problem have plagued agriculture for centuries. 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).

Salinization and waterlogging are not unique to ancient civilizations, however. For example, a critical agricultural and ecological crisis exists in the San Joaquin Valley of California (San Joaquin Valley Drainage Program, 1990). 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. 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. 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). Since the 1983 discovery of selenium poisoning of waterfowl at the Kesterson National Wildlife Refuge in the San Joaquin Valley, the U.S. Department of the Interior has implemented the National Irrigation Water Quality Project. 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). The National Irrigation Water Quality Project has found selenium in detectable quantities in 20 reconnaissance study sites. Of the principal trace elements, selenium appears to pose the greatest potential toxic effects on aquatic biota. 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. 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.

This chapter describes the sources of salts and trace elements and their effects on soils and plants. This chapter also explores alternative management options that can be used to minimize the irrigation-induced water quality problems and sustain irrigated agriculture. Because of the complexities of the salinity, drainage, and toxic element problems, the chapter begins with an overview.

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