Passive Sampling of Inorganics Using Peepers

There is currently a lack of user-friendly guidance documentation and detailed case studies on how to successfully prepare, deploy, retrieve, sample and interpret data from passive samplers for inorganics. The overall objective of this project was to enhance the standardization of, and confidence in, the use of peepers for passive sampling of inorganic constituents such as metals, metalloids, and anions in sediment porewater. This project was based on a field demonstration in which peepers were deployed in surface sediment and surface water at Naval Base San Diego, San Diego, California, in October 2022.

Dialysis samplers, also called peepers, are a type of passive sampler that function by allowing sediment porewater to equilibrate with water contained in a small isolated compartment after insertion of the peeper into sediment. Equilibration occurs over a period of time via passive diffusion through a semi-permeable membrane. Following equilibration with the porewater, the peeper is retrieved, and the liquid is analyzed via standard (i.e., EPA SW-846) methods and reported as a concentration in water that can be easily compared to risk-based criteria in water or used in fate modeling. Peepers have been used for decades; however, due to the lack of standard guidance documents, peeper methods have greatly varied, hindering the use of peepers for regulatory and decision-making purposes. The guidance document and demonstration tests in this project support critical priorities by: 1) improving methods for establishing porewater-based sediment management standards, 2) increasing confidence in sediment cleanup levels, and 3) demonstrating and validating passive sampling-based cleanup levels. 

Over the course of two days, commercially available peepers were deployed at 10 stations, left to equilibrate for approximately 10 days, then retrieved over a course of two days, after which they were processed and shipped to a commercial analytical laboratory.

Ninety percent of the peeper samples were successfully analyzed (peepers at one station were not able to be retrieved) for target metal analytes cadmium, chromium, copper, iron, lead, manganese, mercury, and zinc. Lithium and bromide tracers spiked into the peepers indicated that greater than 70% equilibration was reached for all target metals during the 10-day exposure. Both tracers performed similarly.

In the future, use of the lithium tracer (rather than bromide) is recommended, as the use of a lithium tracer is most efficient in terms of method simplicity and analytical cost savings. Method detection limits for peepers were sufficiently sensitive to detect concentrations lower than the U.S. Environmental Protection Agency’s saltwater Criterion Continuous Concentration for aquatic life. All metals except for mercury (detection limit of approximately 0.1 to 0.4 µg/L) were detected in at least one peeper sample. Differences in concentrations in sediment porewater and surface water were noted for some, but not all, metals. Detailed methods are presented, as well as logistical details and recommendations for planning and executing successful peeper investigations. (Project Completion - 2023)

Risacher, F.F., H. Schneider, I. Drygiannaki, J. Conder, B.G. Pautler, and A.W. Jackson. 2023.  A Review of Peeper Passive Sampling Approaches to Measure the Availability of Inorganics in Sediment Porewater.  Environmental Pollution, 328:121581. doi.org/10.1016/j.envpol.2023.121581.