MASS-BALANCE ANALYSIS OF REACTIVE TRANSPORT AND CATION-EXCHANGE IN A PLUME OF WASTE-WATER-CONTAMINATED GROUNDWATER

Mass-balance calculations were used to quantify reactive transport pro cesses and cation exchange in a plume of groundwater contaminated with septage-effluent wastewater on Cape God, Massachusetts. Of the chlori de mass recharged to the aquifer in effluent, as much as 72% was accou nted for using spatial moment analysis and finite-element integration of groundwater concentrations, which were sampled at less than or equa l to 69 wells and supplemented by borehole electromagnetic-induction l ogging. Comparison of chloride transport and mass balances with transp ort and mass balances of other species indicated that reactive process es substantially altered concentrations of all major chemical constitu ents. Calcium in effluent was exchanged for magnesium on aquifer sedim ents. Potassium also was attenuated, possibly through exchange with ma gnesium, sodium, and/or hydrogen ions. Sufficient hydrogen ions were g enerated by microbial nitrification in the unsaturated zone to consume effluent alkalinity and lower the effluent pH from 7.2 to 5.0 in the recharged groundwater; the resultant acid conditions may have facilita ted anion adsorption and silicate-mineral dissolution. Retardation fac tors (R) calculated from breakthrough curves indicated that calcium (R = 1.4-2.2) and boron (R = 1.3-2.1) were similarly retarded, whereas p otassium experienced greater retardation (R = 1.8-5.2). Retardation of calcium, boron, and potassium was greater in the unsaturated zone tha n in the saturated zone; this may have resulted from spatial heterogen eity in exchange properties and preferential saturated-zone How throug h coarse-grained sediments not present in the unsaturated zone. Althou gh concentrations may stabilize and chemical reactions reach equilibri um at fixed points along paths in the Flume, the mass-balance analysis illustrated that steady-state conditions will not be established thro ughout the aquifer and the cumulative mass of reacted constituents in the plume will increase until the plume reaches its discharge area. Th e analysis also indicates that retrospective study of dissolved concen trations in an established plume after many years of transport may not identify reactive transport and attenuation of plume constituents, if precise data on source concentrations (or masses) and the spatial dis tribution of solutes during plume development are not available. Final ly. transport of the effluent-contaminate groundwater also altered the geochemistry of the aquifer, for example, through cation exchange, su ch that the introduction of clean, uncontaminated water into the aquif er will not immediately restore pre-plume conditions. (C) 1997 Elsevie r Science B.V.