The Colorado River is the most prominent feature of Moab’s waterways, but the key to its existence is the La Sal Mountains. With peaks over 12,000 feet, the La Sal Mountains contain the snowpack that each year recharges several aquifers that produce potable water for Moab.
This week, Science Moab talked with hydrogeologist Tom Lachmar about the path water takes from the high peaks of the La Sals to the Colorado River, several water studies that have been conducted on Moab’s aquifers, and what the results mean for the future.
Science Moab: Can you give us a brief rundown of how water fits into a landscape?
Lachmar: The vast majority of water on the earth is in the oceans. It sits out under the sun, the water evaporates up into the atmosphere, and then winds blow it on land. Then we get precipitation. When it rains, that water travels on the surface, but it also soaks into the subsurface as well. The streams are flowing when it’s not raining, and that’s a result of water that was able to infiltrate into the ground and is reappearing in low places on the earth. So the vast majority of the time that streams are flowing, it’s because of water coming from the ground into the streams.
Science Moab: How does the water seep through the subsurface? Is there a specific way it moves?
Lachmar: Whatever way it can find its way down. Some of it does flow off during events, but the rest seeps down into the subsurface very slowly. The materials that make up the subsurface are porous, so they will absorb water, but their permeability is actually very low, so the water will percolate down through the portion that’s not saturated until you get to the water table.
All the pore spaces, the empty parts between the soil particles or in the cracks in the rock, are full of water. Above that is mostly air and the water is trying to move down. Water is denser than air, so it displaces [the air] as it moves downward until it reaches the water table. Then it’s in the saturated zone, where the pore spaces are filled entirely with water and it moves very slowly. We’re talking centimeters per year. What we’re seeing at the surface usually was precipitated decades or centuries or maybe even millennia ago.
Science Moab: Where’s the water in Moab coming from? Is it snowmelt?
Lachmar: There’s a lot of snow in the La Sals, and that’s where it looks like the majority of the recharge is coming from. It’s snowmelt that percolates down. The majority ends up in the groundwater and then it moves to the Colorado River because that’s the lowest water level. But it would take more than 100 years, I would imagine, for groundwater to move from the La Sals to the river. The La Sals are tertiary, shallow, intrusive igneous rocks, so they themselves are not very permeable. But the older sedimentary rocks around the flanks of the La Sals, they’ve been pushed up, and then they’re exposed so that’s where the water gets in: sedimentary rocks. At a relatively high elevation, those rocks are not horizontal, and actually are dipping towards the Colorado River. So the water gets into those rocks— the Navajo Wingate and Kayenta sandstones—and those feed the main aquifers. Being sandstones, they’ve got good permeability and they store water and it also moves relatively quickly. And every spring when the snow melts in the La Sals, that adds some to compensate for what’s being withdrawn.
Science Moab: Can you talk a bit about the studies on Moab water you were hired to evaluate?
Lachmar: Two studies came to different conclusions. The folks that paid for them wanted to know what we should take as fact. From my perspective, one had a rosy scenario in terms of how much recharge, how much is available, but very little data was actually collected to support it. Everything obeys the fundamental laws and principles and rules in groundwater, but they didn’t actually go out and measure things to get at estimating recharge rates. The other study used a lot of water chemistry, measuring what’s in the water to estimate where the recharge is and roughly how much there is. And that study was less rosy. Moab is growing and water use is increasing. So the second report was saying we’re probably pumping as much or nearly as much water as we should right now. We probably shouldn’t be planning on a large increase in the amount of water removed from the aquifer.
From my perspective, the most important message that I gave to the parties that hired me is that they need to be monitoring the water levels and the pumping rates in the wells that they have, so that they can document if those water levels start going down or the pumping rates start decreasing. They know that in advance and can do something so that it doesn’t get worse.
Science Moab: What is best practice for managing an aquifer?
Lachmar: The water is contained in the pore spaces in the rock or the consolidated loose material. So it’s completely undisturbed. Once all the water in the river has been appropriated, people start putting wells in to get groundwater which would end up in the river. You put in the first well, and you start pumping water out, the water table goes down. It’s an inevitable impact. The second person puts in a well, it goes down a little bit more. There’s no best or worst practice, it’s more a matter of what you can live with, and the state engineer decides what we’re going to live with.
Science Moab is a nonprofit dedicated to engaging community members and visitors with the science happening in Southeast Utah and the Colorado Plateau. To learn more and listen to the rest of this interview, visit www.sciencemoab.org/radio.
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