Table of Contents for this manuscript 24th Salt Water Intrusion Meeting and the 4th Asia-Pacific Coastal Aquifer Management Meeting, 4 –8 July 2016, Cairns, Australia Modeling of unsaturated-saturated groundwater flow in the presence of saltwater intrusion using HYDRUS and MODFLOW numerical codes Adam Szymkiewicz1, Anna Gumuła-Kawęcka1, Jirka Šimůnek2, Bertrand Leterme3, Beata Jaworska-Szulc1 , Małgorzata Pruszkowska Caceres1 , Wioletta Gorczewska-Langner1, Rafael Angulo-Jaramillo4, Diederik Jacques3 1Faculty of Civil and Environmental Engineering, Gdańsk University of Technology, Gdańsk, Poland 2University of California, Riverside, CA, USA 3Institute for Environment, Health and Safety, Belgian Nuclear Research Centre, Mol, Belgium 4LEHNA UMR 5023, Lyon, France / ENTPE, Vaulx-en-Velin, France ABSTRACT Groundwater recharge through the vadose zone can be an important factor affecting the salinity patterns in coastal aquifers, especially in environments such as islands or peninsulas. On the Polish Baltic coast regions such as Hel peninsula, Vistula spit or Wolin island, which host multiple tourist resorts, are potentially vulnerable to the degradation of groundwater quality and quantity due to saline water intrusion, changes of the recharge patterns and contamination from ground surface. There is a need for more extensive including of the complex flow and transport processes occurring in the vadose zone in numerical models of groundwater flow, while keeping their demand for computational power on a reasonable level. One possible solution is coupling one-dimensional models for the unsaturated zone with a three-dimensional saturated groundwater flow model. An example of such a coupled model is given by the Hydrus for Modflow package (Seo et al. 2007). In this work we evaluate the Hydrus for Modflow package in conjunction with SWI2 package (Bakker et al. 2013), which models saltwater intrusion using a simplified approach based on density stratification. We perform numerical simulations for a 2D setting representing a typical cross-section of a peninsula on the Baltic coast. In particular we focus on the effect of uncertainty in the estimation of the vadose zone hydraulic parameters. KEYWORDS: numerical modeling, vadose zone, groundwater recharge, shallow aquifer ACKNOWLEDGEMENTS This work has been supported by National Science Centre, Poland in the framework of the project 2015/17/B/ST10/03233 "Groundwater recharge on outwash plain". REFERENCES Bakker, M. et al. 2013. Documentation of the Seawater Intrusion (SWI2) Package for MODFLOW. USGS Techniques and Methods 6–A46. Seo, H.S., J. Šimůnek, E. Poeter. 2007. Documentation of the Hydrus package for Modflow-2000, the U.S. Geological Survey Modular Ground-water Model. Contact Information: Adam Szymkiewicz, Gdańsk University of Technology, Faculty of Civil and Environmental Engineering, ul. Narutowicza 11/12, 80-233 Gdańsk, Poland Phone: +48-58-347-1085, Email: [email protected] 77.