Goodenough Spring Catchment Area Characterization, Amistad Reservoir, Rio Grande Valley
Goodenough Spring is a major spring discharging more than 100,000 acre-ft/yr into the Rio Grande prior to the construction of Amistad Reservoir and about 51,900 acre-ft/year after its construction along the Texas-Coahuila de Zaragoza border, making it a key component of the area’s water budget (Reeves and Small, 1973; Brune, 1975; Green and Bertetti, 2010) (Figures 1 and 2). Historically, the catchment area for Goodenough Spring was thought to be located north of the Rio Grande, in the United States (Water Treaty of 1944). In recent years, however, anecdotal evidence supports the premise that the source area for Goodenough Spring is actually south of the Rio Grande, in Mexico (Thomas, 1963; Kamps and Groeger, 2006; Kamps et al., 2009). The lack of an established characterization of Goodenough Spring’s catchment area limits effective management of the region’s water quality, since the properties and hydrology of a spring’s catchment area play an integral role in determining the water quality that is issued from the spring. With an improved understanding of the spring’s catchment area, the quality of water that flows into the Amistad Reservoir from Goodenough Spring can be more effectively and efficiently safeguarded. Geologic formations intersected by the Rio Grande consist of carbonate (e.g., limestone) and non-carbonate rocks, such as clastic sandstones, shales, and igneous formations. Goodenough Spring discharges from the karstic carbonate rocks of the Edwards-Trinity Aquifer (Boghici, 2004). This distinction of rock type is crucial, as it affects the quality of water that is introduced into the Rio Grande and the development of high-volume spring discharge. Surface water that recharges non-carbonate rocks moves slowly through the subsurface flow at speeds of tens of feet per year. Comparatively, flow through karstic carbonate rocks can be a kilometer per day or faster. Thus, recharge into karstic carbonate rocks is rapid and unfiltered where it enters through sinkholes and other swallets. This means recharge captured by Goodenough Spring’s catchment area may be discharged into Amistad Reservoir within days or even hours after a storm event. This makes identification and proper management of the spring catchment an area of critical importance for water quality. Development along the Rio Grande will impact the quality of runoff that recharges the river. Further degradation of recharge to Amistad Reservoir and the Rio Grande can be mitigated if the catchment area is delineated and the proper land-use practices are employed. Developing a conceptualization of Goodenough Spring’s catchment area is essential not only to the communities near the Amistad Reservoir, but to a variety of stakeholders living in the Rio Grande Valley downstream from the Amistad Reservoir and near the bays and estuaries of the Gulf of Mexico. According to the Texas Commission on Environmental Quality (TCEQ), more than 3 million people live downstream of the Amistad Reservoir (www.tceq.texas.gov). Degradation of the quality of water discharged into the Amistad Reservoir will directly impact the quality of water discharged from the Amistad Reservoir to the lower Rio Grande and to the bays and estuaries. Efforts that protect the quality of this water will directly impact the environmental and public health of the region, since 1) much of the population relies on the Rio Grande as the principal source of water, and 2) the environmental health of the Rio Grande is contingent on quality of water discharged into the river by upstream tributaries and springs. Given the importance of Goodenough Spring, this Border 2020 Program project focused on constraining the spring’s source area. The key objectives that were undertaken to contribute towards the source area delineation included:
- Compile pre-existing environmental (e.g., hydrologic and geologic) data from the Amistad Reservoir region in Val Verde County, Texas, United States and the State of Coahuila de Zaragoza, Mexico.
- Identify sites for water sampling in both Texas and Coahuila de Zaragoza.
- Conduct a water sampling campaign targeting wells in Texas and Coahuila de Zaragoza as well as Goodenough Spring.
- Send collected water samples to laboratories for water chemistry analyses.
- Evaluate the laboratory analyses results.
- Use the acquired water chemistry data to constrain the Goodenough Spring source area.