Low-Level Mercury

Field Blank Requirements - Don't Skip Them!

Field blanks for mercury are defined as an aliquot of mercury-free reagent water that is placed in a sample container, shipped to the field and treated as a sample in all respects, including contact with the sampling devices and exposure to sampling site conditions, filtration, storage, preservation, and all analytical procedures (definition from s. NR 106.145(9), Wis. Adm. Code).

A field blank is the best way to estimate how much mercury detected in a sample is "real" and how much is contamination. We encourage using a procedure for obtaining a field blank that exposes the blank to the same environmental conditions as the sample. One way to accomplish that uses "clean" procedures with the following basic steps (for a grab sample):

1. Before traveling to the collection site, fill the cleaned containers for the field blank and sample with reagent water. Double bag the bottles, with one bottle per double bag.
2. At the site, remove the field blank container from the double bags and discard the reagent water from the bottle.
3. Remove the sample container from the double bags and pour the water from the sample bottle into the container for the field blank and return it to the double bags. Make sure that the container is labeled appropriately.
4. Collect the sample and return the container to the double bags. Make sure that the sample is labeled appropriately.
5. Package for shipment. Avoid shipping highly contaminated samples with those for low level mercury.

In general, at least one field blank is required for each day and sampling site. Field blank collection should be focused on effluent samples. NR 106.145(9), Wis. Adm. Code identifies the circumstances under which permittees may reduce the frequency for collecting field blanks.

At wastewater treatment plants, the permit may specify a 24-hour composite for the influent sample. The equipment and sample collection container should be cleaned before sampling and the tubing should be replaced regularly. Influent mercury concentrations typically exceed 50 ng/L so it's expected that most, if not all of the bias imparted by ambient contamination will be overwhelmed by the sample concentration. The logistical barriers of collecting a field blank through the composite sampler may be difficult to overcome. It may be necessary to devise other means to assess contamination in these samples.

For more information, contact Tom Mugan.

Back to Top