63 Analyzing Salinity Patterns in the Upper Colorado River Basin

Faculty Mentor: Sara Warix (Geology and Geophysics, University of Utah)

The Paradox Valley located near the Colorado-Utah border, is a geologic anomaly. 300 million years ago, rock was deposited primarily composed of salts including halite and gypsum. Under sandstone in the valley, rests the Paradox Formation which is made up of these salts. The Dolores River, a tributary to the Colorado River, cuts perpendicularly through the valley floor. This river contributes about 6 percent of the total salinity load to the Colorado River, making it one of the most saline tributaries. High salinity levels reduce agriculture yield, damage infrastructure, and increase water treatment costs; annually $348 million dollars are spent to mitigate these negative effects. 40 million people rely on the Colorado River for municipal and industrial uses including irrigating over 5 million acres of land in the West. The salinity of the Dolores River increases abundantly as it moves through the valley, due to dissolution of subsurface salt deposits. Located north of the Paradox Valley, are the La Sal Mountains, which are a large groundwater recharge zone for the Dolores River. As La Sal mountain groundwater flows away from the mountain block and towards the Dolores River, water becomes increasingly saline as it dissolves the Paradox formation. This groundwater degrades the water quality of the Dolores River by adding dissolved salts.

The United States Geological Survey (USGS) has recently begun a project that aims to better understand salinity in the Upper Colorado River Basin. Working with them, I have been analyzing historic water quality data from this region to investigate spatiotemporal water quality trends. I extracted historical geochemistry data from groundwater and surface water sites in the region but primarily focused on two surface water sites with abundant data records. The first site (USGS-0969500) is South of the La Sals, where the Dolores River enters the Paradox Valley. The second site (USGS-09810000), also the Bonderman Field Station, is located North of the La Sals and downstream of the first site, before the confluence of the Dolores and Colorado Rivers. I illustrated seasonal and annual trends at both of these sites using piper diagrams, a common geochemical plotting technique. South of the La Sals, a stronger evaporite signature is present due to higher concentrations of chloride and sulfate. Over time, salt concentrations have been increasing, supporting the idea of continued dissolution of the Paradox formation. North of the La Sals, these waters are more calcium-rich, suggesting more influence from carbonate or shale weathering. Seasonal shifts during low-flow periods (December and January) occur, where samples shift towards increased sulfate and chloride groundwater chemical makeup. Looking ahead, we are hoping to collect more updated water samples in collaboration with the USGS. New data could help us understand where groundwater wells could be installed and find where the groundwater is gaining the most salinity. Overall, this information will help inform more effective water management plans aimed at reducing salinity in the Upper Colorado River Basin.