With the 2016 growing season upon us, many produce growers will soon be collecting water samples from irrigation and postharvest water sources for microbiological analysis. Using proper techniques to collect water samples will help to prevent inaccurate testing results.
When collecting water samples, one should start with the appropriate collection container. Many laboratories will only test water samples that are received in their containers. Consequently, it is important to select a lab, determine their individual requirements, and obtain the appropriate containers prior to collecting samples. Generally, containers used for water sampling will be large enough to hold at least 100 ml of water. The interior should be sterile and the container should be sealed to prevent contamination. Sampling containers may also contain crystals or tablets when received from the lab. These tablets or crystals are made of sodium thiosulfate and are place in the container to neutralize any chlorine that may be in the water sample. They should not be removed.
If irrigation water from a well is being sampled, it is a good idea to collect the sample as close to the water source as possible. This means collecting the sample from the outlet that is closest to the well. Prior to collecting, the rim of the outlet (valve, spigot, etc.) should be sanitized. This can be done using a flame or chlorine. The system should then be allowed to run and water should be allowed to flow out of the outlet long enough to flush the system. A good rule of thumb is to run the system at least 3-5 minutes longer than is necessary to empty the volume of stagnate water remaining from the last use. To collect the sample, the seal on the sample container should be removed or broken and the sample container should be opened only as far as needed to collect the sample. Containers should be filled at least to the fill line and should be closed as quickly as possible. Once the sample is collected, the container should be marked with the date and time of collection and immediately cooled. Samples should be kept as cool as possible by icing or refrigerating until they are delivered to the lab. Many labs have a maximum time interval between collection and sample receipt, usually 24 hours. Samples received too long after collection will not be processed. Those growers who are covered under the Food Safety Modernization Act Produce Rule should pay special attention to time requirements, as the rule specifies EPA Method #1603, which only allows a maximum of 8 hours from sample collection to processing.
If sampling from surface water, such as ponds and lakes, one should try to sample at a depth of 6-12 inches. The container should be submerged prior to opening the lid. The container should then be filled and the lid put back in place prior to removing it from the water. If a dock or other structure is not available for access to deeper water, one can attach a sample container to a pole. Care should be taken not to sample too close to the bottom, as sediments may be collected with the sample. If one finds it necessary walk into the water, sampling should be done ahead of the muddy front that is stirred up by motion. Remember that excess rainfall can also stir up bottom sediments and alter test results. Samples should not be taken immediately after rainfall. Best practice is to collect the sample during a time when the water would normally be used for irrigating. If irrigating from flowing surface water, such as a creek or stream, and it is necessary to wade into the water, be sure to sample from the upstream side, again to avoid collecting stirred up sediments.
Collecting samples of water used for postharvest is similar to collection from an irrigation well. One should select an outlet close to where water lines come into the packing facility. All attachments such as aerators or garden hoses should be removed. The outside rim of the outlet should then be sanitized and water should run through the outlet for 3-5 minutes. The sample container may then be filled, taking care to make sure that the container is open for as little time as possible.
In many cases, unexpected results from a water test can be traced back to poor or inappropriate collection techniques. Details such as not flushing the system, failing to remove attachments, and sampling too near the bottom of surface water can drastically alter water test results. Taking some time to practice proper collection techniques prior to the upcoming season will help to ensure that your water test results are as accurate as possible.