Interesting Graph |
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Rainfall amount and Dissolved Oxygen -2005-
Every year, there are days when the Anacostia River water contains less Dissolved Oxygen than that which won't support fish. The graph shown below illustrates that at least three days in June 2005 the DO values were lower than 2 mg/L, the amount that won't support fish. The DO is measured about 2-3 feet deep from the surface of the river. So, the DO values must have been almost zero at the bottom of the river.
Rainfall amount and Dissolved Oxygen -June, 2003-
Explanation
§ What is Dissolved Oxygen (DO)?
§ Important information for use in interpreting this graph.
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1. Rainwater contains high levels of dissolved oxygen. 2. The rainwater falling in the Anacostia watershed eventually flows into the Anacostia River. 3. The rainwater entering into the Anacostia increases the DO in the river. 4. Fish and other organisms use the DO to breathe. Therefore, an adequate level of DO in the water is necessary in order for many kinds of aquatic organisms to survive. 5. In a healthy (rural/pervious) watershed, rainwater carries lower amounts of organic compounds from forests, plains, and so on. 6. Contrary to a healthy watershed, an unhealthy (urban/impervious) watershed conveys a huge volume of organic (and inorganic) compounds into the river from such sources as the combined sewer overflow (CSO) system, malfunctioning septic tanks, cracks in the sewer pipes, illegal sewer line connections, etc. Oil leakage from automobiles is another large source of pollutants for the river. These contaminants are all brought into the River by rainwater. |
§ The graph shown above illustrates the increase in DO after a rainfall event. For example, on June 17, there was approximately 0.8 inches of rainfall. On the next day, the DO value went up from about 3.0 to 5.5 mg/L.
§ In healthy watersheds, the DO value remains high long after the rainfall event. However, this is not the case for the Anacostia River. It has been heavily abused for a long period of time, and is one of the unhealthiest watersheds in the U.S.A. As mentioned above, rainwater carries huge amounts of organic compounds into the Anacostia River. Many kinds of microorganisms emerge to eat these organic compounds and, at the same time, they consume dissolved oxygen to breathe, dramatically lowering the DO levels in the river. Organic compounds are also oxidized naturally by chemical reactions.
§ The graph above illustrates this pattern of decreasing DO as a result of organic consumption. After the rainfall event on the 20th of June, the DO value decreased daily to the level of about 2.3 mg/L, which is stressful for fish.
§ Insufficient DO levels can cause great numbers of fish to be killed.
§ From this graph, it can be seen how the Anacostia River is adversely impacted.
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