Major Ions in Freshwater Systems

Megan Kelly, Arrupe College of Loyola University Chicago

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Initial Publication Date: September 21, 2021 | Reviewed: August 4, 2022


Dissolved ions are present in all freshwater systems, but humans can change the chemical composition of freshwater in several ways. In this activity, students will examine the concentration of major ions in freshwater systems over time and reason about potential drivers of these changes.

Strengths of Module

This module asks students to consider the environmental impact of cold-weather road safety practices (salt/brine application) and make a recommendation to a government agency. Detailed instructions for creating scatter plots, bar charts, and pivot tables in Microsoft Excel are included. By completing the modules, students will practice technical skills in Excel as well as interpretation of graphical displays of a large dataset.

What does success look like

Students will describe seasonal variations in the chloride concentration of freshwater streams and compare seasonal and interannual changes across gradients of climate and urbanization. Students will create time series graphs, pivot tables, and bar charts, and reason about the patterns they observe, paying particular attention to identifying the role humans play in changing aquatic systems.

Context for Use

This module was created for an introductory Environmental Science course for non-majors at a two-year college. The module could be adapted for courses in hydrology, limnology, or geography at similar or higher levels.

How Instructors Have Used This Module

Using the Project EDDIE Major Ions in Freshwater Systems module in Environmental Processes, Challenges, and Methods
Megan Kelly, Loyola University Chicago
This module introduces students to a common and important source of freshwater pollution that invites debate about how to address the problem. By examining publicly available data, students can discover what types of environments are likely to suffer from road salt pollution, compare environmental concentrations of chloride to concentrations of legal and biological concern, and either defend the use of sodium chloride for road deicing or propose another solution, based on their understanding of the data.

Description and Teaching Materials

Why this Matters:

Water chemistry changes seasonally due to natural and anthropogenic influences. Changes in water chemistry affect ecosystem health by influencing the biological community and physical characteristics of the system.

Quick outline/overview of the activities in this module

  • Pre-module work: Read or listen to clips from Chicago news media about road salt and water quality. Additional readings depending on the level of the students could be assigned.
  • Activity A:Examine one data source (Metropolitan Water Reclamation District of Greater Chicago) using spatial and graphical tools.
  • Activity B: Compare Chicago stream data to rural Illinois stream data (cold, rural) and Atlanta stream data (warm, urban)
  • Activity C: Consider the Chicago area data with Illinois' water quality standards in mind, and examine concentrations over time for one Chicago-area site at a time.

Pre-Module Work

Activity A: Chicago - web-map exploration, time series, regression, pivot tables

Part 1: Examine a web-based GIS with water quality information for Chicago streams, paying particular attention to chloride concentration. Students find the minimum, maximum, and median concentrations for a one year period at one site, calculate the range, and compare with classmate.
Part 2: Plot chloride concentrations in multiple locations in the Chicago area over time to look for overall patterns and trends. Add trendline - reason about why the r-squared is so low and what other factors could be involved.
Part 3: Make a pivot table to more closely examine seasonal changes by plotting the average monthly chloride concentration in Chicago streams - should reveal a strong seasonal pattern.

Activity B: Rural Illinois and Atlanta - time series, regression

Download USGS data and compare temporal chloride concentration trends and patterns in streams in rural Illinois and Atlanta to those in Chicago.

Activity C: Chicago - water quality standard exceedences and average annual chloride concentrations of individual sites (number filtering, pivot tables, plotting)

Consider the Chicago area data with Illinois' water quality standards in mind. Make a pivot table showing the number of exceedances at each site. Choose two sites to examine more closely and plot average annual chloride concentration over time. Reason about the differences between Chicago area monitoring stations.

Teaching Materials:

Teaching Notes and Tips

Notes on the student handout:

The student handout is written for Excel 365 on Windows 10. In places where I remembered that things work differently on Macs, I indicated so in the handout. For classes where students bring their own computers, I have had some success asking students to work in groups with students running the same operating system.

Measures of Student Success

Students have the opportunity to check their understanding with peers at several points throughout the lab.

Potential exam questions could ask students to predict changes (increase/decrease/none) in water chemistry in unfamiliar locations varying in urbanization and climate.

An embedded assessment asks students to review the graphs they made and a table of deicing alternatives from the Cary Institute report, "Road Salt: Moving Toward the Solution," and make a recommendation to the commissioner of Chicago's Department of Streets and Sanitation. This assessment could be moved out of the lab and into an exam as a short answer if desired.

Another follow-up assignment could ask students to design an experiment to test the toxicity of NaCl to seeds at a range of concentrations.

References and Resources