Groundwater recharge with Honouliuli wastewater irrigation: Ewa plain, Southern Oahu, Hawaii.
Lau, L. Stephen
Groundwater replenishment by irrigation with primary chlorinated effluent from the Honouliuli Wastewater Treatment Plant was tested in a three-year demonstration project for the Ewa limestone aquifer, island of Oahu, Hawaii. Among the six options tested with different combinations of crops (California grass and sugarcane), 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. Yield of 1 mgd recharge water required a 14.6-acre plot for this option. For all options, recharge through 3 feet of vegetated, fairly permeable soil overlying 30 feet of permeable reef limestone rock freshened groundwater chlorides to 245 mg/l, stripped virtually all effluent nitrogen, and inactivated effluent bacteria. Toxics analyzed (pesticides and heavy metals) were all below action or detectable levels. The natural system, which acts as a surface living filter and a subsurface trickling filter, outperformed secondary treatment in improving the water quality. No adverse environmental effects were identified-no surface runoff, no insects, and no groundwater contamination. Soil clogging was not evident. Plots were free of shallow standing water except for a few hours after 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 water spread in the aquifer several hundred feet from the site after the irrigation phase of a California grass growth cycle. The state of Hawaii is presently planning a large-scale water reclamation facility that will upgrade the Honouliuli primary effluent by this system to a quality suitable for subsequent reuse.