1 Fate of arsenic, phosphate and ammonium plumes in a coastal aquifer affected by saltwater intrusion N. Colombani a , M. Mastrocicco b.# , H. Prommer c.d.e , C. Sbarbati a and M. Petitta a a Department of Earth Sciences, “Sapienza” University, Roma, Italy b Department of Physics and Earth Sciences, University of Ferrara, Ferrara, Italy c CSIRO Land and Water, Private Bag No. 5, Wembley WA 6913, Australia d School of Earth and Environment, University of Western Australia, 35 Stirling Hwy, Nedlands WA 6009, Australia e National Centre for Groundwater Research and Training (NCGRT), Flinders University, Adelaide, GPO Box 2100, SA 5001, Australia # corresponding author, e-mail: mtm@unife.it Abstract A severe groundwater contamination with extensive plumes of arsenic, phosphate and ammonium was found in a coastal aquifer beneath a former fertilizer production plant. The implementation of an active groundwater remediation strategy, based on a comprehensive pump and treat scheme, now prevents the migration of the dissolved contaminants into the marine environment. However, due to the site’s proximity to the coastline, a seawater wedge was induced by the pumping scheme. Additionally the groundwater flow and salinity patterns were also strongly affected by leakage from the site’s sewer system and from a seawater-fed cooling canal. The objective of this study was to elucidate the fate of arsenic and its co-contaminants over the site’s history under the complex, coupled hydrodynamic and geochemical conditions that prevail at the site. A detailed geochemical characterisation of samples from sediment cores and hydrochemical data provided valuable high- resolution information. The obtained data were used to develop various conceptual models and to constrain the development and calibration of a reactive transport model. The reactive transport simulations were performed for a sub-domain (two-dimensional transect) of an earlier developed