The Colorado Supreme Court affirmed last week the State Engineer’s authority to impose strict volumetric limits on non-tributary groundwater withdrawals, a decision that could reshape water management practices in Douglas County, across the Front Range, and in upstream mountain communities like Estes Park.
The case highlights the growing pressure on groundwater resources in a region increasingly strained by development, climate change, and interconnected water demands.
For municipalities along the Front Range, many of which rely heavily on groundwater from the Denver Basin aquifers, the decision in Parker Water and Sanitation District v. Rein underscores both an imperative to plan for adequate water supplies and to carefully manage the quantity.
But the ruling may is not likely to have significant implications for communities that participate in trans-mountain diversion projects that also supply the growing Front Range and who, like Estes Park, have firm water rights to surface sources.
The state high court addressed an argument by the Parker area’s water supplier that, although the State Engineer can limit annual withdrawals from an aquifer, he cannot also limit those withdrawals to a 100-year period. The justices, in a 6-1 vote, rejected that claim.
“All non-tributary Denver Basin well permits, whether they explicitly state a total limit or not, necessarily include an implicit total volumetric limit of one hundred times the allowed average annual amount of withdrawal,” wrote Justice Maria Berkenkotter in the supreme court’s opinion.
Groundwater use in Colorado
Groundwater plays a crucial role in meeting the water needs of the region that stretches from Fort Collins to Pueblo. Some cities, particularly Aurora, Castle Rock, Greeley, Lone Tree, and Parker, and some densely developed unincorporated areas, such as Highlands Ranch, depend significantly on the aquifer-borne water.
The deep aquifers of the 300-million year old Denver Basin, an underground structure that extends into Wyoming to the north, to the vicinity of Limon in the east, and to the
Pueblo area on its south edge, hold as much as 300 million acre-feet of water. But not
all of the water is accessible. “[T]he usable bank account is estimated to be as much as
150 million acre feet of water,” according to a 1978 publication of a Colorado State
University research institute still considered an authoritative study of the basin.
Because some of the Front Range urban corridor is distant from surface water sources,
use of groundwater has become well-established since the settling of the eastern front
expanse and has grown in recent decades with population.
The trend is especially evident in the fast-growing south suburban Douglas County. According to a 2020 U.S. Geological Survey study, “most wells showed declines on the basis of statistically significant trends and the relative differences in static groundwater-level elevations between” measurements obtained seven years apart.
But cities and towns in Adams County, on the north edge of the Denver Metropolitan Area, are also heavily reliant upon groundwater. According to a 2023 study, 52 out of 76 water providers there primarily depend on groundwater sources, compared to an average of 17% that do so statewide, and, since 1980, groundwater use in the county has more than tripled.
Even in cities that are geared toward surface water use to meet domestic and other needs, such as Greeley and Aurora, groundwater serves a “backup” role. In fast-growing Aurora, about 1.25 billion gallons of water can be expected to be drawn from the aquifers of the Denver Basin each year, while in Greeley, the proposed Terry Ranch Project would assure that city of about 1.2 million acre feet of stored water.
How groundwater is extracted and managed
Extraction and use of groundwater involves drilling deep wells and using pumps to draw
water to the surface, where it is treated and distributed for municipal and residential use.
The Denver Basin system consists of multiple aquifers layered beneath the surface,
each with varying recharge rates and storage capacities. Regulations for groundwater
extraction in Colorado are defined under state law, with oversight provided by the State
Engineer and local water management districts.
Groundwater extraction poses sustainability challenges. Unlike surface water
replenished by snowmelt and precipitation, many of Colorado’s aquifers recharge slowly
or, in some cases, barely at all. USGS has estimated that “natural recharge to the
bedrock aquifers is small, estimated to be about 4,000 acre-feet per year” and that
“groundwater withdrawals from the Denver Basin aquifer system have resulted in water-
level declines of as much as 400 feet in some areas.”
In Colorado, the General Assembly established a statewide system of groundwater regulation in 1965, then updated it several times in subsequent years, most recently in 1985.
Legislators delegated authority to the State Engineer to write conditions for the
granting of required pumping permits.
The Rein Case and Its Impact
The state high court’s ruling in the Rein case reinforces the State Engineer’s power to include total volumetric limits on well permits for non-tributary groundwater. These limits cap the total amount of water that can be extracted over the permit’s lifetime, a practice intended to prevent over-extraction and ensure sustainability.
“The State Engineer’s mandate to set volumetric limits aligns with the broader objective
of preserving Colorado’s vital water resources for current and future generations,”
Berkenkotter wrote for the court majority.
She explained that the legislature explicitly compelled the State Engineer to limit groundwater removal to a 100-year expected well life, rejecting an argument by the Douglas County-based Parker Water and Sanitation District that a permittee could continue to draw out of the aquifer the maximum annual amount authorized by a permit in perpetuity.
For municipalities that lean on groundwater for most of their water supply, the ruling means that water districts must carefully plan their usage to avoid exceeding the volumetric limit and the annual 1% allocation from that limit mandated by the Rein decision. These volumetric restrictions will also prompt local governments and water providers to explore alternative water sources and bolster conservation efforts.
Some have already done so. For example, the Parker Water and Sanitation District, which supplies its city namesake and parts of Lone Tree and Castle Pines, financed the construction of Reuter-Hess Reservoir east of I-25 and Castle Rock has joined with PWSD and the Lower South Platte Water Conservancy District to build reservoirs on the Eastern Plains.
Castle Rock has also committed itself to a system of 100% renewable water by 2065 and has embarked on projects to expand a reservoir near Sedalia and to increase the capacity of its Plum Creek Purification Facility.
Connections to mountain communities and Larimer County
But the consequences of the Rein decision are not likely to be limited to the Front Range and will probably not be confined to efforts to conserve water or take advantage
of water rights on eastern Colorado rivers and streams.
Mountain communities like Estes Park and others in both the Estes Valley and greater Larimer County depend on surface waters that originate in the Rockies and that are often carried across the mountains by means of federally-financed division projects.
Prominent among them is the Colorado-Big Thompson Project, which routes water from
the headwaters of the Colorado River all the way to the South Platte River.
Estes Park’s water supply is significantly dependent on the C-BT, with nearly half of its annual assured quantity delivered at Mary’s Lake. According to a water master management plan finalized in 2015, the city’s 608.5 acre-foot “firm yield” from the C-BT, which represents 50% of the allotted flows at Mary’s Lake, can be cut in drought to a 30% fraction of the quota.
C-BT water can be used only within the project’s service area, which includes parts of Adams, Boulder, Broomfield, Larimer, and Weld counties, but the Rein case’s termination of any prospect of cities counting on perpetual use of aquifers may induce some of them to seek firmed up supplies from surface water sources.
“Each of those communities will have to decide for themselves whether this will affect
their” supply, said Jeff Stahla, a spokesperson for Berthoud-based Northern Water. ‘For
many of them, C-BT and Windy Gap are part of that portfolio.”
Stahla was referring to the Windy Gap Reservoir located near Granby. Estes Park relies on that reservoir for some of its water, which is first pumped to Shadow Mountain Reservoir and Grand Lake and then moved under the mountains via the Alva B. Adams Tunnel.
One response cities that are now significantly dependent on groundwater may continue
to deploy is water rights acquisition. “We do see communities looking for native water rights,” said Stahla. He emphasized that cities that have already quantified water rights
should not be concerned about that possibility. “The water rights for existing
communities should not be affected,” Stahla continued.
Estes Park is among those Colorado municipalities who will not be directly impacted by the Rein case. “Estes Park does not have aquifers,” said Kate Miller, a spokesperson for the town in an email. The state supreme court decision, therefore, “does not affect the security of Estes Park’s surface water rights, including CB-T water rights.”