by Markus Flury, Department of Crop & Soil Sciences, Washington State University, Pullman, WA 99164, and Puyallup, WA 98371

In 2017, we initiated a multi-year study to examine the accumulation of metal nanoparticles and microplastics from biosolids in agricultural soils. Our study is expected to continue through 2023. Results of our nanoparticles investigation were published late last year.

Biosolids can be a source of metals and metal nanoparticles. In our nanoparticles investigation, we quantified the amount of silver accumulated in biosolids-amended soils over the past 20 years to see to what level silver has accumulated in soil and whether silver can migrate through the soil. We analyzed archived soil and biosolids samples to construct a time series and mass balance of silver concentrations in both biosolids and soils from 1996 to 2017. Soils samples were taken from the Douglas County, Washington, GP-17 long-term biosolids application research site (Figure 1).

Silver is a non-regulated metal in biosolids, but it can form nanoparticles in form of silver-sulfide when present in biosolids and soils. We detected silver-sulfide nanoparticles in biosolids samples, but the silver concentrations were too low in the soil samples collected from biosolids-amended fields  to allow for identification of silver-sulfide nanoparticles .

The silver concentrations in the biosolids-amended soil increased steadily from 1996 until 2007, after which the concentrations leveled off at about 1.25 mg silver kg-1 soil (Figure 2). The leveling off of the silver concentrations coincided with a decrease in the silver concentrations in the biosolids themselves, likely caused by less industrial use of silver in the last decade. The majority of the Ag (82%) in soil was confined to the top 10 cm of the soil, small amounts (14%) were detected in 10 to 20-cm depth, and trace amounts (4%) in 30 to 40-cm depth. This suggest that silver is not leaching out from the soil. The overall mass balance for silver measured in 2017 was 90%.

Our study shows that, in a real-world field scenario, biosolids applied at agronomic rates represent a long-term, economically viable source of crop nutrients without increasing the concentration of total Ag in soil above a maximum of 1.5 mg silver kg-1. This concentration is below estimated ecotoxicity limits for silver nanoparticles in soil.

We will use the results from this silver study and expand the analyses for quantification of the amounts of microplastics in soils due to biosolids applications. As silver is easier to analyze than microplastics, the silver study allowed us to determine how accurate we can measure a mass balance of silver applied by biosolids and silver measured in soil.

Original article: Taylor, S. E., C. I. Pearce, I. Chowdhury, L. Kovarik, S. Baum, A. I. Bary, and M. Flury, Long-term accumulation, vertical distribution, and speciation of silver nanoparticles in biosolids-amended soils, J. Environ. Qual., 49, 1679–1689, 2020a. (



Figure 1: Taking soil samples for silver analysis at the Douglas County, Washington,  GP-17 long-term biosolids application site.
Figure 1: Taking soil samples for silver analysis at the Douglas County, Washington,  GP-17 long-term biosolids application site.
fig 2
Figure 2: Total silver concentration measured in biosolids (top) and biosolids-amended soils (bottom) over time in a dryland wheat farming system in Douglas County, Washington. Concentrations refer to mass of Ag per dry mass of biosolids or soil. The green dots represent the times when biosolids were applied to the field.