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1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />3.2.1 SALTWATER ENCROACHMENT <br />Saline water contains high concentrations of undesirable dissolved solids <br />such as chlorides. These substances strictly limit its use by causing the <br />water to be nonpotable, toxic to irrigated plants, and corrosive. <br />Fresh water with as little as two percent sea water will not meet recent <br />state drinking water standards. It is imperative that waters of this <br />nature be kept from entering water supply wells. <br />Salt water is slightly heavier than fresh water because it contains more <br />dissolved solids. Along coastal zones the salt water moves inland as a <br />wedge until balanced by sufficient freshwater head. One approximation of <br />the configuration of the saltwater wedge is the Ghyben - Herzberg model (2) <br />which computes the steady-state location of a sharp interface assuming <br />steady, horizontal freshwater flow. This model, which is shown on Figure <br />3-1, simply stated, says that the pressure produced by the fresh water on <br />one side of the interface must equal the pressure produced by the salt <br />water on the other side. <br />This means that for every foot of fresh water above mean sea level, the <br />stationary interface is located 40 feet below sea level. Thus, each foot <br />of fresh water above mean sea level (msl) ideally represents 40 feet of <br />fresh water below, although, under actual dynamic conditions, this <br />relationship may differ. <br />As water is pumped from a well, the water table around the well is lowered, <br />forming a cone of depression. This depressed cone causes the groundwater <br />to flow laterally by gravity toward the well from the surrounding area. As <br />more water is withdrawn from a well or group of wells, the cones of <br />depression will become greater, affect a larger area, and perhaps overlap. <br />Along the coast of Florida, one of the greatest dangers to public water <br />supply wells is to allow the cone of depression to become so large as to <br />induce the saltwater wedge to migrate inland. This can occur during low <br />water table conditions caused by droughts, improper drainage, overpumpage <br />of the well, or a combination of these conditions. Many communities in <br />IRC.VB4 <br />6/4/87 <br />