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 catchme