An evaluation of the effects of hot spring discharge on the Rio Grande in Big Bend National Park, Brewster County, Texas

The Rio Grande forms the southern boundary of Big Bend National Park, and is the international boundary between the United States and Mexico. The natural condition of the river has been degraded significantly. Of particular significance is the current low flow status of the river, a condition that has generally existed since approximately 1996. Along the Rio Grande between Santa Elena Canyon and Rio Grande Village there are several thermal springs, which are thought to contribute to the total flow of the river. The purpose of this study was to assess the discharge and chemical contributions of thermal springs on the river system.; Observations were made at a particular hot spring on the eastern boundary of the park. During the study, field measurements and water samples were collected, which included discharge measurements and basic chemical evaluations of the spring and river. Water chemistry samples were taken at three locations relative to the hot spring: upstream, the hot spring itself, and downstream. The water chemistry data exhibit the expected mixing relationship with the downstream sample plotting as a mix of the upstream sample and the hot spring sample. The upstream sample shows a seasonal variation with the winter samples having higher Total Dissolved Solids (TDS) and the summer samples having lower TDS. This is due to the influx of dilute local precipitation during the wetter summer season. The hot springs showed a relatively constant TDS value throughout the study. This value falls between the winter high and summer lows of the upstream river sample. The implication of this is that the hot springs "improve" the water quality (lower the TDS) of the river only in the winter months.