A UNIQUE APPROACH TO CALIBRATING A VARIABLE-DENSITY FLOW AND TRANSPORT MODEL

Communities in Florida have disposed of wastewater by injection into deep aquifers since the 1950s. The largest capacity deep-well injection plant in the United States, built in south Florida during the 1970's, injects about 100 Mgal/d of secondary-treated fresh effluent into a saline but highly-transmissive, fractured dolomite aquifer lying 2500 ft below land surface. A 700-ft-thick carbonate-rock middle confining unit separates the zone of injection from an overlying protected-resource aquifer. Ammonia concentrations that exceed background conditions have been observed in monitoring wells open in and above the confining unit, suggesting upward vertical migration of effluent into the overlying aquifer. Vertical movement of the effluent is probably the result of salinity- and/or temperature-induced buoyancy effects created by injection of the wastewater into cooler saline water.

A variable-density numerical flow and transport model of the area is being developed to identify vertical flow pathways for the effluent. Because deep-well injection serves as a large aquifer test, measured hydraulic stress, flow, and hydro-chemical data are being used to aid in model calibration and prediction of injectate movement. Sophisticated parameter estimation techniques in PEST are being used to calibrate the model. Zones of high vertical hydraulic conductivity (fractures) are revealed by defining points in the model domain where hydraulic properties are estimated as a result of the calibration process. Spatial interpolation is then used to assign hydraulic properties to areas between the points.

Preliminary simulations suggest effluent could discharge upward through vertical fractures within the middle confining unit. However, some high hydraulic conductivity zones closely correspond to the location of injection wells. Vertical migration could also occur through anthropogenic pathways such as casing and well borehole annular space, pilot borehole misalignment, or casing perforations caused by corrosion or poor well construction. Conversely, vertical effluent migration could be a result of both carbonate rock heterogeneity and anthropogenic pathways.