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The third and present phase of ground- water development on Long Island began in the early 1950's with the introduction of large-scale sewer systems in the more heavily populated areas. The purpose of the sewers was to prevent domestic wastewater from entering the aquifer system because contaminants from this source were being detected in deep public-supply wells. Even though the sewers protect the aquifers from further contamination, they also prevent the replenishment (recharge) that the wastewater had provided to the ground-water reservoir through the domestic wastewater-disposal systems. The wastewater is now diverted to sewage-treatment plants, whose effluent is discharged to the bays and oceans. The decrease in recharge has caused the water table in the sewered areas to be sub- stantially lowered, the base flow of streams to be reduced or eliminated, and the length of perennial streams to be decreased. In Nassau and Suffolk Counties, about 200 cubic feet per second of wastewater (ground water that has been pumped and used) was discharged in 1985 by the three major sewer districts to the surrounding saltwater bodies (Spinello and Simmons, 1992). As previously noted, the only source of freshwater to the system is precipitation. Therefore, the water required to balance the loss from the ground-water system must come primarily from decreases in ground- water discharge to streams and to surrounding saltwater bodies. Capture of ground-water evapo- transpiration, spring flow, and some surface runoff are also possible, but each of these sources is limited to a maximum of approximately 25 cubic feet per second (Figure 9). As the flow to the streams decreases, the headwaters of the streams dry up and the streams become shorter. As the discharge of ground water to surrounding salt- water bodies decreases, saline ground water moves landward as saltwater intrusion. Thus, this case is an example in which the determination of sustainable yields cannot be based solely on predevelopment water budgets. The specific response of the ground-water system to develop- ment must be taken into account in determining the appropriate limits to set on ground-water use. HIGH PLAINS AQUIFER The High Plains is a 174,000-square-mile area of flat to gently rolling terrain that includes parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming. The area is characterized by moderate precipita- tion but generally has a low natural recharge rate to the ground-water system. Unconsolidated allu- vial deposits that form a water-table aquifer called the High Plains aquifer (consisting largely of the Ogallala aquifer) underlie the region. Irrigation water pumped from the aquifer has made the High Plains one of the Nation's most important agricultural areas. During the late 1800's, settlers and specula- tors moved to the plains, and farming became the major activity in the area. The drought of the 1930's gave rise to the use of irrigation and improved farming practices in the High Plains (Gutentag and others, 1984). Around 1940, a rapid expansion in the use of ground water for irrigation began. In 1949, about 480 million cubic feet per day of ground water was used for irrigation. By 1980, the use had more than quadrupled to about 2,150 million cubic feet per day (U.s. Geological Survey, 1984). Subsequently, it declined to about 1,870 million cubic feet per day in 1990 (McGuire and Sharpe, 1997). Not all of the water pumped for irrigation is consumed as evapotranspiration by crops; some seeps back into the ground and recharges the aquifer. Nevertheless, this intense use of ground water has caused major water-level 26