Nearly 40 years ago, after watching aquifers below Douglas County plunge amid fast growth and heavy use, Colorado lawmakers adopted a “sip slowly” management process that required communities such as Parker and Castle Rock to pump out fixed amounts of nonrenewable groundwater each year in an effort to make the resource last at least 100 years.Fast forward to 2020. That year, the state directed well owners to sip even more slowly, explicitly stating how much water their permits entitled them to, and requiring them to stop pumping at the end of that 100-year period if they have fully used the water to which they were entitled when the original well permits were issued.But Parker and Castle Rock objected, suing the state over the new permitting language. They argued that the original volume estimates used to calculate their annual pumping rates were never meant as formal, total volume limits. Those limits, they argued, could sharply limit their future water supplies because they were essentially a best guess, based on measuring technology that has changed considerably since then. This Fresh Water News story is a collaboration between The Colorado Sun and Water Education Colorado. It also appears at wateredco.org/fresh-water-news.Aurora and Greeley joined the case, siding with the state. A special water court ruled against Parker and Castle Rock, which together appealed to the Colorado Supreme Court. The high court is expected to issue a ruling in the case before the end of the year, according to spokeswoman Suzanne Karrer.Under Colorado’s so-called 100-year rule, well owners can extract no more than 1% of the water under their lands each year, pumping all the water within 100 years of the issuance of their permits. But prior to use of the new permitting language, the total volume of water that could be taken out over the life of the permit was never explicitly stated on the permits themselves, though it was used to calculate the annual extraction rate.State officials said they added the water volumes to ensure wells are regulated in a uniform way and that well owners are informed at the start of that 100-year clock how much actual water they can pump.Deputy State Engineer Tracy Kosloff explained in an email. “If the amount pumped is less than the annual maximum, the length of time it takes to reach the total allowed withdrawal will be more than 100 years. For instance, if one pumps half of the maximum each year, it will take 200 years to reach the total.”However, if the maximum allowed each year is pumped, then the permit will expire at the end of the 100 years, and the well owner would have to stop pumping and find other water sources, Kosloff said.But Parker and Castle Rock argue that water levels in the aquifer vary and that over that 100-year period more water might actually be available to them. Establishing a lifetime limit, especially one based on an estimate and old measuring technology, could deprive them of water to which they are entitled.Colorado is home to several aquifer formations, some of which can be easily recharged via rainfall and snowmelt, and are considered renewable. Others cannot be readily recharged and thus are considered to be nonrenewable. These are known as nontributary aquifers and wells drilled in these areas are at the heart of the dispute.Sean Chambers, Greeley’s director of water and sewer utilities, supports water regulators’ effort to more closely manage nonrenewable underground supplies by including a specific volume on permits because it will better protect everyone over the long run.Greeley is planning a major new aquifer storage facility on the Wyoming border known as the Terry Ranch. The city wants to ensure water it stores underground isn’t inadvertently tapped by other users whose pumping could siphon off the city’s supplies, Chambers said via email.When it became clear in the 1980s and 1990s that the aquifers were in decline, Douglas County communities began reducing the amount of water they were taking out of the aquifers, adding surface supplies from the South Platte River and Cherry Creek, and building multimillion dollar water recycling plants so they can reuse the water they already own.Parker once relied on nonrenewable groundwater for more than 90% of its supplies, but has since reduced that use to roughly 35%. By 2050, it hopes to drop that amount to 25% of its supplies, according to Ron Redd, manager of the Parker Water and Sanitation District.Ultimately, Redd said, it’s likely that the state laws on the books now will have to be changed as a result of the dispute.“If we lose, we will try to run legislation upholding our interpretation of the law,” he said. “We were surprised by this. No one knew it was coming until suddenly we saw this condition on our well permits.”First published by The Colorado Sun.