CP-1994-07
Artificial recharge of ground water for all purposes?
Lau, L. Stephen
Along a dry, hot coast on the island of Oahu, Hawaii, several needs-water demand for a new city, salinization control for a limestone aquifer, and management of wastewater effluent from an ocean outfall discharge-can be met through artificial recharge of groundwater by irrigation with a primary effluent. Among the six options tested in a three-year demonstration project with different combinations of crops (California grass and sugarcane), irrigation methods, and effluent application rates, the most acceptable was California grass grown in 0.2-ha plots bordered by an earth berm and flood-irrigated intermittently at an average rate of 508 mm/wk. A yield of 0.044 m3/s recharge water required 5.9 ha of land for this option; an even higher yield was possible with a minor change in percolate quality. For all options, recharge through 0.9 m of vegetated, fairly permeable soil overlying 9.1 m of permeable reef limestone rock freshened groundwater chlorides to 245 mg/l, stripped virtually all of the effluent nitrogen, and inactivated effluent bacteria. Toxics analyzed (pesticide and heavy metals) were all below maximum contaminant levels. The natural system, which acts as a surface living filter and a subsurface trickling filter, outperformed the secondary treatment system in improving the water quality. No adverse environmental effects-surface runoff, insect infestation, and groundwater contamination-were identified. Soil clogging was not evident. Plots were free of standing water a few hours after effluent application. The mild odor noted only infrequently on site was less than that at the Honouliuli Wastewater Treatment Plant. The biomass produced was of usable quality and adequate quantity. The simulated plume by the recharge water spread in the aquifer several hundred feet from the site after the irrigation phase of a California grass growth cycle. This paper summarizes the technical results on which the planning of a large-scale facility can be based and addresses the institutional difficulties and land-use economics, both of which are immense obstacles yet to be overcome.