Saltwater intrusion and salinization are characterized by the increase of chloride ion concentrations in freshwater aquifers. This occurs predominantly along coastlines, though there are instances of inland salinization. Saltwater intrusion can have several causes, some of which are natural and some are induced by human activities. Once saltwater intrusion has occurred, it is almost impossible to reverse, making this a significant threat to freshwater resources. Mitigation strategies that are designed to slow or halt the rate of saltwater intrusion can be expensive but are necessary to protect the water resources from more damage. Among the natural causes of saltwater intrusion are storm surges caused by hurricanes and other tropical systems. This was seen in 2005 with Hurricanes Katrina and Rita along the Gulf Coast of the United States. In these cases, tidal flushing removed some of the introduced salinity in the freshwater marshes before substantial damage was done. In areas away from the influences of tidal flushing, and in areas where precipitation did not flush the salt water from the water table, there was considerable damage to the freshwater systems (Steyer, et al., 2007). Human induced saltwater intrusion can occur in a couple of different way. One way is by dredging canals in coastal zones that allow saltwater to migrate farther inland than it had been able to previously. This introduced saltwater can wreak havoc on freshwater systems. Areas in Louisiana have in particular experienced this type of saltwater intrusion. One of the most dramatic forms of saltwater intrusion occurs in coastal areas that are dependent upon groundwater for their potable water and irrigation needs. The most common scenario involves the overpumping of the freshwater aquifer. This reduces the head difference at the saltwater-freshwater interface and induces the flow of saltwater into the freshwater system. This is often exacerbated by insufficient recharge to the freshwater aquifer, which can occur in times of drought. The freshwater aquifers “float” on top of the saltwater at the interface due to density differences in the two respective water sources. The saltwater tends to form a wedge under the freshwater that extends inland. As saltwater intrusion occurs, this wedge extends further inland and is seen at shallower depths. The result is that wells that previously produced freshwater can see an increase in chloride concentration that makes the well unusable for irrigation or potable uses. In southwest Florida, the Southwest Florida Water Management District established the Southern Water Use Caution Area to mitigate for saltwater intrusion, among other concerns, all due to the overpumping of the Floridan Aquifer. The saltwater intrusion cannot be stopped, but the rate at which it occurs can be slowed. The Water Management District’s Recovery Strategy includes reducing pumping of groundwater from the Floridan Aquifer. Efforts to find alternative water sources to the Florida Aquifer have proved costly and in some cases not reliable in an area that receives most of its rainfall in four months of the summer. Programs and funding have been established to reduce the pumping from this freshwater source (Southwest Florida Water Management District, 2004). California, and San Luis Obispo County in particular, has been quite innovative in their attempts to stem saltwater intrusion. They have implemented a strategy that uses treated wastewater to recharge aquifers to mitigate for saltwater intrusion. Other elements of their plan include replacing groundwater with treated effluent for agricultural purposes and urban reuse. A mitigation plan for saltwater intrusion in coastal communities would involve a multi-pronged approach. In areas where the salinization is due to storm surge, coastal barriers could be built to prevent or slow the storm surge. Where saltwater has been introduced due to storm surge, some manner of opening the barriers would be needed to allow the saltwater to be flushed from the freshwater system. This would entail a substantial engineering requirement. Opponents to this tactic would argue that engineering a natural process would cause more harm due to the impacts the structures would have on natural systems. However, the loss of wetland habitat affects many things including tourism in coastal communities. The jobs that could be created by the need to engineer, build, and monitor the structures would also help the coastal communities. In areas that are experiencing saltwater intrusion due to overpumping of the overlying freshwater aquifers, the mitigation solution involves cooperation at multiple levels. The main problem of pumping groundwater at a rate that is not sustainable must be addressed. Conversely, the impact of the saltwater intrusion must be quantified. This could be done by estimating how many wells have already been impacted and what the projection is for future impacts given current pumping rates. It may be possible that the cost of reducing groundwater withdrawals is much more than the cost of replacing wells that have been impacted by saltwater intrusion. The development of alternative sources of water would need funding for both research and capital improvements in communities where saltwater intrusion is an issue. With potential sea level rises from climate change and global warming, saltwater intrusion and salinization will continue to be an important water resource issue. Advances in technology and innovations in how we use our water resources will be key in the mitigation of saltwater intrusion.

References
Southwest Florida Water Management District, 2004. Southern Water Use Caution Area Recovery Plan. Retrieved February 17, 2009, from http://www.watermatters.org

Steyer, G., 2007. Potential Consequences of Saltwater Intrusion Associated With Hurricanes Katrina and Rita. Retrieved February 17, 2009, from http://pubs.usgs.gov/circ/1306/pdf/c1306_ch6_c.pdf