Residents of this area were advised not to drink water in December.

The December water contamination that closed four schools and prevented Shaw residents from using their water for drinking, showering, or dish washing will just have to remain a mystery.

On Dec. 17, DC Water issued a Do Not Drink Advisory for a portion of the city, shown in the map above, that encompassed parts of Shaw and Logan Circle. A day later, tests came back positive for petroleum in the water, and the advisory remained in place. DC Water lifted the advisory on Dec. 19.

Now the water authority has completed its investigation into the matter, and its conclusions are… inconclusive.

“DC Water did not identify a source of the petroleum odor during the inspection of cross-connections at over 140 sites including buildings, homes, construction sites, gas stations, and underground fuel storage tanks in the area,” DC Water reports in a press release.

The investigation worked to identify cross-connections, which are links between the water distribution system and a customer’s water system that could contaminate the drinking water distribution system; reviewed construction activity that could have caused contamination; and surveyed underground fuel storage tanks in the area.

So how did the investigation fail to yield any answers? For that DC Water helpfully provides a list of frequently asked questions, which we’re pretty sure no one has asked yet:

Frequently Asked Questions

How is it possible that the laboratory analysis and DC Water’s investigation did not determine where the petroleum odor originated? Petroleum products are highly volatile, meaning they quickly move from a liquid phase to a gas phase. Because they do not stay in water long it is difficult to “catch” the contamination in water samples collected after initial reports. It is also possible that the petroleum odor originated from outside the DC drinking water system, but the initial investigation suggested that drinking water contamination was possible. DC Water issued the advisory until tests could confirm that the water was safe. Public safety is our first priority, and we acted conservatively to protect customers from any potential health risk.

Could this situation happen again? The probability of contamination of the drinking water system is low because the system is pressurized, meaning the water is always pushing outwards. We urge customers to report any unusual odors or observations of the water or dumping of any type of chemical in the sanitary sewer by calling the 24-hour Emergency Command Center at 202-612-3400.

While no water system is risk-free, the Washington DC metropolitan area has put a lot of thought, resources, and practice into making the water system as safe as possible. DC Water regularly participates in planning exercises and coordinates with regulators and emergency response agencies to prepare for contamination scenarios. DC Water is proactively working with District partners to expand laboratory and emergency response capabilities for quicker testing and response for future incidents. Review information about preparing for water emergencies on DC Water’s website at http://www.dcwater.com/education/prepared_for_water_emergencies.cfm.

How did DC Water decide to issue and lift the advisory? DC Water received customer reports of a petroleum-type odor and conducted field testing with the DC National Guard Civil Support Team and the DC Fire Department. Tests indicated the possible presence of an unknown petrochemical. Based on this information, DC Water and the US Environmental Protection Agency Region III determined that it was necessary to issue the Do Not Drink advisory during the investigation. DC Water sent samples to laboratories at the Washington Aqueduct and EPA Region III Environmental Laboratory at Ft. Meade for further analysis. Water samples were tested on Thursday morning after extensive flushing was performed in the distribution system in the affected area on Wednesday night. Flushing the pipe system involves releasing water from hydrants to remove any possible contaminants and draw fresh water into the system. The distribution system was also flushed on Thursday evening. DC Water, in coordination with EPA Region III, lifted the advisory after these test results confirmed that the drinking water met EPA drinking water standards and indicated that there was no contamination present.

What is the best way to stay informed about DC Water service disruptions that may affect customers? Customers can sign up for email or text alerts by entering contact information on DC Water’s website at http://www.dcwater.com/news/alert_form.cfm. Although some alerts are sent to all customers, DC Water can send alerts to customers in specific locations during repairs or emergency work. Customers can also follow us on Twitter and Facebook to review helpful information and frequent updates.

Map via DC Water