Part II: South Metro cities starting to diversify water sources

Castle Rock and Parker 25 years ago were almost entirely dependent upon groundwater. They are diversifying, and one plan is to import water from far down the South Platte River Valley.

This is Part II of a four-part series about groundwater basins in Colorado. The story was commissioned by Water Education Colorado and appears in a variant form in the summer 2025 issue of Headwaters magazine. Photos by Allen Best unless otherwise noted.

by Allen Best

Unlike the sparsely populated Republican River Basin, the south metro area of the Denver Basin has large and still-growing cities. Most of the south metro area lies within Douglas County, whose population ballooned between 1980 and 2025 from 25,200 to nearly 400,000.

Castle Rock, the county’s largest city, has 87,000 residents. Based on approved development, the city expects to grow to a population of 120,000 to 140,000. Parker, the second largest city, has 68,000 residents and has zoning for 80,000. Utilities serving these two cities in 2005 were almost 100% dependent upon extractions from the underlying Dawson, Denver, Arapahoe and Laramie-Fox Hills aquifers. Both cities as well as other jurisdictions have lessened their dependence, but they have much work to do.

How much water remains? That’s not an easy answer to deliver, as a consultant told the Castle Rock City Council in 2005. A council member asked him: “Just how much water remains?” Perhaps leery of trying to offer easy answers that required a half-hour explanation, he simply smiled and said: “It’s dark down there.”

That absence of total certainty was at the heart of a Colorado Supreme Court decision handed down in late 2024. Parker Water and Sanitation District, Castle Rock Water and others had squared off in water court beginning in 2021 with the Colorado Division of Water Resources. Parker Water has 33 wells that are 515 to 2,745 feet deep. State-issued permits for the newest five wells limit the volumes to what could be withdrawn during 100 years at a rate of 1% a year. Parker Water and several other south-metro jurisdictions disputed the state’s authority to attach this stipulation.

The stipulation was premised on a 1973 law in which state legislators ordered a “slow sip” of Denver Basin aquifers. Later legislation and rulemaking clarified that withdrawals were not to exceed 1% of total recoverable water in that portion underlying the land of the permittee’s well in any given year.

Castle Rock believes it has underlying water in the Denver Basin aquifers to satisfy its needs for 300 years but is also making efforts to reduce per-capita use while also diversifying sources. It has 87,000 residents now but expects to grow to between 120,000 and 140,000.

This dispute is about the future. When the cities reach those 100-year limits and the total volumetric limits associated with their wells, will they be able to continue pumping. Must they cease pumping even if water remains in the aquifer?

Aurora, which lies within a half-mile of Parker Water wells, argued its water rights could be harmed if Parker pumped more than the total volume of water found to be available for its wells.

It’s crucial to understand that water underground knows no property lines, no signs saying “Welcome to Parker.” Water could, in theory, flow from below Aurora’s land to Parker’s wells. Underground, there are no fences.

Colorado Supreme Court justices, in their November 2024 majority opinion, warned of a “race to the bottom of the aquifer, with earlier permittees receiving a significant head start.” What would happen if Parker Water, Castle Rock Water and others had their druthers? “Absent a total volumetric limit, a permittee who continues to pump at the maximum permitted rate for more than 100 years would end up pulling water to its well that would not otherwise be underlying its land,” said the justices in their majority opinion.

In his dissent, Justice Brian Boatright came to the opposite conclusion, siding with the south-metro jurisdictions.

A study by the U.S. Geological Survey published in 2011 used a model that found 1% to 2% of precipitation becomes water in the bedrock aquifers and 7% in the alluvial aquifer. For urban irrigation, such as at the Watercolor subdivision in Castle Rock, 2.5 inches of water makes it back to underlying aquifers each year.

Some south-metro entities may seek state legislation that reflects what they believe is the best policy. As it stands now, a permit-holder that has withdrawn the total volumetric amount identified on a well permit must cease pumping, says Jason Ullmann, the state engineer and director of the Colorado Division of Water Resources. He has authority to notify users in writing of their violations. Could he shut down wells? They would be given “time as may reasonably be necessary to correct deficiencies,” he says. But yes, they would be “subject to enforcement.”

Just how much water remains in the Denver Basin aquifers? The Division of Water Resources issues well permits, and in doing so, estimates the potential volume of water underlying the applicant’s parcel. But the state agency does not track changes in volume over time, nor does it track the amount of water that wells pump. It requires well owners to maintain pumping records.

When asked how much water remains in Castle Rock’s wells, Mark Marlowe, director of the city’s water utility, suggested consultation of a hydrogeologist, perhaps from the U.S. Geological Survey. Pressed further, he said Castle Rock’s groundwater supply will last more than 300 years “from a legal standpoint” based on current rates of use.

The practical effect of the Supreme Court ruling on Castle Rock? Very little in the short term, Marlowe says. In 2005, Castle Rock set out to create a pathway to dramatically lessen groundwater dependence. “We’ve been headed down this road for a long time,” he says. So why participate in Parker’s lawsuit? Because, he replied, the city wants to make long-term use of its investment in groundwater extraction. And as a practical matter, the city commonly extracts less than the 1% allowed annually.

Marlowe’s answer is not totally satisfying, but the work done by Castle Rock since 2005 must be acknowledged. It was 100% dependent on groundwater extraction then. It is adding new impoundments to store surface water, pumping water upstream from Chatfield Reservoir, and doubling the daily capacity for treating wastewater. Castle Rock already has lessened its dependence on groundwater to less than 69% over the last four years and Marlowe says he’s confident that by 2050 it will lessen to 25%.

Several of Castle Rock’s successes have involved working with other south-metro jurisdictions, including the Parker Water and Sanitation District. In 2013, when Ron Redd was hired by Parker Water as general manager, the utility was still 90% groundwater reliant. He was given a mission: transition to renewable sources.

A key project has been water reuse. Water introduced into the South Platte River from other basins or from groundwater can be reused. Aurora Water set out to do so in 2003. The $680 million Prairie Waters Project pumps water from the river-side aquifer near Fort Lupton to a reservoir in the southeast metropolitan area. From there, in 2010, Parker Water, Castle Rock and eight other south-metro communities joined Denver Water and Aurora Water in a partnership called WISE (Water Infrastructure and Supply Efficiency) to further manage infrastructure cooperatively and deliver the reclaimed water to their members.

Making this possible was a new 75,000-acre-foot impoundment called Rueter-Hess Reservoir. Completed in 2012, it is a core asset for Parker Water and three other utilities who share its use.

Jim Yahn, left, manager of the Prewitt Reservoir, which might become part of the Platte Valley Water Partnership, speaks with Ron Red, manager of Parker Water and Sanitation District, and Joe Frank, general manager of the Lower South Platte Water Conservancy District, which is part of the proect. There is still hope that Prewitt would be part of the plan,” says Yahn. “The decree that Parker and Lower South Platte are seeking still has Prewitt Reservoir as a component of the plan.”

The Platte Valley Water Partnership is even more ambitious. Parker Water and Castle Rock Water have joined with the Lower South Platte Water Conservancy District.

They plan to detain South Platte River water that currently flows downstream into Nebraska during winter and spring runoff. The South Platte River Compact allows the use of this water. Little excess exists in many years, but when there is, such as in 2023, no place exists to store that water. The project plans to use Prewitt Reservoir and a new reservoir northwest of Akron in the capture and storage of those flows before pumping some of that water 125 miles to Rueter-Hess Reservoir.

Farmers will also have access to a cut of this “new” water — with agricultural users receiving 50% of the captured water and municipalities receiving 50%. Construction is set to begin around 2035, at an anticipated cost of $780 million.

As of mid-July, it’s not clear how the Nebraska lawsuit against Colorado involving water for Nebraska’s proposed Perkins Canal might affect this project.

A final important component of the path forward for the water utilities who mine Denver Basin aquifers lies in conservation, particularly for outdoor landscaping. The prevailing theme at one time was use as much as you want — but pay for it. That thinking has shifted to limits and goals of reduced use.

Parker has reduced groundwater dependence to 60% and has goals to reduce it to 25%. Might that be achieved in tapping the aquifers of the San Luis Valley? The idea has provoked outrage for more than 30 years.

“Thanks, but no thanks,” is how Redd describes Parker’s response to the idea of a lengthy straw sucking water from two river basins away.

“We have our project, and financially it makes a lot more sense to go that route.”

For that matter, the San Luis Valley aquifers have their own problems.

Part III: Declines in flows of the Rio Grande parallel those of the Colorado River during the 21st century. There were problems anyway for the potato and other growers around in the eponymously named San Luis Valley farm community of Center. Simply put, less water must be pumped from underground. Easier said than done. You can also download the entire story here in a magazine format.