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

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).