NEW INSIGHTS INTO THE HYDRAULIC CONDUCTIVITY (K) OF FRACTURED TILL BASED ON 30 YEARS OF FIELD, LABORATORY, AND MODELING STUDIES IN IOWA

In glaciated regions, estimation of the bulk hydraulic conductivity (K_b) of till, usually with slug tests, is standard practice for site investigations. Since the early 1990s, students in the ISU Hydrogeology Research Group have moved beyond standard slug testing to understand how till fractures impact K_b values. Phase 1 of this work involved installation of vertical and angled piezometer nests in the Walnut Creek watershed (WCW) near Ames, IA and the Northeast Iowa Research Center (NERC) near Nashua, IA. The loamy till units ranged from late Wisconsinan (WCW) to Pre-Illinoian (NERC) age and exhibited fractures to about 4 m depth (WCW) and 24 m depth (Nashua). Slug and pumping tests in till in the near-surface oxidized till (≤4 m) produced nearly identical K_b values – 10e-5 m/s (WCW) and 10e-6 m/s (NERC) – suggesting a dense, interconnected fracture network. Phase 2 involved extraction of 0.43-m-diameter by 0.4 to 0.45-m-long, intact, undisturbed cores from the Phase 1 site. Fracture intensity and orientation were mapped in the field trenches. Fractures were visible on the outside of the large cores and dyes showed that water flowed primarily within the fractures. In addition, K_b values of the cores were nearly the same as K_b values measured in Phase 1, confirming that till fractures control K_b at the field scale. Mobile-Immobile and FracMan (MAFIC) model simulations confirmed fractures as the primary pathway for water and solute transport in the till. Phase 3 involved model simulation (HydroGeoSphere) of till at WCW to estimate the representative elementary volume (REV). Results showed that the REV of the till is related directly to both fracture density and transmissivity. REV size ranges from 4-5 m³ at depths of 1-2 m, where fractures are densest and most transmissive, to 2-3 m³ at depths of 3.3-3.7 m where fractures are the least dense and least transmissive. Values of K in the fractures (K_f) are up to five orders of magnitude greater than matrix (K_m). The smaller REV for till units with sparse fracture networks and smaller aperture sizes likely reflects a larger component of flow in the till matrix, which, in turn, would produce the lower K_b values seen in sparsely fractured till. Therefore, a critical threshold for fracture intensity and aperture size may exist, below which the impact of fractures on K_b is obscured by low matrix K_m values.