Waste discharge regulations can be concentration- or mass-based except for:

Waste discharge regulations often have two primary ways of measuring pollutants: concentration-based limits, which focus on the amount of a substance present in a given volume of water, and mass-based limits, which consider the total amount of a substance being discharged over a specific time period. The correct answer indicates a category of pollutants that doesn't typically fit neatly into either concentration or mass-based frameworks.

pH and temperature stand out because they are specific parameter values that need to be maintained within certain limits rather than measured in concentrations of harmful substances. Unlike other pollutants, pH levels indicate the acidity or basicity of the water, while temperature can reflect thermal pollution effects. These parameters are critical for maintaining aquatic life and overall water quality, hence they are regulated as discrete thresholds rather than through concentration or mass calculations.

Different classes of pollutants, such as suspended solids, metals, organics, and oils, can be directly measured in concentration or as total mass in a discharge over time. However, the nature of pH and temperature makes them unique, requiring specific values rather than a variable range that could be interpreted through concentration or mass limits. This regulatory approach ensures that the integrity of aquatic ecosystems is upheld, as imbalances in pH or temperature can have significant ecological impacts.