Field Assessment of Monitoring and Water Supply Well Seals

Monitoring and water supply wells are sealed with low hydraulic conductivity grout placed in the annulus between the casing and the drillhole. The well seal protects groundwater from pollution by restricting fluid flow through the annulus. A defective well seal compromises public health. Field assessment of well seals, failed and intact, will improve the methods and materials used for well seal construction.

Well seal performance was evaluated using a downhole, ultrasonic device developed at the University of Wisconsin-Madison in the Department of Civil and Environmental Engineering. Sixteen monitoring and water supply wells were field tested. Factors influencing seal integrity, including type of sealant, seal placement method, and influence of geology and
hydrogeology, were considered.

Results from the study demonstrate the effectiveness of the ultrasonic probe for use in well seal inspection. All wells showed seal degradation near the ground surface where desiccation cracking and frost action is the greatest. Monitoring well seals constructed with bentonite slurries performed the best overall when the water table was shallow. Bentonite slurry based seals performed poorest for wells with deep water tables (> 10 m). This was especially apparent for the water wells logged in this study. Poor water well seal performance is attributed to dilution of the drilling mud, settling and infiltration into the adjacent coarse grained formation, and bridging of cuttings shoveled into the annulus.