Research about Upper Santa Cruz Basin

USGS2016

Bedrock morphology and structure, upper Santa Cruz Basin, south-central Arizona, with transient electromagnetic survey data

The upper Santa Cruz Basin is an important groundwater basin containing the regional aquifer for the city of Nogales, Arizona. This report provides data and interpretations of data aimed at better understanding the bedrock morphology and structure of the upper Santa Cruz Basin study area which encompasses the Rio Rico and Nogales 1:24,000-scale U.S. Geological Survey quadrangles. Data used in this report include the Arizona Aeromagnetic and Gravity Maps and Data referred to here as the 1996 Patagonia Aeromagnetic survey, Bouguer gravity anomaly data, and conductivity-depth transforms (CDTs) from the 1998 Santa Cruz transient electromagnetic survey (whose data are included in appendixes 1 and 2 of this report). Analyses based on magnetic gradients worked well to identify the range-front faults along the Mt. Benedict horst block, the location of possibly fault-controlled canyons to the west of Mt. Benedict, the edges of buried lava flows, and numerous other concealed faults and contacts. Applying the 1996 Patagonia aeromagnetic survey data using the horizontal gradient method produced results that were most closely correlated with the observed geology. The 1996 Patagonia aeromagnetic survey was used to estimate depth to bedrock in the upper Santa Cruz Basin study area. Three different depth estimation methods were applied to the data: Euler deconvolution, horizontal gradient magnitude, and analytic signal. The final depth to bedrock map was produced by choosing the maximum depth from each of the three methods at a given location and combining all maximum depths. In locations of rocks with a known reversed natural remanent magnetic field, gravity based depth estimates from Gettings and Houser (1997) were used. The depth to bedrock map was supported by modeling aeromagnetic anomaly data along six profiles. These cross sectional models demonstrated that by using the depth to bedrock map generated in this study, known and concealed faults, measured and estimated magnetic susceptibilities of rocks found in the study area, and estimated natural remanent magnetic intensities and directions, reasonable geologic models can be built. This indicates that the depth to bedrock map is reason-able and geologically possible. Finally, CDTs derived from the 1998 Santa Cruz Basin transient electromagnetic survey were used to help identify basin structure and some physical properties of the basin fill in the study area. The CDTs also helped to confirm depth to bedrock estimates in the Santa Cruz Basin, in particular a region of elevated bedrock in the area of Potrero Canyon, and a deep basin in the location of the Arizona State Highway 82 microbasin. The CDTs identified many concealed faults in the study area and possibly indicate deep water-saturated clay-rich sediments in the west-central portion of the study area. These sediments grade to more sand-rich saturated sediments to the south with relatively thick, possibly unsaturated, sediments at the surface. Also, the CDTs may indicate deep saturated clay-rich sediments in the Highway 82 microbasin and in the Mount Benedict horst block from Proto Canyon south to the international border.

Arizona↗

USGS2000

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).

Arizona↗