New Mexico Geological Society Annual Spring Meeting — Abstracts

Underflow, the water moving in zones of saturation perched in unconsolidated material on bedrock beneath slopes or dry stream channels, can be an important part of the hydrologic cycle. For example, the phenomenon has long been recognized in Mortandad Canyon at Los Alamos National Laboratory, NM.

Underflow is of special interest at environmental-restoration sites, where it can be responsible for generating leachate or transporting contaminants. All means of mobilizing contaminants by water should be included in site characterizations, but underflow is often overlooked. The occurrence of underflow may be determined with a simple piezometer installed at the colluvium/bedrock or alluvium/bedrock contact and equipped with a continuous water-level monitoring device. Ideally, such piezometers are installed upgradient of contamination to determine if underflow is entering the site.

The problem has been addressed at two environmental-restoration sites. One site is the Tererro Mine, north of Pecos, NM, where underflow could escape conventional measures to deny water access to the waste-rock pile and produce acid runoff. The other is a Sandia National Laboratories, NM, test site, where underflow could potentially mobilize contaminants at shallow depth and cause the point of their entry into regional ground water to be at some distance from their source.