EXAMINING HYDROCHEMISTRY IN TEXAS AND COAHUILA DE ZARAGOZA TO CONSTRAIN THE SOURCE AREA OF GOODENOUGH SPRING, AMISTAD RESERVOIR, U.S.-MEXICO BORDER REGION

Goodenough Spring is a major karstic spring which historically discharged more than 100,000 acre-ft/year at the base of the International Amistad Reservoir along the U.S.-Mexico border, making it a key component of the area’s water budget. Historically, the Goodenough Spring catchment area was thought to be located north of the Rio Grande in the United States. Recent observations, however, support the premise that the spring’s source area is south of the Rio Grande, in Mexico. The lack of an established characterization of Goodenough Spring’s source area limits effective management of the region’s water budget and water quality, since the properties and hydrology of the spring’s source area are integral to determining the water quality issued from the spring, into the reservoir, and into the downstream portions of the Rio Grande basin. Given that this portion of the Rio Grande serves as a water resource to over 3 million people in the region, thorough understanding of the spring systems is critical. In order to constrain the spring’s source area, research has been undertaken to investigate the hydrochemistry of groundwater sites in both the U.S. and Mexico. In the summer of 2021, the research team sampled five wells in the Mexican state of Coahuila de Zaragoza, four wells in Texas, and Goodenough Spring itself. By analyzing the samples for a comprehensive suite of parameters (major ions, metals, and isotopes), and comparing the results with meteorologic and geologic data, discerning potential transboundary linkages and constraining Goodenough Spring’s source area is possible. The results also provide insight into a poorly studied area of Coahuila de Zaragoza. Future work will focus on sampling additional wells in nearby watersheds as well as sampling surface-water sites and other springs in the study area. Acquisition of continuous water-quality data from Goodenough Spring by deploying a sonde at the spring over the course of several months will also augment scientific understanding of seasonal and long-term spring dynamics.