High salinity can limit water available for agriculture, drinking water, aquatic life and infrastructure, with significant impacts to the economy and human health. Salt occurs naturally in water, but salt loads are influenced by irrigated agriculture, geology, land cover, land-use practices and precipitation. Salinity can exacerbate corrosion of lead pipes and increase lead levels in drinking water and mobilize other metals or pollutants as well. High salinity levels in the Colorado River reduce agricultural yield, damage infrastructure and are estimated to cause $348 million per year in damage to infrastructure and crop production.

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“This study shows us how irrigation and climate work together to influence salts going into streams,” said USGS hydrologist Olivia Miller, lead author on the study. “Future climate change in the Southwest, combined with changes in irrigation, may affect stream water quality, but we don’t yet understand how these interactions will play out, so our next step is developing a model to test scenarios of future climate change.”

Wet periods have higher salinity loads because increased runoff from rain and melting snow and increased groundwater movement bring more salts into rivers. In contrast, drier periods have lower salinity loads. Irrigation also plays an important role, contributing salts to the river more efficiently than any other source. 

“Salt loading to the Upper Colorado River and tributaries is a significant economic and environmental concern which limits the utility of the Colorado River and creates economic damages to downstream water users,” said Don A. Barnett, Executive Director, Colorado River Basin Salinity Control Forum.

For the new study, USGS scientists created a dynamic model that simulates the flow of water and salts throughout the whole Upper Colorado Basin between 1986 and 2017, allowing them to estimate salinity in the river and identify its sources for every year over that time.

The study confirmed previous findings that salts come primarily from groundwater (66-82%), with smaller portions attributed to runoff and springs. The salts in groundwater may initially come from infiltration of irrigation water, but once dissolved in groundwater, tracing the source is difficult. Groundwater is stored for long periods underground, meaning that there can be a time lag between when the salts enter the groundwater and when they end up in the river. As a result, while salinity management efforts focused on surface runoff processes may produce small results in the short term, larger impacts may take longer to work through the groundwater system.

"The Upper Colorado River Basin States are taking actions to reduce salinity in the Colorado River for the benefit of the 40 million people who use the River’s water,” said Paul Kehmeier, Salinity Program Coordinator, Colorado Department of Agriculture. “This study helps clarify that the sources of salt vary over time and it will help inform managers on strategies to continue improving the quality of water in the Basin."

The study was published in the journal Environmental Research Letters.

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