A reliable in-situ probe for dynamic metal speciation in natural waters has been a dream for all scientists that study trace metals in the environment. The objective of this project is to advance toward that goal by developing a better in-situ measurement probe based in the recent advances of electroanalytical techniques. The difficulties connected with development of these probes arise first from the very low concentrations of the trace metals in natural waters (typically below 10-8M); second from the need of bringing the equipment to the field (autonomy problems), third the problems posed by the myriad ligands existing in the natural waters that are able to bind metals (speciation problems) and last but by no means the least the fact that natural waters are generally subject to changing conditions and are practically never at chemical equilibrium (dynamic problems). In natural waters only a small portion of the total dissolved metal exists as free hydrated cations because metal ions form stable complexes with a large variety of dissolved inorganic and organic ligands and also adsorbs onto colloids and suspended matter and can interact with micro-organisms. Most of these show a polyfunctional and a polyelectrolytic character and, thus, have a broad, net range of free energies of complex formation, formation/dissociation rate constants and sizes, hence controlling the bioavailability, toxicity and mobility of the metal ions. As of recently it also become important to take into account the impact of anthropogenic stabilized nanoparticles. A fundamental aspect that arise from the system not being in chemical equilibrium is that a correct interpretation of the fate and environmental impact of metal complexes must consider the importance of the reactivity and fluxes of the metal compounds and the relative time scales of these processes. Due to their environmental relevance lead, cadmium, copper, zinc and silver will be the main trace metals focused in our study. Our approach to develop a better in-situ dynamic sensor is to use the potential of recently developed stripping electroanalytical techniques, Scanned stripping chronopotentiometry (SSCP) and Absence of gradients and nernstian equilibrium stripping technique (AGNES). To validate an analytical procedure it is necessary to compare its results with a well-established method. For this purpose we will include in our in-situ probe a Donnan Membrane device (DMT). It is necessary to understand the signal obtained and validate the analytical methods used. In complex matrices this implies a three way approach: first the analysis of increasingly more complex model systems in the laboratory, second the study of a pilot-scale environment spiked with the model systems and finally in-situ investigation of different field situations (usually different seasons). To understand the results it is fundamental to apply models that can describe and predict the behavior of the system. Thermodynamic modeling will include the NICA-Donnan model to account for metal ion organic ligand interactions and the CD-MUSIC model to account for metal binding onto mineral surfaces. Dynamic modeling will also be applied to develop a flux transport model to discriminate which parameters control metal ion speciation in complex aquatic systems. In summary, the objectives of the project are: 1.-Build and develop an in-situ measurement probe based in SSCP/AGNES/DMT, 2.-Perform laboratory studies in well characterized complex mixtures of polymers, nanoparticles and particles to understand the response of our techniques, 3.-Test our in-situ probe during a pilot-scale study 4.-Method validation of the in-situ probe in field samples taken in different seasons, 5.-Describe the metal binding data obtained in the field studies using thermodynamic and kinetic models.