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Waifer Supply a 4 Pollution Control 3-26. SALT -WATER INTRUSION second edition Salt -water contamination of fresh -water aquifers presents a serious water -quality problem in island locations; in coastal areas; and occasion- ally inland, as in Arizona, where some aquifers contain highly saline waters. -Because fresh water is lighter than salt water (specific . ravity of sea water is about 1.035) it.%vill usually float above a layer-of.sall water. When an aquifer is pumped, the original equilibrium'•is-disturbed and salt, water replaces the fresh water. Under cquilibriuni:c6nditions a drawdown of L ft in the fresh -water table will result -in* a rise -or approximately 40 ft by the salt water. Pumping rates of wells subject to salt -water intrusion are therefore seriously limited. In coastal areas, recharge. wells are somtimes used in an attempt to main:+in a sufficient head to prevent sca-water intrusion. Injection wells have been used effectively in this manner in Southern California. A prime example of fresh -water contamination .by sea •water is noted in Long Island, New York.30 During the first part of this eentuiy the rate of pumping far exceeded the natural recharge rate. -The problem was further complicated because storm -water -runoff from the highly' de- veloped land areas was transported-directly'to the sea. This precluded the opportunity for this water-to-rcturn-to the -ground. A-s"pumping con- tinued, the water table dropped well below sea level and saline. water in— truded the aquifer. The result was such .a.serious -impairment of the local water quality that Long Island was forced to transport its water supply from upper_New York State. 3-27. GROUNDWATER RECHARGE The volumes of groundwater replaced annually through natural mechanisms are relatively small because of the slow rates of movement of groundwaters and the limited opportunity for surface waters to penetrate the earth's surface. To supplement this natural recharge process, a recent trend toward artificial recharge has been developing. In 1955 over 700 million gallons of water per day were artifically recharged in the United States." This water was derived from natural surface sources, returns from air conditioning, industrial wastes, and municipal water supplies. The total recharge volume was equal, however, to only about 1.5 percent of the groundwater withdrawn that year. In California, for example, arti- ficial recharge is presently a primary method of water conservation. Dur- ing the 1957-1958 period a daily recharge volume of about 560 million gallons was reported for 63 projects in that state alone.26 Numerous methods are employed in artificial recharge operations. One of the most common plans. is the utilization of holding basins. The usual practice into impound the water in a series of reservoirs arranged -so that the overflow of one will enter the next, and so on' These artificial storage works are generally -formed by the -construction of -dikes -or levees. A second method is the modified stream bed which makes use of the nat- ural water supply. The stream channel is widened, leveled, scarified, or treated by a combination of methods to increaseits recharge .capabilities. Ditches and furrows are also used. The basic types of arrangement are the contour type, in which the ditch follows the contour of the ground; the lateral type, in which water is diverted into a number of small furrows from the main canal or channel; and the tree -shaped or branching type, where water is diverted from the primary channel into successively smaller canals and ditches. Where slopes are relatively flat and uniform, flood- ing provides an economical means of recharge. Normal practice is to spread the recharge water over the ground at relatively small depths so as not to disturb the soil or native vegetation. An additional method is the use of injection wells. Recharge rates are normally less than pumping rates for the same head conditions, however, due to clogging which is often encountered in the area adjacent to the well casing.25 Clogging may result from the entrapment of fine aquifer particles, from suspended ma- terial in the recharge water which is subsequently strained out and de- posited in the vicinity of the well screen, from air binding, from chemical reactions between recharge and natural waters, and from bacteria. For best results the recharge water should be clear, contain little or no so- dium, and be chlorinated. Richter and Chun have outlined several factors worthy of consid- eration relative to the selection of artificial recharge project sites.26 Es- sentially they are availability and character of local and imported water supply; factors relating to infiltration rates of the native soils; operation and maintenance problems; net benefits; water quality; and legal con- siderations. MAR 9 1,97 John W. Clark Professor of Citi/ F. ngineerirrg V.ttexico.Stntr University Warren Viessman, Jr. Director, Water Resources Research Institute and Professor of Civil F.ngindering University of Xe hmsko Mork J. Hammer Professor of C"111 Engineering University ofN broska International Textbook Company Cnlleoe nivisinn - Inle•rt Educational Publishers Scranton - Toronto - London