Investigating the Decline of Dissolved Oxygen in the Anacostia River

Earlier this year, as we celebrated the 50th anniversary of Earth Day, the Anacostia Watershed Society released its 2020 State of the Anacostia River Report. The Anacostia River received a passing grade, its highest score yet, with most parameters showing improvement. However, Dissolved Oxygen (DO) continues to show a mysterious decline. DO is critical for the survival of aquatic life since almost all living creatures need oxygen to live.  At least 5 mg/L of DO is needed to sustain healthy communities of aquatic life. When nutrients or organic matter content is too high, it can cause DO levels to fall to dangerous levels.  DO levels below 3 milligrams per liter (mg/L) are a concern, and waters with levels below 1 mg/L are considered hypoxic (lacking oxygen) and usually devoid of life. 

DO was expected to increase/improve after DC’s Combined Sewer Overflows (CSO) started to decrease. High amounts of organic matter entering the river through CSOs are decomposed by microorganisms and deplete oxygen in the river water. Thus, an expected CSO reduction would improve the dissolved oxygen amount, however, our data analysis has shown a surprising decline, instead of improvement in DO values. In 2009, DC Water reduced 40% of sewage overflows. By 2011, CSO reduction reached 60%.  When Anacostia River Tunnels came online in March 2018, the overflows were reduced by 90% (a great achievement for reducing harmful fecal bacteria, too, in the river).  The reduction significantly reduced organic matter input into the river, but dissolved oxygen levels continued to decline.  

In the State of the River Report Card, AWS assigns a %Score that shows the percentage of time when DO was above or equal to 5 mg/L, the ideal level needed to maintain healthy aquatic life. DO %Score is declining after 2009 as shown in the graph below. In the 2020 State of the River Report Card, the score for dissolved oxygen was 33% as an average across the three sections of the river, compared to 70% prior to 2009. 

*AWS’s %Score calculation shows the percentage of time when DO was above or equal to 5 mg/L.

This decline causes concern about DO in the Anacostia River, and we can hypothesize why it is trending down.  Listed below are the possible reasons of decline:

1. Legacy organic matter is accumulated at the bottom of the river.  
   CSOs carry a lot of organic matter into the river. There is a significant amount of organic matter deposited at the bottom of the river. That organic matter is now being decomposed.  It may be consuming oxygen significantly. We know that the DO values are zero as we measure DO deeper in the river. 
2. CSO events actually added some oxygen temporarily.
   CSOs are a mixture of sewage and rainwater.  Rainwater is oxygen-rich,and the mixture is agitated vigorously in the pipes before being discharged, which allows oxygen to get into the mixture, so discharge itself is oxygen-rich. This could mean that the discharge into the river raised the oxygen level at least for a few days causing the Anacostia River water to have low oxygen except during such rainfall events.
3. Increased organic matter from other sources.
   There are still a lot of organic matter inputs, such as glass clippings and leaves on impervious surfaces, into the river. 

If legacy organic matter at the bottom of the river is the cause, the first remedy that comes to mind is to dredge the entire bottom of the river; however, that is a costly undertaking that requires federal and local government cooperation.  Dredging the entire bottom of the river is not a cost effective solution; therefore, it may be better to allow nature's ability to heal.  The tidal Anacostia River is “washed” by the cleaner Potomac River water twice a day by tidal action.  So, it is expected the sediment at the bottom is oxidized gradually.

That CSO events add oxygen temporarily might not be too bad. Now, DO may be consumed by the legacy organic matter at the bottom.  That is probably the reason that the Anacostia River always has low oxygen during dry weather.  As the sediment is  oxidized, it is expected that the average DO values will go up eventually  Thus, without the DO input by CSO events, the river would have higher DO values all the time in the long run.  A good thing is that the significant CSO reduction means significant organic matter reduction into the river.  Much less organic matter is now added to the river.

Now let’s take a look at #3, that the decline is caused by an increase in organic matter getting to the river from other sources.

AWS has had a unique experience maintaining a screen type trash trap called in Nash Run since 2009.  Because it is a screen type, it captures most types of trash as well as other debris such as twigs, branches, and even huge downed trees.  One type of organic matter that AWS is concerned to find in the trash trap is the discarded grass clippings and large amounts of leaves.

Grass clippings are captured as soon as the growing season starts.  One might think that grass is natural, and there is no problem to see it in streams.  However, though the grass is natural, grass clippings are generated by human activities - mowing, and too much of it is coming to the river.  The large amount of organic matter will be decomposed in the river and the decomposition consumes dissolved oxygen. The amount of organic material (leaves, grass) that enters the river due to landscaping activities is far beyond the amount that the river can support.  

Pictures below show the trapped grass clippings at Nash Run Trash Trap.

Those grass clippings are washed away by stormwater runoff to streams.  Example photos of grass clippings left on impervious surfaces are shown below.

One can also observe that as soon as autumn comes, the trap receives an excessive amount of fallen leaves.  Though the leaves are natural material, if the land is all forested, most of them would remain on land.  It is the impervious surface that humans created that causes the problem.  Most leaves that fall on the impervious surfaces will be washed into streams and eventually to the river.

Though there is no practical method to significantly reduce the amount of leaves coming into streams, home and property owners in the Anacostia watershed have the ability to reduce the amount of grass clippings washed into streams.  Here are some measures that can be taken to reduce the organic matter input into the river.

How to reduce grass clippings from washing away into streams.

1. Collect grass clippings and compost it
   This may sound impractical but some contractors collect grass clippings as they mow along streets, with a net in place to collect grass clippings from the mower.  Government agencies can mandate the use of such nets, or give contracts in favor of landscape companies who collect grass clippings.
2. Leave the grass clippings on the mowed lawn.
   After you mow your lawn, you can sprinkle the grass clippings over the mowed lawn.  “Simply put, grass clippings are good for lawns because they turn into natural fertilizer.”  It will add nutrients, moisture, and carbon that are needed for the healthy growth of the grass.  Visit the linked page shown at the footnote to learn more.

Additionally, watershed residents can collect leaves as much as possible and compost them.  In the long run, as impervious surfaces are removed, the amount of leaves coming to streams will be reduced.  

Grass and fallen leaves are natural matter. When the watershed was all forested, nature’s own system was to have grasses and other debris make its way slowly to the river when it rains. In smaller quantities, these provide food for many aquatic animals.  However, when the watershed is covered with impervious surfaces such as roads, roofs, and parking lots, and the lawn is mowed and the grass clippings are left on the impervious surfaces, too much of them flow into the river.  When one wades into some parts of the river, you encounter feet of organic rich mud that traps your feet.  This may be one reason that the Anacostia Watershed Society is seeing low DO levels even after the CSO reduction.  Residents can help reduce the amount of organic matter coming into the river by leaving grass clippings on mowed lawn and by collecting fallen leaves for composting.