[A-CG43] Water and sediment dynamics from land to coastal zones

[ACG43-09] The partial truth of suspended sediment particle size

*Dhruv Sehgal¹, Sjoukje de Lange², Nuria Marinez Carreras³, Kryss Waldschläger², Victor Bense², Christophe Hissler³, Ton Hoitink² (1.Institute of Industrial Science, The University of Tokyo, Tokyo, Japan, 2.Hydrology and Environmental Hydraulics Group, Wageningen University and Research, Wageningen, The Netherlands, 3.Catchment and Eco-Hydrology Research Group, Environmental Research and Innovation Department, Luxembourg Institute of Science and Technology, Belvaux, Luxembourg)

Keywords:suspended sediment, particle size, laser diffraction, flocculation

Acquisition of accurate and reliable sediment particle size distribution (PSD) measurements is important for understanding ecological and geomorphological processes and tackling technological challenges. However, achieving an unbiased PSD is challenging due to various influencing factors, such as the choice of measurement method (in the field or in the laboratory), instrument, particle shape, measurement location and frequency, and data processing decisions. When measuring samples ex situ (in the laboratory), even more factors can play a role in the measured PSD, such as sample collection, sample storage, and sample treatment. Using PSD measurements without considering the limitations associated to these factors, can have significant impacts on study and modelling results that are based on grain size statistics (e.g., median grain size D₅₀ or D₉₀/D₁₀). Additionally, the methodological choice to measure PSD in situ or ex situ is often a logistical consideration or depends on the aim of the study. However, the differences in measured PSDs when using these two methodologies can also be leveraged to understand the differences between natural processes influencing PSD in the field and the more controlled laboratory conditions. Being aware of the differences, rather than simply accepting any given PSD as the true grain size, can bring us closer to objective, unbiased PSD measurements. To date, a comparison of in situ and ex situ measurements by one single instrument has not been made yet.

In this study, we compare the results between in situ and ex situ measurements using a LISST-200X mounted in a stream for two days, and simultaneously grabbing water samples for post measurements. Water samples were spit and stored at room temperature (18-23 °C) while exposed to light and at 5 °C in the dark. After one, two, and three weeks respectively, a water sample was taken from each storage, gently agitated and the ex situ particle size distribution measured in a LISST-200X and a Malvern Mastersizer-3000. The data revealed that the D₅₀ of the ex situ samples was larger than the in situ D₅₀. Particle images showed that flocs had formed in the samples while stored. The D₅₀ of the ex situ samples did not change significantly with longer storage and it was impossible to return the ex situ samples to their original in situ state. The study concludes that ex situ measurements are only useful for obtaining the particle size distribution of the primary particles. It is intrinsically difficult to measure the size distribution of flocculated particles.

A secondary objective was to examine the measurement time required to obtain an accurate D₅₀. The analysis showed that, for the dataset in question, an average time of one minute was required to obtain an accurate D₅₀. The reason is that large particles are few and far between in situ and the sample volume of the instrument becomes important: At a given concentration, a small sample volume is statistically less likely to contain a large particle than a larger sample volume. Hence averaging is needed to observe and properly account for the larger particles.