SAN FRANCISCO — THOSE of us who live in the United States are fortunate; generally we don’t have to give a lot of thought to the safety of our tap water. This makes our collective experience with water very different from that of hundreds of millions of people across the globe who lack access to clean water.

But twice this year the water supply for a major American city was interrupted for days by water pollution. In January, a chemical used in the processing of coal leaked from a ruptured storage tank into the Elk River, contaminating the water supply for about 300,000 people in and around Charleston, W.Va., the state’s capital and largest city. Then, last weekend, the water supply for over 400,000 people in Toledo, Ohio, was declared unsafe because of the presence of microcystin, a toxin released by algae blooms in nearby Lake Erie, the source of the city’s water.

While the circumstances in each situation are different, there are notable similarities. In each case, the pollution could not be adequately treated by the local water plants. Sudden “do not drink” (and, in some cases, “do not bathe”) warnings resulted. And in each case, activities in or near the communities caused, or partially caused, the problem. In Charleston, it was an upstream industrial spill; in Toledo, polluted runoff, including from agriculture, along the Great Lakes stoked the slimy, fluorescent algae blooms that sent residents flocking to supermarkets for bottled water.

Those events offer two important reminders about water in the United States.

The first is that while our country has made huge strides in reducing water pollution since the 1970s, when Congress passed federal laws like the Clean Water Act and the Safe Drinking Water Act, controlling water pollution is not a “set it and forget it” endeavor. Those statutes set broad goals but depend on states and the Environmental Protection Agency to design and update programs to keep the water clean.