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The salinity of the water supply of Barbers Point Naval Air Station has increased markedly since 1983. The Naval Air Station obtains its water, about 3 million gallons per day, from Barbers Point shaft, a water shaft that taps the Waianae part of the Pearl Harbor aquifer underlying the dry, southeastern flank of the Waianae mountains on the island of Oahu, Hawaii. From 1983 to 1985 the chloride concentration of the water, indicative of the overall salt content, increased from 220 to 250 mg/L (milligrams per liter) and has remained near that level through 1986. The U.S. Environmental Protection Agency has established 250 mg/L as the maximum recommended chloride concentration in drinking water because above that level many people can taste the salt. Barbers Point shaft draws water from a large lens-shaped body of freshwater, which floats on the saltwater that has intruded into the island from the ocean. The lens is several hundred feet thick and is separated from the underlying saltwater by a thick transition zone in which the chloride concentration increases with depth. The shaft is designed to skim water from the surface of the freshwater lens to minimize the possibility of drawing water from the transition zone. Prior to 1983, the top 100 to 200 feet of the lens near Barbers Point shaft contained water with a chloride concentration of about 200 mg/L. The high chloride concentration in shallow ground water at all wells in the area indicates that most of the salts in the freshwater lens are contributed by rainfall, sea spray, and irrigation return water. At Barbers Point shaft, pumping may draw a small amount of saltwater from the transition zone and increase the chloride concentration in the pumped water by about 20 mg/L. Salinity of the lens decreases progressively inland in response to recharge from relatively fresher water and in response to an increasing lens thickness with increasing distance from the shoreline. The increase, in 1983, in the chloride concentration of water at the shaft was most probably the result of saltier recharge water reaching the water table, and not the result of increased mixing of underlying saltwater with the freshwater. The chloride concentration of the recharge water has probably increased because, in 1980, the drip method of irrigation began to replace the furrow method on sugarcane fields near the shaft. Drip irrigation applies water directly to the base of the plant at a low but continuous rate through small tubes. With the furrow method, the plants receive water about twice a month when the rows of furrows that cross the fields are flooded. The amount of water and dissolved salts applied to the fields by the two methods of irrigation differs by only about 15 percent, but when the drip method is used, much less water percolates past the root zone of the plants and recharge to the aquifer is reduced. While the plants use a large fraction of the irrigation water, most of the salts remain in the unused fraction of irrigation water that recharges the aquifer. Relative to the recharge resulting from furrow irrigation, the recharge water resulting from drip irrigation has a higher chloride concentration because the salts are dissolved in less water. The recharge from irrigation return flow provides a large fraction of the ground water that flows through the aquifer, and as a result an increase in the salinity of the irrigation return water has resulted in a measurable increase in the salinity of ground water. A mixing-cell model was used to estimate the effect of drip irrigation on the chloride concentration of the ground water in the vicinity of Barbers Point shaft. The model predicted an increase in chloride concentration of about 50 mg/L. The observed increase was about 30 mg/L and the chloride concentration is presently stable at 245 to 250 mg/L; hence, the chloride concentration is not expected to increase significantly more. The processes that control the chloride concentration at Barbers Point shaft are not fully understood, and it is possible that the abrupt increase in chloride concentration from 1983 to 1985 is a passing phenomenon, in which case, the concentration would decline in the next few years. However, the concentration will not decline below the ambient concentration level in the aquifer (200-220 mg/L). The chloride concentration of water from Barbers Point shaft can be lowered significantly only if the aquifer is recharged by fresher water or if a source of fresher water is blended with the Barbers Point shaft water.

Source of salts in the Waianae part of the Pearl Harbor aquifer near Barbers Point water tunnel, Oahu, Hawaii