Saltwater intrusion

The term saltwater intrusion ( intrusion, noun from the Latin verb intrudere, dt literally Into shock ) the intrusion of salt water is called into coastal freshwater aquifers. This process occurs because of the difference in density of salt and fresh water in a natural way on almost all coasts from which adjoin the sea water. Also a natural way can cause the intrusion of salt water into coastal areas storm surges. In many cases, this process is beyond today produced by Groundwater extraction from coastal wells, or caused by the construction of navigation channels. The channels offer the salt water one way to come into contact with fresh water in coastal plains.

Effects on drinking water

When groundwater is removed faster by taking, as it can flow, the water table is lowered. This reduction decreases the hydrostatic pressure. If this happens near a seashore, then the underground inflow of water from the sea possible, and the groundwater is contaminated with salt water. This process takes place today in many coastal locations from, for example, in the coastal states of the United States.

Hydrology

Salt water intrusion is also not disturbed by groundwater extraction condition a normal process in many groundwater bodies in the vicinity of the ocean. The salt water has a higher density than fresh water, so that the pressure under a salt water column is slightly higher than that with the same high freshwater column. If the groundwater body and the salt water bodies are connected to each other, this results in depth to a water flow from the denser (salt water) to the less dense (fresh water) medium until the pressure ratios are balanced.

This landward directed Ausgleichsfluß of salt water is confined to areas near the coast. Further inland, the level of groundwater is higher because the land surface is higher there as a rule, so that the higher pressure of the fresh water enough there to resist the landward pressure of the salt water. The higher water level of the fresh water further inland also generates an ocean northward directed flow in the upper part of the aquifer. At the land-sea boundary flows as in the upper part of the aquifer freshwater seaward, in the lower part sea water is in hydrostatic equilibrium with the overlying freshwater. The saltwater intrusion is therefore wedge-shaped.

The extraction of fresh water from the aquifer disturbs this balance in that it reduces the pressure of the fresh water so that salt water penetrates landward. This can cause the salt water reaches the sampling points, so that promote these brackish water and are no longer usable for drinking or irrigation purposes. To avoid such effects, the groundwater in coastal areas in many countries is closely monitored, and the flow of groundwater is estimated by numerical models.

The Ghijben - Herzberg equation

After the Frenchman Joseph Commun you had the principle described in a paper in 1828 for the first time, the physical formula has been independently to calculate the salt water intrusion from each other 1888/89 by the two Dutch military and Willem J. Drabbe Badon Ghyben ( 1845-1907, also Willem Badon Ghijben written ) and published in 1901 by the German government building officer Alexander Herzberg. They developed this analytical solutions as well as possible approximate description of the behavior of intrudierendem salt water based on a number of assumptions, which do not, however, apply in all cases. The formula found of them is called Ghijben - Herzberg equation, isolated as DGH effect. The effect follows the Archimedean principle.

The diagram illustrates the formula :; are doing

• : For the amount of fresh water above sea level
• : For the amount of fresh water below sea level

The two values ​​and are on their respective densities (fresh water: 1.0 grams per cubic centimeter (g/cm3 ) at 20 ° C) and connected ( 1.025 g/cm3 salt water). The equation can be simplified to the expression. In this form shows the Ghijben - Herzberg equation that in a non-limited aquifer every meter of fresh water above sea level a water column of 40 m below the same equivalent - similar to the appearance of floating in salt water iceberg, where in each case only a fraction of its mass visible is.

The modeling by computer nowadays allows the application of numerical methods ( in general, the method of finite differences or finite elements ) that are better adapted to the specific conditions of a site.

Modeling of salt water intrusion

The modeling of salt water intrusion is difficult for several reasons. Typical problems in the modeling are:

• The incidence of fractures and joints in water-bearing rock. Their presence or absence has a major influence on the penetration of salt water, their size and position in space is not exactly known, however.

• The occurrence of different hydraulic characteristics on a small scale. You may have also a great influence on the behavior of the aquifer, but can not be detected by the model due to their small size.

• The change in the hydraulic properties of the salt water intrusion. A mixture of salt and fresh water is often undersaturated with respect to calcium, dissolved in the mixing zone of calcium from the solution and thus changes the hydraulic properties.

• Known as the cation-exchange process, the advance and retreat of salt water slows down, and makes accurate calculations difficult.

• The fact that a saltwater intrusion is usually in motion, not in equilibrium, complicates the control of modeling sets of data for water level or the pumping rates.

• In long-term modeling the long-term evolution of the climate is not known. The models, however, react as sensitive to sea level and the rate of groundwater recharge, the change can not be predicted accurately according to current knowledge changes.

Prevention of salt water intrusion at locks

Salt water intrusion can be a problem in the field of locks, where salt water meets with fresh water. Special locks such as the Hiram M. Chittenden Locks in Washington are equipped with a reservoir from which the pumped out of the locks salt water is collected and can be pumped back into the salt- carrying part of the water. At the Hiram M. Chittenden Locks in addition, a small part of the salt water is pumped to the fish ladder to make the migratory fish for upriver attractive.

Areas with active salt water intrusion

Active salt water intrusion is taking place in many coastal areas of the world. Examples of problem areas in terms of water supply can be found for example in Benin, Cyprus, Morocco, Pakistan, Tunisia and the Gulf of Bohai in China. In the United States, for example coastlines of Florida and Georgia, the surrounding area of ​​San Leandro, California, and the area of Lake Pontchartrain in Louisiana are affected.