The actinides comprise a group of 15 metals (with atomic numbers ranging from 89 to 103) that are all radioactive and occur as cations in natural surface waters. Only the first 10 actinides are covered in this study, as these are the most environmentally relevant, where the dominant oxidation states are as follow: actinium(III), thorium(IV), protactinium(V), uranium(VI), neptunium(V), plutonium(IV) and (V), americium(III), curium(III), berkelium(III), and californium(III). The physicochemical form, or speciation, of an actinide (e.g., free metal ion, or complexes with inorganic or organic ligands) in natural surface waters can be determined using a range of analytical techniques. However, such methods are seldom commercially available and rarely determine a complete distribution of all metal species. A complementary approach, which is more cost effective, time efficient, and predictive, is the application of geochemical speciation modeling, which calculates the percentage distribution of all actinide species based on known or postulated chemical reactions. The general consensus is that geochemical speciation models can provide useful results if applied correctly with an understanding of the differences between simulated and real systems. This is the first study to use an internally consistent equilibrium database within a geochemical model (WHAM) to calculate the speciation of the first 10 actinides across fresh, estuarine, and marine waters that incorporates natural dissolved organic matter (DOM) (i.e., fulvic acid). The speciation of a metal (actinide) largely governs its uptake and/or toxicity (bioavailability) in aquatic organisms. The general consensus is that bioavailability is best predicted by the concentration of the free metal ion (M^z+) and that metals complexed with most inorganic ligands (e.g., carbonate or sulfate) or natural DOM, typically have low bioavailability. There is also evidence to suggest that colloidal thorium is bioavailable to freshwater organisms. The study correlates observed actinide speciation with bioavailability (where available) or utilizes predicted actinide speciation to determine the likely magnitude of bioavailability, as applied to aquatic organisms. This study also addresses the likely effects of global ocean acidification and increased natural DOM concentrations in fresh surface waters on actinide speciation and bioavailability.