The contribution of dissolved organic matter (DOM) to alkalinity in
estuarine waters and its relationship to CO2 chemistry has not
received much previous attention. In this paper, we present some of the
first organic alkalinity measurements in the context of estuarine mixing,
focusing on three rivers in the Southeastern US. The simple model presented
here demonstrates that the organic contribution to alkalinity in estuarine
waters is largely controlled by the dramatic pH change in the early stage of
the mixing. As mixing continues, organic alkalinity becomes nearly
conservative with respect to salinity change. Although we did not detect any
DOM removal during estuarine mixing, this work provides an alternative
approach to evaluate the issues of colloids and small particles formation
and coagulation during mixing. This paper demonstrates that a large part of
the chargeable sites of humic substances are either completely protonated or
deprotonated during the estuarine mixing processes and therefore do not
contribute to alkalinity nor proton transfer reactions. One class of
chargeable sites may be adequate to describe the changing acid-base
properties of humic substances during the mixing process. Furthermore,
dependence of acidty on ionic strength is of second order importance. The
data shall be valuable for evaluating the more elaborate models of humic
substances, which either have multiple sites or continues site distribution
and take ionic strength into consideration.