Mass-balance analysis of reactive transport and cation exchange in a plume of wastewater-contaminated groundwater

Mass-balance calculations were used to quantify reactive transport processes and cation exchange in a plume of groundwater contaminated with septage-effluent wastewater on Cape Cod, Massachusetts. Of the chloride mass recharged to the aquifer in effluent, as much as 72% was accounted for using spatial moment analysis and finite-element integration of groundwater concentrations, which were sampled at less than or equal to 69 wells and supplemented by borehole electromagnetic-induction logging. Comparison of chloride transport and mass balances with transport and mass balances of other species indicated that reactive processes substantially altered concentrations of all major chemical constituents. Calcium in effluent was exchanged for magnesium on aquifer sediments. Potassium also was attenuated, possibly through exchange with magnesium, sodium, and/or hydrogen ions. Sufficient hydrogen ions were generated 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 facilitated anion adsorption and silicate-mineral dissolution. Retardation factors (R) calculated from breakthrough curves indicated that calcium (R = 1.4-2.2) and boron (R = 1.3-2.1) were similarly retarded, whereas potassium experienced greater retardation (R = 1.8-5.2). Retardation of calcium, boron, and potassium was greater in the unsaturated zone than in the saturated zone; this may have resulted from spatial heterogeneity in exchange properties and preferential saturated-zone flow through coarse-grained sediments not present in the unsaturated zone. Although concentrations may stabilize and chemical reactions reach equilibrium at fixed points along paths in the plume, the mass-balance analysis illustrated that steady-stale conditions will not be established throughout the aquifer and the cumulative mass of reacted constituents in the plume will increase until the plume reaches its discharge area. The analysis also indicates that retrospective study of dissolved concentrations 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 distribution of solutes during plume development are not available. Finally, transport of the effluent-contaminated groundwater also altered the geochemistry of the aquifer, for example, through cation exchange, such that the introduction of clean, uncontaminated water into the aquifer will not immediately restore pre-plume conditions.