Arsenic, long synonymous in people's mind with poison exhibits a varied, fascinating and dynamic biogeochemistry. Chemically and biologically reactive, its chemical form, or speciation, changes with slight variations in chemical or biological conditions. Depending upon the extent to which any arsenic containing system is dominated by physical/chemical or biological process, the forms of arsenic may change between the various in organic and methylated species, and may alter rapidly with varying conditions. Early research revolved around the formulation of pigments, and later in the development of effective medicines. Later still, thanks due to its long history as a poison, arsenic was included in numerous agricultural practices, mainly as a herbicide or pesticide. It has also seen service in the rather more specialised field of chemical warfare, and still poses threats as a result of improper disposal. Much of the recent research has focused on the identification of previously unknown organoarsenic species found in estuarine and marine waters. This work is building up an understanding of the biological pathways involved in the biochemical cycling of arsenic. Little work has been carried out with respect to the cycling of arsenic in freshwaters in comparison to that in marine and estuarine waters. Similarly, there has been less work performed on the speciation of arsenic in freshwater sediment interstitial waters, than there has on marine sediments, or intertidal sediments. The characterisation of arsenic in dynamic porewater poses a set of unusual and difficult problems, not the least being the procurement of representative, discrete samples. A number of potential sampling methods are reviewed, and variations on the thin film gel sampling technique are drought to provide perhaps the best option, although this will depend upon the type of intertidal sediment being investigated, and the information sought. It may be impossible to propose a general model of arsenic cycling either at a local scale or at a global level. This is of course due to the great diversity in ecosystems, each having different controls over arsenic speciation, and containing different biological communities. Once a given system has been described, the patterns of arsenic speciation (both spatially and temporally) are explainable, and potential impacts can be identified, but (hey cannot be transferred to another system. The continuing accumulation of information regarding arsenic speciation in different systems is helping in the unravelling of the larger global arsenic cycle. Such an understanding can only be a benefit in the development of safe and efficient remediation schemes for contaminated soil and aquatic systems
