This article was originally published in the May 2016 issue of Cottage Life magazine.
You asked: how bad is it for my lake? Ratings: 1 (whateves) to 5 (superbad).
Fireworks
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Badness level 2.5: Bad like a Katherine Heigl movie. Your lake will recover. But it might want two hours of its life back, dammit!
Why: Some studies found perchlorate, a chemical in pyro technics, present in lakes adjacent to fireworks displays— but it was undetectable over time. (In one study, the per chlorate was gone nine days later.)
What to do: You can use perchlorate free fireworks (but they’re more expensive).
Urine
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Badness level 2: Mostly, it’s just plain gross.
Why: Urine is almost all water, but it can contain traces of pharmaceuticals, and it does contain nitrates. In theory, a lot of pee in a small lake could promote algal blooms. (In 2012, Time reported that swimmers in Germany’s Eichbaum Lake killed 500 fish by peeing too much.)
What to do: Go before you hit the water. Even little kids can learn this.
Golf balls
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Badness level 4: Bad if you don’t remove them.
Why: According to research from the Danish Golf Union, golf balls release heavy metals such as zinc, and take up to 1,000 years to decompose.
What to do: Collect your balls, Tiger. Or, obviously, don’t hit them into the water in the first place.
DEET and sunscreen
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Badness level tbd: Badish, partially because the amount that washes off your body is small.
Why: Research shows that DEET and some components of sunscreens can accumulate in the swimming areas of lakes. The impact? “Minimal risk” according to a lot of scientists, but the impact is still under investigation.
What to do: Stay tuned! Meanwhile, wear products as directed, not to excess. Apply DEET; don’t marinate in it.
Soap
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Badness level 5: Bad. To the bone.
Why: It’s well documented: shampoos and soaps add phosphates to the water, which increases nutrients and pro motes the growth of algae.
What to do: Use EcoLogo-certified products, and don’t lather up or rinse off in the lake. 
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