Zero-valent iron can be used to remove inorganic arsenic from aqueous solutions. The mechanism for arsenic removal appears to be a surface precipitation or adsorption of arsenic with iron. This mechanism differs significantly from the removal mechanisms of chlorinated hydrocarbons (reductive dechlorination) and chromium(VI) reduction accompanied by hydroxide precipitation. Arsenic removal efficiencies of greater than 95% are observed in laboratory and field-column studies. Arsenate is removed more effectively than arsenite; however, effective arsenite removal occurs even under anoxic conditions. The removal efficiency is related to the surface area or the type of iron used and improved over time, possibly due to pitting of the iron surface and increased surface area for sorption due to iron corrosion and ferrous iron adsorption/precipitation. Preliminary results indicate that arsenic is tightly bound to the iron filings, and only a small percentage is readily leached. This finding coupled with the results of field experiments and spectroscopic analysis (SEM/EDX and XPS) provide evidence that surface precipitation is the predominant removal mechanism. Key words: Arsenic removal; zero-valent iron

Temporary streams are those water courses that undergo the recurrent cessation of flow or the complete drying of their channel. The structure and composition of biological communities in temporary stream reaches are strongly dependent on the temporal changes of the aquatic habitats determined by the hydrological conditions. Therefore, the structural and functional characteristics of aquatic fauna to assess the ecological quality of a temporary stream reach cannot be used without taking into account the controls imposed by the hydrological regime. This paper develops methods for analysing temporary streams' aquatic regimes, based on the definition of six aquatic states that summarize the transient sets of mesohabitats occurring on a given reach at a particular moment, depending on the hydrological conditions: Hyperrheic, Eurheic, Oligorheic, Arheic, Hyporheic and Edaphic. When the hydrological conditions lead to a change in the aquatic state, the structure and composition of the aquatic community changes according to the new set of available habitats. We used the water discharge records from gauging stations or simulations with rainfall-runoff models to infer the temporal patterns of occurrence of these states in the Aquatic States Frequency Graph we developed. The visual analysis of this graph is complemented by the development of two metrics which describe the permanence of flow and the seasonal predictability of zero flow periods. Finally, a classification of temporary streams in four aquatic regimes in terms of their influence over the development of aquatic life is updated from the existing classifications, with stream aquatic regimes defined as Permanent, Temporary-pools, Temporary-dry and Episodic. While aquatic regimes describe the long-term overall variability of the hydrological conditions of the river section and have been used for many years by hydrologists and ecologists, aquatic states describe the availability of mesohabitats in given periods that determine the presence of different biotic assemblages. This novel concept links hydrological and ecological conditions in a unique way. All these methods were implemented with data from eight temporary streams around the Mediterranean within the MIRAGE project. Their application was a precondition to assessing the ecological quality of these streams.