A multiple-aquifer system in the bayfront area of Palo Alto, California is being intruded with seawater from San Francisco Bay. In order to combat this potential degradation of the ground-water supplies in the area, a sea-water intrusion barrier is being constructed consisting of a series of injection wells uses to inject 2.0 million gallons per day (7.6 X 106 l/d) of reclaimed wastewater into a shallow aquifer. The injected water is subsequently removed by a similar system of extraction wells to avoid any possible degradation of the water-supply aquifers from this source, and to allow reuse of the reclaimed wastewater. The investigation phase included test drilling,, aquifer testing and injection testing to determine the feasibility of the injection/extraction (I/E) concept. The number, spacing and location of I/E doublets were optimized using a digital computer model. The double-cased, double-screened wells were constructed using corrosion-resistant materials and were designed for ease of routine maintenance. In operation, injection and extraction will be computer controlled by sensing piezometric levels in a series of monitor wells. Water pumped from the extraction wells will be sold for industrial and agricultural purposes. The I/E well system has been approved for 87 1/2 percent Federal and State grand funding.