A STUDY OF HYPORHEIC WATERS FROM BIG BEND NATIONAL PARK SANDBARS

Hyporheic conditions and hydrologic parameters are being investigated at two locations adjacent to the Rio Grande in Big Bend National Park. Two sandbars along the Texas shore were chosen for their proximity to two USGS gaging stations. These sites are roughly 80 river miles apart, near Castolon and Rio Grande Village. The installation of piezometers in these sandbars to gain information on the subsurface near-shore conditions has been the focus of the first stage of the study. These piezometers were emplaced with a water-jetting tool, using river water, to minimize disturbance to the biogeochemistry of the hyporheous. Piezometers were completed in fine sediments to the point of refusal offered by underlying cobbles. The bottom three feet is the screened interval.

There are a total of 14 piezometers at the eastern site, located adjacent to the river access at Rio Grande Village. These range in depth from 6 feet closest to the river, to 18 feet furthest from the river. A total of six pressure transducers have been installed in these piezometers to monitor barometric pressure, temperature, and water levels within the wells. These transducers have been collecting data continuously for more than a year. At the western site, located upstream from the confluence of Blue Creek and the Rio Grande, 12 wells have been installed. This site is lower and closer to the river, consequently the depths attained by the water-jetting tool before hitting cobbles are not as great. These wells range in depth from 6 to 9 feet. Water levels in the wells are measured periodically and data from the transducers is gathered. These data can be compared with stage heights in the river to give information on the hydrologic properties of the sandbars. Hydrogeologic tests have been conducted on the wells, and will continue, to gather information such as hydraulic conductivity and aquifer transmissivity.

This is the first study of its kind on the Rio Grande in the Big Bend region. Results will be valuable in furthering the knowledge of surface- groundwater interactions in arid regions, as well as providing data on hyporheic conditions as they relate to increased channel sedimentation, riparian restoration projects, and the control of exotic riparian vegetation.