If you like your tap water clean and your water bills low, you might want to care a bit more about the trees upstream. That’s the message behind a new study published in PLOS Water by researchers at North Carolina State University. The research looked at how changing land use—specifically deforestation to convert into farms or suburbs—impacts water quality in the southeastern U.S., and the findings are, well, a bit murky.

Using a sophisticated simulation tool called the Soil and Water Assessment Tool, researchers modeled four different land-use scenarios through the year 2070. The authors focused on the Middle Chattahoochee watershed, a major source of drinking water in Georgia and Alabama. The results show that when forests disappear, clean water tends to go with them.

“In terms of aspects of water quality that we have long-term data on, two of the biggest are nitrogen levels and the amount of sediment in the water,” explained Katherine Martin, co-author of the study. “Looking at those two, in places where we’re losing forest cover, we see both of those increasing. Those are both detrimental to the quality of drinking water, and they require more filtration.”

Forests act like giant Brita filters. Their roots stabilize soil, their canopy buffers heavy rainfall, and their spongy floors let water slowly trickle into the ground. Replace all that with concrete, asphalt, and tilled farmland, and things change fast and for the worst. Rainwater no longer seeps into the soil—it races across hard surfaces, picking up nitrogen from fertilizers and loose sediment along the way, and dumps it all into nearby streams and rivers.

That extra gunk in the water doesn’t just bother fish—it clogs up filtration systems and boosts the cost of water treatment. “It also increases the cost of managing water treatment plants,” said Martin. “For facilities that do not serve large populations, this can lead to large per-capita price increases that end up being passed on to residents.”

In other words: more runoff equals dirtier water. Dirtier water equals higher treatment costs. Higher treatment costs eventually show up on your water bill.

The study looked at 15 water intake points and combined future climate projections with likely socioeconomic trends to assess long-term risks. Unsurprisingly, the areas most vulnerable to rising treatment costs are those on the urban fringe—places where population growth is likely and forests are still abundant. The researchers aren’t arguing against growth. In fact, they’re pretty clear that both farming and city development have their place.

“Agriculture and urban development are beneficial, and this study does not say otherwise,” Martin noted. “What we are seeing is that there are trade-offs when we lose forest cover, and we need to open up the conversation about those.”

Translation: we need to manage our lands better including decreasing deforestation and be more mindful and scientific about the effects of human development and how it will affects water downstream.

From a public policy standpoint, this kind of data can help guide smarter land-use decisions. From a consumer perspective, it’s a reminder that even if you live in a city, the health of a forest a hundred miles away could still impact your wallet and your water glass.

Want cleaner, cheaper water? Save forests and plant more trees.

ET Gay et al, Projected land use changes will cause water quality degradation at drinking water intakes across a regional watershed. PLOS Water (2025). DOI: 10.1371/journal.pwat.0000313