. See the Floodplain
Analysis for details.
2 https://gis.colorado.gov/dnrviewer/Index.html?viewer=dwrwellpermit
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7. Environmental Impacts
An environmental impact analysis has been conducted by Environmental Solutions, Inc. and
can be found in Appendix 2.
8. Nuisance
A detailed discussions of all potential impacts to adjacent land from generation of vapor, dust,
smoke, noise, glare or vibration, or other emanations can be found under the Additional
Standards for Gravel Operations section further on in this SUP application.
9. Hours of Operation
Hours of operation at the North Hangs Mine will be limited to Monday thru Saturday, 7am to 7
pm. As specified in Article 7-1001.H.