Differences between in situ and ex situ suspended sediment particle size measurement choices

Abstract

Measurements of particle sizes suspended in a water column are crucial for understanding various aspects of river ecology and morphology. These measurements can be obtained either directly in the field using a submersible sensor (in situ) or by collecting samples and analyzing them in a laboratory (ex situ). Typically, the choice between these methods is based on logistical considerations, often overlooking the potential impact on the measurements. This study investigates the disparities between in situ and ex situ measurements. The findings reveal that ex situ measurements yield particle sizes that are, on average, 112% larger than those obtained in situ, possibly due to the formation of flocs—aggregates of mineral and organic particles—in riverine environments. It's observed that flocs tend to grow during storage and manipulation in the laboratory but break apart during ex situ measurements. Consequently, ex situ measurements are deemed inadequate for accurately determining natural particle sizes. The study also suggests the duration required for measurements to representatively capture particle sizes, indicating longer measurement periods for in situ measurements.