Incidence and Timing of Low Dissolved Oxygen Events in the Squamscott River: 2005-07. Addendum to Impacts of Wastewater Treatment Facilities on Receiving Water Quality

摘要

The Squamscott River has had extended episodes of low dissolved oxygen (DO) that have been recorded at a site near its mouth during the past few years. These episodes were recorded as a result of temporally intensive monitoring by a datasonde deployed through most of each year. Whereas low DO events can occur during April-November, events during the colder months are typically less frequent and are often caused by unusual natural or severe weather conditions. Low DO events occur most frequently during July-September when elevated levels of nutrients are most likely to contribute to their cause, and are thus of most concern. The study found the warm season time period of July-September to be the time of year when low DO events were most frequent and pervasive. In comparisons between each year from 2005 to 2007, 2005 had less frequent and pervasive low DO events compared to 2006 and 2007. Relative to tidal cycle conditions, low DO conditions were most likely to occur during neap tide conditions, as indicated by the least shallow depth readings for the data sonde. Beyond the seasonal and tidal time periods, the time of day where conditions are most likely to cause low DO events is also critical for focusing field efforts. The most frequently observed time of day when either a low DO event was initiated or the lowest DO reading was recorded was in the morning, especially before 8:00 AM. Much less frequent occurrence of these events was observed during the second half of days. It appears that the predicted conditions for conducting water measurements and sampling during 2005 were relatively accurate. The study should have been more successful except that 2005 was a year in which low DO episodes were less frequent and pervasive. Future studies in the Squamscott River area near the data sonde can benefit from use of the results reported herein.