Impacts of Seawater Rise on Seawater Intrusion in the Nile Delta Aquifer, Egypt by Ahmed Sefelnasr 1 and Mohsen Sherif 2 Abstract Several investigations have recently considered the possible impacts of climate change and seawater level rise on seawater intrusion in coastal aquifers. All have revealed the severity of the problem and the significance of the landward movement of the dispersion zone under the condition of seawater level rise. Most of the studies did not consider the possible effects of the seawater rise on the inland movement of the shoreline and the associate changes in the boundary conditions at the seaside and the domain geometry. Such effects become more evident in flat, low land, coastal alluvial plans where large areas might be submerged with seawater under a relatively small increase in the seawater level. None of the studies combined the effect of increased groundwater pumping, due to the possible decline in precipitation and shortage in surface water resources, with the expected landward shift of the shore line. In this article, the possible effects of seawater level rise in the Mediterranean Sea on the seawater intrusion problem in the Nile Delta Aquifer are investigated using FEFLOW. The simulations are conducted in horizontal view while considering the effect of the shoreline landward shift using digital elevation models. In addition to the basic run (current conditions), six different scenarios are considered. Scenarios one, two, and three assume a 0.5 m seawater rise while the total pumping is reduced by 50%, maintained as per the current conditions and doubled, respectively. Scenarios four, five, and six assume a 1.0 m seawater rise and the total pumping is changed as in the first three scenarios. The shoreline is moved to account for the seawater rise and hence the study domain and the seaside boundary are modified accordingly. It is concluded that, large areas in the coastal zone of the Nile Delta will be submerged by seawater and the coast line will shift landward by several kilometers in the eastern and western sides of the Delta. Scenario six represents the worst case under which the volume of freshwater will be reduced to about 513 km 3 (billion m 3 ). Introduction Groundwater represents the main source of freshwater in many countries around the globe. This is typically true in arid and semi-arid regions, where rainfall is scarce, ran- dom and insignificant and surface water bodies are limited and sometimes absent. Because of the high population density near shorelines and the tendency of people to live in, and develop the, coastal regions, coastal aquifers are generally exposed to extensive groundwater pumping which, in most cases, exceeds natural replenishment rates. Seawater intrusion in coastal aquifers is a common problem and is encountered, with different degrees, in almost all coastal aquifers. It is regarded as a natural process that might be accelerated or retarded by external factors such as increase or decrease in the groundwater pumping, irrigation and recharge practices, land use, 1 Geology Department, Faculty of Science, Assuit University, 71516 Assuit, Egypt; ahmed.sefelnasr@yahoo.com 2 Corresponding author: Department of Civil and Environ- mental Engineering, College of Engineering, United Arab Emirates University, P.O. Box # 15551, Al Ain, United Arab Emirates; (971) 3 7621694; msherif@uaeu.ac.ae Received October 2012, accepted February 2013.  2013, National Ground Water Association. doi: 10.1111/gwat.12058 and possible seawater rise due to the impacts of global warming. The seawater intrusion problem has been under inves- tigation for well over a century (Werner and Simmons 2009; Post and Abarca 2010). A comprehensive review on different aspects of seawater intrusion assessment, moni- toring, and modeling is provided by Bear et al. (1999). The Intergovernmental Panel of Climate Change estimated that the global mean seawater levels have risen by 10 to 20 cm during the last century. It is also estimated that during this century the seawater level rise will be in the range of 11 to 88 cm (IPCC 2001, 2007). The increase in the seawater level, due to climate change, will have major impacts on groundwater resources in coastal aquifers. First, the shoreline will shift to a new landward position and depending on the land topography this shift might be significant and the groundwater in the affected zone will become completely saline. Second, the increase in the seawater level would cause additional pressure head at the seaside and, hence, the seawater water would advance more inland. Third, climate change may cause variations in rainfall which would affect the natural replenishment of groundwater. Fourth, due to the anticipated reduction in rainfall and surface water resources in arid and semi-arid regions, the reliance on, 264 Vol. 52, No. 2 – Groundwater – March-April 2014 (pages 264 – 276) NGWA.org