Investigators with the Arlington, Virginia, Department of Environmental Services have determined that swimming pool water was responsible for killing nearly 100 fish in Four Mile Run.

Arlington Dept. of Environmental Services spokesman Peter Golkin said that overflow from the pool at a “multi-family property swimming pool” an apartment or condo complex “got into the storm drain” and made its way to the stream.

Officials first reported the incident on September 7, saying that they were investigating a hazmat situation. Following their investigation, they confirmed the cause was “flawed seasonal pool care.”

Golkin stressed that allowing the pool water into the storm drain “was not a malicious act.” It was a multi-family property swimming pool. The owners and their service people have been very cooperative with the investigation and in making followup improvements so such an incident isn’t repeated.

The department said that in the fall, large amounts of water from swimming pools and spas are often discharged into the storm drain system. This can cause problems, due to the fact that the storm drain system in Arlington drains directly into its streams.

“Swimming pool and spa water can have devastating effects on the health of our streams if not disposed of properly,” officials stated on the department website.

“The chlorine, bromine, algaecides, cleaning chemicals, and low oxygen levels can kill fish and other aquatic life in streams. Also, draining large volumes of water quickly can cause stream bank erosion.”

Only freshwater that is dechlorinated, pH neutral, chemicalfree, and clean may be slowly discharged into the storm drain system.