The human footprint on the environment may have affected one of the Earth’s most precious resources — our drinking water — in a major way throughout the last century, according to new research. A study published Monday in the journal Proceedings of the National Academy of Sciences suggests that population growth and land use changes since the year 1900 have increased pollution in urban watersheds around the world and driven up the cost of water treatment in the process.

In the past, case studies have suggested that certain cities around the world have seen an increase in pollution and degradation of the watersheds they rely on for tap water, thanks to an increase in human populations and activities nearby, said Robert McDonald, a lead scientist with the Nature Conservancy and the lead author of the new paper. When this happens, cities must sometimes build more complex — and more expensive — water treatment plants to make sure their water supplies stay safe for human use.

“The goal of the paper was to say how much is that going on globally,” McDonald said. “How many cities have had degradation in their source watershed and how much does that cost us?”

To find out, McDonald and a group of researchers from the Nature Conservancy, Yale University and Washington State University combined data on where cities around the world get their drinking water with data on human population growth and land use changes in those regions. They focused on watersheds for more than 300 cities worldwide, all with populations exceeding 750,000 people.

The researchers used these data sets to reconstruct the degradation of watersheds between the years 1900 and 2005 using a model that builds on previous research examining the way human activities affect water quality. Specifically, they focused on changes in nitrogen, phosphorus and sediment levels in the water. As these types of water pollution increase, they can require more complex forms of water treatment to make the water safe.

Finally, the researchers compiled a data set with information on the type of water treatment technology used in certain cities around the world, including 100 cities in the United States and 30 cities in other nations. They used this information to inform their estimates of the relationship between changes in water quality and changes in water treatment costs over time.

Overall, the researchers estimated that declines in water quality — because of human activity — caused water treatment costs to rise by 50 percent in nearly a third of all large cities worldwide. The data suggested that populations rose in just about all of the studied cities, although the amount of population growth varied significantly from one place to the next. Similarly, the actual land use changes to the studied watersheds — such as the expansion of agriculture — also differed by location.

The researchers found that about 90 percent of all urban watersheds experienced some degree of degradation as a result. And 44 percent of these experienced moderate or even severe declines.

On average, the model suggested that sediment in the watersheds increased by about 40 percent between 1900 and 2005. Phosphorus increased by 47 percent, and nitrogen increased by a whopping 119 percent. Each of these pollution spikes is associated with an increase in the complexity of water treatment required. And more complex water treatment facilities come with an increase in costs, both for constructing them in the first place and operating them once they’re built.

Altogether, the researchers estimate that 29 percent of cities worldwide have suffered a significant increase in their water treatment costs over the past century because of the degradation of their watersheds. Whether those costs increased for any given city depended largely on what type of water treatment facility originally existed and how much it had to be altered, McDonald noted (which explains why the percentage of cities suffering significantly increased treatment costs is slightly smaller than the percentage of cities that experienced major declines in water quality).

By extrapolating their estimates to all large cities globally, the researchers suggested that the total cost of watershed degradation worldwide — in terms of water treatment costs, that is — comes to about $5.4 billion annually.

“It depends on your perspective whether that’s big or not,” McDonald said. “For impacted cities, it’s probably about 50 percent more than they would typically pay for water treatment.”

From the researchers’ perspective, in any case, it’s enough to open up a conversation about how to protect urban watersheds in the future. In many cases, utilities have little say over how farmers and other landowners utilize the land around a watershed — but new policies could be written to help come to some sort of a compromise on that issue, McDonald suggested.

“The one thing that some cities do is develop a source watershed protection plan and then make investments in it,” he said. These investments could involve strategies like new conservation initiatives or incentives to prevent erosion around bodies of water.

Additionally, he suggested that utilities taking extra measures to prevent water waste — through leaks, for example — can help them make better and more efficient use of the water they already have at hand.

Such measures may only become more important as the human population continues to skyrocket (likely exceeding 9 billion people by midcentury) and other factors — such as climate change — threaten to damage existing water supplies in the future.

“The alternative is more spending on water treatment plants,” McDonald said. “And that’s what I worry about when I see these stats.”