The Impact Of Flocculation on In Situ and Ex Situ Particle Size
Measurements by Laser Diffraction
Abstract
Accurate particle size distribution (PSD) measurements of suspended
particulate matter composed of flocs and aggregates are important to
improve understanding of ecological and geomorphological processes, and
for environmental engineering applications. PSD 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 are not acknowledged. In this study, a
laser-diffraction instrument (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 suspended particulate matter. This
study provides recommendations for optimizing PSD measurements by
calculating the measurement times required to obtain robust PSD
measurements (exceeding three minutes per sample), which are larger for
field samples with coarser particles and wider PSDs.