Estuarine mixing behavior of colloidal organic carbon and colloidal mercury in Galveston Bay, Texas

Abstract

Mercury (Hg) in estuarine water is distributed among different physical phases (i.e. particulate, colloidal, and truly dissolved). This phase speciation influences the fate and cycling of Hg in estuarine systems. However, limited information exists on the estuarine distribution of colloidal phase Hg, mainly due to the technical difficulties involved in measuring it. In the present study, we determined Hg and organic carbon levels from unfiltered, filtered (<0.45 μm), colloidal (10 kDa–0.45 μm), and truly dissolved (<10 kDa) fractions of Galveston Bay surface water in order to understand the estuarine mixing behavior of Hg species as well as interactions of Hg with colloidal organic matter. For the riverine end-member, the colloidal fraction comprised 43 ± 11% of the total dissolved Hg pool and decreased to 17 ± 8% in brackish water. In the estuarine mixing zone, dissolved Hg and colloidal organic carbon showed non-conservative removal behavior, particularly in the low salinity (<15 ppt) region. This removal may be caused by salt-induced coagulation of colloidal matter and consequent removal of dissolved Hg. The particle–water interaction, K_d ([particulate Hg (mol kg⁻¹)]/[dissolved Hg (mol L⁻¹)]) of Hg decreased as particle concentration increased, while the particle–water partition coefficient based on colloidal Hg and the truly dissolved Hg fraction, K_c ([colloidal Hg (mol kg⁻¹)]/[truly dissolved Hg (mol L⁻¹)]) of Hg remained constant as particle concentration increased. This suggests that the particle concentration effect is associated with the amount of colloidal Hg, increasing in proportion to the amount of suspended particulate matter. This work demonstrates that, colloidal organic matter plays an important role in the transport, particle–water partitioning, and removal of dissolved Hg in estuarine waters.