Moab’s watershed is complex, and understanding it is crucial to good development decisions. On April 19, the Grand County Commission hosted the first of what it hopes will be quarterly information sessions with Marc Stilson, regional engineer for the southeastern regional office of the Utah Division of Water Rights, and Chris Wilkowske, hydrologist for the U.S. Geological Survey. The public is encouraged to attend and ask questions at these sessions.
Stilson gave an overview of recent groundwater studies in the Moab area and their findings and implications. The main point he emphasized is that the issue needs more research:
“There’s a lot more study that needs to be done to understand the dynamics of this system and the movement of water through it,” Stilson said.
Recent reports
Stilson summarized three reports that have been completed in recent years. In 2019, the USGS completed a comprehensive, “big picture” study to estimate the groundwater supply in the Spanish Valley watershed. That report produced estimated amounts in ranges: between 14,000 and 16,000 acre-feet of water in the entire watershed (from the mountain tops to the river valley), and between 4,500 and 7,800 acre-feet of water that’s available within the valley aquifers.
“It might be a little less, might be a little more,” Stilson said, “but we have to narrow down into what’s available down here that we can tap through springs and wells in the valley.”
It’s an important number to know because it’s used to determine safe yield. By Utah state code as well as for prudent development and resource management, a community should plan not to draw more out of its aquifer than is recharged annually.
Dipping into stored groundwater—called mining the aquifer—is “like overdrawing your bank account,” Stilson said. “It’s not sustainable.”
Water managers and development planners need to know not just how much water is being recharged into the aquifer, but also how much is being used.
The valley’s largest water users—Moab City, the Grand Water & Sewer Service Agency, and the recently-formed San Juan Spanish Valley Special Service District—reported a combined total use of 3,676 acre-feet of water in 2021. No one knows how much additional water is being drawn by private well owners; they’re not required to report their use, and there’s no tracking system in place. Stilson hopes to one day collect that data, but he said it will be a “heavy lift,” requiring cooperation from private well-owners and expensive metering equipment.
The Utah Division of Water Rights has recently adjudicated all the wells in the valley, identifying all water rights filed and determining whether they are in use, abandoned, or not being used; that information may give a hint of how much water is used by private well-owners.
With an estimated range of numbers for both water availability and water use, it’s hard to say how close the Moab valley is to maxing out its safe yield. According to 2020 estimates, Stilson said the amount of water left available for development is between 1300 and 3500-acre-feet. To get a more precise number, researchers will have to conduct more studies.
“Are we on the bottom end of the range, or are we on the top end of the range? If we’re on the top end of the range we have a little more breathing room,” Stilson said. “We think there’s still a little bit of groundwater to develop, but probably not a lot.”
Another report commissioned by the City of Moab in 2018 looked into city-specific questions, conducting three phases between 2018 and 2020. The report is specific to the city but has information relevant to the whole watershed.
In 2020, the USGS published a research paper that further analyzed points of the agency’s 2019 study.
Areas for future study
Stilson identified three main areas that need more detailed examination: the Ken’s Lake area, the Moab Golf Course area, and the Scott and Norma Matheson Wetlands Preserve. Each of those areas serves a key function in how water moves through the valley—at Ken’s Lake, water is diverted from Mill Creek into the reservoir, and from there it’s stored until some is drawn out for irrigation. The Moab Golf Course has springs and wells tapped into the Glen Canyon Group Aquifer, the higher-quality and larger of Moab’s two aquifers. Those springs are being used by Moab City, but it’s unclear how much water is flowing to them or how much is available for development. The wetlands preserve, Stilson said, is important to watch because it could be affected by water development higher up in the valley. The first year of a three-year study to examine those impacts has been completed by the Utah Geological Survey.
Stilson said more monitoring wells are needed and described an instance when the lack of them prevented him from finding answers. A private well owner in the valley called the Division of Water Rights worried that his well had gone dry. Stilson probed the wells around it: one seemed to have the same reading observed a few years ago; the other was designed for pumping water rather than monitoring it, and technicians weren’t able to get their instruments into it.
“There was no monitoring well close enough to tell us what was going on with this well,” Stilson said—whether it had indeed gone dry, or something else had happened, like the well collapsing or rusting out. The existing network of wells used for monitoring, Stilson said, was not designed, but is instead an ad-hoc network including wells that already existed and could be linked, or where other circumstances facilitated the installation of a monitoring well. There are gaps in the network that he hopes can be filled.
The county recently received a grant from the National Ground-Water Monitoring Network, which helps bodies contribute their water monitoring data to a centralized database. Membership in the network may open up future funding for more infrastructure like monitoring wells.
Stilson also hopes to learn more about the relationship between the Valley Fill aquifer, which sits in a small sand and gravel layer on top of the larger Glen Canyon Group aquifer, which is in a sandstone layer and has higher-quality water.
Another topic for future study is how Pack Creek recharges the Valley Fill aquifer. The 2019 USGS study suggested that the waterway plays an important role in that recharge, meaning it could potentially be used to supply “artificial recharge” projects in the future. In wet years, excess water in Pack Creek likely flows to the Colorado River instead of into the aquifer; through artificial recharge infrastructure, that water could be diverted back into the aquifer. A gage on Pack Creek, anticipated to be installed this summer, can collect more information on how much water the creek is delivering to the aquifer, and also give flood warnings in the summer.
Stilson told the commission that even though the community may be close to maxing out its safe yield from the local aquifers, they’re in a good position. There’s more data on groundwater in the Moab area than in other parts of southern Utah, and it doesn’t look as though the Moab area has surpassed its safe yield. Other Utah communities, such as Cedar City, are already past their safe yields and are having to cut back existing water uses.
“You’re doing a good thing, focusing on this issue, trying to understand it, trying to figure out how to make this work before you get into a position where we’re mining,” Stilson said.
