HIGH RESOLUTION VERTICAL GRADIENTS FOR IDENTIFYING KV CONTRASTS AND DELINEATING HYDROGEOLOGIC UNITS AT A CONTAMINATED SEDIMENTARY ROCK FIELD SITE

In standard practice, existing lithostratigraphic units are often assumed to be hydrogeologic units (HGUs). The boundaries of these HGUs are then used to guide the length and position of well screens for collection of hydraulic data, which, in turn, are used to inform numerical models. There is an inherent assumption that lithostratigraphic units will have uniform hydraulic properties within them and hydraulic contrasts at their boundaries. This study describes a unique approach to HGU delineation that relies on the 3-D distribution, magnitude, and direction of the vertical component of hydraulic gradient to delineate HGUs for a contaminated sedimentary rock study area in southern Wisconsin. Seven hydraulic head profiles obtained from depth-discrete and detailed (average of 3.4 monitoring zones per 10 m) multilevel systems monitoring to depths between 90 and 146 m are used to examine the three-dimensional distribution of vertical gradients. Inspection of the vertical gradients along two cross-sections 3.9 and 3.7 km long revealed 11 laterally correlatable vertical gradient zones. These vertical gradient zones indicated laterally extensive contrasts in bulk vertical hydraulic conductivity (K_V) and connectivity, the boundaries of which did not coincide with the lithostratigraphic units. Further investigations revealed that the K_v contrasts were strongly associated with regionally extensive sequence stratigraphic units: maximum flooding intervals and unconformities. Integration of the vertical gradients and sequence stratigraphy allowed for robust 3-D delineation of HGUs for the 16 km² study area. The high resolution head data and HGUs interpreted from the vertical gradients are critical to the construction and calibration of numerical models used to predict contaminant transport and fate at the site and evaluate potential remedial options.