Where can you find federally approved sample holding times and preservation techniques?

The correct response indicates that both 40 CFR 136 and 40 CFR 141 contain important information regarding federally approved sample holding times and preservation techniques.

40 CFR 136 primarily deals with guidelines for the establishment of test procedures and analytical methods for the National Pollutant Discharge Elimination System (NPDES) under the Clean Water Act. This includes critical information about how to handle and preserve samples to ensure accurate testing results.

On the other hand, 40 CFR 141 focuses on drinking water regulations, which also includes sample holding times and preservation methods pertinent to contaminants that may be present in drinking water sources.

By recognizing that both of these federal regulations encompass valuable details related to sample handling, it is clear why the answer indicating both is correct. Understanding the contents of these regulations is vital for anyone involved in water quality analysis, ensuring they follow standardized practices for reliable and valid results.