Over lunch recently, a friend mentioned that she'd heard of people emptying their swimming pools into local streams and rivers. "That can't be good, can it?" she asked. I agreed it sounded troubling, though I couldn't say for sure. Pool chlorine is basically just solid bleach, after all.

Because I love a good question, I promised to investigate further.

But Googling "Maine swimming pool discharge rules" got me in over my head very quickly. Government regulations aren't easy to read. I'm as lay as the next man; I needed help.

One of the great things about working at a center for science and education and conservation is that, even if no one on staff knows an answer, someone will most likely know a partner who does. Very quickly, I connected with Gregg Wood, who works on Water Quality Management at the Maine Department of Environmental Protection.

What Gregg Told Me

"Swimming pool water is toxic." Thats how it stays so clean (and how we swimmers keep from getting microbially mauled every time we take a dip). The chlorine that owners add to their pools is often in the range of 5 milligrams per liter, which is also known as 5,000 parts per billion.

In fresh water, the maximum concentration of chlorine allowed by Maine DEP is 19 parts per billion. In salt water, its even less: 11 parts per billion.

So, swimming pool water has 260-450 times the allowable limit of chlorine. My eyebrows nearly flew off my forehead. Yes, that really shouldn't be poured into one's backyard stream.

DEP strongly advises against discharging one's pool into any body of water. However, they readily admit that they cant find or stop everyone from dumping into Maines rivers, streams, or even the ocean.  

Maine DEP Recommendations

1. Leave a pool to sit untreated for 7-10 days. This allows the chlorine to break down and the water to revert to allowable concentration levels. Then the pool can be emptied into a vegetated area (still preferably a buffered area that does not run off into a body of water).
2. Add dechlorination chemicals to the pool that, within hours, break down the chlorine in the water to < 10 ppb. Then the water can be dumped as in #1.
3. Discharge the pool water into the local sewer system, but only after calling the Town or water utility and receiving permission. Ideally, the water's chlorine levels will first be decreased, either through Option #1 or #2 above.

Bottom line: Dumping chlorinated water into local water bodies, fresh or salt, is BAD. Its toxic and will likely kill fish, invertebrates, and beneficial bacteria and plankton. Don't do it, at least not without dechlorinating it first.

Thanks, Gill P., for the question.

This public service announcement brought to you by curiosity. "THE MORE YOU KNOW..."