HIGH RESOLUTION HEAD PROFILES REVEAL NEW INSIGHTS ABOUT AQUITARD MECHANISMS IN SEDIMENTARY ROCKS

In porous media, aquitards are conceptualized as intervals of geologic materials with low vertical hydraulic conductivity. In rock, the bulk hydraulic properties are typically controlled by fractures. Consequently, changes in fracture network characteristics might give rise to aquitard mechanisms that are distinct from those in porous media. Previous outcrop-based geomechanical studies have shown that joints preferentially terminate at specific stratigraphic horizons but have not provided any hydraulic evidence that these horizons function as aquitards. Here, we use high resolution head profiles to provide direct hydraulic evidence for decreases in bulk vertical hydraulic conductivity across stratigraphic horizons. The research was done at a site near Madison, Wisconsin where dense non-aqueous phase liquid (DNAPL) contaminants impact a sedimentary rock aquifer system. Twelve boreholes were continuously cored to between 53 and 152 m bgs along two, 4 km long orthogonal cross-sections spanning the dissolved phase plume. These boreholes were comprehensively characterized using a variety of high resolution techniques and then instrumented with multilevel systems with between 17 and 46 monitoring intervals. Hydraulic head profiles were collected at each location several times per year over multi-year periods. The head profiles reveal large increases in vertical gradient that occur across thin (< 1 m) stratigraphic intervals or bed contacts rather than across thicker units that may be considered aquitards based on their low matrix hydraulic conductivity. Comparison of these vertical gradients with detailed lithologic information from the cores and geophysical logs suggests these head loss zones are caused by poor connectivity of vertical fractures. Outcrop fracture measurements are in progress and preliminary results indicate strong contrasts in fracture network characteristics across the same stratigraphic intervals where large head loss across very thin intervals is observed. This unique aquitard mechanism is not currently accounted for in conceptual or numerical models of groundwater flow and contaminant transport. The omission could have strong implications for migration of DNAPL and prediction of flow path trajectories and residence times.