Hydraulic/Chemical Changes During Ground-Water Recharge by Injection

Abstract

Ground-water recharge by injection of reclaimed water is a feasible method of improving ground-water quality in the shallow aquifer system in the Palo Alto Baylands along the San Francisco Bay. Ground water was initially more saline than sea water. Reclaimed water was injected at a rate of 10 gallons per minute from June 5, 1980, to July 1, 1980. At the completion of injection, water from an observation well 31 feet from the injection well was 98 percent injected water-in essence, fresh water. An abrupt rise in the water level in the injection well of about 1.5 feet during the initial injection test was the result of a 3.5 percent density difference between injected fresh water and saline ground water. The arrival of injected water at observation wells showed the same effect, allowing monitoring of chemical and hydraulic changes entirely through water-level data. The initially sodic clays in the confining layer were expected to swell as the saline ground water (sodium source) was diluted by recharge water. The sodium ion causes excessive coordination with the hydronium ion (H 3 O + ) in the clay lattice, resulting in expansion as the saline water is diluted. X-ray diffraction analysis of clay samples soaked first in native and then in injected water showed this effect. Calcium replaces sodium and limits expansion. Prior to injection the saline ground water was supersaturated with calcite. Dilution, as injection proceeded, eventually produced an undersaturation of calcite. An increase in well specific capacity indicates that calcite dissolved from the aquifer matrix, improving hydraulic conductivity.