GEOCHEMICAL MODELING OF THE FATE AND TRANSPORT OF INJECTED FRESH WASTEWATER INTO A DEEP SALINE AQUIFER

Treated domestic wastewater traditionally has been discharged offshore in coastal areas via ocean outfalls. In response to environmental concerns associated with ocean outfalls, deep well injection of treated wastewater into non-potable aquifers has become increasingly used as an alternative. These deep aquifers tend to be saline, and the discharge of fresh wastewater into them raises concerns of geochemical reactions as a result of the mixing of the two waters, as well as the buoyant transport of the wastewater upwards into overlying aquifers. The Miami-Dade Water and Sewer Department (MDWASD), Miami-Dade County, Florida, operates two deep-well injection facilities in Miami-Dade County, and injects an average of 430 million liters per day into a deep saline aquifer (Cl^- 600 μmol l^-1) approximately 900 meters below land surface. Although this deep aquifer is separated from overlying brackish aquifers (Cl^- 200 μmol l^-1) by a confining unit 150 meters thick, evidence of migration of injected wastewater has been detected in the overlying aquifers. The injected wastewater is a source of freshwater recharge chemically distinct from the native brackish aquifer water. Ammonium contained in the injected wastewater is significantly higher (average 325 μmol l^-1), than native aquifer water background levels (2 - 4 μmol l^-1), and exhibits a high seasonal variability in response to the wet and dry climatic conditions in south Florida. Geochemical data indicate that the injected ammonium behaves conservatively when mixed with native water, and thus can be used as a tracer of injected wastewater. Ammonium data, in conjunction with major ion chemistry and stable isotope data, were used to identify source and pathways of the recharge. Helium/tritium data were used to obtain apparent ages of groundwater, and to trace injected recharge pathways.