GEOLOGIC SETTING AND VULNERABILITY OF PACIFIC NORTHWEST COMMUNITIES TO TSUNAMI-INDUCED CONTAMINATION OF WATER RESOURCES

On December 26, 2004, a tsunami generated by a large (Mw ~ 9.1) earthquake off the coast of Sumatra, Indonesia, inundated many of the coastal regions bordering the Indian Ocean. This inundation caused widespread contamination of shallow wells and aquifers by seawater. In Sri Lanka and Thailand, rehabilitation efforts were ineffective due to the lack of specific water resources-based recovery plans and information regarding treatment options. While not all coastal communities are similarly limited to shallow, poorly developed water sources, many communities in the Pacific Northwest and across the world are located in areas where they too could face massive water resource contamination in the event of a tsunami. This study evaluates several communities in high tsunami-risk regions on the vulnerability of their water resources to tsunami inundation. If the major water sources in a region are poorly developed wells drawing from shallow, easily infiltrated aquifers, the community's water resources are vulnerable. However, if the water sources are well developed in deep, confined aquifers, or are from alternate non-coastal sources, they might not be at risk. Five Pacific Northwest communities (Bellingham, Port Angeles, and Port Townsend, Washington; Astoria and Seaside, Oregon) were evaluated based on their depth of inundation, soil permeability, and depth to water table. Other qualities, including net recharge (based on regional precipitation), are important in estimating the extent of contamination over time but were not considered in this study. To quantify vulnerability, a relative value system was applied to these three traits and risk maps were created. This report also briefly examines the similar effects of inundation caused by tropical storms. Based on risk maps, Washington and Oregon well construction laws, and communities' primary water source information, it is concluded that these communities have a low risk of widespread tsunami-induced contamination of water resources.