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Decreases in Salinity Noted in Southwestern Streams
Editors Note: Maps, graphs, and reports supporting this information are at: http://water.usgs.gov/nawqa/studies/mrb/salinity.html
The U.S. Geological Survey (USGS) released a report today describing salinity levels in streams and ground water in parts of Arizona, California, Colorado, Nevada, New Mexico, Utah and Wyoming. The report concludes that although salinity varies widely throughout the region, levels have generally decreased in many streams during the past two decades.
Elevated salinity levels, or concentrations of dissolved solids, can limit the suitability of water for many uses, including agricultural production and drinking water.
Trends in dissolved-solids concentrations in streams were evaluated from 1974 through 2003. “The greatest change occurred during 1989 to 2003, when annual dissolved-solids concentrations decreased at more than half of the sites throughout the region,” explained lead author and USGS hydrologist Dave Anning. “The reductions were widespread, as indicated by decreases at nearly all of the sites on the main stem of the Colorado and Green Rivers. We also noted increases at about one-third of the sites, while the remaining sites showed no trends.”
Decreases in salinity are attributed, in part, to natural causes, such as geomorphic changes or climate variations. They also may be associated with human-related factors, such as changes in land and water use, reservoir management, trans-basin exports, and implementation of salinity-control projects. Salinity control projects include activities like using low water-use irrigation systems and re-directing saline water away from streams.
USGS findings show that dissolved solids decreased from 1989 through 2003 at all sites downstream from salinity-control projects, and that the decreases were greater than decreases upstream from projects. For example, estimated annual loads of dissolved solids in the Gunnison River in the Upper Colorado River Basin decreased by about 162,000 tons per year downstream from the Lower Gunnison salinity-control unit, in contrast to a decrease of only 2,880 tons per year upstream from the unit. This net decrease is about 15 percent of the annual load in the lower Gunnison River.
“This is good news,“ said Dr. Robert Hirsch, Associate Director for Water, “and shows successes from the region’s investments in salinity control over the past several decades.”
Salinity-control projects have been implemented since the mid-1970s by the Bureau of Reclamation, U.S. Department of Agriculture, and the Bureau of Land Management to control salinity of water delivered to Mexico, per the 1974 Colorado River Basin Salinity Control Act.
USGS has been monitoring salinity levels in streams and ground water in the southwest for more than 30 years.
Trends in dissolved-solids concentrations were less apparent in ground water than in surface water. Salinity levels varied throughout the basin-fill aquifers underlying the southwest, but generally were below 1,000 milligrams per liter. Concentrations exceeded the U.S. Environmental Protection Agency secondary drinking-water standard of 500 milligrams per liter (established for taste and hardness) throughout about half of the aquifers.
The USGS study also documents the variability of salinity throughout the region – from 22 to 13,800 milligrams per liter in streams.
“We know that both natural factors and human activities affect salinity. Through new geo-statistical modeling techniques, we were able to show that land- and water-use activities, primarily associated with pasture and cultivated land, contribute more than half (56 percent) of the salinity to streams, whereas natural geologic materials provide the remaining 44 percent,” said Anning.
“High quality water is key to meeting the multitude of water uses in the southwest,” Hirsch went on to explain. “This USGS assessment provides a rich source of information about salinity in surface water and ground water, with detailed maps, graphs, and tables. This report provides reference information that will be used by water managers throughout the region for years to come.”
The report, “Dissolved Solids in Basin-Fill Aquifers and Streams in the Southwestern United States,” U.S. Geological Survey Scientific Investigations Report 2006-5315, is available on the Internet at http://water.usgs.gov/nawqa/studies/mrb/salinity.html (printed copies will be available early November). Also available on the Internet site is an interactive, user-friendly mapping tool to evaluate the spatial distribution and sources of dissolved solids in streams and ground water.
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Public Affairs Specialist
U.S. Geological Survey, Office of Communications