VERIFYING THE INTEGRITY OF ANNULAR AND BACK-FILLED SEALS FOR VADOSE-ZONE MONITORING WELLS

Monitoring the movement of contaminants throughout tbe vadose zone req uires the use of wells and the credibility of a monitoring program dep ends on obtaining an adequate seal between the well casing and borehol e 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 in filtration 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 subhor izontal interflow zones) in order to observe water movement during the infiltration test, Bentonite was used between sand intervals to preve nt the borehole (or annular space) from serving as a conduit for verti cal water flow or tracer migration. Neutron probes were used to confir m the presence and locations of each completion interval (sand or bent onite) by distinguishing differences in background-water content of th e 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 sp ectroscopy system, respectively. Results confirm that each well was in stalled 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.