This study quantified the formation and speciation of halogenated organic disinfection byproducts (DBPs) during chlorination as a function of seawater intrusion by spiking fresh groundwater with 0.1% to 2% seawater by volume. The four chlorine- and bromine-containing trihalomethanes (THM4) and the nine chlorine- and bromine-containing haloacetic acids (HAA9) were the DBPs investigated. The bromide concentration increased from 38 mu g L-1 in fresh groundwater to 974 ug L-1 in 2% seawater, and upon chlorination the concentration of THM4 increased from 43 to 206 pg L-1 and the concentration of HAA9 increased from 39 to 75 pig L-1. At 0.4% seawater and higher the THM4 concentration exceeded the U.S. EPA maximum contaminant level of 80 pg L-1. Although the mass concentration of HAA9 increased by nearly a factor of 2 over the seawater range of 0% to 2%, the mass concentration of the five regulated HAAs (i.e., HAA5) remained approximately constant. This is because HAA5 excludes four bromine-containing species. The increase in mass concentrations of THM4 and HAA9 with increasing seawater intrusion was the result of an increased molar formation of DBP species and a shift from chlorinated species to brominated species. (C) 2014 Elsevier B.V. All rights reserved.