U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 2010-3097

2010

The Border Environmental Health Initiative—Investigating 

the Transboundary Santa Cruz Watershed

In 2004 the U.S. Geological Survey 

(USGS) launched the Border Environmental Health Initiative (BEHI), a 

major project encompassing the entire 

U.S.–Mexico border region. In 2009, 

a study of the Santa Cruz River Watershed (SCW), located in the border 

region of Arizona and Sonora, Mexico, 

was initiated as part of the BEHI. In this 

borderland region of the desert Southwest, human health and the ecosystems 

on which humans rely depend critically 

on limited water resources. Surface 

water is scarce during much of the 

year, and groundwater is the primary 

source for industrial, agricultural, and 

domestic use. 

In order to identify risks to water 

resources in the SCW, and the potential 

consequences to riparian ecosystems 

and ultimately human health, the 

USGS is using an interdisciplinary and 

integrative approach that incorporates 

the expertise of geographers, hydrologists, biologists, and geologists to track 

organic and inorganic contaminants and 

their effects from sources to sinks in 

sediment, water, plants, and animals. 

Existing groundwater and surfacewater models are being used and modified to assess contaminant and sediment 

transport. Water samples, as well as 

samples of sediment, aquatic macroinvertebrates, aquatic plants (macrophytes), algae, riparian grasses, fish, 

and birds have been and will continue 

to be sampled at five locations along the 

Santa Cruz River (fig. 1). 

A

B

C

Figure 1. U.S. Geological Survey scientists taking 

samples of (A) plants and soil, (B) water and aquatic 

invertebrates, and (C) environmental parameters in 

the Santa Cruz River watershed.

U.S. - Mexico Border Environmental Health Initiative
á

Figure 2. Map showing location of the Santa Cruz River Watershed at the Arizona-Sonora border and locations of sampling sites in the study area.

NIWWTP, Nogales International Waste Water Treatment Plant.

Field samples are being obtained at 

sites that coincide with historical sampling programs of the U.S. Fish and Wildlife Service (King and others, 1999). Site 

locations (see fig. 2) include (i) the headwaters at San Rafael del la Zanja (which 

should be unaffected by downstream 

contaminant sources); (ii) the Sonoita 

Creek tributary, which passes through the 

abandoned mining district at Patagonia; 

(iii) the binational Nogales Wash tributary that flows through highly urbanized 

areas of Nogales, Sonora, and Nogales, 

Arizona; (iv) effluent downstream from 

the Nogales International Waste Water 

Treatment Plant (NIWWTP); and (v) the 

Santa Cruz River at Tumacácori, downstream of the confluence of these several 

branches. This experimental design allows USGS scientists to accurately determine contaminant contributions from the 

mining district and municipalities; it will 

also provide information on contaminant 

levels in water before entering and after 

exiting the NIWWTP. 

Field data are being used in combination with the Soil and Water Assessment 

Tool (SWAT) to identify contributing 

areas, sources and transport modes of 

contaminants to track their movement 

in surface waters. SWAT is a watershed 

model that quantifies the impact of land 

management practices, and estimates 

of runoff and evapotranspiration from 

SWAT simulations will be used to 

modify boundary conditions in existing 

groundwater-flow models. The resulting 

improved understanding of the effects 

of human activities on aquifer dynamics 

and contaminant transport will enable the 

SWAT model to be used to identify critical subwatersheds where implementing 

management practices could be most effective to abate pollution. This USGS research in the Santa Cruz River Watershed 

will help determine the linkages between 

the condition of the physical environment 

and health issues in the borderlands.

For more information contact

Laura M. Norman, lnorman@usgs.gov; 

James Callegary, jcallega@usgs.gov;

Charles van Riper III, charles_van_riper@usgs.gov; 

and Floyd Gray, fgray@usgs.gov

Reference Cited

King, K.A., Zaun, B.J., and Velasco, A.L., 1997, 

Contaminants as a limiting factor of fish and 

wildlife populations in the Santa Cruz River, 

Arizona: U.S. Fish and Wildlife Service, 

Phoenix, Ariz., 66 p.