USGS science fora changing world Monitoring the Water Quality of the Nation's Large Rivers Rio Grande NASQAN Program The U.S. Geological Survey (USGS) has monitored the water quality in the Rio Grande Basin as part of the redesigned National Stream Quality Accounting Network (NASQAN) since 1995 (Hooper and others, 1997). The NASQAN program was designed to characterize the concentra­ tions and transport of sediment and selected chemical constituents found in the Nation's large rivers including the Mississippi, Colorado, and Columbia in addition to the Rio Grande. In these four basins, the USGS currently (1998) operates a network of 40 NASQAN sites, with an emphasis on quantifying the mass flux for each constituent (the amount of material moving past the site, expressed in tons per day). By applying a consistent flux-based approach in the Rio Grande Basin, the NASQAN pro­ gram is generating the information needed to identify regional sources for a variety of constit­ uents, including agricultural chemicals and trace elements, in the basin. The effect of the large reservoirs on the Rio Grande can be observed as constituent fluxes are routed downstream. The analysis of constituent fluxes on a basin-wide scale will provide the means to assess the influence of human activity on water-quality conditions in the Rio Grande. Environmental Setting Irrigation withdrawals from the Rio Grande site at El Paso is 125 river miles (in the lower Rio Grande Valley, which downstream of Elephant Butte The Rio Grande originates in the San comprises Cameron, Hildalgo, Starr, and Reservoir in New Mexico and 1 .7 miles Juan Mountains of southern Colorado and Willacy Counties) accounted for about 44 upstream of the American Dam at El follows a 1,885-mile course before it percent of the surface-water irrigation Paso. Streamflow in the Rio Grande at flows into the Gulf of Mexico (fig. 1). withdrawals in Texas during 1994 (Texas El Paso is controlled largely by releases Along the way, the river and its tributaries Water Development Board, 1996). from Elephant Butte Reservoir. At drain a land area of 182,200 square miles. American Dam, much of the flow in the This drainage encompasses a widely Site Selection Rio Grande is diverted for irrigation and varied landscape in the United States and municipal uses to the American Canal Mexico, including mountains, forests, and Eight NASQAN sampling sites in Texas and the Acequia Madre Canal deserts. The basin is home to diverse (fig. 1, table 1) were selected in the Rio in Mexico. Downstream of El Paso/ native plants and wildlife as well as some Grande Basin to monitor the fluxes from Ciudad Juarez, the Rio Grande has little 10 million people 8 million in Mexico subbasins. Sites were located specifically or no flow until the waters of the Rio alone. For approximately two-thirds of its to measure inflow and outflow of material Conchos, which originates in the Sierra course, the river also serves as the from the two main-stem reservoirs Madre Occidental in Mexico, join the boundary between the United States (Amistad International and Falcon river near Presidio/Ojinaga. and Mexico. International) that strongly affect the flux Rio Grande at Foster Ranch In this mostly arid to semiarid region, of chemical constituents and sediment in near Langtry (2) is approximately the absence of flow in the river as well as the the Rio Grande. Land use in the subbasins 600 miles downstream of El Paso and presence of flow determines the basin's is dominated by rangeland, with forest, 300 miles downstream of the character. Many of the river tributaries are agricultural, and urban areas constituting confluence of the Rio Grande and Rio intermittent streams. Much of the flow is the remainder (Texas Natural Resource Conchos. Because much of the water controlled by numerous reservoirs in the Conservation Commission, 1994, fig. reaching El Paso is diverted, basin. Throughout the basin, an extensive 1.2). In descriptions of each site below, Streamflow at Foster Ranch is largely system of water structures captures and the numbers in parentheses correspond to from the Rio Conchos. This site, in controls the flow of water in the subbasins site numbers in figure 1 and table 1. conjunction with the Pecos River site, to meet regional needs for flood control, Rio Grande at El Paso (1) reflects provides data to describe the flux of power generation, and storage for domestic, drainage of the entire Rio Grande main constituents and sediment into agricultural, and industrial purposes. stem in Colorado and New Mexico. The Amistad International Reservoir. U.S. Geological Survey USGS Fact Sheet FS-083-98 U.S. Department of the Interior Table 1. Description of NASQAN sampling sites in the Rio Grande Basin [ . not applicable] Incremental ^itp Drainage Incremental Mean increase/ one increase in streamflow decrease in no. Name area (fig- 1) (square drainage area (cubic feet streamflow \ w / miles) (square miles) per second) (cubic feet per second) 1 Rio Grande at El Paso, Tex. 29,267 0 640 640 f 2 Rio Grande at Foster Ranch nearrf., »_ -.Langtry, 0 J ^ Tex. -,r- ._ __ .. 80,742 51,475 1,946 -T«-'- -Ct_1,306 ' 3 Pecos River near Langtry, Tex. 35,179 0 262 262 4 Rio Grande below Amistad Dam near Del Rio, Tex. 123,143 42,401 2,510 302 5 Rio Grande below Laredo, Tex. 132,578 9,435 3,433 923 6 Rio Grande below Falcon Dam, Tex. 159,270 26.692 3,223 -210 " 7 Arroyo Colorado at Harlingen, Tex. 182 -- 247 ~"M^K 8 Rio Grande near Brownsville, Tex. 176,333 1 7,063 2,226 -997 Pecos River near Langtry (3) is on and biological processes to alter the quality data on the outflow from the reservoir, the Pecos River approximately 15 miles of the inflowing water. These processes which can be compared to data on the upstream of its confluence with the Rio include deposition of sediment, evaporative inflow to assess the effect of retention Grande. The Pecos River is the major concentration of solutes, and biological and transformation of material within tributary to the Rio Grande within the removal of nutrients. This site provides the reservoir. United States. The Pecos River originates in the mountains of northern ior New Mexico, flows southward through EXPLANATION eastern New Mexico, and empties into Red Bluff Reservoir at the Texas-New SUBBASIN-ddentified by the location Mexico border. The flow of the Pecos of the most downstream point of the River at Langtry has been regulated by subbasin. except for closed basins) this reservoir since 1937. The Pecos | | Rio Grande at El Paso River joins the Rio Grande at the %f~] Rio Grande at Foster Ranch upstream end of Amistad International | | Pecos River near Langtry Reservoir. 35 Rio Grande below Amistad Dam I"*1 ' | Rio Grande below Amistad Dam near Del Rio (4). Water in Amistad | ,./ ] Rio Grande below Laredo 34° International Reservoir has a mean I | Rio Grande Below Falcon Dam residence time of about 1.6 years, which ELEPHANT RUT" [*'' | Rio Grande near Brownsville allows for numerous chemical, physical, RESERVOIR | | Closed basin NASQAN sampling site (number refers to table 1) MISTAD 1TERK \TIOMAL SERVOIR 28' Laredo \ United States 99° '~ FALCON INTERNATIONAL RESERVOIR LACUNA MWRI .6 r~Nv Mission Willacy N« /'" ^ ville Cameron Hidalgo Figure 1. Rio Grande Basin showing subbasins and NASQAN sampling sites. Rio Grande below Laredo (5). lead, and zinc, many common water- attributed to natural saline springs, About 37 percent of the water that soluble pesticides such as atrazine and irrigation return flows, and evaporation discharges from the Rio Grande Basin metalochlor, and suspended sediment. during the summer months. enters the river between the Amistad Frequency of sampling ranges from 6 to Metals also could be present in International Reservoir and Laredo. This 10 times per year depending on local site the Rio Grande because of extensive reach also has large centers of population characteristics. In the upper Rio Grande mining in New Mexico and in the Rio and industry that could affect water Basin, flow generally peaks in the early Conchos Basin in Mexico. The rapid quality. This site provides data to account summer. In the middle and lower regions development of maquiladoras for the inflow of chemical constituents of the basin, flow is controlled primarily (assembly plants in Mexico) also could and sediment from this major subbasin to by releases from Amistad International contribute to trace element the Rio Grande and to describe the and Falcon International Reservoirs. concentrations in the lower Rio Grande quality of inflow to Falcon International Because these reservoir releases are Basin (Texas Natural Resource Reservoir. dictated largely by irrigation needs, Conservation Commission, 1994). Rio Grande below Falcon Dam sti eamflow in the middle and lower basin Historical data for riverbed sediments (6) is 2.5 miles downstream of Falcon tends to be more e e.ily distributed than in the Rio Grande indicate increasing Dam. The Rio Salado in Mexico, which that in the upper basin and peak flows temporal trends in more trace elements joins the Rio Grande at the upstream end typically occur in the late summer and in the reaches of the river near El Paso of Falcon International Reservoir, is the early fall. The sampling strategy is to and Laredo than in other reaches, major tributary to this reach. This site assess water-quality conditions throughout which could be related to human provides data on the retention and the range of flows, with an emphasis on activities within the subbasins transformation of materials transported high flows. The strategy will be adjusted upstream of El Paso and Laredo (Lee into Falcon International Reservoir.