WRRCTR No.96 Waste Injection into the Hawaiian Ghyben-Herzberg Aquifer: A Laboratory Study Using a Sand-Packed Hydraulic Model

Abstract

Injection of wastes into the Hawaiian subsurface environment presents unique problems because the waste effluents normally are injected into the salt or brackish water underlying the fresh Ghyben-Herzberg lens. Because the waste water commonly has approximately the same density as fresh water, in addition to any ambient groundwater flow effects, a buoyant uplift is produced which causes the injected waste to move upward and outward from the injection point as a buoyant plume. A laboratory sand-packed hydraulic model was used to study the mechanics of buoyant plume movement, and the entrainment of salt water by the plume. Simulated waste effluent was injected into a density-stratified aquifer system under static groundwater conditions, and the effects on plume mechanics of varying several different injection parameters, such as injection depth, injection rate, type of injection source, density of receiving water, etc., were observed. The laboratory studies indicated that although several of the injection parameters (most notably depth of the injection with respect to the saltfresh interface) have an effect on the details of the injection process and the plume movement and configuration, none of the injection parameters exerts a truly significant control on the ultimate fate of the injected plume. That is, for the conditions stipulated in this study, the injection plumes always migrated well up into the freshwater lens, regardless of variations in any of the several injection parameters. Furthermore, these experiments showed little evidence of entrainment of the surrounding salt water into the injected buoyant plumes, which strongly suggests that the principal means of plume movement is by mass displacement rather than by mixing processes.