A Citizen's Guide to Understanding and Monitoring Lakes and Streams

Chapter 4 - From the Field to the Lab

Measuring Nutrients in Lakes and Streams

Field Sampling Considerations

There are no special field sampling concerns associated with nutrient samples other than those described in the section on basic sample collection techniques. It is especially important that sample containers be clean when collecting nutrient samples. Containers used for nutrient analyses should be acid-washed (soaked in dilute hydrochloric acid) and rinsed thoroughly with distilled water. If you are collecting samples for later laboratory analysis, you must properly preserve and store them. Professional labs usually provide properly cleaned bottles, often with preservative already in them. Clarify these procedures with the lab before sampling.

Measurement Methods

Although there may be numerous laboratory techniques for analyzing any one nutrient, this should not be a concern for a volunteer monitoring program. For the purpose of this guide, there are only two methods: analysis by a certified lab, or analysis by the use of field kits. In the first case, volunteers only need be concerned with proper collection and preservation techniques. In the second case, detailed directions will be provided with the kits.

If nutrient samples are collected for later analysis, proper handling and preservation are important. The table below details handling, preservation, and holding times for each of the nutrients. All require the same initial treatment – preservation with sulfuric acid (except for SRP) and storing in the dark at 4^o C. (This means storing them on ice in a cooler until you can get them to a refrigerator.) The amount of time they can be held in this condition before analysis varies. It is the lab’s responsibility to analyze the samples before the time limit expires. They need to report the time and day of analysis. One of the QA/QC checks you should make when data are returned is that samples were run within an acceptable time limit.

Preserve nutrient samples by inserting a few drops of sulfuric acid into the sample bottle. Enough sulfuric acid needs to be added to reach a pH <2. The amount needed will vary depending upon the volume of water being preserved; however, it doesn’t need to be an exact amount. Typically about a half a milliliter or half an eye dropper full is enough to preserve a 250-mL sample. Test the number of drops needed on one sample and use that amount for the rest of the study. Do not test pH on a sample you intend to have analyzed unless you ensure that the sample isn’t inadvertently contaminated.

Because the preservation and handling methods are the same for all but the SPR, one sample bottle can be used for most of the nutrient analyses from one station. This will save time, energy, and space because fewer bottles will need to be washed, fewer samples will need to be collected, and less space will be needed in the ice chest, refrigerator, and whatever other storage containers are used. Usually one 500-mL container plus an additional 125 mL container, if SRP is being analyzed, will suffice for all the nutrients. Have this OK’d with the lab ahead of time.

The Special Case of SRP

SRP (aka orthophosphorus or phosphate) samples must be filtered within 8 hours of collection and then analyzed within 2 days. Because of the delay between collecting samples and getting them to a lab for analysis, the filtering usually must be done by the persons doing the monitoring. Filtering requires the use of a small pump, filtering apparatus, and 0.45um membrane filters that have been soaked in distilled water before use. A minimum of 50 mL of water must be filtered for each sample, and then poured into a fresh, properly cleaned sample bottle (acid washed and distilled water rinsed). The entire filtering apparatus needs to be acid rinsed and thoroughly rinsed with distilled water between each sample.

QA/QC Considerations

In addition to standard lab QA/QC, lab replicates should be collected for 5 to 10 percent of the samples, based on a random selection process. If kits are used for analysis, you may want to check your results against laboratory methods. In this case, 5 to 10 percent of the samples should replicated by the laboratory method to check kit precision against more sophisticated lab methods.

The next section discusses how to measure total suspended solids and turbidity in lakes and streams.

Measuring Temperature | Measuring Dissolved Oxygen | Measuring pH | Measuring Secchi Disk Depth | Measuring Nutrient Concentrations | Measuring TSS | Measuring Chlorophyll a | Measuring Fecal Bacteria

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