Development, agriculture present risks for drinking water quality

(Press-News.org) A new study from North Carolina State University researchers finds that conversion of forests to urban development or agriculture near streams can have harmful effects on water quality downstream, presenting both health concerns and raising the cost of water treatment.

Using a model called the Soil and Water Assessment Tool, researchers mapped out the current and projected future effects of four land-use scenarios at 15 water intake locations across the Middle Chattahoochee watershed in Georgia and Alabama. By combining a series of potential socioeconomic outcomes and climate change models reaching out to 2070, researchers examined several potential land use change scenarios to predict their effects on water quality.

Katherine Martin, associate professor in the NC State University College of Natural Resources and co-author of a paper on the study, said that in models where forest cover was converted to other land uses, water quality suffered.

“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. Looking at those two, in places where we’re losing forest cover, we see both of those increasing,” she said. “Those are both detrimental to the quality of drinking water, and they require more filtration.”

Part of the issue, Martin said, is the relatively high level of fertilizer used in large-scale agriculture. Urban development results in large areas of impermeable surfaces, where rainwater cannot soak into the ground and instead runs off into rivers and streams. This causes the water to carry more sediment into those waterways than it would if it had been absorbed into the ground.

Increased filtration has several knock-on effects, Martin said. Not only is it potentially harmful for aquatic life, but it also increases the cost of managing water treatment plants. 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. These areas are also more likely to see increased development, due to their abundance of open land. The study suggests that more attention should be paid to where development might have serious effects on water quality for people living nearby, Martin said.

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

The study, “Projected land use changes will cause water quality degradation at drinking water intakes across a regional watershed,” is published in PLOS Water. Co-authors include Elly T. Gay of NC State’s Department of Forestry and Natural Resources, and Peter V. Caldwell of the Center for Integrated Forest Science. This work was supported by the U.S. Department of Agriculture Forest Service Southern Research Station agreement number 20-CS-11330180-053.

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Note to editors: The abstract of the paper follows.

“Projected land use changes will cause water quality degradation at drinking water intakes across a regional watershed.”

Authors: Elly T. Gay 1 and Katherine L. Martin, North Carolina State University College of Forestry and Natural Resources; Peter V. Caldwell, Center for Integrated Forest Science.

Published: April 23, 2025 in PLOS Water

DOI: 10.1371/journal.pwat.0000313

Abstract: Forest conversion to development threatens the ability of watersheds to provide stable and clean water supplies. Water managers are targeting forest conservation as a source water protection strategy to maintain healthy watershed function in developing areas, especially upstream of drinking water treatment facilities. Understanding the role of current forest cover in safeguarding these facilities is therefore crucial. We used the Soil and Water Assessment Tool (SWAT) to examine the relationship between upstream forest cover and downstream water resources under both current (2020) conditions and multiple projected land use scenarios for 2070 in the Middle Chattahoochee watershed, USA. We investigated the extent to which existing forest cover benefits water quality at 15 drinking water intake facilities within the watershed. Our analysis of four land use change scenarios revealed that forest conversion and increased development resulted in higher average annual concentrations of total suspended sediment (TSS) and total nitrogen (TN) at 13 out of 15 intake facilities, with potential increases of up to 318% for sediment and 220% for nitrogen. Conversely, concentrations decreased relative to the baseline when upstream agricultural land was converted to forest cover or new, low-intensity development, suggesting that certain types of development may improve water quality compared to maintaining agricultural land. Our results also indicated that extreme nitrogen and sediment concentration events – defined as days exceeding the highest 10th percentile of baseline concentrations – could become 3.6 to 6.6 times more frequent in the future, respectively. Notably, forest conversion to new development upstream of intakes with smaller subwatersheds could reduce water quality for utilities serving smaller towns and rural areas, which may have limited resources for managing this challenge. Our findings highlight vulnerable intake locations and underscore the benefits of forest conservation for source water protection under future land use change.

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