• Study explores groundwater flow and brine discharge in Paradox Valley.
• Brine pumping reduced salinity in Dolores River by 70% since 1996.
• Five scenarios evaluated to guide future brine management.
• Model identifies potential for further research and data collection.

August 21, 2024 — A new report from the Colorado Water Science Center, published on August 19, 2024, delves into the complex interactions between groundwater flow and brine discharge in the Paradox Valley, Montrose County, Colorado. The study is a cooperative effort with the U.S. Geological Survey (USGS) and the Bureau of Reclamation, offering crucial insights into the ongoing efforts to manage salinity levels in the Dolores River, a critical tributary of the Colorado River.  (Full report can be downloaded here.)

Understanding Brine.

Brine is a concentrated solution of water and salt, typically sodium chloride or calcium chloride. The brine concentration can vary from 3.5% to 26%, depending on factors like temperature. This solution can naturally occur underground, in salt lakes, or as seawater. It can also be produced by evaporation of ground saline water or by mining sodium chloride. Brine management is crucial in regions like Paradox Valley, where natural brine discharge significantly impacts water salinity.

Background on the Paradox Valley Unit (PVU).

Since July 1996, the Bureau of Reclamation has employed the Paradox Valley Unit (PVU) to reduce the salinity, or total dissolved solids (TDS), of the Dolores River by pumping brine from shallow wells adjacent to the river. This brine is then injected into deep disposal wells, significantly lowering TDS concentrations by around 70% by 2015. However, recent constraints on injection due to seismic activity have prompted a reevaluation of the PVU’s operations.

Modeling and Key Findings.

The USGS developed a MODFLOW-6 model simulating groundwater flow and TDS transport over 33 years (1987-2020). This model was calibrated to match observed data, including the TDS mass flux withdrawn by the PVU and discharged into the Dolores River. Despite generally accurate simulations, some discrepancies were noted, particularly during drought conditions when the model overestimated TDS mass flux to the river by about 41% post-PVU.

Scenario Analysis.

To inform future brine management strategies, the study explored five scenarios covering the years 2021-2025. These scenarios included variations in pumping schedules and the impacts of reduced irrigation return flows. Notably, the study found that ceasing pumping during high-flow periods could be nearly as effective in reducing river salinity as year-round pumping, offering benefits such as reduced seismic risk.

Future Research and Recommendations.

The report concludes that further research is needed, while the current model reasonably approximates conditions. This includes field data collection on hydrogeologic properties and applying advanced modeling tools. Enhancing the model’s accuracy could significantly improve understanding and management of brine discharge into the Dolores River, contributing to more effective salinity control in the region.

As water scarcity and quality remain pressing issues in the Western United States, this study’s findings underscore the importance of continued research and innovation in groundwater and brine management practices.

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Citation Information:

Simulation of groundwater flow and brine discharge to the Dolores River in the Paradox Valley, Montrose County, Colorado.

DOI 10.3133/sir20245038

Authors: Charles E. Heywood, Suzanne S. Paschke, M. Alisa Mast, Kenneth R Watts

Image:

Paradox Valley and Dolores River, looking down into Paradox Valley.  Mark Iverson, 2008.  Licensed under the Creative Commons Attribution-Share Alike 2.0 Generic license.

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