1. Access Denied
STATUS: For a huge percentage of earth's population, turning on a faucet would be as miraculous as turning water into wine. That's because 1.1 billion people don't have access to clean water, and two-fifths of the world's inhabitants—2.6 billion people—lack access to sanitation facilities, resulting in regular exposure to human waste, particularly in local water sources. The deadly result: As many as five million people a year—most of them children—die from waterborne maladies like cholera, typhoid, dysentery, hepatitis, and diarrhea, which by itself is estimated to kill one child every 15 seconds.

According to some studies, 80 percent of the developing world's health problems are related to contaminated water. "It's not a supply or technology issue—there's clean water all over the world," says Ted Kuepper, executive director of Global Water, a California-based nonprofit. "It's a matter of getting it to people."

SOLUTION: Simple gravity-fed spring-catchment systems, $10,000 to $20,000 each, can transport clean water to villages within a few miles of natural springs; these work well in places like Central America. In drier climes, wells are often the only source of clean water, though rainwater-collection systems can also be used. Simply providing villages with communal taps and latrines can reduce disease by more than 75 percent.

And they're hot causes, too. Hip-hop mogul Jay-Z recently filmed an MTV documentary on water issues, and while shooting a movie in the Sahara last year, actor Matt Damon created the H₂O Africa Foundation, to bring water access to the filming locations in Senegal, Mauritania, Mali, Libya, and Egypt. But the United Nations has the most ambitious mission: One of its Millennium Development Goals is to cut in half by 2015 the number of people without access to clean water and basic sanitation, a goal the World Health Organization says could cost $11.3 billion per year.

2. Oceans in Peril
STATUS: Big blue is in deep trouble. Fish populations are tanking due to overfishing (a recent study in Science estimates that by 2048 there will be no commercially harvestable seafood left), and pollution has created about 200 deoxygenated areas in places like the Gulf of Mexico, where there's a New Jersey-size dead zone in summer. Meanwhile, bottom trawling, in which nets are dragged along the seafloor, is destroying fragile ecosystems like coral gardens, and longlines are snagging animals like turtles and albatrosses, decimating their populations. Overdevelopment is imperiling coastlines, and CO₂ emissions, which concentrate carbon in the water and wreak havoc with pH balances, are causing ocean acidification, which could destroy a vast spectrum of sea life—from diatoms to oysters to coral reefs.

SOLUTION: For ecologist Carl Safina, founder of the Blue Ocean Institute and a MacArthur fellow, the global priority right now is restoring marine wildlife, making fish populations as healthy as possible. In this regard, the U.S. has a good track record: From the 1996 Sustainable Fisheries Act to the creation of no-fishing-allowed reserves—such as last June's establishment of the 137,792-square-mile North-western Hawaiian Islands Marine National Monument—the nation is giving fish a fighting chance. "The U.S. has a whole suite of recovering species—striped bass, king mackerel, summer flounder, and swordfish," says Safina. Worldwide, there are some similar moves to create reserves, such as the 71,000-square-mile Phoenix Islands Protected Area, near Kiribati. But the vast majority of coastal waters still have few or no protections.

The silver lining? "Oftentimes it takes a sense of crisis to get countries and individuals mobilized," says Josh Reichert, director of the environmental division of the Pew Charitable Trusts, whose focus includes ocean health. "And we are now faced with a crisis."

3. Mass Pollution
STATUS: Toxic sludge. Raw sewage. Animal excrement. Deadly farm chemicals. All of these ingredients find their way into the world's water supply. In developing nations, as much as 90 percent of sewage and 70 percent of industrial waste pour into local waters without any treatment whatsoever. Even in the U.S., sewage pollution is problematic. "Most treatment plants are old and inefficient, and their technology is from 1916," says Nancy Stoner, a senior attorney at the Natural Resources Defense Council. If fixes aren't made, she warns, "by 2025 we can expect to have as much sewage pollution as we had in 1968, before the Clean Water Act."

But agriculture is the crisis at our doorstep. Irrigation runoff and massive amounts of animal waste from factory farms—the nation's top water-pollution sources—have fouled more than 173,000 miles of waterways. According to Worldwatch Institute, once contaminants reach groundwater, they are "essentially permanent," since on average they remain there for 1,400 years.

SOLUTION: "Primary and secondary treatment of waste is the most inexpensive thing we can do," says Paul Faeth, former managing director of the World Resources Institute, an environmental think tank with a focus on water. That means building water-treatment plants and cleaning up industrial waste around the world, an effort led by groups like the Global Water Partnership, an international network of water agencies that connects developing nations to technical expertise. In the U.S., Maryland and other states around the Chesapeake Bay have reached a comprehensive agreement to clean up the bay and have already restored more than 3,000 miles of natural streamside buffer zones to filter toxins and fertilizers that would otherwise seep into the Chesapeake. Bad practices on factory farms—which hold animal waste in lagoons or spray liquefied manure on crops—have been successfully fought on the state and county levels, and a 2003 lawsuit brought by the Natural Resources Defense Council, Sierra Club, and Waterkeeper Alliance against the Environmental Protection Agency has forced the federal office to put some teeth into its factory-farm rules. The EPA is now hashing out new permit requirements that will likely go into effect this summer.