A High Resolution Vertical Gradient Approach to Hydrogeologic Unit Delineation in Fractured Sedimentary Rocks
Prediction of contaminant transport and fate relies on robust delineation of hydrogeologic units (HGUs), which serve as the framework for all conceptual and numerical models. In layered sedimentary rock systems, contrasts in bulk vertical hydraulic conductivity (Kv) are expected to refract groundwater flow lines and be indicative of distinct HGUs. However, HGU delineation typically relies on data indirectly related to hydraulic properties or hydraulic data insensitive to contrasts in Kv. Flow system theory shows that the distribution of hydraulic head reflects contrasts in Kv. Therefore, depth-discrete and detailed (i.e. high resolution) hydraulic head profiles should identify contrasts in Kv in layered systems. This research develops, applies, and tests a high resolution head/vertical gradient profile approach to HGU delineation for sedimentary rock groundwater systems. First, the repeatability and characteristics of head profiles in sedimentary rocks were evaluated by collecting data from three contaminated field sites with contrasting geologic and flow system conditions. The shapes of the head profiles were reproducible in time and geometric in nature. The head profiles displayed thick zones with no or minimal vertical gradient separated by thinner zones with large vertical gradient indicating contrasts in Kv that did not coincide with lithostratigraphic units. Next, the method was applied at the plume scale to a site in Wisconsin with seven vertical gradient profiles collected along two cross-sections. The vertical gradient cross-sections revealed nine laterally extensive zones with contrasting Kv. The contrasts in Kv were closely associated with key sequence stratigraphic units and integration of the two data sets resulted in delineation of eight HGUs for the site. Last, comparison of these HGUs to the site contaminant mass and phase distributions, including detailed rock core contaminant profiles, provided additional verification for the HGU delineation and added insight regarding important flow and contaminant migration pathways. Application of the high resolution head/vertical gradient profile method at the Wisconsin site resulted in hydraulic measurement based, geologically integrated, and more accurately delineated HGUs. The revised hydrogeologic unit conceptual model will improve predictions of contaminant transport and fate and evaluation of remediation system designs.