Verifying the integrity of annular and back-filled seals for vadose-zone monitoring wells
1997
Dunnivant, F.M. | Porro, I. | Bishop, C. | Hubbell, J. | Giles, J.R. | Newman, M.E.
Monitoring the movement of contaminants throughout the vadose zone requires the use of wells and the credibility of a monitoring program depends on obtaining an adequate seal between the well casing and borehole wall. The credibility of monitoring well installation was evaluated during the Idaho National Engineering Laboratory's Large Scale Pumping and Infiltration Test. Wells were drilled in and around a 6.5 acre infiltration basin with an air rotary rig using a downhole hammer, cased with PVC or steel, and the annular space back-filled with alternating layers of bentonite and sand. The purpose of completing the wells in this manner was to isolate fractured intervals (subvertical and subhorizontal interflow zones) in order to observe water movement during the infiltration test. Bentonite was used between sand intervals to prevent the borehole (or annular space) from serving as a conduit for vertical water flow or tracer migration. Neutron probes were used to confirm the presence and locations of each completion interval (sand or bentonite) by distinguishing differences in background-water content of the back-filling materials. Upon flooding of the infiltration basin with water containing radioactive tracers, water flow and tracer transport were monitored using neutron probes and an in situ, downhole gamma spectroscopy system, respectively. Results confirm that each well was installed correctly; water and tracers flowed through natural fractures in the subsurface and arrived at the monitoring sites located at sand completion zones. Significant water or tracer flow through the annular space between the well casing and borehole was not observed.
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