Identification of Alternatives for Environmental Review
The Bureau of Land Management (BLM) is leading the federal evaluation process for the proposed Wild Horse Reservoir under the National Environmental Policy Act (NEPA) that will evaluate environmental, cultural, and local community impacts. The BLM is conducting this evaluation process and is responsible for preparing a detailed report, called an Environmental Impact Statement (EIS). This report will evaluate and disclose the impacts of the proposed project as well as alternatives that have the potential to meet Aurora’s needs.
Through the process to date, two alternative sites in Park County have been identified for further evaluation: a site just south of the proposed project and a possible expansion of Spinney Mountain Reservoir. With landowner permission, natural resource and cultural surveys will be conducted on the alternatives this spring and summer of this year (late April through August). Additionally, crews may be collecting geotechnical data to assess construction feasibility and potential constraints. Stakeholders will have an opportunity to provide feedback during the EIS process on the project after the BLM issues a Notice of Intent (NOI) to formally begin the study, which is tentatively scheduled for 2026.
Project Alternatives
Wild Horse Reservoir (Proposed Action)
Aurora Water is proposing to build the Wild Horse Reservoir in Park County, Colorado, approximately six miles west of Spinney Mountain Reservoir near Hartsel. The reservoir—which would cover a surface area of approximately 1,700 acres and store 95,000 acre-feet of water—would be off channel, meaning no dams would block existing rivers or streams. Water would be supplied into the reservoir through a new inlet pipeline connected to the existing Otero pipeline. A new outlet pipeline would transport water from Wild Horse Reservoir to Spinney Mountain Reservoir as needed.
Importantly, water stored in the proposed reservoir would be supplied from Aurora Water’s existing Colorado and Arkansas River water rights so no new water rights or decrees would be required.
Wild Horse South Reservoir Alternative
The Wild Horse South Reservoir Alternative would consist of a reservoir storing 95,000 acre-feet of water in Park County, Colorado, approximately 10 miles south of Hartsel and approximately 10 miles southwest of Spinney Mountain Reservoir. The reservoir would be off channel, meaning no dams would block existing rivers or streams. Water would be transported to the reservoir through a new inlet pipeline connected to the existing Otero pipeline. A new outlet pipeline would transport water from the reservoir to Spinney Mountain Reservoir as needed. The reservoir would require one dam. Similar to the proposed action, no new water rights or decrees would be pursued for this alternative.
Small Wild Horse Reservoir + Denver Basin Aquifer Storage and Recovery (ASR) Alternative
The Small Wild Horse Reservoir + Denver Basin Aquifer Storage and Recovery (ASR) Alternative would consist of:
- A reservoir storing 45,000 acre-feet of water located in Park County, Colorado, about six miles west of Spinney Mountain Reservoir near Hartsel.
- A system of deep underground wells and pipelines located within the city of Aurora, Colorado, in Arapahoe County.
The smaller reservoir would be off channel, meaning no dams would block existing rivers or streams. Water would be transported to the reservoir through a new inlet pipeline connected to the existing Otero pipeline. A new outlet pipeline would transport water from the reservoir to Spinney Mountain Reservoir as needed. Similar to the proposed action, no new water rights or decrees would be pursued for this alternative.
The ASR system would include 21 ASR well sites and a network of pipelines for storage in the Denver Basin Aquifer. The ASR wells would be deep aquifer wells that would allow water to be injected and withdrawn from the aquifer. A new pump station at Griswold Water Purification Facility (WPF) would pump the treated water through a bidirectional pipeline network to the ASR wells to inject for storage. The ASR wells would withdraw stored water as needed and then pump the water to a desalination facility for treatment and back to Griswold WPF for treatment and distribution.
Spinney Mountain Reservoir Enlargement Alternative
The existing Spinney Mountain Reservoir is located in Park County below the confluence of the Middle Fork and South Fork of the South Platte River, approximately three miles upstream from Eleven Mile Canyon Reservoir. The Spinney Mountain Reservoir Enlargement Alternative consists of increasing the storage capacity of the existing on-channel (South Platte River) reservoir by an additional 130,000 acre-feet to a total of 184,000 acre-feet. The Spinney Mountain Reservoir Enlargement Alternative would require the existing dam to either be raised by 22 feet or replaced with a new dam constructed downstream of the existing dam. A new saddle dam on the northern end of the reservoir would also be required to complete the enlargement. Similar to the proposed action, no new water rights or decrees would be pursued for this alternative.
No Action Alternative—Denver Basin Aquifer Storage and Recovery (ASR)
The No Action Alternative consists of an ASR system located within the City of Aurora, Colorado, in Arapahoe County. The ASR system would include 21 ASR well sites and a network of pipelines for storage in the Denver Basin Aquifer. The ASR wells would be deep aquifer wells that allow water to be injected and withdrawn from the aquifer. A new pump station at Griswold Water Purification Facility (WPF) would pump the treated water through a bidirectional pipeline network to the ASR wells to inject for storage. The ASR wells would withdraw stored water as needed and then pump the water to a desalination facility for treatment and back to Griswold WPF for treatment and distribution.
