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Castle Rock to buy phase 1 of Tallgrass Box Elder project after treatment-plant constraint
Summary
Town staff told council that East Cherry Creek Valley cannot treat Castle Rock’s groundwater under the proposed arrangement, and the town will proceed with a $15,000,000 purchase of phase 1 infrastructure while evaluating alternatives with Aurora and WISE to avoid duplicate desalination facilities.
Water department staff (Mark Marlowe) updated the council on the Tallgrass Box Elder project and said new information about the East Cherry Creek Valley (ECCV) treatment plant changed options for phase 1b and phase 2.
Marlowe explained ECCV's plant is not permitted to treat "groundwater under the influence of surface water," which describes Castle Rock's collected groundwater; that limitation requires the town to consider on-site treatment or other routing options. Tallgrass completed phase 1 infrastructure (rehabbed wells, collection piping and controls) and provided cost estimates for further phases ranging from approximately $41 million to $55 million, depending on financing.
Because the parties could not finalize agreement terms by the contract deadline, Marlowe said the town will move forward with its planned $15,000,000 purchase of the phase 1 infrastructure that is already budgeted. He also discussed alternatives, including partnering with Aurora Water and the WISE partners to treat water regionally to avoid building two separate desalination facilities; Marlowe cited a $270,000,000 estimate for a 12-million-gallon-per-day desalination plant and said a regional approach could reduce Castle Rock's share of that cost.
Marlowe also noted interim leasing opportunities: the town can lease water while infrastructure integration continues; staff cited typical municipal lease rates of about $400 per acre-foot and oil-and-gas industry rates around $3,100 per acre-foot and referenced a $400,000 take-or-pay lease component in the Tallgrass proposal.
Council asked clarifying questions about treatment options, distances involved (the project spans many miles, with some collection points 60+ miles away), and the budget/closing timeline. Marlowe said the town expects to close on the phase 1 purchase in the coming weeks and will continue negotiations about routing water to Aurora's campus and other options.
