Calculating Resilience, Tipping Points, and Restoration for Lakes at Risk from Acid Rain
In the first part, students titrate a buffered solution made from dissolving Alka-Seltzer tablets in water. They monitor the pH of the solution as drops of hydrochloric acid are added. A tipping point occurs when the pH drops sharply upon addition of one added drop of acid. By measuring the amount of acid added, they calculate the buffering capacity of the Alka-Seltzer solution.
In the second part, they create a spreadsheet model for the acidification of Big Moose Lake in the Adirondack Mountains of New York State. Given input data for the rate of acid deposition on watershed soils and the rate of replenishment of base cations through the natural weathering of bedrock, they calculate the time trends of the acidification event. The model shows the progressive decline in the soil's Cation Exchange Capacity, and the number of years it took for acid deposition to trigger a tipping point in the Lake's pH. The model also shows the impact of the Clean Air Act Amendments of 1990 on restoring the resilience of acid-buffering capacity in the watershed, and the estimated time it will take for complete recovery to pre-acidification levels.
Students will learn about delayed and unanticipated environmental impacts of industrial pollution. They will gain insight into how trends in pollutant inputs differ from trends in the response of ecosystems to the inputs, viz. the inputs are linear while the response is strongly non-linear. This non-linearity can lead to catastrophic tipping points in the system's ability to function. They will brainstorm about how to develop the means for early warning detection of such catastrophes.
By and large we are taught to think that the future will be merely a linear extrapolation of the past. In contrast, this activity highlights non-linear behavior and tipping points in the functioning of ecosystems. Students will better understand "systems thinking," which grasps the idea that an important quality of any interacting system is non-linear outcomes.
Context for Use
Description and Teaching Materials
My description is provided in files 1 and 2 below.
- Part 1: How Alka-Seltzer Works to Reduce Acid Indigestion (Acrobat (PDF) 239kB Apr10 14)
- Part 2: How a Watershed Protects Its Lake from Acid Rain: Example of Big Moose Lake (Acrobat (PDF) 311kB Apr10 14)