Arapahoe County may triple the amount of water developers will be made to bring to any new subdivision they build, as a historic drought continues to grip the region and demographers project the county’s population to surge to more than 800,000 by 2050.
The stricter limit, which would increase the required groundwater allocation for new development from the state minimum of 100 years to 300 years — known among water managers as the “300-year rule” — will be considered as part of an 18-month, $500,000 water study Arapahoe County is launching this month.
Any new regulations or directives from the county’s study, the first of its kind in 20 years, would apply only to unincorporated parts of the county.
“Arapahoe County recognizes that growth is going to occur,” said Bryan Weimer, the county’s public works and development director. “How we manage that growth in a sustainable manner is what we hope the study will tell us.”
The county would join several others in Colorado, like Adams, Elbert and El Paso, that have adopted the 300-year rule as demand on metro area aquifers has shot up over the decades. The population in the Greeley/Boulder/Denver metropolitan statistical area, under which the Denver Basin water table lies, has leaped from less than 2 million in 1985 to nearly 3.6 million last year.
It could jump to 4.4 million people by mid-century, according to state demography data.
And much of that new development is headed to the eastern periphery of metro Denver, just beyond the E-470 beltway. Sixteen of the top 20 best-selling residential developments in the metro area are in Adams, Arapahoe and Douglas counties, which accounted for 76% of all metro area lots under development, according to 2021 data from real estate analytics firm Zonda.
But just how much growth is constrained by stricter water supply requirements in Arapahoe County, with a population of 655,000, is not clear. According to state demography numbers, the county’s projected population will increase to just over 800,000 over the next 28 years, which would translate to an additional demand of 21,200 to 53,300 acre-feet of water a year.
An acre-foot amounts to about 326,000 gallons, a year’s worth for two typical families of four.
“I think there is adequate supply to support future growth,” said Ralf Topper, former senior hydrogeologist for the Colorado Division of Water Resources.
Topper helped author the “Citizen’s Guide to Denver Basin Groundwater,” in which it is estimated that there are more than 200 million acre-feet of recoverable water in the Denver basin. At an assumed withdrawal rate of 350,000 acre-feet a year, that would peg the life of the basin at approximately 570 years.
But the devil is in the details, Topper said. Not all of the aquifers in the Denver basin — the Dawson, Denver, Arapahoe, and Laramie-Fox Hills (from shallowest to deepest) — share equal water quality, quantity or ease of access.
He applauded Arapahoe County for taking a closer look at what lies beneath its feet.
“The hydrologic characteristics of the aquifer are not uniform throughout the Denver basin,” he said. “Because of the variability of the aquifer by location, it’s critical you know you can quantify the volume of water at the location where you want to develop.”
Without adequate water, development dries up. Already communities in California, Arizona and Utah are facing restraints on growth as the Colorado River — and the reservoirs it fills — dries up amid a historic drought.
Closer to home, homebuilders recently raised alarms that if Thornton can’t access water it owns in the Cache la Poudre River, development in the fast-growing suburb could slow to a trickle in just a few years. Since 2019, Larimer County has blocked Thornton from building a pipeline across the county to a point west of Fort Collins, where it would draw water from the Poudre.
Kevin Reidy, state senior water efficiency specialist with the Colorado Water Conservation Board, said it’s important to remember that water supply is just one half of the equation. Lowering demand and use is fundamental to making the supply last.
“There’s not enough work on that demand side,” he said. “We need to have everything on the table.”
Controlling demand comes in many forms, Reidy said, including charging water customers higher rates for heavier use and requiring that developers install drought-tolerant landscaping when they build. Arapahoe County’s largest city, Aurora, approved a measure in September that prohibits cool-weather grass for new golf courses and reduces the amount of grass in new developments.
Communities can also incorporate dual-pipe systems into residential neighborhoods that take wastewater from a home, clean it up and re-use it for irrigation.
“There are things we’re doing today that 20 years ago people said we would never do,” Reidy said.
That’s what the 920-home Independence neighborhood near Elizabeth is doing, said Elbert County Commissioner Chris Richardson. The subdivision, which is still under construction, uses a “purple pipe” system that repurposes water consumed in households for outdoor use.
It’s imperative the county of 26,000 save water where it can, given the fact that it’s dwarfed in population by adjacent counties like Douglas, El Paso and Arapahoe, all of which tap the same underground water sources.
Elbert County imposed the 300-year rule 20 years ago after wells near Elizabeth began to dry up, Richardson said.
“Water has always been a high priority out here,” he said.
El Paso County was first out of the gate in Colorado to impose the 300-year rule, which it did in 1986. It was done, according to the county’s water plan, “with the intent, at least in part, of encouraging land developers to bring in additional renewable water sources.”
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To meet the 300-year rule, county spokeswoman Deborah Contreras said, developers must produce a report that documents their sources of water along with a letter from the Colorado Division of Water Resouces vouching that the supply is adequate for three centuries.
Adams County imposed its 300-year rule for groundwater in 2002. County spokesman William Porter said several developers “have had difficulty demonstrating their water rights” to state water officials but he wasn’t aware of any developers that had abandoned projects because of water concerns.
While the county doesn’t provide water or manage water rights, Porter said, it does “require the demonstration of adequate water for new development.” And that will become an increasing challenge as the drought endures and the population continues to boom.
“Without proper management and conservation of water resources, there may come a time where a developer cannot obtain adequate water rights,” he said.
