A grab sample must be collected for stormwater discharge permit applications within how many minutes after discharge?

A grab sample must be collected for stormwater discharge permit applications within how many minutes after discharge?

• A. 10 min
• B. 20 min
• C. 30 min
• D. 60 min

Answer: (C)

Timing matters because a grab sample is meant to reflect the water quality during the active discharge. For stormwater permit requirements, the sample must be collected within 30 minutes after discharge begins, so the data represent conditions while the runoff is actually flowing from the site. Waiting longer than 30 minutes allows changes to occur—dilution, dilution from additional flows, or shifts in pollutants—that can make the sample non-representative of the discharge event. The 60-minute option exceeds the permitted window and wouldn’t meet the rule. While shorter times like 10 or 20 minutes fall within the allowed period, the standard specifies the upper limit of 30 minutes, which is why 30 minutes is the best answer.

Timing matters because a grab sample is meant to reflect the water quality during the active discharge. For stormwater permit requirements, the sample must be collected within 30 minutes after discharge begins, so the data represent conditions while the runoff is actually flowing from the site. Waiting longer than 30 minutes allows changes to occur—dilution, dilution from additional flows, or shifts in pollutants—that can make the sample non-representative of the discharge event. The 60-minute option exceeds the permitted window and wouldn’t meet the rule. While shorter times like 10 or 20 minutes fall within the allowed period, the standard specifies the upper limit of 30 minutes, which is why 30 minutes is the best answer.