It’s been a big news year for drinking water. In January, a coal-processing chemical called MCHM leaked from a tank farm above the banks of the Elk River just upstream of Charleston, West Virginia. The MCHM entered the intake of the city’s water plant, leading a few hours later to a massive do-not-drink water advisory for nine counties and more than 300,000 residents. In August, residents of Toledo, Ohio, were warned not to drink their water because of a toxin called microcystin caused by an algal bloom in Lake Erie. And Tuesday marks the 40th anniversary of the Safe Drinking Water Act, our nation’s most important law governing tap water. The law has achieved much and provides an event worthy of celebration. But is our drinking glass half-empty or half-full?

Everyone cares about drinking water and yet, despite episodes such as those in Charleston and Toledo, we take its convenience and safety remarkably for granted. That most certainly is not the case in much of the world, where “Is it safe to drink the water?” is a very real question. Almost 2 billion people don’t have access to treated water. Yet I can go virtually anywhere in the United States and take a sip from the tap without a second’s thought. Even better, tap water regularly outperforms bottled water in blind taste tests. More people can drink safely from American faucets today than ever before. In historic terms, though, the assumption of safe drinking water is a very recent development.

New Yorkers justifiably boast about the quality of their tap water (possibly the secret behind their fine bagels), but it did not have a public water system until 1832, long after its rival port cities of Philadelphia and Baltimore. Indeed, the Swedish botanist Peter Kalm, in a put-down that Rodney Dangerfield would have loved, reported after visiting New York in the mid-18th century that its water was so bad that horses from out of town refused to drink it. As America grew, so, too, did its water systems. On the eve of the Civil War in 1860, there were more than 400 public water systems, expanding to more than 3,000 by 1900.

Delivering water, though, is not enough. It also has to be safe to drink. Prior to the understanding of germ theory in the late 19th century and realization of the dangers bacteria posed, cholera and typhoid epidemics were commonplace and little-understood. The practice of chlorination, introduced to America by the water supplier for Jersey City in 1908, eradicated these threats within decades. Ironically, Jersey City actually sued the water company, arguing that the older system of sand filtration was needed to provide the pure and wholesome water required in the supply contract. The judge decided in favor of progress, finding for the company. The New York Times report of the decision was remarkably prescient about the importance of chlorination: “so successful has been this experiment that any municipal water plant, no matter how large, can be made as pure as mountain spring water.” Written a century ago, it almost reads like an ad for bottled water today.

The first standards for drinking water in America were developed by the Public Health Service in 1914, two years after the famed aviation brother Wilbur Wright died of typhoid.* The federal standards addressed bacteriological threats, but the PHS’ powers were limited, so the standards applied only to interstate common carriers such as trains, buses, and ships. Water providers to these carriers had to use chlorination, and this soon covered all the major cities.

The PHS standards were neither comprehensive nor revised very often. Rachel Carson’s Silent Spring heightened the country’s consciousness about pesticides and chemicals, and advances in science revealed whole classes of dangerous viruses, but only 28 of these contaminants were covered when Carson’s book was published in 1962. And even where there were federal standards, there was scant implementation. By 1970, the PHS standards directly applied to just 650 out of the nation’s 35,000 public water suppliers. Only 14 states had adopted the standards for themselves.

This was clearly evident in an influential 1969 PHS study examining public water suppliers in Vermont, West Virginia, Colorado, California, and Washington. One-third of the samples exceeded the PHS standards; more than 40 percent of the citizens were drinking “substandard water;” and nationwide, up to 8 million Americans were drinking “potentially dangerous water.” In Washington state alone, two-thirds of the systems had not tested for chemicals in their water in the past year and, of the 127 systems surveyed, only seven would pass the state’s bacteriological standards.

These numbers were alarming, and they helped lead to the passage of the Safe Drinking Water Act, or SDWA, in 1974. At that time some parts of the country enjoyed very good tap water but many did not, and worse, we didn’t really know which were which. The SDWA was amended in 1986 and 1996.

Taken together, the law has been groundbreaking in three key respects. First, it created uniform drinking water standards for a wide range of contaminants that were enforceable throughout the country. This may seem like common sense today, but it was a radically original idea, recently introduced in the Clean Air Act of 1970 and Clean Water Act of 1972. Second, it provided badly needed government funds through loans and grants for infrastructure. More times than not, poor water quality was due to lack of resources. Third, it directly engaged the public by making the state of our tap water much more transparent. For the first time, water suppliers were required to send out regular reports on water quality and, perhaps more important, immediately notify customers when serious violations occurred.

The SDWA has worked extremely well. America enjoys among the safest and most reliable drinking water in the world. According to EPA Administrator Gina McCarthy, more than 90 percent of water customers enjoy drinking water that meets all standards all the time, a far cry from the worrying landscape of the 1969 PHS report—glass half-full.

And yet, while the 40th anniversary of the SDWA surely warrants praise, significant challenges remain. Since 1997, the State Revolving Fund has distributed more than $26 billion in grants and loans to improve water treatment and delivery systems. That’s impressive, until one realizes that the EPA has estimated $384 billion in infrastructure needs through 2030. The American Water Works Association has estimated the cost at $1 trillion. Why so high? Consider that, in our nation’s capital, water mains are an average of about 80 years old. Some were buried before the Civil War. In an era of shrinking budgets, where will the money come from?

The SDWA must also face the challenge of “emerging contaminants” from medications, personal care products, and other newly recognized pollution sources, including compounds that act like hormones. Our water treatment systems were not designed for many of these chemicals and don’t eliminate them. A 2008 study by the Associated Press found 56 medications or their byproducts in the treated drinking water of metropolitan areas serving more than 40 million people. They are present in very small concentrations, measured in parts per million or lower. But they’re there, and assessing their effects on human and ecosystem health can challenge the outer bounds of toxicology. Should they be regulated? If so, are we willing to pay for the added treatment costs this will require?

And there is the challenge of new contaminants, most notably in the case of hydraulic fracturing. Fracking raises two types of threats to aquifers: the fracking fluids that are injected deep underground to fracture the shale formations, and the methane and polluted fluids that come back up. Perhaps surprisingly, the 2005 Energy Policy Act effectively exempted fracking from coverage under the SDWA. The involvement of Vice President Cheney, former CEO of the Halliburton Corp., in passage of the law led some to call this the Halliburton Loophole.

And this raises what, ironically, is the SDWA’s greatest challenge—how much it does not cover. The SDWA has increasingly turned to source protection. We are far better-off keeping our water from being contaminated in the first place than worrying about how to treat it afterward, but the SDWA doesn’t reach very far. The immediate cause of the drinking water contamination in Charleston, West Virginia, was due to inadequate oversight of a chemical tank farm. That lies outside the SDWA’s domain. The immediate cause of the toxins in Toledo was an algal bloom caused by agricultural runoff into Lake Erie. That lies outside the SDWA. So, too, does fracking.

The challenges to providing safe drinking water to the population are very real, and that has always been the case, from ancient Rome through today, yet the scale of these challenges should not blind us to the tremendous achievement represented by the 40th anniversary of the SDWA. Our tap water is safer than it has ever been. But continued protection of our drinking water for the next 40 years will require vigilance and perhaps a transformation. It should not be surprising that something as fundamental and pervasive as drinking water cannot be fully protected by a single statute. We are used to enjoying safe water and paying monthly bills as “consumer drinkers.” Fundamental protection of our drinking water will not occur, however, unless we take on the role of “citizen drinkers,” using our political process to demand effective protection through better enforcement of our laws and renewed scrutiny of activities threatening our source waters.

*Correction, Dec. 16, 2014: This article originally misspelled Wilbur Wright’s first name.