Southwest Streams Show Improved Water Quality

Streams and other surface water sources in the American Southwest are showing declining levels of salt and other dissolved solids, a sign that salinity-control projects in the region are working, according to news
from the U.S. Geological Survey (USGS).

USGS scientists studied dissolved-solids measurements taken from 1974 through 2003 at sites in Arizona, California, Colorado, Nevada, New Mexico, Utah and Wyoming. They found that, while salinity levels in streams and other groundwater sources still vary widely, the amount of salt and other dissolved solids has generally dropped over the past two decades.

"The greatest change occurred during 1989 to 2003, when annual dissolved-solids concentrations decreased at more than half of the sites throughout the region," said Dave Anning, a USGS hydrologist and lead author of the study. "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."

"This is good news, and shows successes from the region's investments in salinity control over the past several decades," said Robert Hirsch, the USGS's associate director for Water.

High levels of dissolved solids can make water unsuitable for agriculture or drinking. Several government agencies -- including the Bureau of Reclamation, the U.S. Department of Agriculture and the Bureau of Land Management -- have implemented salinity-control projects in the Southwest since the mid-1970s. Their efforts are aimed at complying with the 1974 Colorado River Basin Salinity Control Act, which was created to control the salinity of U.S. waters flowing into Mexico.

Water salinity can be controlled, for example, by using low water-use irrigation systems or by redirecting high-saline water from streams.

"We know that both natural factors and human activities affect salinity," Anning said. "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."

The USGS study also found that declining salinity levels were more noticeable at the surface than in underground aquifers. About half of the aquifers examined had dissolved-solids concentrations higher than 500 milligrams per liter, the standard established by the U.S. Environmental Protection Agency for acceptable secondary drinking-water.

U.S. Geological Survey, "Decreases in Salinity Noted in Southwestern Streams." URL: (http://www.usgs.gov/newsroom/article.asp?ID=1781)

"High-quality water is key to meeting the multitude of water uses in the southwest," Hirsch said. "This USGS assessment provides a rich source of information about salinity in surface water and ground water ... that will be used by water managers throughout the region for years to come."