Inland water chemistry: the Nordic Lake Survey 1995

Tom Andersen
University of Oslo, Norway, Department of biosciences

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Initial Publication Date: May 15, 2019 | Reviewed: December 10, 2020 (see revision history: 2 events)

Summary

While the ionic composition of surface seawater is basically the same anywhere in the world's oceans, the chemistry of inland waters can vary by orders of magnitude over short distances. In this activity we explore a data set on surface water chemistry in almost 5000 lakes across the Nordic countries (Norway, Sweden, Finland). Water chemistry of lakes in this mostly sparsely populated region does not carry a strong signal from local human activity. This allows us to explore large-scale gradients related to distance to the ocean, soil and landscape characteristics, post-glacial history, and effects of long-distance pollutant transport processes.

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Learning Goals

Develop skills for exploring data sets with thousands of records by recasting and aggregating data, investigating relationships between variables, and visualizing spatial patterns
Get an overview the main ionic constituents of inland waters, their relative abundances, and how they covary with each other and with environmental gradients
Become familiar with different concentration units (mass, molar, charge equivalents) and how these can be used to compute derived quantities like acid neutralizing capacity (ANC) and predicted conductivity
Learn how chloride can be used as a proxy for the sea-salt contribution to inland water composition, and visualize how this influence declines with distance from the ocean

Context for Use

Variations of this lab exercise has been used in both undergraduate and graduate level courses. The computer lab activity takes 2-4 hours depending on how many activities will be included, with at least the same amount of time for writing up a report answering questions posed in the exercise. The exercise requires familiarity with the R computing environment, and that R (and preferably Rstudio) is installed on the lab computers or the student's own computing devices. Basic knowledge of chemistry and physical geography or limnology will be useful. The exercise works best if 2-3 students work together and discuss the questions between them as they come up. Each student group should work at their own pace and request assistance from teachers if they get stuck with the coding or if they cannot find answers to questions. The exercise could be adapted to other geographical regions where similar inland water survey data are available (e.g., from USGS).

Description and Teaching Materials

The students are given a text file with the data and a HTML document (created with Rmarkdown) with instructions for how to proceed with exploring the data set. This document contains hints and critical snippets of code, but the students will have to compose most of the coding by themselves. The exercise can be broken down into 4 activities all based on the same data set, but with increasing complexity.

Activity A: Read the data file and do basic familiarization and exploration (what do the different columns represent, what are their summary statistics, what do the distributions look like, which ions are most / least abundant, etc.)
Activity B: Plot spatial distributions of different ions using the leaflet package. Interpret distribution patterns in terms main geographical gradients of sea-salt influence, long-distance air pollution, etc.
Activity C: Investigate the sea-salt contribution to inland water composition. Discuss non-marine sources of ions like SO₄, Na, and Ca. Calculate acid neutralizing capacity (ANC) as the difference between strong base cations and strong acid anions. Discuss possible explanations for discrepancies between ANC and alkalinity.
Activity D: Compute specific conductivity from ionic composition and first principles (more challenging than the previous, especially with weak physical chemistry background). Discuss possible explanations for the discrepancy between observed and predicted conductivity at high ionic strength or in low-pH lakes.