In 2001, King County initiated a comprehensive study of the Green-Duwamish watershed, called the Green-Duwamish Watershed Water Quality Assessment (GDWQA) Comprehensive Monitoring Program. The primary purpose of the program was to collect and analyze water quality data within the Green-Duwamish watershed. As part of the GDWQA, King County implemented the current microbial source tracking (MST) study described herein to address bacterial contamination in the watershed and to better understand the relationship between land use and bacteria levels in the water.

Water samples for the study were collected in 2003 and 2004 during eight storm events and seven base flow events. The samples were collected at nine study locations - four on the main stem Green River and five at the mouth of tributaries of the Green River. A total of 1,503 E. coli isolates were obtained from the samples and used in a ribotyping analysis to determine their source. Bacteria concentration data were evaluated to test for differences in terms of hydrologic and seasonal conditions, as well as for compliance with water quality standards, to determine how bacteria concentrations vary in the Green River watershed. Bacteria data were also evaluated to assess the influence that land-use activities and sample location have on bacteria concentrations in the Green River watershed.