New Mexico Geological Society Annual Spring Meeting — Abstracts

The Bosque del Apache Wildlife Refuge is in the Rio Grande river valley, within the Rio Grand rift. Heat flows in this part of the Rio Grande rift are relatively high: 70 -100 mW m^-2. Preliminary work in the Bosque del Apache area suggests that subsmface temperature are pelturbed, most probably ground-water flow.

A warm-water well on the Refuge suggests a high geothel'IDal gradient. Estimates based on the water temperature and depth of this well (now inaccessible) yield a temperature gradient of 180 -200°C km^-1. Subsurface temperatures at nearby abandoned wells on the Bosque define much lower temperature gradients: 8 -32°C km^-1 (lower than we would expect in the Rio Grande rift). These low-gradient wells are not as deep as the the warm-water well. The presence of lower temperature gradients at shallow depths suggests that downward ground water flow is sweeping heat out of the near-surface environment. We suggest that ground-water recharge, either streamflow losses from the Rio Grande or percolation from irrigation and other water use at the Refuge, is cooling near-smface temperatures and masking higher heat flows at greater depths.

The estimated temperature gradient at the warm-water well suggests a heat flow in excess of 200mW m^-2, which is much higher than typical Rio Grande rift heat flows. This site may represent anomalously high subsurface temperatures caused by upward ground water flow. There are a number of possible causes for such upflow. Some evidence (the driller's log of an old oil test) suggests a basement high near this site which could cause the Socorro basin (through which the Rio Grande flows) to become quite shallow and constricted. Such a constliction could force deep ground water to flow upward, elevating subsurface temperatures. Another possibility is that the warm-water well reflects a local geothermal anomaly caused by warm water from a deeper aquifer system leaking up through a fracture or fractures.

We have measured anomalously high subsurface temperatures at another site, east of the Refuge boundary, up out of the Rio Grande river valley. We speculate that upward ground-water flow may be responsible for the high temperature gradient observed at this site. In this case, ground-water flowing westward from the Jornada del Muerto into the Socorro basin may be forced upward by a subsurface hydrologic barrier, or ground water may leak upward through fractures associated with the eastern boundruy of Socorro basin.