Massachusetts Wastewater Practice Exam

Grab samples are used when testing for:

1. A. Sulfides

2. B. Metal concentration

3. C. Dissolved gases

4. D. Nitrate

The correct answer is: C. Dissolved gases

Grab samples are particularly useful when testing for dissolved gases because they provide an instantaneous measurement of the gas concentration in the water at a specific point in time. Since dissolved gases can fluctuate rapidly due to changes in environmental conditions, grab sampling captures the immediate concentration of those gases, allowing for accurate analysis. This method is highly effective for dissolved gases such as oxygen, carbon dioxide, and methane, where rapid shifts in concentration may occur. Continuous monitoring or composite sampling might average out these rapid changes and may not accurately represent their levels at a given moment, which is essential for certain assessments. In contrast, while grab samples can be used for metals, sulfides, and nitrates, these substances often require more controlled sampling methods or monitoring over time to obtain a representative understanding of their concentrations. Dissolved gases, being influenced by environmental conditions, benefit significantly from the straightforward nature of grab sampling.