An investigation of future saltwater intrusion into the coastal aquifer, Partido de la Costa, province of Buenos Aires

CARRETERO SILVINA; RAPAGLIA JOHN

Congreso; XVIII Congreso Geologico Argentino; 2011

According to the IPCC fourth assessment report (2007) sea level rise is one of the more certain effects of global climate change. Salt water intrusion into coastal aquifers is an important impact of sea level rise, which has received some attention among the coastal scientific community in recent years (e.g. Döll 2009, Ranjan et al. 2009, Sherif and Singh 1999, Werner and Simmons 2009). Sandy coastal aquifers, especially those on barrier islands, have a strong hydraulic connection with the sea. High permeability and hence high conductivity allow for the penetration of salt water into the aquifer bottom. Ghyben (1888) and Herzberg (1901) describe the width of the freshwater lens to be about 41 times the height of the water table above mean sea level, with freshwater floating on denser saltwater. If more saltwater penetrates below the island, and the freshwater lens cannot migrate, the freshwater lens will shrink thereby reducing the quantity of potable water available. As these aquifers are often the most important/only source of potable water for coastal residents. It is, therefore, of critical importance to understand how changing sea levels will impact the groundwater quality in coastal aquifers. In addition to sea level rise increasing population is likely to increase the demand for potable water in these coastal aquifers, further exacerbating their susceptibility to salt water intrusion. We follow the methodology proposed by Werner and Simmons (2009) following Custodio (1987) to quantify the increase in the salt water intrusion length into the coastal aquifer at Partido de la Costa, Buenos Aires, Argentina. Partido de la Costa is located in a barrier system with a fringe of coastal dunes underlain by a sandy phreatic aquifer with a thickness between 7 and 18 m. From a hydrologic point of view, the area belongs to the Costera Region (González, 2005).Currently the more than 70,000 residents inhabit Partido de la Costa, 100% of whom are dependent upon the shallow aquifer for their potable water supply. The fresh water lens is limited by sea water to the east and brackish continental water to the west. System recharge is solely from precipitation (avg. 1000 mm y-1) and precipitation minus evapotranspiration is calculated to be 230 mm yr-1.Utilizing known hydrogeological parameters and an estimate of population growth we are able to calculate the future salt water intrusion length pending different values of sea level rise. Two algorithms were used to calculate the salt water intrusion length. The constant flux model, which does not decrease the freshwater lens but shifts it upward, suggests that the salt water intrusion length will increase by 25, 33, and 38 meters pending these aquifer recharge scenarios and considering a one meter sea level rise. Results from the constant head model, which is based upon a reduction in the size of the aquifer with sea level rise, find a much greater increase in the salt water intrusion length; 298, 333, and 357 meters, for these parameters respectively.With this information we attempt to calculate the reduction in the volume of potable water and suggest whether the coastal aquifer will be able to continually supply Partido de la Costa with sufficient potable water. If the constant flux model is the case the coastal aquifer under investigation will not be greatly affected by different sea level rise or water consumption scenarios. However, if the constant head model is the case (which is more likely due to the low elevation of the land surface) an increase in salt water intrusion length by more than 200 meters will severely degrade the aquifers, forcing coastal managers to utilize a different water source for their residents