The killer of dozens of fish in Four Mile Run? Pool water, investigators say.

We now know the likely culprit that killed nearly 100 fish in Four Mile Run last week: pool water.

“Investigators say flawed seasonal pool care involving chlorine and overflow led to last week’s fish kill in Four Mile Run,” Arlington Dept. of Environmental Services spokesman Peter Golkin tells ARLnow. “Recent rains have now cleared the stream. Reminder: No filters on our storm drains. Please be careful.”

Golkin said overflow from the pool at a “multi-family property swimming pool” — in other words, an apartment or condo complex — “got into the storm drain” and made its way to the stream, between S. Walter Reed Drive and S. Taylor Street.

Pool water, it turns out, is deadly.

“Swimming pool and spa water can have devastating effects on the health of our streams if not disposed of properly,” the county’s website says. “The chlorine, bromine, algaecides, cleaning chemicals and low oxygen levels can kill fish and other aquatic life in streams.”

“Only freshwater that is dechlorinated, pH neutral, chemical-free and clean may be slowly discharged into the storm drain system,” says the website. Otherwise, pool water must go into the sewer system.

Golkin noted that the county’s rules around swimming pool drainage are “especially timely as this is prime season for closing out pools for the year.”

It is illegal to drain untreated pool water directly or indirectly into storm drains, though it’s not clear whether anyone will face any fines or other consequences in this case.

“It was not a malicious act,” Golkin said. “It was a multi-family property swimming pool. The owners and their service people have been very cooperative with the investigation and in making follow-up improvements so such an incident isn’t repeated.”

Another pool-related drainage issue that comes up around this time each year: pool drainage that flows into neighboring properties, flooding yards, killing grass and sparking neighborhood disputes. The county considers such disputes to be out of its regulatory control.

More from the county website:

Chapter 26-7 makes it unlawful for any person to discharge directly or indirectly into the storm sewer system or state waters, any substance likely, in the opinion of the County Manager, to have an adverse effect on the storm sewer system or state waters. Failure to comply with code requirements may result in enforcement action, including the issuance of civil penalties as outlined in Chapter 26-10 of the Arlington County Code. Enforcement action may also be taken by state and federal authorities in the event of a fish kill. Please share this information with pool service companies. You may be held responsible for the results of their actions.

If pool or spa water is to be released over-land, the release should be:

• At least 10 feet from the property line
• Monitored and controlled to prevent flooding or erosion of neighboring properties

Conflicts between neighbors that arise due to the release of pool or spa water are considered civil in nature. The Property Drainage webpage contains further information about residential drainage concerns and the potential conflicts that can arise.

For more information on swimming pools and how to properly manage pool water discharge, call 703-228-4488.

Update: Investigators say flawed seasonal pool care involving chlorine and overflow led to last week's fish kill in Four Mile Run. Recent rains have now cleared the stream. Reminder: No filters on our storm drains. Please be careful. @ArlingtonVaFD https://t.co/1HhF74Y8HX

— Arlington Department of Environmental Services (@ArlingtonDES) September 12, 2022