New Mexico Geological Society Annual Spring Meeting — Abstracts

A low-head weir was installed in lower Los Alamos Canyon to eliminate off-site transport of contaminant-bearing sediment by fire-enhanced runoff. Perched water occurs in basalt beneath the area. To monitor the impact of ponding at the weir on subsurface water quality, three boreholes (one vertical, one at 43° and One at 34°) were drilled. As the basalt is highly fractured and unstable, holes had to be drilled by casing-advance methods. To keep the angled holes open when casing was retrieved, but still permit the flexible liners used for monitoring to contact the basalt, a shield was inserted. It consists of 6-inch PVC in which three 30-inch scallops were cut out per 10-ft length. The ideal orientation for the shield is with scallops on the bottom. However, cutting scallops resulted in sufficient weight loss that the shield rotated upon insertion down the casing, placing the scallops on top. This would have allowed rock to fall in when the casing was pulled. Alternating blank and scalloped sections of PVC solved the problem in the 43° hole (but not the 34° hole). When the liner was everted into the 43° hole by inflation, it tried to go out into the annulus through the first scallop encountered. To prevent this, the liner was installed un-inflated and bundled with its bottom in a sleeve tied to the tip of a small diameter PVC rod used to push the material into the cased hole. A centralizer at the end of the rod guided it past scallops. When the bottom of the hole was reached, the liner was partially inflated. Weak thread selected to bundle and attach the liner to the rod broke upon inflation, permitting the rod and centralizer to be readily retrieved. Plans call for drilling out the shield in the 34° hole and casing it with perforated PVC along most of its length. The annular space shall be filled with material to provide continuity between the highly fractured basalt and the PVC. Such construction will permit frequent neutron-probe moisture surveys, as well as sampling soil water with an absorbent liner.