Most people in Tucson who rely on groundwater for their drinking water are not aware that the quality of groundwater varies considerably throughout the metropolitan area and from season to season. This is mainly due to the fact that water quality is largely determined by the geological make-up of the area from which it is pumped. Our groundwater has accumulated over thousands of years of rainfall, snowmelt, and streamflow percolating through the ground. The type of soils that the water moved through affected the amount and type of minerals which dissolved into the water as it accumulated beneath the surface.
Tucson Water pumps groundwater from more than 175 wells which are spread out from the far east side of Tucson to Avra Valley, west of the Tucson Mountains. The quality of water varies from area to area in temperature, hardness, sodium content, pH, total dissolved solids, metals, and trace constituents like fluoride.
Differences also can occur because of the way Tucson Water manages its wells for peak efficiency. Not all wells pump all the time, but are turned off and on depending on demand or to perform necessary maintenance. If demand for water is high in one area of town and low in another, Tucson Water can move water to where it's needed. During the winter, less efficient wells are shut down to save pumping costs.
All of these variables can change the pattern of water delivery so that customers may receive groundwater from different sources at different times, therefore accounting for changes in water quality.