  Cohesive effects between fine-grained sediment particles greatly influence
their effective settling rate and erodibility. Many studies have observed a
qualitative difference in settling dynamics between clays in freshwater, where
particles remain dispersed, and in saltwater, where aggregates form and settle
rapidly. The critical coagulation concentration (CCC) of salt that separates
the two regimes however remains under-investigated, even though knowledge of
the CCC is crucial to understanding aggregation in settings such as estuaries,
where large salt concentration gradients occur. Furthermore, no simple
criterion exists to predict the CCC for clay suspensions. In this study,
systematic experiments are performed to determine the CCC, by measuring
transmitted light intensity through clay suspensions. To investigate the effect
of ion valence, sodium chloride (NaCl) and calcium chloride (CaCl$_2$) are
used. For kaolin clay, the results show a CCC of 0.6mM NaCl ($\approx 0.04$ppt
NaCl $=$ 0.04 PSU), and of 0.04mM CaCl$_2$ ($\approx 0.004$ppt CaCl$_2$).
Because these salinities are lower than those commonly observed in nature,
these findings indicate that kaolin clay should flocculate in nearly all
natural aquatic environments. Furthermore, due to the fact that tap water often
has salinities higher than this threshold, these results imply that great care
is needed in experiments, especially in large facilities where using distilled
water is not feasible. In addition, a simple criterion to estimate the CCC for
a kaolin clay suspension is derived. This criterion predicts that flocculation
occurs at extremely low salt concentrations and is approximately independent of
clay concentration, in agreement with the experimental observations and
consistent with experimental evidence from the literature.
