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<br />seepage of excess irrigation water from fields, ditches and canals onto ancient <br /> <br />marine deposits beneath the irrigated lands. <br /> <br />Extremely saline aquifers <br /> <br />~ <br />~ (groundwater deposits of concentrations reaching 10,000 mg/ll overlying a <br /><::I <br />(.oj marine-deposited. formation known as Mancos Shale are the primary source of <br />farm-derived salinity. Salts contained in the formation are dissolved by water <br /> <br />entering aquifers and coming into chemical equilibrium with the shale en route <br />to the river's channels. The number of salinity sources (largely nonpoint), <br />the variety of potential technical and institutional solutions for their <br />abatement, and the fact that those adversely affected by this externality are <br />separated spatially, temporally, and administratively from the principal <br />pOllutant sources create a unique and complex basin-wide water quality <br /> <br />management problem. <br /> <br />(Miller, Weatherford, and Thorson provide an excellent <br /> <br />overview of the Colorado River salinity issue.) <br />Salinity emerged as a major policy issue in the Colorado River Basin in <br />the early 1960's. An irrigation project in western Arizona began discharging <br />highly saline waters from a new drainage system. The filling of Lake Powell, <br />the reservoir behind Glen Canyon Dam, sharply curtailed water supplies <br />available to dilute salts downstream. These two events drastically reduced the <br /> <br />quality (and quantity) of water ava1lable to those receiving Colorado River <br />supplies in Mexico and in parts of the U.S. The Environmental Protection <br />Agency (1971) reported a significant level of economic damages from salinity, a <br />conclusion supported later by the U.S. Bureau of Reclamation (1980). Concerns <br />over potential further water quality decline on both sides of the border led to <br />the Salinity Control Act of 1974. This act provided for abatement of both <br /> <br />natural and agricultural sources, and authorized construction of four Upper <br />Basin salinity control units and the study of twelve others. The Sol1 <br /> <br />2 <br />