areas such as Washington State, intrusion only reaches portions of the aquifer, affecting only certain water supply wells. Other aquifers have faced more widespread salinity contamination, significantly affecting groundwater supplies for the region. For instance, in Cape May, New Jersey, where groundwater extraction has lowered water tables by up to 30 meters, saltwater intrusion has caused closure of over 120 water supply wells since the 1940s. Ghyben–Herzberg relation The first physical formulations of saltwater intrusion were made by in 1888 and 1889 as well as in 1901, thus called the Ghyben–Herzberg relation. They derived analytical solutions to approximate the intrusion behavior, which are based on a number of assumptions that do not hold in all field cases. In the equation, the thickness of the freshwater zone above sea level is represented as and that below sea level is represented as . The two thicknesses and , are related by and where is the density of freshwater and is the density of saltwater. Freshwater has a density of about 1.000 grams per cubic centimeter (g/cm3) at 20 °C, whereas that of seawater is about 1.025 g/cm3. The equation can be simplified to . The Ghyben–Herzberg ratio states that, for every meter of fresh water in an unconfined aquifer above sea level, there will be forty meters of fresh water in the aquifer below sea level. In the 20th century the vastly increased computing power available allowed the use of numerical methods (usually finite differences or finite elements) that need fewer assumptions and can be applied more generally. Modeling Modeling of saltwater intrusion is considered difficult. Some typical difficulties that arise are: The possible presence of fissures and cracks and fractures in the aquifer, whose precise positions and extents are unknown but which have great influence on the development of the saltwater intrusion The possible presence of small scale heterogeneities in the hydraulic properties of the aquifer, which are too small to be taken into account by the model but which may also have great influence on the development of the saltwater intrusion The change of hydraulic properties by the saltwater intrusion. A mixture of saltwater and freshwater is often undersaturated with respect to calcium, triggering dissolution of calcium in the mixing zone and changing hydraulic properties. The process known as cation exchange, which slows the advance of a saltwater intrusion and also slows the retreat of a saltwater intrusion. The fact