Why do I have to dispose of waste water/use the bathroom 200 feet from water?

The primary objective for disposing of human waste and/or wastewater (dish water, toothpaste, etc.) a minimum of 200 ft from water. is to minimize the impact to water sources. Water is a precious resource and recreationists should take every reasonable precaution to protect the quality of all water sources. In most environments, when wastewater is deposited on the ground, it is filtered through the organic material and vegetation before entering the soil. This process helps lessen the impact to water sources. As for human waste, the 200 ft. barrier gives an added layer of protection to water sources by allowing the waste to decompose with little chance of introducing pathogens into springs, rivers, lakes and creeks.
Dish Washing: For dishwashing, collect water in a clean pot or expanding jug and take it to a wash site at least 200 feet away from water sources. This lessens trampling of lakeshores, riverbanks and springs, and helps keep soap and other pollutants out of the water. Use hot water, elbow grease, and as little soap as necessary to do the job. Strain dirty dishwater with a fine mesh strainer before scattering it broadly well away from camp, especially if bears are a concern. Pack out the contents of the strainer in a plastic bag along with any uneaten leftovers.
Human Waste: If no facilities are available, deposit solid human waste in “cat holes” dug 6 to 8 inches deep at least 200 feet from water, camp, trails, and drainages. This practice is acceptable for most environments. However, check with the Center or appropriate land management agency if unsure. Bring a trowel to dig the hole, and disguise it well after use. The microbes found in soil will break down feces and the pathogens they contain. If possible, it is recommended that used toilet paper be packed out. Otherwise, use as little as necessary and bury it deeply in the hole.
Soaps and lotions: Soap, even when it’s biodegradable, can affect the water quality of lakes and streams, so minimize its use. Always wash yourself well away from shorelines (200 feet), and rinse with water carried in a pot or jug. This allows the soil to act as a filter. Where fresh water is scarce, think twice before swimming in creeks or potholes. Lotion, sunscreen, insect repellent and body oils can contaminate these vital water sources. Unscented soaps and lotions are strongly recommended when traveling and camping in bear country. Consider rinsing yourself off 200 ft from water sources before entering any water source to remove bug repellent, lotion, sunscreen, etc.