PFAS-NTN Subnetwork (Beta)
The PFAS-NTN subnetwork, (PFN) officially launched on April 7, 2026. The subnetwork measures weekly concentrations and deposition fluxes of more than 40 per- and polyfluorinated (PFAS) compounds in wet deposition (precipitation) from NTN samples with sufficient excess volume (i.e., >50 mL remaining after NTN analysis).
PFN was established as a provisional NADP subnetwork on January 1, 2024 after several years of method development and long-term validation at a suite of existing NTN sites. See the NTN site for NTN specific sample data. The PFN operated as a provisional network until April 2026, when the NADP Executive Committee finalized and approved the QA documentation and SOPs. All sites that participated in the provisional subnetwork transitioned to the official NADP subnetwork. Data from the provisional network (i.e., before April 2026) will be available online separately from the official network data pending manuscript publication and sponsor approval.
There is growing concern that PFAS contamination in surface and ground waters, ambient and indoor air, soils, and vegetation causes adverse health impacts to humans, wildlife, and ecosystems through a myriad of different exposure pathways. The NADP PFN subnetwork provides key data to evaluate the atmospheric transport and transformation pathways and fate of a large suite of PFAS compounds. PFAS in wet deposition can lead to contamination of surface and drinking water sources, soil and vegetation, and in certain environments atmospheric deposition represents the largest source of PFAS to those ecosystems. Leveraging the existing NTN, with more than 230 wet deposition monitoring sites across North America, allows for local, regional and national comparisons of PFAS concentrations and depositional fluxes.
PFAS measurements can be added to existing NTN site operations, or a new NTN site can be established. Information on joining the network is available here.
Method Development and Evaluation Period
Since 2018, the WSLH in partnership with federal and state agencies, has been evaluating and refining methods for measuring more than 40 PFAS compounds in wet deposition samples collected at existing NTN sites. By expanding the suite of measurements at NTN sites beyond the routine NTN analytes, NADP has the potential to increase overall participation in the NTN program. The potential growth of the NTN will benefit the program by expanding site coverage, welcoming new network partners, and addressing data gaps in emerging contaminants of concern.
- 2020 – 2021: Precision has been evaluated at co-located sites (NC30, NC96, NC97) – typically RSD <10%
- Over 200 QA samples (field and trip blanks, field analyte spikes) collected during 10 events over 3 years across 16 sites confirm exceptional method performance
- Bag sampling has been tested in both laboratory and field trials and shows no contamination or PFAS sorption issues
- The 10-site trial network supported by EPA/ORD has resulted in nearly 22,000 observations, the largest database of PFAS levels in precipitation
- External data quality audits completed by WSP for 2020-2022 and in progress for 2023-2024
Field Methods
Every Tuesday morning at 9:00 a.m., NTN site operators across the country head to their monitoring stations to retrieve samples of the previous week’s precipitation, whether rain, snow, or sleet.
Samples are collected continuously over the week in a wet deposition collector. The collector opens automatically during wet weather, allowing the precipitation to fall into a collection bucket lined with a sample bag, and then closes as soon as the precipitation stops.
At the site, the operator replaces the collection bucket and bag with one a new each week. The operator then takes the sample to a field laboratory, where it is weighed. Next, the sample is transferred to a shipping bottle and sent to the NADP Analytical Laboratory (NAL) in Madison, Wisconsin.
There are minimal changes to the routine NTN field procedures once a site joins the PFAS-NTN subnetwork. The site operator will be asked to replace the collector lid with an NADP-supplied PFAS-free liner. During the weekly sample changeout the operator will be asked to wear vinyl gloves and a Tyvek jacket (both supplied by NADP) to minimize potential contamination of the precipitation sample. The operator will ship the NTN sample back to the NAL. Semi-annually, the operator will be asked to process field and trip blanks, and field spikes for quality control.
Each site is also equipped with a rain gauge that provides a continuous record of rainfall amounts. Rainfall is recorded to the nearest 0.01 in. The rain gauge ensures the wet deposition collector operated properly throughout the week.
Lab Methods
Upon receipt of the NTN sample at the NAL, a sub-sample (100 mL) is taken for measurement of the routine NTN analytes in the NAL, and excess volume is transferred to the PFAS laboratory. PFAS in the precipitation samples are quantified using isotope-dilution liquid chromatography tandem mass spectrometry (ID-LC-MS/MS). An automated solid-phase extraction protocol (WAX-SPE) is performed prior to quantification to isolate and pre-concentrate the 30+ PFAS compounds.
Following secondary data review, the laboratory PFAS analysis is joined with peripheral sample details from NTN. After basic checks for inconsistent data is performed, the data is uploaded into a static database table. The data in this table is used for data downloads from the web.
The current listing of laboratory SOPs are available. Copies are available upon request.