The effects of mineral reactions on trace metal characteristics of groundwater in desert basins of southern Arizona

Abstract

The geochemistry, of groundwater collected from deep wells in the western section of the lower Santa Cruz basin of southeastern Arizona, was studied in order to determine the extent to which geochemica] analysis can be used to assess fluid flow and major and trace element migration patterns along hydrologic flowpaths in desert basins. Interaction between groundwater and enclosing sediments, and mixing between chemically distinct basin groundwater is found to exert a significant control on the chemical patterns that have evolved in the system. Activity-activity diagrams of the Na-Si-O-H system show that groundwater throughout the basin clusters near the three phase boundary between fluid, kaolinite, and montmorillonite and trends along the boundary to higher log (aNa⁺/aH⁺) values. A reaction model generated with computer program PHREEQE, that combines silicate weathering, kaolinite-Na-beidellite equilibrium, calcite equilibrium, and solution mixing, can simulate trends in groundwater composition along flowpaths in the basin. Trace metals introduced into the basin by the weathering of a buried porphyry copper deposit become spatially separated upon migration. Metal concentrations are found to be correlated to major cation concentrations. Cu is associated with high Na concentrations and a high ratio of Carbonate:Ca, whereas Zn is associated with high Ca concentration and a low ratio of Carbonate:Ca. Behavior of Cu and Zn during low-temperature transport can be controlled by the effects of mineral alteration on groundwater composition. Computer analysis of early basin diagenesis shows that changes in major solute composition that accompany weathering, constrained by equilibrium with clays and calcite, can produce the metal segregation pattern observed in the basin. Because the aquifer is strongly influenced by silicate and carbonate mineral equilibrium, the introduction of Central Arizona Project recharge, which is not in equilibrium with alluvial minerals, into the basin, requires a response by mineral reactions that attempt to restore the system to a state of equilibrium with kaolinite, montmorillonite, and calcite. A reaction model is developed to predict the consequent effects of outside recharge on groundwater quality.