Arapahoe County proposal could reduce groundwater-based development capacity
Aquifer-specific production factors could lower the number of homes supported by groundwater in central and eastern Arapahoe County, but the draft would not change existing approvals.
Arapahoe County is considering aquifer-specific production factors that could reduce the number of homes supported by groundwater in parts of the county. The draft proposal has not been adopted and would apply to future rezonings, planned developments, subdivisions and special-use permits that rely on Denver Basin groundwater.
County commissioners discussed the proposal Sept. 21 during an informational study session but took no action. A Planning Commission hearing is scheduled for Oct. 20, followed by a Board of County Commissioners hearing Nov. 24.
The draft Water Supply Element, part of Arapahoe County’s required update to its 2018 Comprehensive Plan under Senate Bill 24-174, would apply aquifer-specific factors to the state’s 100-year groundwater calculation during development review.
The draft says Petra hydrogeologic modeling found that the state calculation can overestimate sustainable well production because it does not fully account for regional drawdown, well-to-well interference and declining aquifer flow capacity. The modeled physical availability averaged about 60% of the amount calculated under state Denver Basin allocation rules.
The proposed factors are 0.60 for the Denver aquifer, 0.42 for the Upper Arapahoe, 0.27 for the Lower Arapahoe and 0.71 for the Laramie-Fox Hills. The Lower Arapahoe would face the largest reduction: 839 acre-feet per year modeled as physically available, compared with 3,145 acre-feet under the state calculation.
A county presentation used a hypothetical residential example to illustrate the potential effect: An allocation that supported 100 homes under the state calculation would support about 59 homes using the proposed factors. The example is a planning calculation, not a denial or reduction of an existing project.
The proposal primarily targets Region 2, the central and eastern growth area where groundwater and local alluvial wells supply much development outside centralized systems. Region 1, the predominantly urban western area, relies mainly on imported renewable surface water and municipal providers; the factors would provide a countywide baseline there, subject to site-specific review. Region 3, the eastern rural and agricultural area, relies largely on individual wells and irrigation and is not included in the Petra production-factor analysis. The Dawson aquifer in southwestern Arapahoe County also would be excluded because the draft says its water is already tied to municipalities or older subdivisions.
The 2024 Arapahoe County Water Supply Study says the state’s 100-year allocation remains the operative legal framework and that the county currently requires a water-supply plan at the zoning stage. A 2018 county policy called for limiting annual withdrawals to one-third of the state allocation, a 300-year concept, but the study says that policy never became a regulation.