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In case you haven't seen this before, worth a look: https://en.m.wikipedia.org/wiki/Hetch_Hetchy#Hetch_Hetchy_Project https://en.m.wikipedia.org/wiki/Hetch_Hetc

• toephu2 9y ago
  "Water from Hetch Hetchy is some of the cleanest municipal water in the United States; San Francisco is one of six U.S. cities not required by law to filter its tap water, although the water is disinfected by ozonation and, since 2011, exposure to UV.[58] The water quality is high because of the unique geology of the upper Tuolumne River drainage basin, which consists mostly of bare granite; as a result, the rivers feeding Hetch Hetchy Reservoir have extremely low loads of sediments and nutrients."