An Investigation of the Potential for Side Sewer Infiltration to Local Freshwater Systems - Thornton Creek, Seattle WA: A Case Study

Abstract

As the wastewater infrastructure throughout our urbanized land continues to age, pipe failures leading to sanitary backups and contamination of the surrounding areas will become more frequent. Most private side sewers throughout the United States have life expectancies of less than 100 years and are swiftly reaching the end of their design life. It is necessary for municipalities to understand the scope of potential private side sewer failure that may be occurring now or in the near future within their jurisdiction in order to properly plan prevention and mitigation. This research explores Thornton Creek, a stream system located in the northeastern corner of Seattle, Washington that has been impacted by fecal contamination. The City of Seattle has yet to identify the primary source of this pollution, but it is hypothesized that it may be partially caused by failing side sewer systems throughout the watershed. In Seattle side sewers are privately owned, hindering the city’s ability to collect adequate information about their conditions. Because of this lack of direct data on side-sewer condition, I investigated whether other existing data sources could be used to estimate the potential conditions of the side sewers in the City. Using City of Seattle Department of Planning and Development side sewer repair and replacement data, three questions were evaluated: (1) is there evidence of correlation between pipe age and the number of line repair/replacement permit requests? (2) is there evidence of correlation between pipe proximity to a creek and the number of line repair/replacement permit requests? (3) is there evidence of correlation between slope and the number of line repair/replacement permit requests? Results of this study indicate that there may exist a correlation between the pipe age and the number of line repair/replacement permit requests as well as a correlation between distance from steep slopes and the number of line repair/replacement permit requests. Yet, due to the assumptions and a lack of data no definitive conclusions can be made. This study indicates possible ways the City can utilize available data to investigate the hypothesized source of contamination due to side sewer conditions and the additional data on the side sewer system necessary to complete a more rigorous analysis.