COMPLEX STRATIGRAPHY NEAR PUGET SOUND: BUILDING A 3D FRAMEWORK FOR GROUNDWATER FLOW

The unconsolidated deposits that make up the Puget Sound Basin were primarily deposited during multiple periods of glacial advance and retreat and resulted in numerous discontinuous stratigraphic layers, with up to 3,000 feet in total thickness. In Pierce and King Counties alone, over 30 geologic units of glacial and interglacial origin have been identified. These units were simplified and grouped into 9 hydrogeologic units based on similar hydraulic characteristics. The complexity of these deposits makes construction of a three-dimensional (3D) hydrogeologic framework model especially challenging. These 9 hydrogeologic units have variable horizontal continuity due to their depositional environments, but are also cross-cut by subsequently incised stream valleys filled with two alluvial units and a mudflow deposit, for a total of 12 units. On the basis of geologic maps of the land surface, geologic well logs, and the land-surface topography, we constructed a 3D hydrogeologic framework model for the purpose of simulating groundwater flow. This 3D framework model is represented numerically by the top and bottom altitudes of each hydrogeologic unit in the study area. Interpolation was used in areas where observed data were absent. Discontinuous units pose the largest computational challenge in building a 3D framework that ensures correct juxtaposition of all units. While software is available to assist in constructing complex 3D frameworks, each software package has particular strengths and limitations. Therefore, a combination of different software packages was used to construct the 3D hydrogeologic framework model. This framework model was then translated into a finite-difference grid for groundwater-flow modeling, which required additional modifications to assure correct hydraulic connectivity between all units.