MODELING GROUNDWATER FLOW IN SAPROLITE AND BEDROCK IN NORTH CAROLINA PIEDMONT

Modeling groundwater flow in fractured rock continues to push the limits of groundwater modeling techniques, even in the third decade of widespread use of finite-difference groundwater models. The Piedmont terrain of North Carolina provides a perfect setting to apply Modflow to simulate groundwater flow patterns in fractured crystalline bedrock aquifers. An understanding of the fracture porosity in the modeled terrain is critical to the success of using Modflow. A series of three-dimensional pumping tests provided this understanding. Familiarity with the geology of the almost 170,000,000 square-foot model area led to the selection of two characteristic areas for pumping tests. The pumping test areas were selected based on three key factors: (1) stratigraphy of saprolite, weathered bedrock, and unweathered granite and gneiss crystalline formations; (2) location of regional and local fracture sets; and (3) the position of linear diabase intrusive bodies. Detailed observations during well installation contributed to accurate pumping test evaluation using various analytical methods. These observations and analyses indicated the existence of seven distinct hydrogeologic regimes: (1) saprolite; (2) moderately weathered to unweathered bedrock; (3) intermediate unweathered bedrock; (4) deep unweathered bedrock; (5) moderately productive fracture zones; (6) highly productive fracture zones; and (7) diabasic hydraulic barrier zones. Rather than defining a conventional, layered sequence as is typical of Modflow model construction, the three-dimensional distribution of these zones required that all four layers representing the vertical extent of the model be transected by fracture zones and diabase barriers. This required a regular and small, 50-foot node spacing throughout the model. The model construction allowed accurate calibration of vertical and horizontal gradients observed in an extensive array of dozens of wells. The groundwater flow model of the North Carolina Piedmont terrain illustrates how Modflow can be used to simulate flow in a complex terrain marked by multiple fracture sets, varying degrees of weathering with depth, and the presence of igneous intrusions.