IMPROVED AQUIFER SUSCEPTIBILITY AND INFILTRATION MAPPING, PUGET SOUND, WASHINGTON

We have been preparing new products derived from digital geologic maps for evaluating aquifer susceptibility and infiltration. The geologic maps and derivative products are based on a rapidly growing geologic borehole database, currently containing over 66,000 sites with surface and subsurface data accessible via ArcIMS over the Internet or on agency intranets.

Two recent mapping projects in western Washington display the benefits of new digital geologic mapping using this geologic database: 1) a regional-scale map of infiltrative soils and 2) an aquifer-susceptibility map. Runoff and consequent lack of infiltration is a major challenge for large projects seeking to mitigate increased downstream runoff into salmon-bearing streams; conversely, surface water-groundwater interactions are also of significant environmental concern because of the potential for groundwater contamination. A regional-scale map of infiltrative soils has been prepared for the planning of a major interstate freeway widening project. Substantial cost savings are realized by locating key structures near areas of infiltrative soils. The old geologic maps of the area show large, undifferentiated expanses of glacial till with low infiltration rates. The new geologic maps, based on new field work and the borehole database, reveal substantially less till and more widespread deposits with greater infiltration potential.

An aquifer susceptibility map of one of the major islands of Puget Sound has been based entirely on new geologic mapping and the geologic database supplemented with groundwater data. It replaces an older version that was developed using twenty-year old geologic mapping made with traditional methods. Queries of the database provided spatial information regarding surface and near-surface geologic materials, depth to groundwater, and whether the groundwater was confined or unconfined. ArcMap facilitated viewing LIDAR topography, aerial photography, old and new field mapping data, and cultural features to inform the geologic mapping. The new aquifer susceptibility map shows substantially more detail, and has demonstrably greater accuracy, than the old map. New mapping reveals substantially less till at the land surface than previously mapped, which translates into substantially greater aquifer susceptibility.