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Upper Santa Cruz River Watershed Project

Using Interdisciplinary Earth Science to Study Environmental Impacts on the Upper Santa Cruz River Watershed and Regional Aquifer

Members of the project (in alphabetical order)

USGS — Denver: Byron R. Berger, Laurie Chrzanowski, Carol A. Finn, Trude V.V. King, Jeffrey D. Phillips, W. Ian Ridley, Wayne C. Shanks III, Richard B. Wanty

USGS — Tucson: Mark E. Gettings, Floyd Gray, Brenda B. Houser

Overview

Southern Arizona, including Tucson and Nogales, is an economic growth area as well as an increasingly popular region for retirement. Population growth and economic development increase the demand for quality potable and non-potable water, thus placing an increased burden on the regional aquifer systems. The Upper Santa Cruz River is also the site of one of the most diverse riparian habitats in the southwest with some species on the endangered species list and others proposed for the list. In southern Arizona aquifer systems are confined within a series of small sedimentary basins, which define a part of the Upper Santa Cruz River Watershed. The small basins are fed by surface and ground water from the surrounding mountains. Water quality is effected by the nature of the rocks in these mountainous areas.

The mountainous recharge areas are heavily mineralized and hydrothermally altered. World-class porphyry copper systems have been identified in this region, and many have been commercially exploited, e.g. Sierrita, Pima, Mission, Twin Buttes. Small precious metal deposits are numerous, and many have been commercially exploited and subsequently abandoned. Such mineralized regions are potential sources of toxic metals to surface and ground waters. No attempt has been made to identify and understand the various natural and anthropogenic controls on water quality in this regional aquifer system.

To understand a complex geologic system, such as all or part of a watershed, and involve three dimensions, a number of research tools must be employed and the information integrated. In the case of the Upper Santa Cruz River Watershed, examining the surface provides information on mineralogic variability, and hence, potential chemical contributions to fluids. Examination of the subsurface may provide information regarding pathways for groundwater flow. In order to achieve these goals the following tools are being used:

Geologic Information

Field checking of abandoned metal mines in the various mountain ranges surrounding the sediment filled basins, e.g. Patagonia Mountains, Santa Rita Mountains. Mapping of surface fractures and faults that may have been sites of mineralization and recent fluid flow. More geologic information.

Geophysical Information

Collection of high resolution airborne magnetics and electromagnetics (300 meter spaced flight lines) over each mountain block and within the sediment-filled basins. Collection of high resolution land-based geophysical data. The intent is to provide subsurface geologic information, particularly with regard to distribution of subsurface fractures and faults, and to better understand the structure of the sedimentary basins. Airborne magnetics have been colected for most of the area under study and airborne electromagnetcs have been collected over the sedimentary basin regions. More geophysical information.

Remote Sensing Information

Collection of high resolution AVIRIS data (continuous coverage with ground resolution of 17 meters) over the study area. This data, following processing, provides surface information on the distribution of minerals and vegetation. AVIRIS data has been collected, processed and interpreted using imaging spectroscopy methods,over approximately 600 sq. miles in the southern part of the study area. More remote sensing information.

Geochemical Information

Collection of precise trace element and stable isotope data for rocks, minerals and fluids throughout the mountainous recharge areas. Stable isotopes are used here to fingerprint sources of chemical elements dissolved in fluids and to track fluid evolution. Trace element data can be used to identify the sources, and specific mineralogic siting, of potentially toxic metals. More geochemical information.