June 22, 2023 Ι Syracuse University researchers co-authored a study exploring the extent to which human activities are contributing to hydrogeochemical changes in U.S. rivers.

Studies have found that human activities and urbanization are driving salinization (increased salt content) of freshwater sources across the country. In excess, salinity can make water undrinkable, increase the cost of treating water, and harm freshwater fish and wildlife. A team of researchers from Syracuse University and Texas A&M University have applied a machine learning model to explore where and to what extent human activities are contributing to the hydrogeochemical changes, such as increases in salinity and alkalinity in U.S. rivers.

Human activities contribute to rising salinity

Along with the rise in salinity has also been an increase in alkalinity over time, and past research suggests that salinization may enhance alkalinization. But unlike excess salinity, alkalinization can have a positive impact on the environment due to its ability to neutralize water acidity and absorb carbon dioxide in the Earth’s atmosphere—a key component to combating climate change. Therefore, understanding the processes at play which are affecting salinity and alkalinity have important environmental and health implications.

The group used data from 226 river monitoring sites across the U.S. and built two machine learning models to predict monthly salinity and alkalinity levels at each site. These sites were selected because long-term continuous water quality measurements have been recorded for at least 30 years.

From urban to rural settings, the model explored a diverse range of watersheds, which are areas where all flowing surface water converges to a single point, such as a river or lake. It evaluated 32 watershed factors ranging from hydrology, climate, geology, soil chemistry, land use and land cover to pinpoint the factors contributing to rising salinity and alkalinity. The team’s models determined human activities as major contributors to the salinity of U.S. rivers, while rising alkalinity was mainly attributed more to natural processes than human activities.

On the influence of human activities

The results from the group’s sodium prediction model, which detected human activities such as the application of road salt as major contributions to the salinity of U.S. rivers, were consistent with previous studies. This model specifically revealed population density and impervious surface percentage (artificial surfaces such as roads) as the two most important contributors to higher salt content in U.S. rivers.

With the salinity results confirming the accuracy of the team’s model, they then turned their attention to alkalinity. Their model identified natural processes as predominantly contributing to variation in river alkalinity, a contrast to previous research that identified human activities as the main contributor to alkalinization. They found that that local climatic and hydrogeological conditions including runoff, sediment, soil pH and moisture, were features most affecting river alkalinity.

Critical to the carbon cycle

„Rock weathering is the primary source of alkalinity in natural waters and is one of the main ways to bring down carbon dioxide in air,“ says Wen. Think of it as a feedback loop: when there is too much carbon dioxide in the atmosphere, temperatures increase leading to enhanced rock weathering. With more rock being dissolved into watersheds due to enhanced rock weathering, alkalinity rises and in turn brings down carbon dioxide.

„While we found that natural processes are the primary drivers of alkalinization, these natural factors can still be changed by humans. We can alter the alkalinity level in rivers by changing the natural parameters, so we need to invest more to restore the natural conditions of watersheds and tackle global warming and climate changes to deal with alkalinization in U.S. rivers“, says Wen

The results from the team’s study can help inform future research about enhanced rock weathering efforts. By distributing rock dust across large areas, it increases the amount of contact between rain and rock, which enhances carbon removal from the atmosphere. Wen says the team’s model can help answer questions about the evolution of natural conditions in different regions—an important step needed to implement enhanced rock weathering more effectively.

The Study

The team, which included Syracuse University researchers Tao Wen, assistant professor in the College of Arts and Sciences‘ Department of Earth and Environmental Sciences (EES), Beibei E, a graduate student in EES, Charles T. Driscoll, University Professor of Environmental Systems and Distinguished Professor in the College of Engineering and Computer Science, and Texas A&M assistant professor Shuang Zhang, recently had their findings published in the journal Science of the Total Environment.

Beibei E et al, Human and natural impacts on the U.S. freshwater salinization and alkalinization: A machine learning approach, Science of The Total Environment (2023). DOI: 10.1016/j.scitotenv.2023.164138