Installation of the Monitoring Site at the Los Alamos Canyon Low-Head Weir

The Cerro Grande fire of 2000 had an enormously adverse impact on and around Los Alamos National Laboratory (LANL). Immediately there were concerns about the potential for enhanced runoff/offsite transport of contaminant-laden sediments because of watershed damage. In response to this concern, the U.S. Army Corps of Engineers installed a low-head weir in Los Alamos Canyon near the White Rock ''Y.'' However, the occurrence of fractured basalt at the surface and ponding of runoff behind the weir enhance the possibility of downward migration of contaminants. Therefore, three boreholes were drilled on the south bank of the channel by LANL to provide a means of monitoring the impact of the Cerro Grande fire and of the weir on water quality beneath the canyon. The boreholes and associated instrumentation are referred to as the Los Alamos Weir Site (LAWS). The three boreholes include a vertical hole and two angled holes (one at approximately 45{sup o} and one at approximately 30{sup o}). Since the basalt is highly fractured, the holes would not stay open. Plans called for inserting flexible liners into all holes. However, using liners in such unstable ground was problematic and, in the angled holes, required deployment through scalloped or perforated polyvinyl chloride (PVC) shield. The vertical hole (LAWS-01), drilled to a total depth of 281.5 ft below ground surface (bgs), was completed as a 278-ft deep monitoring well with four screens: one targeting shallow perched water encountered at 80 ft, two in what may correspond to the upper perched zone at regional groundwater characterization well R-9i (1/4 mi. to the west), and one in what may correspond to the lower perched zone at R-9i. A Water FLUTe{trademark} system deployed in the well isolates the screened intervals; associated transducers and sampling ports permit monitoring head and water quality in the screened intervals. The second hole (LAWS-02), drilled at an angle of 43{sup o} from horizontal, is 156 ft long and bottoms at a depth of 106 ft bgs. The shallow perched water seen at LAWS-01 (at 80 ft) was not encountered. A scalloped PVC shield was installed to keep the hole open while permitting flexible liners to contact the borehole wall. It was initially instrumented with a color-reactive liner to locate water-producing fractures. That was later replaced by an absorbent liner to collect water from the vadose zone. The third hole (LAWS-03), drilled at an angle of 34{sup o} from horizontal, initially had a length of 136 ft and bottomed at a depth of 76 ft bgs. However, the PVC shield rotated during installation such that scallops were at the top and rock debris repeatedly fell in, preventing liner insertion. While pulling the scalloped PVC to replace it with a perforated PVC shield that did not require orientation, the scalloped PVC broke and only 85 ft was recovered. The hole was blocked at that position and could not be drilled out with the equipment available. Thus, LAWS-03 was completed at a length of 85 ft and a depth of 40 ft bgs. An absorbent liner was installed at the outset in preparation for the 2002 summer monsoon season. The entire monitoring site is enclosed inside a locked, 8-ft-high chainlink fence for security. The liners used in the angled boreholes carry electrical wire pairs to detect soil-moisture changes. Surface-water data are provided by stream gages above and below the weir site. Depth of ponding behind the weir is provided by a gage installed just behind the structure.