### Ground-water quality in the upper Santa Cruz Basin, Arizona, 1998

By: Alissa Coes**Dorinda J. GellenbeckDouglas C. TowneMaureen C. Freark

                            https://doi.org/10.3133/wri004117

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#### Abstract

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

Twenty-nine percent of the ground-water samples collected had concentrations of at least oneconstituent that exceeded a Federal or State water-quality standard. The Maximum Contaminant Levels ofthe U.S. Environmental Protection Agency and the aquifer water-quality standards of the State of Arizonawere exceeded for arsenic, fluoride, and nitrite plus nitrate. The Secondary Maximum ContaminantLevels 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 naturalfactors that have the most effect on ground-water quality in the basin are depth in the aquifer and distancefrom 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 significantlyhigher in samples collected from wells less than 2 kilometers from major faults than in samples fromwells greater than 2 kilometers from major faults (p≤0.05). Previous studies have attributed this relation tothe upward migration through faults of ground water from gypsiferous mudstones. Ground-water qualitywas not significantly different among the various basin-fill units; between parts of the basin fill that differin thickness, lateral extent, and composition north and south of an inferred fault; or among areas thatdiffer in distance from stream alluvium (p>0.05).

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

Pre-existing ground-water quality data forsix wells indicated that from the 1980s to 1998,concentrations of nitrite plus nitrate and dissolvedsolids significantly increased at a well in anagricultural area, concentrations of nitrite plusnitrate significantly increased at a well where theland use had changed from rangeland to urban,and concentrations of nitrite plus nitrate anddissolved solids significantly decreased at a well inan urban area (p≤0.10). Constituents did notsignificantly increase or decrease from the 1980sto 1998 at an additional well in an agriculturalarea, at an additional well where the land use hadchanged from rangeland to urban, and at a wellwhere the land use had changed from agriculturalto urban (p≤0.10).

#### Study Area

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