Arsenic (As) contamination in groundwater has received significant attention recently. Natural and
anthropogenic sources contribute to the worldwide occurrence of As contamination. As speciation is an
important factor related to its toxic and mobile behavior. The release of As from soils and sediments into
groundwater is governed by several geophysicochemical processes, of which, As sorption behavior is of
principle significance. This review paper summarizes existing information regarding the effects of natural
organic matter (NOM) on the fate and mobility of As species in the environment. NOM may enhance the
release of As from soils and sediments into the soil solution, thereby facilitating As leaching into the
groundwater. The main influencing mechanisms include competition for available adsorption sites, formation
of aqueous complexes, and/or changes in the redox potential of site surfaces and As redox speciation.
NOM may also serve as binding agents, thereby reducing As mobility. However, comparably little
research has been performed on this aspect. Since most investigations have been done on purified minerals
under laboratory conditions, further research involving various geological materials under natural environmental
conditions is required. Development of proper geochemical conceptual models may provide
means of predicting the role of NOM in arsenic leaching and/or immobilization.