Q: Are soap and shampoo safe to use in the lake?
A: No! Never!
That shampoo bottle or soap package says “all natural” but that doesn’t mean it’s harmless. Surfactants and other natural ingredients harm the chemical composition of lake water and can have a deleterious effect on the microscopic and macroscopic critters in the food chain. Interrupt that food chain and you make it difficult for the larger critters, including fish, to feed.
But the packaging says it’s biodegradable. That may be true, but biodegradable soaps and shampoos need soil and the bacteria found in that soil to break down. NOTHING is biodegradable in water. Every drop of shampoo you put into the water is there for good. All the living creatures in the water will be drinking that soap and it will negatively affect their health. Shampoo could end up in your drinking water and you probably don’t want to drink that.
If you feel you just must clean off in the lake, use a washcloth and scrub. Water is an excellent solvent that attaches to dirt and mud and washes it away. No soap required! Shampooing in an outdoor shower is fine so long as it is at least 30 m (98 feet) away from the water’s edge.
Better yet—play it safe. Wash inside and let your septic system do its job of slowly releasing its contents into the soil where it can biodegrade over time.