A Portland Water Bureau Facebook fan posted a comment during the recent storm event in late November:
Hey, so I thought that since you guys are in the water business and all, you might have an inside track on this whole rain deal. Can you pass on a message to whomever manages the rain that while we appreciate their gumption and generosity, we all are in desperate need of some sunlight? Any assistance in this matter would be greatly appreciated. :)
This humorous query made a few of us here at the bureau ask, do Portland Water Bureau customers know how rain storms can affect their drinking water, and what we do to ensure that potential impacts are minimized?
Portland Water Bureau staff constantly reviews weather and rainfall patterns and forecasts, not only to predict water supply, but also because very wet conditions and storms can affect one important aspect of Bull Run water: turbidity. Turbidity is the cloudiness or haziness caused by individual particles (suspended solids) that are generally invisible to the naked eye, similar to smoke in the air. Normally, turbidity levels in Bull Run water are very low, but, during storms, the Bull Run River and its tributaries can carry dirt and other organic debris into the Bull Run reservoirs.
Federal regulations set allowable turbidity levels at less than 5 nephelometric turbidity units (or NTU). A water treatment plant that uses filtration would trap and detain most of the solids that cause turbidity, but Bull Run water is very high quality and does not need to be filtered. Therefore, we monitor turbidity not only in the Bull Run reservoirs, but also in all of the major streams that feed the reservoirs.
When a storm starts, the Portland Water Bureau Operations group focuses its efforts on collecting and analyzing this stream flow data to predict whether the flows into the river, reservoirs, and intake are vigorous enough to churn up sediment and affect water quality. These analysts also look at the rate of change of the turbidity. Interestingly, we have found that, when the daily average flow on the largest tributary exceeds 5,000 cubic feet per second (cfs), there is a strong probability that the turbidity at the intake—the point where water leaves the reservoirs for treatment and transport—will exceed 5 NTU (the allowable limit).
If this exceedance is likely, the staff who operate the Columbia South Shore Well Field will be put on alert in case the Bull Run supply must be shut down. When turbidity levels reach Portland Water Bureau’s threshold (lower than the 5 NTU regulatory limit), the staff at the well field start pumping groundwater as the Bull Run Supply is shut down. Groundwater initially blends with the Bull Run water already in distribution system pipes and tanks until, over the course of a few days, water served is 100% groundwater. Our Community Information staff let our customers know that we will be serving groundwater, water quality staff alert sensitive water users of expected water quality changes, and our Customer Service staff gear up for questions from customers.
So during the next storm, when most Portlanders are sitting cozy at home or their favorite beverage establishment, know that the employees at the Portland Water Bureau are working to ensure that it delivers high-quality water to its customers.
Terry Black
Sr. Community Outreach and Information Representative