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 geol ogic conditions and water-level elevations, largely determine the move ment of deep-percolation water below the water table. Significant fact ors in determining the rate and direction of ground-water flow are the layering of alluvium and the contrast between the horizontal permeabi lity of coarse-grained deposits and the vertical permeability of inter bedded, fine-grained deposits, or confining beds. Once the physical or hydraulic component of the ground-water-flow system is understood, kn owledge of geochemical processes can be used to predict the mobility o f various trace inorganic chemical constituents in deep-percolation wa ter. Some of these constituents can move at the same rate as the groun d water itself, while others are relatively immobile. The trace inorga nic constituents of greatest concern relative to water-quality standar ds are derived from natural geologic deposits, and most are present at oxyanions. These constituents have been mobilized by irrigation pract ices in some localities and are important in terms of ground-water or surface-water uses in the alluvial basins.