County groundwater report: PETRA modeling finds more Denver Basin volume than SB5 allocations; wells show small declines with local margin concerns

Consultants working on the Douglas County Water Plan presented a draft groundwater analysis that compares modern PETRA-based geophysical data with the legal quantification method under Senate Bill 5 and extended an earlier USGS trend analysis through 2025.

"Based on current data and current tools, what is the quantification of the Denver Basin groundwater," said Bill Fronczak of LRE Water, summarizing the technical objectives. Fronczak said the team used PETRA geologic modeling and compiled more than 10,000 well records countywide to assess storage and saturated thickness across the Denver Basin aquifers.

The consultants reported 10,375 wells in the county’s inventory after including mountain/forested wells and fractured-granite completions. They said municipal providers more often access deeper aquifers (Denver and Arapahoe) while many individual on-lot wells tap shallower units such as the Dawson. The team analyzed roughly 555 wells for geophysical cross-sections and used about 98 wells with reliable single-aquifer completion data for a trends analysis.

Fronczak said PETRA-based gross-availability results indicate more recoverable water in the Lower Dawson, Denver and Arapahoe aquifers than the SB5 (and SB213-era) legal allocations reflect. He attributed the difference to newer and denser geophysical logs and subsurface data that were not available when the SB5-derived calculations were made.

"We’re seeing more water availability based upon our analysis," Fronczak said, but he cautioned that PETRA quantities are gross numbers that are not synonymous with sustainably extractable annual yield. He noted the analysis removes pre-1973 (pre-SB213) rights and certain nontributary actual volumes from gross totals to avoid double-counting protected rights.

On groundwater-level trends, Fronczak reported relatively small net declines across the county in the Upper and Lower Dawson over the past 15 years based on the usable well records reviewed, with larger localized declines nearer the hogback (the western margin). He said confined and unconfined behavior varies by aquifer and by location, and that water-level interpretation is complicated by wells that are pumped at the time of measurement and by a scarcity of dedicated Denver Basin groundwater-monitoring wells.

The report flagged a practical concern for many individual well owners: a large share of private wells do not penetrate the full aquifer thickness and are screened or pumped at shallower levels. That increases the risk that individual property owners will need to redrill or lower pumps to access deeper, higher-quality production zones — a costly prospect.

Fronczak gave county-scale numbers but emphasized caveats. He cited an example of roughly "north of 300,000 acre-feet" in gross estimated Denver Basin water in unincorporated areas and a countywide gross figure discussed in the presentation; he said those numbers represent modeled storage estimates rather than an annual sustainable yield. Commissioners and staff said they still expect to overlay provider-specific supply and demand data in the final county report to determine whether and where renewable surface supplies will be necessary for long-term planning.

Commissioners asked the consultants to finalize the groundwater chapter, provide cleaned GIS outputs and deliver the complete supply-and-demand analysis so the board and county staff can evaluate policy options, including whether the county should encourage shared deeper wells or prioritize connections to municipal renewable-water systems in growth areas. Consultants said they expect the draft groundwater chapter and interactive GIS to be available within a few weeks and will respond to follow-up data requests.

The commission took no formal vote on the groundwater results; members discussed next steps for briefing county commissioners and the county’s legislative lobbyists about permitting and groundwater policy implications.