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w CJ1 ..e:. .J;,.. B.259297 Background The salinity of the Colorado River increases dramatically as the river makes its way along its 1,400-mile journey from its headwaters in Wyoming and Colorado to its termination in Mexico. Nearly half of the salinity is caused by nature, when, for example, groundwater flows through salt formations and enters the river or when saline springs contribute their salt to the river. But another major contributor to the river's salinity is the use of the water for agriculture. Simply put, when water is diverted from the river for irrigation, the salinity increases as the level of water in the river is depleted. Some of the diverted water, once applied to crops, then seeps into the ground, picks up salt from the soil, and returns-now with a much higher saline content-to the river. Because there is less water remaining in the river to dilute the salt, salinity increases. Two major pieces of legislation address the salinity of the Colorado River. The first, the Clean Water Act, as amended (33 U.S.C. 1251, 1313), required national water quality standards. In response to the requirements of this act, the EPA approved numeric criteria for salinity levels at three monitoring stations along the Colorado River. The salinity of the water passing these stations is not supposed to exceed these criteria As part of its treaty of February 3, 1944, and an agreement of August 30, 1973, with the Republic of Mexico, the United States agreed to take measures to ensure that the water flowing into Mexico from the Colorado River would have an average annual salinity concentration based on that of the Colorado River water arriving at the Imperial Dam.2 The Imperial Dam, near Yuma, Arizona, is the last U.S. station at which salinity standards have been set before the river enters Mexico. The second act, the Colorado River Basin Salinity Control Act of 1974, as amended (43 U.S.C. 1571, 1591), was passed to enhance and protect the quality of water delivered to users in the United States and Mexico. Title I of the act primarily authorized the Secretary of the Interior to construct a desalting plant to enable the United States to comply with its treaty obligation to Mexico. Title II of the act directed the Secretary to proceed with a salinity control program. Specifically, title II authorized the Secretary, through BOR, to proceed with the construction of four specific salinity control projects and to continue the planning of several other projects. The 1984 amendments to the act required two additional agencies-BLM and USDA-to implement salinity control programs. The 2Specifically, Minute No. 242 of the International Boundary and Water Commission, United States and Mexico, states that the salinity concentration of Colorado River water entering Mexico will not exceed, by more than 115 parts per million (plus or minus 30) of total dissolved solids, the average annual salinity concentration of the water at the Imperial Dam. Page 3 GAOIRCED-95-58 Salinity Control Projects in the Colorado River Basin