MODELING THE HYDRAULIC CONNECTIONS OF ALLUVIAL VALLEY AQUIFERS AND ADJACENT GLACIAL DEPOSITS NEAR PUGET SOUND

A groundwater model for the southeastern lowlands surrounding Puget Sound encompasses an area of 887 square miles in King and Pierce counties, Washington. Groundwater supplies approximately half of the total water demand in this study area that is home to more than one million people. The model was constructed with the MODFLOW-NWT open-source code and developed for the purpose of examining impacts on groundwater levels and connected surface water bodies due to stresses on the hydrologic system such as increased water use, population growth, and drought conditions. The model consists of 13 layers and a uniform grid with 416 rows, 433 columns, and a horizontal cell length of 500 ft. Areas of concern in the development of this model were along the steep slopes and bluffs of the Puyallup, White, and Green River valleys. These valley walls were particularly challenging to model because the coarse 500 x 500 ft grid prevents an accurate representation of the valley slopes. In particular, MODFLOW-NWT does not allow for horizontal connections between alluvial valley-fill aquifers and adjacent hydrogeologic units when these units are in different model layers. Vertical flow into the upper alluvial aquifer (AL1) was also limited by a mudflow deposit that separates AL1 from the lower alluvial aquifer (AL2). The mudflow bypass method was developed to address concerns about the lack of both vertical and horizontal hydrologic connections between AL1 and adjacent glacial aquifers, without applying a major revision to an unstructured grid. This method involved adjusting model cells to allow for vertical flow between AL1 and horizontally adjacent hydrogeologic units. After comparing model versions with and without the mudflow bypass method, it was determined that while these modifications better simulated reality, they had little impact on model results. This leads us to conclude that at the regional scale the increased flow to AL1 due to the mudflow bypass method is negligible but may be useful in detailed local-scale models.