The dissolved oxygen (DO) values in 2013 were very low in the Upper DC Anacostia (Section 2) and the Lower DC Anacostia (Section 3). Therefore, we looked deeper into the weather pattern to see if a correlation might exist.
Because the District has a Combined Sewer Overflow system that carries raw sewage and rainwater in the same pipe, the mixture will be discharged when it rains approximately more than 0.5 inches. After an intense rainfall, AWS has observed from our monitoring that DO values become very low for approximately 2 weeks. Then, we counted the number of days in a year that would fall into such a 2 week period. The result is summarized in the table below.
|Number of days that had intense (equal to or greater than 1 inch) precipitation in the previous 14 days||104||48||55||58||93|
It turned out that we had intense rainfall events in 2009 and 2013 frequently from April through October, the time duration from which we use the DO data for our assessment.
The graphs for each year are shown below. The shaded areas represent the days that had intense rainfall events for 14 days prior to those day.
In 2009, eleven more days were influenced negatively by intense rainfall than in 2013, however, DO values were better than those in 2013 (i.e., %Sore in 2009 was higher than that in 2013). This is probably because the intensity of rainfall was stronger in 2013 than in 2009 though the frequency was similar. Also, the tide might have influenced DO values. If there is an intense rainfall when the tide is coming in, the CSO discharge will be carried to upstream portion of the Anacostia, thus, the negative influence will remain longer.
There are many factors that influence DO values. The weather pattern is definitely one of the most influential factors.