Technical Report No. 107
A LABORATORY STUDY OF WASTE INJECTION INTO A GHYBEN-HERZBERG GROUNDWATER SYSTEM UNDER DYNAMIC CONDITIONS
Duane L. Heutmaker, Frank L. Peterson, and Stephen W. Wheatcraft
Injection of wastes into a Ghyben-Herzberg groundwater system presents unique problems because the waste effluents are normally 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 dynamic groundwater conditions, and the effects on plume mechanics of varying several different injection parameters, such as injection depth and rate, type of injection source, strength of the ambient flow field, and density of ambient receiving water, were observed.
In every experiment conducted during this study, a buoyant plume of injected effluent, which was clearly distinct from the resident aquifer liquids, formed and rose vertically into the lower portion of the freshwater lens, where it was subjected to the freshwater flow field and migrated downgradient with the fresh water, still keeping its identity as a distinct plume of injected effluent.
Three of the injection parameters exerted significant control on the movement of the injection plumes. The rate of effluent injection and the strength of the ambient freshwater flow field significantly influenced both the vertical and upgradient dimensions of the plumes, and also appeared to influence downgradient mixing of the plume with the ambient fresh water; and the depth of effluent injection, with respect to the relative density of the resident liquids, influenced the upgradient migration of the plumes.
Although there was some initial evidence of salt water entrainment within the salt zone injection plumes, it was a transient phenomenon, and little evidence of salt water entrainment was observed once the plumes reached steady-state conditions. Downgradient dilution of the effluent plumes by the ambient fresh water was apparent, however, and increased with downgradient distance from the injection well.