NCEES Fundamentals of Engineering (FE) Environmental Practice Exam 2025 - Free Environmental FE Practice Questions and Study Guide

Approximate alkalinity is generally understood as a measure of the capacity of water to neutralize acids, which is primarily due to the presence of bicarbonate (HCO3-) and carbonate (CO3²-) ions. In terms of chemical species, alkalinity can be represented by the sum of these species because they play key roles in buffering and maintaining the pH of aquatic systems.

The correct option combines the concentrations of bicarbonate and carbonate ions, with carbonate being multiplied by two to account for its higher contribution to alkalinity per mole compared to bicarbonate. This aligns with the stoichiometry of how these species interact in aqueous systems, particularly during acid-base reactions.

When a strong acid is introduced, bicarbonate can react and release protons, while carbonate can also react, effectively neutralizing the acid and thus contributing to the solution’s alkalinity. The expression captures this buffering capacity accurately.

The inclusion of hydroxide ions or hydrogen ions in other options would misrepresent the primary contributors to alkalinity as they do not increase the buffering capacity in a straightforward addition. Therefore, focusing on the relevant bicarbonate and carbonate concentrations gives a clear and direct measure of alkalinity, making it essential for assessing water quality and ecosystem health.