IDENTIFICATION OF AQUITARDS IN FRACTURED SEDIMENTARY ROCK

In sedimentary rock sequences, aquitards are commonly designated on the basis of lithology, such as shaley units within sandstones or carbonates. This study shows detailed profiles of hydraulic head versus depth provide the most relevant indication of aquitard units in fractured sedimentary rock. Since 2003, 19 detailed Westbay™ multilevel systems (MLSs) have been installed in comprehensively investigated coreholes at 3 contaminated fractured rock field sites near Los Angeles, CA; Madison, WI; and Toronto, ON. These sites include sandstones, siltstones, shales, and dolostones. The MLSs incorporate an average of 29 head measurement zones or about 3 zones per 10 m. At 2 of the 3 sites, data collected from these MLSs show the vertical component of the hydraulic gradient, referred to here as the vertical gradient, is generally downward. Both upward and downward vertical gradients are observed at the third site. The plots of hydraulic head versus depth have simple geometric shapes with sections of minimal vertical gradient separated by sharp inflections. The sharp inflections indicate intervals with relatively low vertical hydraulic conductivity, or aquitards. The sharp inflections often represent vertical gradients much greater than 1 and the maximum vertical gradient calculated for this study is 48. The maximum vertical gradient represents a loss of 43.6 m of hydraulic head over the length of a packer seal, 0.9 m. The head profiles are compared to other complimentary datasets collected from the continuous cores and coreholes. Several conclusions follow from these comparisons. First, the sharp inflections often occur across much thinner intervals than represented by lithology indicated by core and/or geophysical logs. Second, the sharp inflections do not always occur across lithologies, such as shale, typically associated with the term aquitard. Rather, some inflections likely represent a poor vertical hydraulic connection between the fracture networks of adjacent aquifers. Aquitard units delineated using the detailed vertical head profiles are being incorporated into numerical models of groundwater flow and contaminant transport. Therefore, appropriate delineation of these aquitard units is critical to producing realistic simulations that will be used to evaluate plume dynamics and remedial options.