Comparing dry and wet sieving with laser diffraction to the hydrometer method for particle size analysis of sandy bioretention soil media

Abstract

Standardization of particle size analysis (PSA) is crucial to ensure the proper blending of stormwater filter media such as bioretention soil media (BSM). BSM typically contains >80% sand and is amended with organic matter and fines (silt and clay) to support pollutant removal. However, there is currently no standardized PSA method to verify whether BSM meets design specifications. This study compares three PSA methods—hydrometer, dry sieving with laser diffraction (DS + LD), and wet sieving with laser diffraction (WS + LD)—to evaluate their accuracy and repeatability for analyzing sandy BSM. Twenty-seven BSM samples were collected from three bioretention cells in Ohio. Substantial variability in results was observed depending on the BSM sample mass used in the hydrometer method. Triplicate BSM analyses by both DS + LD (83.9 ± 1.7% sand, 9.6 ± 3.0% silt, 6.5 ± 2.7% clay) and WS + LD (84.1 ± 1.7% sand, 10.6 ± 2.2% silt, 5.3 ± 1.7% clay) demonstrated high intra- and inter-method agreement, especially for sand content. The hydrometer and WS + LD methods were the most consistent for measuring clay content. We recommend measuring the sand and fines fractions separately after appropriate dispersion when conducting PSA on sandy soils like BSM. Although PSA requires additional time and cost, ensuring the proper delivery of well-characterized BSM outweighs the costs of potential bioretention cell clogging and reconstruction. The method used for BSM textural analysis should be reported by laboratories and on delivery of BSM. Future research should focus on BSM pretreatment methods prior to analysis of the mineral fraction to establish a standardized method for BSM PSA.