A two-year investigation by Seattle Public Utilities (SPU) has confirmed human fecal bacteria are likely entering North Seattle’s Thornton Creek at multiple locations.

Funded by the Washington Department of Ecology and led by SPU stormwater scientist Jonathan Frodge, the study was based on samples collected at 45 sites throughout the watershed, under a variety of conditions

It has been known for years that fecal coliform bacteria concentrations in Thornton Creek exceed the state water quality standard and pose a potential threat to public health. The new study confirms human bacteria are present and contribute to the water quality problem. The study is also the first to identify sub-basins (general areas) where bacteria appear to be entering the stream.

The study is seen as an important step toward identifying and correcting bacterial sources in the creek.

Among the most urbanized streams in King County, Thornton Creek consistently ranks among the highest for fecal coliform bacteria.

SPU stormwater investigators are continuing their work to locate sources of bacteria in the creek, and will be working in the Thornton Creek watershed this summer.

“Human waste shouldn’t be in this creek,” said SPU Director Ray Hoffman.

“While we don’t yet know the exact sources of the bacteria, this study will help us zoom in on the likely problem areas. Our next step is to find the sources — and control or eliminate them,” Hoffman said.

Hoffman said SPU will be posting signs throughout the watershed, advising people to stay out of Thornton Creek. Public Health — Seattle & King County suggests that people avoid coming into contact with water in any urban stream, including Thornton Creek. If water gets on the skin, wash with soap and warm water.

Public Health noted that Seattle’s Matthews Beach Park, on Lake Washington to the north of the mouth of Thornton Creek, is regularly tested for bacteria, and there is nothing in the report that raises concerns for safety park users’ safety.

“We have the right starting point — knowing what kind of source to look for,” said Joan Nolan, Ecology’s water-quality improvement planner for the Lake Washington basin. “Further investigation should pinpoint specific sources to control and thus correct Thornton Creek’s bacteria problems.”

Previous efforts to locate bacteria in Thornton Creek have focused on finding illicit sewage inputs —cross connections — to the city’s stormwater drainage system, which empties into the stream at a number of locations. (In 2010, two cross connections were found by SPU’s Illicit Discharge Detection and Elimination (IDDE) team and corrected.)

Guided by the new study, the IDDE team will now look for sources that enter Thornton Creek directly, in addition to sources that first enter SPU’s stormwater drainage system before entering the creek. The investigation could extend into the wet season, as the rising and falling water table may affect investigative techniques used in finding the sources.