Some of the water penetrates the soil or the underlying formations, reemerging as stream flow farther downstream or recharging the groundwater basin again, some of the minerals that come into contact with the water on the way are dissolved and carried along. It dissolves small amounts of minerals from the riverbed and soils with which it comes into contact, slowly cutting out a channel along its path. Because of the complexities of the salinity, drainage, and toxic element problems, the chapter begins with an overview.ДЇorce. This chapter also explores alternative management options that can be used to minimize the irrigation-induced water quality problems and sustain irrigated agriculture. This chapter describes the sources of salts and trace elements and their effects on soils and plants. The ecological hazards of potentially toxic trace elements-such as selenium, boron, arsenic, molybdenum, mercury, vanadium, and uranium-are magnified when agricultural drainage waters are disposed into hydrologically closed basins and sinks and accumulate in the food chain. The primary source of selenium in the drainage waters of the San Joaquin Valley is the Cretaceous-period marine sedimentary shales of the Coast Range mountains. Of the principal trace elements, selenium appears to pose the greatest potential toxic effects on aquatic biota. The National Irrigation Water Quality Project has found selenium in detectable quantities in 20 reconnaissance study sites. Twenty-six sites in 15 western states are being investigated to ascertain whether selenium and other trace elements in irrigation drainage water are causing ecological damage (Engberg et al., 1991). Department of the Interior has implemented the National Irrigation Water Quality Project. Since the 1983 discovery of selenium poisoning of waterfowl at the Kesterson National Wildlife Refuge in the San Joaquin Valley, the U.S. Adding to the difficulties in managing irrigation-induced water quality problems was the discovery of selenium and other toxic trace elements in subsurface drainage waters in the San Joaquin Valley's west side (National Research Council, 1989b). About 54,000 ha (133,000 acres) of the drainage-impacted lands have tile drainage systems, but only about 21 percent of the entire west side, however, can discharge its saline subsurface drainage into the San Joaquin River for eventual disposal into the Pacific Ocean. Of the nearly 1 million ha (2.5 million acres) of irrigated cropland on the valley's west side, about 38 percent is waterlogged and 59 percent has increased levels of salt. For example, a critical agricultural and ecological crisis exists in the San Joaquin Valley of California (San Joaquin Valley Drainage Program, 1990). Salinization and waterlogging are not unique to ancient civilizations, however. Historical records from the past 6,000 years reveal that many ancient civilizations whose existence was based on irrigated agriculture have failed, for example, the Sumerian civilization in the Mesopotamian Plains in Iraq, the Harappa civilization in the Indus Plain region in India and Pakistan, the inhabitants of the lower Viru Valley in Peru, and the Hohokam Indians in the Salt River region in Arizona (Tanji, 1990). Problem have plagued agriculture for centuries. Worldwide, in the arid-to-semiarid regions where irrigation is practiced, soil salinization and the frequently accompanying waterlogging Szabolcs (1989) reports that, worldwide, some 10 million ha (25 million acres) of irrigated land is abandoned annually because of salinization, sodification, and waterlogging. Saline and sodic (sodium-containing) soils cover about 10 percent of the world's arable lands and exist in 100 countries. According to Szabolcs (1989), about 1 billion ha (25 billion acres) of the world's soils are affected by salt. Soil salinization is a worldwide problem. Salinity problems also occur in humid regions through, for example, seawater intrusion into low-lying coastal farmlands. Halvorson (1990) estimates that nearly 1 million ha (2.5 million acres) of productive agricultural dryland in the western United States has been salinized through saline seeps. Salinity problems are not restricted to irrigated areas and arid climates. About 5.2 million ha (14 million acres) of this irrigated land is currently affected by salt. Some constituents of salinity, such as sodium, may have a deleterious impact on the physical condition of the soil, impairing crop production.ДЄbout 19 million ha (47 million acres), or 10 percent of U.S. Waterlogging causes poor aeration in the root zone, with effects ranging from reduced growth to death of plants. Salinity affects germination as well as seedling and vegetative growth. The salt content (salinity) of soils and the frequently associated drainage problems are pervasive in agriculture.

40 different types of salt