Solution of Salt Water Intrusion in Ground Water in Coastal Areas

Salt Waters and Fresh Waters Relations in Coastal Area:Along coastal belt, a natural equilibrium between fresh and salt ground water is developed. The fresh water floats on the sea water as the specific gravity of sea water as the specific gravity of sea water is about 1.025. Hydrostatic equilibrium would require a fresh water column about 1.025 times as high as a salt water column, i.e. 1 ft of fresh water would exist above sea level for each 40 ft below sea level. Condition of hydrostatic equilibrium do not occur, however, because of hydraulic gradient imposed by the sloping water table. Magnification of the interfaces near sea level shows that fresh water is flowing out of the fresh water aquifer through a seepage face and across a portion of the ocean bottom into the ocean.

Thus the true shape of the interface is governed by hydrodynamic balance of the fresh and salt water. For most conditions, however, the 1/4o ratio rule may be applied without introducing serious error.

Problems Associated with Pumping:

Equilibrium is disturbed, if the water table is lowered by pumping and an inverted core of salt water rises under the well. For equilibrium the salt water rises approximately 40 ft for each fact of drawdown in the fresh water. This severely limits the pumping rates of wells along coastlines.

Solution :

Horizontal collectors or radial wells which operate with a small drawdown need not extend below sea level to avoid pumping salt water. This technique is widely used for the development of water supply on islands. Overdraft of the fresh ground-water may reduce the seaward gradient and permit the salt water to advance inland. Recharge wells along the coast have tried as a means of maintaining adequate fresh water to avoid saltwater intrusion. To prevent clogging from bacterial slimes and thus maintain satisfactory recharge, the water is chlorinated to about 6 mg/l . Some of the recharged water is wasted to the sea as part of the cost of salt water control. A similar problem is encountered in inland areas where salt water may remain in sedimentary rocks formed beneath the ocean or may have formed by solution of salts from surrounding rocks. Under these conditions pumping in adjacent freshwater aquifers must be limited to amounts which will not permit intrusion of the mineralized water.