Figure 1. Water Resources in the Upper Santa Cruz River Basin
Figure 1 shows the geography of the Santa Cruz River Basin. The river flows from the San Raphael Valley in eastern Arizona, dips into Mexico, then runs northward until it converges with the Gila River about 12 miles southwest of Phoenix. The river is 225 miles long, with 35 miles of it flowing through Mexico. Several tributaries and creeks empty into the Santa Cruz River on the Arizona side.
Figure 1. Water Resources in the Upper Santa Cruz River Basin
Land Use. The land around the river is used for a variety of purposes. High density residential areas predominate in Nogales, Sonora, while low and medium density residential areas are more prevalent in Arizona. Other uses for land include industry, commerce, transportation, recreation, and agriculture. Agricultural use of land has declined a great deal over the past 20 years, while residential and industry use has increased. Land needed for transportation and recreation purposes has also increased with the population and industry growth. Figure 2 shows types of land use in Ambos Nogales.
Water Resources. The water necessary to support the populations and industries of Ambos Nogales comes primarily from underground water sources called aquifers. These aquifers are naturally occurring storage units which hold water in the porous substrates such as sand and gravel. This water is accessed only by wells or springs. Aquifers along the Santa Cruz River and its tributaries are tapped by dozens of municipal wells to supply water for Ambos Nogales, as well as for agriculture and mining.
Ecology. The Santa Cruz River supports a great many species of plant and animal life. Rivers and other natural watercourses and the land around them, called riparian areas, are extremely important to Arizona's wildlife. Forty percent of Arizona's endangered birds rely on these areas for survival. In addition, a full 75% of Arizona wildlife is dependent at some point during their life cycles on riparian areas. Not only do riparian areas nurture animal life, but the vegetation growing around the waterways helps with stream bank stabilization, erosion control, water quality, cooler temperatures for wildlife, and groundwater recharge.
The IWWTP, built just downstream of the convergence of the Nogales Wash and the Santa Cruz River (see Figure 1) is located on the Arizona side, about 12 miles north of the U.S.-Mexican border. Built in the 1950s by both countries and jointly operated since, the plant currently processes over 12 million gallons of sewage from Ambos Nogales, with about seven million gallons flowing from Nogales, Sonora, and four million from Nogales, Arizona. Approximately 6,000 acre feet of treated sewage annually flow from the plant, emptying directly into the Santa Cruz River. The IWWTP operates at just below 75% capacity, which means whatever impact the plant has on the river will increase as human population and sewage volume increase in Ambos Nogales.
Although nitrogen and ammonia levels were high just below the treatment plant, the researchers found that these levels greatly decreased downstream. This seems to indicate that these pollutants were being broken down by natural processes like chemical transformation, absorption by riparian vegetation, or adsorption (adhesion to solids) by sediment particles. Although pollution levels were reduced several kilometers below the treatment plant, they often were elevated again near downstream contamination sources such as the town of Tubac. Researchers attributed the failure of the river to return to the lower pollution levels found upstream of the IWWTP to the contaminants added by septic systems, agricultural runoff, and other sources downstream.
Vegetation. Vegetation in the arid Ambos Nogales area depends primarily on water availability, although it also requires nutrients like nitrogen and phosphorus to flourish. Therefore, while it may appear worrisome that the plants absorbed the pollutants, the riparian vegetation actually thrive on the higher levels of nutrients that are discharged by the sewage treatment plant. The more plentiful supply of water downstream of the facility also helps support the native vegetation of cottonwood, willows, and mesquite forests, which are more plentiful where the river is perennial. Researchers found that the cottonwood forests downstream of the IWWTP have larger stem diameters and more plentiful foliage than the cottonwood stands upstream of the plant, indicating more fully developed and robust vegetation. This suggests that not only does the IWWTP have a positive effect on riparian vegetation, but the vegetation may be helping the quality of the water as well, by absorbing some of the polluting nutrients found in the river.
Groundwater. Other determinants of the health of the Santa Cruz River Basin include the groundwater levels. A balance must be maintained between water pumped from the aquifers and water deposited into the aquifers by rivers and streams. When more water is withdrawn from the groundwater supplies than can be naturally replenished, extraction becomes more difficult and costly and frequently the quality of the water deteriorates. This reduces the supply of water for human consumption. Another consequence of excessive water withdrawal is the loss of crucial plant life, which not only supports various wildlife species, but as in the case of the cottonwoods downstream of the IWWTP, may also help clean the river of pollutants.
In many places in the river basin, excessive groundwater pumping has resulted in the river no longer flowing year-round. Discharge from the IWWTP has improved water levels, however, and the river is once again perennial for about 19 miles downstream from the plant. In addition to surface water levels increasing, groundwater levels have also improved. Aquifers below the plant are replenished by discharge from the plant, helping to maintain the groundwater balance necessary for adequate water supply.
Care must be taken as well to maintain a balance in groundwater pumping for human consumption and the natural replenishing of the groundwater. As the population of Ambos Nogales continues to increase, city officials will face the challenge of preserving these water sources by not over-pumping them.
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Last updated 03/21/05