Oronoco Bay in Alexandria.  Image by the author.

Every year, the city of Alexandria ejects 11.3 million gallons of sewage into its surrounding waterways including the Potomac River.  This is nearly enough to fill the Reflecting Pool by the Lincoln Memorial twice.  State lawmakers are now threatening to strip the city of funding if they don’t make major reductions to the pollutant level.  The city is working to fix the problem, but that will take several decades.

Old infrastructure is where the problem starts

Of Alexandria’s 15.4 square miles of sewer system, six percent of it, located in Old Town, is an  older infrastructure called Combined Storm and Sewer (CSS). With CSS systems, clean rain water runs into a sewer system and mixes with sewage, and the mix is then transported to a wastewater treatment facility to be cleaned. In Alexandria’s case, that facility is just north of the Beltway.

During rainstorms, CSS systems often cannot handle the increased water level, and the mixture of rainwater and sewage overflows into local waterways. This problem is not unique to Alexandria— it affects 800 communities across the nation.

In Alexandria, there are three places where sewage overflows into Hunting Creek. There’s also a place where it goes into Oronoco Bay on the Potomac River in Old Town.

Where sewage overflows in Alexandria. Image by City of Alexandria

Last month in Richmond, two downstream state senators in Fairfax County introduced a bill to get Alexandria to fix this problem. If passed, Alexandria would be required to eliminate sewer discharge by 2020 or the state would strip approximately $125 to $150 million in state funding to the city.  The Alexandria Chamber of Commerce responded saying they were committed to solving the issue but that doing it in three years isn’t possible.

It'd be best to totally separate storm water and sewage, but that isn't realistic for Old Town

Since 1995 the Virginia Department of Environmental Quality (VDEQ) has overseen sewage overflows, meaning it looks looks closely at how much wastewater is getting out and what can be done to curb that in the future. In 1999, Alexandria drew up a Long Term Control Plan (LTCP) for reducing overflows over the coming decades. Based on the Environmental Protection Agency Combined Sewer Overflow Policy, the plan calls out multiple combined strategies to reduce the number of pollution overflows into Hunting Creek by 96%. The plan received an update in 2016.

The most effective system for preventing sewage overflows is what’s called a Separate Sewer System. The approach here is to hold clean rainwater where it falls using green infrastructure like retaining ponds and green roofs. This prevents the rainwater from ever mixing with sewage, and it’s an approach that 94% of Alexandria’s sewer infrastructure takes.


The difference between sewer systems. Image by DC Water

Unfortunately, since the older infrastructure is already in place in Old Town, changing to a Separate Sewer System would be very disruptive and costly.  It would result in Old Town being turned into a continuous construction zone, with crews ripping up roads and surgically separating the storm lines from the sewer lines through 2035. The process would also cost an estimated $525 million, which is far more expensive than other potential options.  

The current solution proposed in the LTCP involves reducing the discharge by using a combination of tunnels and tanks as a means to temporarily store the sewage overflow during heavy rainstorms.  Would-be overflows that would have ended up in local waterways would now be diverted into those tunnels and tanks.  

The plan is to build a tunnel at two of the places where sewage is currently overflowing into Hunting Creek. The 10-foot diameter tunnels would run approximately 100 feet under the ground and retain 1.6 million gallons during storms.  A holding tank would be added to the third spot and retain about 3.6 million gallons of sewage. Following the storms, the tanks and tunnels would be pumped out so the sewage could be treated.

DC is installing a similar system, a 13 mile tunnel to store sewage and rainwater and reduce overflow by 96%. The District’s $2.6 billion dollar project is slated to be completed in 2022.

The current proposed timeline for the tunnels and tanks solution opts to tackle the projects consecutively rather than construct them concurrently, resulting in the work going out into the 2030s. This solution is far less disruptive than separating the storm from the sewer because the locations are largely contained to the areas in the immediate vicinity and well below the ground of the four overflows being augmented.  

As a complementary strategy to these tunnel and tanks, new developments will include separate storm and sewer lines along with other green infrastructure.

Typical tunnel sewer construction. Image by DC Water

The estimated cost for tunnels and tanks for the the three discharge points on Hunting Creek is up to $187 million.  Preliminary estimates show the project would increase the average homeowner’s sewage bill in Alexandria from $45 to $60 dollars.  Similar strategies could be employed for the Oronoco Bay discharge point at a cost of between an estimated $70 to $100 million.  

In 2010, VDEQ issued bacteria guidelines for Hunting Creek, which required additional action by the city.  As a result, the city planned to address the three discharge points on Hunting Creek first and at the completion of those projects assess the one at Oronoco Bay around 2032.  However, last November the city council moved up that date for addressing Oronoco Bay with a directive to begin the environmental study in 2018 and project planning in 2026.

This is a slow process, but it’s moving forward

Local groups, like the Potomac River Runners for example, have grown impatient with the length of the process. The point to the city’s own records, which show unsafe fecal levels in more than half of the Oronoco Bay samples taken between 2007 and 2012.  Future guidelines proposed in the LTCP limiting the city to an average of four discharges per year, but it's hard to picture that given the current average of 40 to 70 times year.  It can be even more frustrating for some that the proposed solutions have an implementation timetable spread over two decades.

Nobody involved in this denies that Alexandria really needs to cut down on incidents of sewage overflow. The major challenge now is planning and installing the infrastructure solutions efficiently in the years to come.

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Mike Grinnell has worked in the design and construction industry since 1998, and relocated to DC in 2001.  He has a BS in Building Construction from Auburn University and studied Economic Development in grad school at Virginia Tech.  Mike served two terms on the Potomac Yard Design Advisory Committee before moving to West Springfield with his wife and two daughters.