Reference: Robison, A. L., & Scanlon, T. M. (2018). Climate change to offset improvements in watershed acid‐base status provided by Clean Air Act and Amendments: A model application in Shenandoah National Park, Virginia. Journal of Geophysical Research: Biogeosciences. https://doi.org/10.1029/2018JG004519
The Clean Air Act: It’s not just for the air!
In the United States, the Clean Air Act and accompanying laws were passed with the intent of reducing the amount of air pollution that can be released by different industries and activities. As the name implies, we can thank these laws for making our air a lot cleaner. What is less obvious is that these laws have also helped to make our rivers, lakes, and streams into healthier habits for plants and animals.
Fossil fuel use, particularly burning coal, releases two air pollutants: sulfur dioxide and nitrous oxide. When these two air pollutants are released into the atmosphere, they can form acids that can make their way back down to the earth’s surface as acid rain, which, as it sounds, is rain that is more acidic than usual. This can cause a number of problems on land, but can also be really harmful when that acid rain gets in our fresh surface waters. Thankfully, the Clean Air Act has limited the release of these air pollutants, leading to a decrease in acid rain. Unfortunately, that has not solved all the problems of acid in rivers and lakes.
The base-ics of watershed chemistry
Acid rain can be a big problem when it falls onto watersheds. A watershed is the land that surrounds a lake or river such that rain that falls onto that land will eventually make it into that lake or river. If acid makes it into the lake or river, it can be harmful to any plants or animals that live in there. Before rainwater makes it to the river, however, it interacts with the soils and existing water in the surrounding land. The degree to which acid rain poses a risk depends on a number of characteristics of the watershed.
In some cases, there is a natural protection of compounds that can act as a base to neutralize the acid before it can get to any plants or animals, similar to an antacid that you would take if you had heartburn (acid reflux). In a watershed this is referred to as alkalinity. You can think of the amount of alkalinity as being similar to the dose of antacid – some watersheds have higher “dose” of these acid-neutralizing compounds, while other watersheds may have a lower “dose.” Another important characteristic is how easily acid moves through the soils in a watershed. In some types of soils, the acid is more likely to get “stuck,” and in those watersheds, the acid will build up over time and then slowly be released over a longer period of time.
The Clean Air Act has dramatically decreased the amount of acid rain in the United States. Because of the varied ways that acid rain interacts with watersheds, individual watersheds have responded differently to this decrease. Some watersheds with higher alkalinities have recovered to more neutral conditions quickly. Other watersheds where there is the tendency for soils to hold onto acids have recovered much more slowly. This can be worrying since there are other factors, such as temperature and the amount of water that flows through a watershed, that can impact this acid-base balance. These factors are expected to change with climate change, so it is also possible that this could have an impact for the chemistry of some watersheds.
A model solution: Predicting a future under climate change
In this study, Andrew Robison and Todd Scanlon wanted to find out what might happen to the acid-base chemistry of a watershed as climate change affects the rainfall and temperature of the watershed. They focused on a watershed in Shenandoah National Park in Virginia, USA, which has been slow to recover from all the acid left by acid rain prior to the Clean Air Act. Furthermore, this watershed has the tendency to hold onto acids in the soil and slowly release them. In order to find out what will happen in the future in this watershed, they used a computer model that predicts the chemical reactions that will occur in the watershed and how much acid will end up in the water as a result. They compared scenarios both with and without impacts of climate change to see how changes in the climate could affect the acidity of the water.
When they analyzed their results, they found that acid in the watershed is likely to continue to be a problem in the future. In both cases, the water became more acidic and there was a decrease in the alkalinity (the neutralizing “dose”). However, in the case where climate change was considered, the impacts were more severe. This suggests that climate change has the potential to counteract the benefits of the Clean Air Act. They estimated that more than 50% of the benefits to watersheds from the Clean Air Act could be counteracted by the negative impacts of climate change. The scientists thought this was because of the higher flows of water that are predicted with climate change. This can have a double impact: one, by drawing more acid out of the soils into the water and two, by washing out some of the compounds that contribute to the alkalinity that can buffer against the acid.
The takeaway: Changing climate = changing watershed acidity
In this scenario, we can see how the complex interactions between air, water, and the physical environment can lead to unexpected impacts of climate change. We often hear about how climate change will cause higher temperatures, but just as there are hidden benefits of the Clean Air Act, there are likely to be hidden consequences of climate change. This study showed how climate change has the potential to exacerbate issues of acidification of watersheds, with serious implications for plants and animals in these systems. With more studies like this, we can gain a better understanding of these interactions and what the future may hold.