A Review of Upward Migration of Effluent Related to Subsurface Injection at Miami-Dade Water and Sewer South District Plant

McNeill, Donald

From Author Introduction: In summary, the problem of upward migration of effluent at the South District plant has resulted from the inadequate geological characterization of the subsurface lithologies and their associated petrophysical properties. As a result, 10 of the 17 injection wells are not cased deep enough to properly inject effluent below the important confining unit at the top of the Oldsmar Formation. As a result, effluent enters permeable ·rocks which allow buoyancy-driven upward migration. The Middle Confining Unit is not of sufficiently low permeability to confine the injected effluent. The degree of effluent movement offsite is unknown at this time, but below the confining unit onsite flow is probably landward. Above the confining unit, the combination of the potentiometric influences and buoyancy likely influence the flow, although upward flow is dominant as judged by the detection of ammonia in numerous monitor wells immediately overlying the injection zone. Currently, the ultimate fate of the injected effluent is unknown. The fact that effluent has reached the upper Floridan Aquifer some 30 times faster than first predicted, bodes poorly for quantitatively predicting the ultimate fate of these injected fluids. Furthermore, the considerable degree of uncertainty in the subsurface geology, hydrogeologic properties of the "confining" unit, and efficiency of effluent plume migration clearly demonstrates that no reasonable degree of certainty exists for the protection of underground sources of drinking water and nearby fragile marine and terrestrial ecosystems (Biscayne National Park, Everglades National Park, and the Florida Key National Marine Sanctuary).