On Wednesday September 18, 2013, the Portland Water Bureau (PWB) learned that a water sample collected September 17 at SW 27th Avenue and Nevada Court was positive for total coliforms. As required by the Total Coliform Rule (TCR), additional samples were collected at the same location and at locations upstream and downstream from there. Within two weeks, a total of 83 drinking water samples were tested for total coliforms from this area. Forty-two of these samples tested positive for total coliforms. All of these samples, and an additional 301 samples collected throughout the system in September, were all negative for E coli.
A positive test for total coliforms is an indicator that contamination is possible but does not necessarily indicate an immediate health risk. Had the bureau detected E. coli, an indicator of more serious contamination and an immediate health risk, the public would have been notified as soon as possible.
If more than 5% of all water samples taken in a given month are positive for total coliforms, the TCR requires public notification within 30 days. During September, 12% of our samples showed the presence of total coliforms. About 17,000 homes and businesses were in the area where total coliforms were found. We will mail a notification to these customers by November 10, 2013.
Total coliforms can indicate an operational issue in the drinking water system. Our investigation, and additional monitoring of water in the area, showed unusually warm water and a seasonal decrease in drinking water demand. Lower demand increases the amount of time water spends in the distribution system, which can reduce the amount of disinfectant in the water. Warm temperatures further lower disinfectant levels. Under these conditions bacterial activity can increase.
To resolve this, we increased the amount of disinfectant used to treat the water, and then we increased the water flowing through the pipes in this part of the system, also known as “flushing”. In addition, sediments and organic matter that might have reduced the effectiveness of the disinfectant were removed from the pipes in this area through high-velocity flushing. This increased the disinfection levels in the water in this area and also brought in fresh cool water.
On October 23, 2013, sampling results at multiple locations in this area were all negative for total coliforms, indicating that the corrective actions were effective. We will continue to monitor water quality and test for bacteria in this area.
While detections for coliforms are not unusual at this time of year, detections of this duration have been very rare. We will be looking very closely at this event and implementing long-term changes to avoid a similar situation in the future.