WRRCTR No.182 Groundwater recharge with Honouliuli wastewater irrigation: Ewa plain, southern Oahu, Hawaii

Abstract

High-rate groundwater replenishment by irrigation with primary chlorinated effluent from the Honouliuli Wastewater Treatment Plant was tested in a 3-yr demonstration project for the Ewa limestone aquifer, Oahu. Among the six options tested with different combinations of crops, irrigation methods, and effluent application rates, the most acceptable was California grass, which was grown in plots (0.5 acre each) surrounded by an earth berm and border-flood irrigated intermittently at an average rate of 20 in./wk. For this option, the production of 1 mgd of recharge water requires a 14.6-acre plot. For all options, recharge through 3 ft of vegetated, fairly permeable soil overlying 30 ft of permeable, reef limestone rock stabilized groundwater chlorides to 245 mg/l, stripped virtually all effluent nitrogen, and inactivated effluent bacteria. Toxics analyzed (pesticides, solvents, and heavy metals) were all below action or detectable levels. The natural system, which acts as a "living filter", outperformed secondary treatment in improving the water quality. No adverse environmental effects were identified: no surface runoff, no insects, and no groundwater contamination. Except for slight clogging in the California grass plots toward the end of each harvest cycle, surface soil clogging was not evident. Plots were free of shallow standing water except for a few hours on the day of effluent application. The mild odor noted only infrequently on site was less than that at the treatment plant. The biomass produced was of usable quality and adequate quantity. The simulated recharge plume spread in the aquifer several hundred feet from the site after the irrigation phase of a California-grass growth cycle. Project results favor a large scale water reclamation facility which will upgrade the Honouliuli primary effluent to a quality suitable for subsequent reuse.