Vertical temperature and chemical gradients in groundwater in the Tucson Basin, Arizona

Abstract

A study of vertical temperature and chemical gradients in groundwater was conducted, primarily to determine whether or not temperature gradients could be used as an index of vertical mixing in well bores and, hence, as a guide in the interpretation of point samples taken at various depths for chemical analysis. Measurements of temperature variation with depth in well bores were made with a pre-calibrated thermistor. Water samples were taken with point samplers at specific depths. Graphical comparisons of profiles of temperature, electrical conductivity, and bicarbonate and chloride contents indicate agreement with the assumed hypotheses of vertical flow and/or mixing in well bores for 50% of the wells sampled. Anomalies were found in the chemical gradients. Incomplete chemical analysis was noted as an important factor of the chemical anomalies. The water chemistry was relatively uniform and the variation with depth was not pronounced. pH studies indicated that the water is in, or close to, equilibrium with respect to calcite in most of the wells. The water is predominantly of sodium or calcium bicarbonate type in irrigation water class C2-S1. It was preliminarily concluded that vertical temperature gradients can be used as a guide in the interpretation of the chemistry of samples taken at various depths in a well bore.