WATER WELL ANNULAR SEAL PERFORMANCE AND STRATIGRAPHIC CHARACTERISTICS IN THE UNSATURATED ZONE: IS THERE A RELATIONSHIP?

Water well annular seals are an important part of effective water well construction because they prevent surface water infiltration and cross contamination between aquifers. Previous studies have shown that bentonite-based grout slurry seals tend to desiccate when placed in the unsaturated zone, causing cracking or settling. It has been suggested that the moisture content of the geologic formation surrounding the well is the primary cause of potential seal desiccation. Other stratigraphic properties that may also influence seal behavior are cation exchange capacity, carbonate concentrations, and the presence of buried soils although corresponding hypothesizes remain to be fully tested. Previous research on bentonite slurry seals has focused primarily on laboratory simulations because of the inherent difficulties involved with studying annular seal conditions at significant depth without disturbance in the field. We believe this study is the first of its kind to utilize digital down-hole videos from clear-cased wells to study annular seals. These clear cased wells were placed in geologically distinct areas of Nebraska. Continuous core samples were collected at each site in an effort to determine the relationship between physical and chemical properties of the borehole stratigraphy and corresponding areas of desiccation in the annular seals. Initial core analyses suggest that buried soils, increased soil organic carbon content, and decreased soil cation exchange capacity affect bentonite slurry seals at specific depths. Particle size analysis also suggests a site-specific relationship with seal condition. Contrary to the findings of previous researchers, however, we have not established a direct correlation between moisture content of the formation and grout desiccation.