The U.S. Geological Survey (USGS) has provided an independent quality assurance project to the National Atmospheric Deposition Program (NADP) for over 44 years. Since 1997, USGS has operated a Field Audit Program (Program) to evaluate contamination and stability of dissolved chemical constituents in NADP National Trends Network (NTN) samples. The Program prepares synthetic precipitation solutions that replicate precipitation-level concentrations of major cations and anions and ammonium using chemical reagents, and the samples are shipped to the NTN site operators for processing in the field. After a week without precipitation (dry week), site operators pour 75 percent of their Field-Audit samples into a sample-collection container that was deployed to the field during the dry week, and a lid is used to seal the container for a 24-hour residence time. After the residence time, the sample is processed and containerized for shipment and analysis. The samples are shipped to the NADP Analytical Laboratory (NAL) for chemical analysis. The resulting concentrations from the set of two samples are analyzed to determine positive or negative percent differences. A positive difference represents assumed contamination, and a negative difference implies loss of dissolved ions, which may occur by adsorption to the container walls, degradation by bacteria, and(or) other chemical transformations such as oxidation or degassing.

Evaluation of Program data (1997 - 2023) indicate no trends in NADP/NTN sample contamination or stability attributable to changes in climate despite prolonged drought in the southwestern U.S. and increasing record high temperatures in many places nationwide. Other factors which could affect sample integrity include upgrading approximately 40 percent of the NTN to more sensitive collectors (~2010 - 2012), changing laboratories (2017), and the transition from unlined to plastic-lined collector buckets (2019). While these changing protocols have shifted trends in measured concentrations, they have not resulted in detectable variations in contamination or stability.

¹ U.S. Geological Survey, Observing Systems Division, Hydrologic Networks Branch, Denver, CO

² U.S. Geological Survey, New York Water Science Center, Troy, NY

³ U.S. Geological Survey, Wyoming-Montana Water Science Center, Cheyenne, WY

*Corresponding Author: ndeyette@usgs.gov