Hydrochemical facies study of ground water in the Tucson Basin

Abstract

The concept of hydrochemical facies is used to study the distribution and, indirectly, to identify the origin of the chemical character of ground water in the basin-fill aquifer of the Tucson Basin in relationship to the hydrogeologic framework. Hydrochemical fades of ground water is defined operationally in terms of the lateral (horizontal) variation of chemical quality. The following chemical constituents are included in this study: total dissolved solids, calcium, magnesium, sodium, potassium, chloride, sulfate, bicarbonate, pH, nitrate, fluoride, strontium and zinc. A conceptual process-response model relates the regional distribution of dissolved constituents to the following hydrogeologic controls: (a) the chemical composition of the rock and soil in the drainage area before recharge to the ground-water basin and conditions at the recharge sites, (b) the lithology of the basin-fill aquifer, and (c) the direction of groundwater flow within the aquifer itself. Trend surface analysis suggests that the regional distribution patterns of total dissolved solids, calcium, sodium, sulfate and strontium show a tendency to parallel the direction of ground-water flow. The distribution pattern of chloride ions based on old analyses shows a trend opposite to the distribution pattern of chloride ions based on new analyses from the same area. Nitrate content of ground water and specific capacity of wells seem to be related. Q-factor analysis of data from the basin-fill aquifer demonstrates that the overall chemical character of the ground water does not change substantially as it moves through the basin. It is concluded that the chemical character of ground water in the basin-fill aquifer of the Tucson Basin was acquired mainly during contact with various rock types in the drainage basin before recharge. The lithology of the aquifer, presumably, only plays a secondary role in determining the overall chemical composition of the ground water. After recharge to the basin-fill aquifer the distribution of dissolved constituents is controlled primarily by the flow pattern. A chemical equilibrium model of calcite and water is used to approach the problem of determining whether precipitation or dissolution of calcite takes place in the aquifer. Measured calcium ion concentrations and pH values are compared to calcium ion concentrations and pH values computed for the equilibrium model. Assuming that the equilibrium model represents actual conditions in the aquifer, departures from the equilibrium model may be used to predict the chemical behaviour of calcite In the basin-fill aquifer.