Fifty-eight ground-water samples were collected and analyzed in 1998 by the U.S. Geological Survey and the Arizona Department of Environmental Quality to assess ground-water quality and to identify factors affecting ground-water quality in the Upper Santa Cruz Basin. In addition, pre-existing groundwater quality data for six wells were analyzed to determine changes in the ground-water quality of the basin over time.

Twenty-nine percent of the ground-water samples collected had concentrations of at least one constituent that exceeded a Federal or State water-quality standard. The Maximum Contaminant Levels of the U.S. Environmental Protection Agency and the aquifer water-quality standards of the State of Arizona were exceeded for arsenic, fluoride, and nitrite plus nitrate. The Secondary Maximum Contaminant Levels of the U.S. Environmental Protection Agency were exceeded for fluoride, iron, manganese, pH, sulfate, and dissolved solids.

Ground-water quality in the basin is affected by natural factors and human activities. The natural factors that have the most effect on ground-water quality in the basin are depth in the aquifer and distance from major faults. Ground-water temperatures and pH significantly increased with well depth (p≤0.05). Concentrations of dissolved solids, alkalinity, calcium, potassium, chloride, and sulfate were significantly higher in samples collected from wells less than 2 kilometers from major faults than in samples from wells greater than 2 kilometers from major faults (p≤0.05). Previous studies have attributed this relation to the upward migration through faults of ground water from gypsiferous mudstones. Ground-water quality was not significantly different among the various basin-fill units; between parts of the basin fill that differ in thickness, lateral extent, and composition north and south of an inferred fault; or among areas that differ in distance from stream alluvium (p>0.05).

Human activities have a substantial effect on ground-water quality in the basin. Ground water that contained recent (post-1953) recharge from urban areas had significantly higher concentrations of nitrite plus nitrate than ground water that did not contain recent recharge from the land surface (p≤0.05). Ground water that contained recent recharge from present agricultural areas had significantly higher concentrations of nitrite plus nitrate, calcium, and potassium than ground water that did not contain recent recharge from the land surface (p≤0.05). Ground water that contained recent recharge from present agricultural areas also had significantly higher concentrations of calcium, potassium, alkalinity, and dissolved solids than ground water that contained recent recharge from urban areas (p≤0.05).

Pre-existing ground-water quality data for six wells indicated that from the 1980s to 1998, concentrations of nitrite plus nitrate and dissolved solids significantly increased at a well in an agricultural area, concentrations of nitrite plus nitrate significantly increased at a well where the land use had changed from rangeland to urban, and concentrations of nitrite plus nitrate and dissolved solids significantly decreased at a well in an urban area (p≤0.10). Constituents did not significantly increase or decrease from the 1980s to 1998 at an additional well in an agricultural area, at an additional well where the land use had changed from rangeland to urban, and at a well where the land use had changed from agricultural to urban (p≤0.10).