Sample Handling for NPDES
The accuracy of a sample result begins well before the start of an analytical procedure. It begins with sample collection. No matter how accurate you are with you analyses, or how thorough you are in method QA/QC, if your samples have been improperly collected, your results can be, and most probably will be, wrong. Additionally, for all samples collected for NPDES reporting purposes, you must follow the requirements outlined for sample collection in Title 40 Code of Federal Regulations part 136 (40 CFR 136) Table II ““Required Containers, Preservation Techniques, and Holding Times.”
While it is impossible to cover specific collection techniques for every analyte, there are some general considerations that can be addressed in this article.
Most samples can be collected in either polyethylene or glass containers, but some samples must be collected specifically in one or the other. It’s important to know what the exceptions are. For instance, volatile organic compounds (VOCs) must be collected in glass containers (with Teflon-lined caps with septums), not plastic. Some of the VOCs can dissolve into the walls of a plastic container, or may leach organic compounds out of the container. Both of these situations will produce erroneous VOC results.
All containers and sampler tubing should be cleaned according to the type of analyses done on the sample. At a minimum, all containers should be washed with lab-grade detergent and hot water, rinsed several times with tap water, followed by several rinses of distilled or deionized water. There may also be additional rinses depending on the analyte being sample for. For instance, containers for trace metal collection should be washed with a lab-grade detergent and tap water, rinsed with 1:1 nitric acid:water, rinsed with 1:1 hydrochloric acid:water, then rinsed with deionized water. Any additional cleaning procedures for specific analytes can be found in Standard Methods for the Examination of Water and Wastewater, EPA’s Industrail User Inspection and Sampling Manual for POTWs (April 1994), and to a lesser extent, EPA’s Methods for Chemical Analysis of Water and Wastes.
The purpose of sample preservation is to minimize chemical and biological degradation until the sample can be analyzed. Every parameter listed in your NPDES permit has a specific preservation technique and is outlined in 40 CFR 136. Preservation techniques most often include pH adjustment, chemical addition (sodium thiosulfate as a dechlorinating agent, for example), cooling to 4 degrees C, or a combination of these. If a sample is not analyzed immediately after collection, it must be properly preserved or the results of that sample cannot be accepted for NPDES reporting purposes. If ever in doubt about which preservation technique to use, as a minimum, cool the sample to 4 degrees C.
The holding time of a sample is the maximum time allowed between the moment a sample is collected to when analysis on that sample must begin. Following both the proper holding times, as well as required preservation, will help to retain the integrity of the sample so that accurate, legally defensible results can be obtained.
Holding times for NPDES samples are outlined in 40 CFR 136. Any sample collected for NPDES purposes must be analyzed within the proper holding time or the results of the analyses can not be accepted for reporting purposes.
It is always best to start an analysis on a sample as soon as possible. Some samples collected for such parameters as pH and chlorine residual have no acceptable holding times and must be analyzed immediately (“immediately” usually means within 15 minutes of collection). Other samples can be stored, once properly preserved, for varying lengths of time to accommodate transportation to the lab or for other scheduling demands. The length of time varies depending on what analyses must be performed.
Obviously, the holding time for a grab sample starts at the moment of collection. When the holding time starts for a composite sample is somewhat ambiguous. The EPA’s national office does not have an official interpretation on this yet, However, EPA Region 1 interprets the regulations to mean that the holding time starts the moment the first aliquot is collected. For example, 40 CFR 136 states that the maximum holding time for a sample collected for BOD analysis is 48 hours. If you start a 24-hour composite sampler to collect wastewater at 8:00 am on Monday and collect the composited sample at 8:00 am on Tuesday, then you have until 8:00 am Wednesday to start the BOD analysis. Even though the holding time is 48 hours, you’ve used up 24 of those hours in the compositing process.
If there is any discrepancy in recommended container types, preservation, or holding times between what is listed in a test method and what is outlined in 40 CFR 136, you must follow 40 CFR. It’s also a good idea to post Table II ““Required Containers, Preservation Techniques, and Holding Times” in your lab for quick and easy reference.
Sample collection using the proper containers, preservation, and testing within the required holding times will add to the accuracy of your results. It’s from these results that wastewater treatment process decisions are made, permits are written, and environmental policies are developed. Therefore, it’s in your best interest to produce the most accurate and precise data that you can – starting from sample collection through to the reporting of the data.
The information in this article is based on general information under the NPDES program. As usual, check your federal, state, and local regulations. You may have additional regulations or requirements that you must meet.
If you have any questions, suggestions, or comments, please contact NEWEA Lab Practices Committee Chair Tim Loftus at (508) 949-3865 (firstname.lastname@example.org).