The graph shows the annual amount of salt (sodium chloride or NaC)

The graph shows the annual amount of salt (sodium chloride, or NaCl) used on roads in Nation X, for the purpose of melting snow and ice. The natural deposition is the average amount of salt that would be deposited on the surface of the country (by, for example, rainfall), regardless of human activities.



This use of salt has resulted in considerably increased concentrations of both sodium (Na) and chloride (Cl) in the country's surface water (rivers, streams, lakes)–sufficient to be toxic in both plants and fish, and for significant adverse human health effects due to increased salt intake. However, because sodium passes less easily through soils than chloride passes, the levels of extra sodium in the country's aquifers (underground water) are notably lower than the levels of extra chloride, and likely negligible in most locations.

Milkweed, an essential source of food for all monarch butterflies, absorbs a significant amount of sodium from the soil when growing next to salt-treated roads. The graph shows results of an experiment comparing monarch butterflies that ate only milkweed that was not next to roads (prairie milkweed) to those that ate milkweed that was next to salt-treated roads (road milkweed). The interval defined by the vertical line segment at the top of each bar is the experimental error. Differences are statistically significant when the experimental error intervals do not intersect. For example, the difference in measurement of thoracic protein was statistically significant for males but not for females. The experimenters attributed the statistically significant differences to the differences in sodium intake.

Two residents of the Nation X municipality where the experiment described under the Butterflies tab was conducted are discussing the use of salt on roads. Resident 1 asserts that this use has had an effect on animal life. The annual amount should therefore be reduced everywhere, such that the total amount used per year across the country never exceeds the average natural deposition across the country. Resident 2 recommends a similar reduction, but only because of adverse health effects on people due to elevated levels of sodium in drinking water.