The Impact of Flocculation on In Situ and Ex Situ Particle Size Measurements by Laser Diffraction

• Published in: Water resources research 2024-01, Vol.60 (1), p.n/a
• Main Authors: Lange, S. I., Sehgal, D., Martínez‐Carreras, N., Waldschläger, K., Bense, V., Hissler, C., Hoitink, A. J. F.
• Format: Article
• Language: English
• Subjects: Aggregates; Clay minerals; Diffraction; Environmental engineering; Flocculation; Geomorphology; in and ex situ; In situ measurement; Laboratories; laser diffraction; Lasers; LISST‐200X; measurement time; Mineral particles; Particle size; Particle size distribution; Particulate matter; River ecology; Rivers; Sedimentation & deposition; Sediments; Sensors; Shear stress; Size distribution; Stirring; Submersibles; Suspended particulate matter; Water circulation; Water column
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2023WR035176

Accurate particle size distribution (PSD) measurements of suspended particulate matter (SPM) composed of flocs and aggregates are important to improve understanding of ecological and geomorphological processes, and for environmental engineering applications. PSDs can be measured in situ (in the field) using a submersible sensor, or ex situ (in the laboratory) using samples. The methodological choice is often guided by logistical factors, and the differences in PSDs acquired by in situ and ex situ measurements is of concern. In this study, a laser‐diffraction instrument (the LISST‐200X) was used to compare in situ and ex situ PSD measurements. Samples measured ex situ were stored for three consecutive weeks and measured each week in a laboratory using different stirrer speeds. We observed that ex situ measurements display a higher D50 (median particle size) than in situ measurements of the same sample (up to 613% larger, 112% on average). Our experiments show that the difference between in situ and ex situ measurements can be explained by flocculation of the riverine sediments during the first week of storage. During the subsequent ex situ measurements, the stirring results in a significantly lower D50. Ex situ measurements are therefore unsuitable for flocculated SPM. This study provides recommendations for optimizing PSD measurements by calculating the measurement times required to obtain robust PSD measurements (exceeding 3 min per sample), which are larger for field samples with coarser particles and wider PSDs. Plain Language Summary Measurements of the size of particles suspended in a water column are important for understanding many processes related to river ecology and morphology. It is possible to measure these particles directly in the field using a submersible sensor (in situ), or by taking samples and transporting them to a laboratory (ex situ). The choice between these options often depends on logistics, with little recognition of the impact that this choice can have on the measurements. In this research, the differences between in situ and ex situ measurements are explored. We find that ex situ measured particle sizes are on average 112% larger than in situ measurements, which can be related to flocculation of the riverine particles. Flocs are a combination of mineral particles (such as silt or clay) and organic particles, forming larger aggregates. Our results show that flocs grow when a sample is taken to the laboratory and stored. During e