USGS Science⌕ Search
Home/Archive/USGS

USGS · sir20105126

Hydrogeologic framework of the middle San Pedro watershed, southeastern Arizona

Jesse E. Dickinson·Jeffrey R. Kennedy·D. R. Pool·Jeffrey T. Cordova·John T. Parker·J. P. Macy·Blakemore Thomas
View original source ↗Export metadata ↓

Abstract

Water managers in rural Arizona are under increasing pressure to provide sustainable supplies of water despite rapid population growth and demands for environmental protection. This report describes the results of a study of the hydrogeologic framework of the middle San Pedro watershed. The components of this report include: (1) a description of the geologic setting and depositional history of basin fill sediments that form the primary aquifer system, (2) updated bedrock altitudes underlying basin fill sediments calculated using a subsurface density model of gravity data, (3) delineation of hydrogeologic units in the basin fill using lithologic descriptions in driller's logs and models of airborne electrical resistivity data, (4) a digital three-dimensional (3D) hydrogeologic framework model (HFM) that represents spatial extents and thicknesses of the hydrogeologic units (HGUs), and (5) description of the hydrologic properties of the HGUs. The lithologic interpretations based on geophysical data and unit thickness and extent of the HGUs included in the HFM define potential configurations of hydraulic zones and parameters that can be incorporated in groundwater-flow models. The hydrogeologic framework comprises permeable and impermeable stratigraphic units: (1) bedrock, (2) sedimentary rocks predating basin-and-range deformation, (3) lower basin fill, (4) upper basin fill, and (5) stream alluvium. The bedrock unit includes Proterozoic to Cretaceous crystalline rocks, sedimentary rocks, and limestone that are relatively impermeable and poor aquifers, except for saturated portions of limestone. The pre-basin-and-range sediments underlie the lower basin fill but are relatively impermeable owing to cementation. However, they may be an important water-bearing unit where fractured. Alluvium of the lower basin fill, the main water-bearing unit, was deposited in the structural trough between the uplifted ridges of bedrock and (or) pre-basin-and-range sediments. Alluvium of the upper basin fill may be more permeable than the lower basin fill, but it is generally unsaturated in the study area. The lower basin fill stratigraphic unit was delineated into three HGUs on the basis of lithologic descriptions in driller?s logs and one-dimensional (1D) electrical models of airborne transient electromagnetic (TEM) surveys. The interbedded lower basin fill (ILBF) HGU represents an upper sequence having resistivity values between 5 and 40 ohm-m identified as interbedded sand, gravel, and clay in driller?s logs. Below this upper sequence, fine-grained lower basin fill (FLBF) HGU represents a thick silt and clay sequence having resistivity values between 5 and 20 ohm-m. Within the coarse-grained lower basin fill (CLBF) HGU, which underlies the silt and clay of the FLBF, the resistivity values on logs and 1D models increase to several hundred ohm-m and are highly variable within sand and gravel layers. These sequences match distinct resistivity and lithologic layers identified by geophysical logs in the adjacent Sierra Vista subwatershed, suggesting that these sequences are laterally continuous within both the Benson and Sierra Vista subwatersheds in the Upper San Pedro Basin. A subsurface density model based on gravity data was constructed to identify the top of bedrock and structures that may affect regional groundwater flow. The subsurface density model contains six layers having uniform density values, which are assigned on the basis of geophysical logs. The density values for the layers range between 1.65 g/cm3 for unsaturated sediments near the land surface and 2.67 g/cm3 for bedrock. Major features include three subbasins within the study area, the Huachuca City subbasin, the Tombstone subbasin, and the Benson subbasin, which have no expression in surface topography or lithology. Bedrock altitudes from the subsurface density model defined top altitudes of the bedrock HGU. The HFM includes the following HGUs in ascending stratigr

Explore related subjects

Scientific Investigations ReportReport

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

Jesse E. Dickinson, Jeffrey R. Kennedy, D. R. Pool, Jeffrey T. Cordova, John T. Parker, J. P. Macy, Blakemore Thomas. 2010. Hydrogeologic framework of the middle San Pedro watershed, southeastern Arizona. https://doi.org/10.3133/sir20105126

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related USGS reports

USGS2026

Monitoring recreation on federally managed lands and waters—Aspects of visitor use

Federally managed public lands and waters receive around 1 billion recreational visits each year. Data on these visitors can aid in guiding policy decisions, managing resources effectively, and communicating the economic contributions of lands and waters. This report explores the methods used by agencies to collect data on aspects of recreational visitor use to Federal lands and waters (apart from visitation numbers, which are the focus of a companion publication). Aspects of recreational visitor use include visitor demographics, recreational activity participation, visitor satisfaction, visitor attitudes and experiences, trip characteristics, and economic contributions. We review practices used to understand aspects of visitor use across seven Federal agencies, revealing similarities such as the use of visitor intercept surveys and coverage of similar topic areas, and differences in how survey programs are operationalized and how specific questions on visitor surveys are worded. We also evaluate emerging technologies, such as geolocated social media and mobile device location data, for their potential to aid in understanding aspects of visitor use. This report concludes with potential opportunities to enhance data collection and coordination, ensuring cost-effective data collection and informed decision making.

Scientific Investigations Report↗
USGS2026

Evaluation of best management practices at an edge-of-field site in the Eagle Creek watershed, Ohio, 2012–20

In 2010, the U.S. Geological Survey worked in partnership with the Great Lakes Restoration Initiative and the Natural Resources Conservation Service to identify farm fields in priority watersheds—watersheds critically important to Great Lake health. Intensive best management practices were implemented in five States to test the efficacy of best management practices, also referred to as agricultural conservation practices, to reduce sediment and nutrient runoff on agricultural fields. These fields were chosen as representatives of the priority watersheds because their farming practices and geographical conditions were common among farms in those watersheds. A farm site located near Findlay, Ohio, in the Eagle Creek watershed used two conservation practices, cover crop (Natural Resources Conservation Service practice standard 340) and variable rate technology (VRT; Natural Resources Conservation Service practice standard 590), which represent common practices in the Maumee priority watershed. This study monitored surface runoff and subsurface tile runoff at the Eagle Creek watershed site. The effects of cover crop and VRT nutrient application on sediment and nutrient runoff were assessed from October 2012 to September 2020. Cover crops were applied in fall of 2016, 2017, and 2018, coinciding with VRT applications. The following parameters were analyzed as part of the study: runoff discharge; peak discharge; and concentrations and loads of total phosphorus, particulate phosphorus, dissolved reactive phosphorus, and suspended sediment. High peak runoff events disproportionally affected nutrient concentrations and loads in runoff. A threshold of the 85th percentile of the peak discharge was chosen to assess the effect of cover crop and VRT nutrient application on nutrient mitigation at each gage. Runoff events below the 85th percentile were considered small, and runoff events over the 85th percentile were considered large. Concentrations from collected water-quality samples were used for the analysis; estimated concentrations were excluded from the statistical evaluation. All surface runoff parameters during small runoff events, except surface runoff volume, were significantly lower in the period with cover crops and VRT implementation. Contrarily, subsurface runoff parameters during small events were not statistically different between the periods with and without cover crops and VRT implementation. Surface and subsurface parameters during large events were also not statistically different between the periods with and without cover crops and VRT implementation. These results indicate cover crops and VRT may improve water quality during small runoff events, but additional best management practices that mitigate large runoff events may lead to greater water-quality improvement given the contribution of large events to overall losses.

Ohio↗
USGS2026

Continuous monitoring and temporal variability of fluorescence of dissolved organic matter in karst groundwater of the Edwards aquifer, south-central Texas, 2019–24

Widespread urbanization on the Edwards aquifer recharge zone in south-central Texas has prompted the development of new approaches and tools for evaluating the current and future status of water quality in the San Antonio and Barton Springs segments of the Edwards aquifer. The U.S. Geological Survey, in cooperation with the San Antonio Water System and the City of Austin, applied continuous monitoring of fluorescence of dissolved organic matter (fDOM) to characterize the sources and transport of dissolved organic matter and associated constituents in the Edwards aquifer. Continuous fDOM time-series data were adjusted using empirically derived temperature, turbidity, and inner-filter-effect correction factors prior to analysis and interpretation. Fully corrected fDOM time-series data from the study sites were evaluated for short- and long-term temporal variability in fDOM in the context of the hydrologic and climatic conditions that occurred during the 5-year study period to understand vulnerability of the aquifer to potential contaminants in recharge. Study results indicate that fDOM is an effective proxy for dissolved organic carbon that, in turn, is indicative of the influx of recent surface water and associated contaminants, and thus, aquifer vulnerability to those contaminants. Although other continuously monitored water-quality parameters provide insights into changes in water quality in response to varying hydrologic conditions, the quantitative relation between fDOM and dissolved organic carbon directly indicates the timing and magnitude of pulses of organic constituents derived from the land surface, whereas other water-quality parameters do not.

Texas↗