HYDROGEOLOGIC FACTORS AFFECTING MOBILITY OF TRACE INORGANIC CONSTITUENTS

The aquifers beneath irrigated lands in the southwestern United States are composed of alluvial deposits that, together with subsurface geologic conditions and water-level elevations, largely determine the movement of deep-percolation water below the water table. Significant factors in determining the rate and direction of ground-water flow are the layering of alluvium and the contrast between the horizontal permeability of coarse-grained deposits and the vertical permeability of interbedded, fine-grained deposits, or confining beds. Once the physical or hydraulic component of the ground-water-flow system is understood, knowledge of geochemical processes can be used to predict the mobility of various trace inorganic chemical constituents in deep-percolation water. Some of these constituents can move at the same rate as the ground water itself, while others are relatively immobile. The trace inorganic constituents of greatest concern relative to water-quality standards are derived from natural geologic deposits, and most are present at oxyanions. These constituents have been mobilized by irrigation practices in some localities and are important in terms of ground-water or surface-water uses in the alluvial basins.