Calculating Resilience, Tipping Points, and Restoration for Lakes at Risk from Acid Rain

This page authored by Bill Stigliani, University of Northern Iowa
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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.

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Learning Goals

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.

The activity directly addresses risk in the context of lake water acidification caused by a tipping point in the watershed soil's ability to buffer the inputs of acid rain. It addresses resilience in the context of the watershed soil's capacity to avert lake water acidification for 70 years after the onset of acid rain inputs. Resilience is also addressed by the soil's ability to restore the depleted buffering capacity by natural replenishment of base cations. It addresses geoscience because resilience, risk, tipping points, and replenishment can only be understood in light of the geochemical process occurring in soils.

Context for Use

This activity is applicable to undergraduate university students who have completed a year of freshman chemistry, or a vigorous high school chemistry class. The class size is ideally between 20 and 30 students, but it could be scaled to larger classes. The titration of the buffered Alka-Seltzer solution is conducted in a lab, but it could be presented as part of a lecture. The spread sheet model is a take-home exercise. Time required for this activity is a few hours. Students will need some knowledge of chemistry and they should know how to set up spread sheets. The activity should be presented when discussing the qualities of ecosystem resilience and restoration. The activity can be taught at different levels of sophistication, so it is quite amenable to adaptation in other settings where the students are not primarily science majors.

Description and Teaching Materials

My description is provided in files 1 and 2 below.

Teaching Notes and Tips

This activity is proposed, but not yet implemented in a classroom setting. No practical tips at this time.


When the students submit their reports, they will be graded and reviewed for errors and misconceptions. Students will be given the opportunity to improve their grades by resubmitting the reports with corrections based on the instructor's review.

References and Resources

This activity is an original document developed by the participant. There are currently no online resources describing this activity.