A Nitrogen-Rich Septage-Effluent Plume in a Glacial Aquifer, Cape Cod, Massachusetts, February 1990 through December 1992

Desimone, Leslie; Barlow, Paul; Howes, Brian

A nitrogen-rich septage-effluent plume was monitored to quantify the physical, chemical, and microbial processes that affect the transport and fate of selected constituents in the plume within the aquifer. The plume of contaminated ground water resulted from the infiltration of wastewater at the Tri-Town Septage Treatment Facility in Orleans, Cape Cod, Massachusetts. The septageeffluent plume was monitored from the beginning of effluent discharge in February 1990 through December 1992. Lithologic, hydrologic, and predischarge water-quality data were used to characterize the glacial aquifer. The physical transport and chemical characteristics of the septageeffluent plume were monitored by analyzing water-quality samples from 83 wells. The physical development of the plume also was delineated with borehole elec tromagne tic-induction logging. Chemical and microbial processes affecting constituents in the plume were inferred from water-quality data, mass-balance analyses, and the direct measurement of reactive processes involving nitrogen (ammonium sorption and denitrification of nitrate). By December 1992, the septage-effluent plume extended about 200 feet westward and 450 feet northwestward of the infiltration beds. The primary direction of transport was toward aquifer discharge Areas in a nearby coastal marsh and Cape Cod Bay. The plume preferentially flowed through a 5- to 60-foot thick lithologic unit of medium to very coarse sand at a mean linear velocity of 0.4 to 0.5 foot per day. Concentrations of total dissolved nitrogen ranged from more than 40 milligrams per liter as nitrogen in the center of the plume to less than 10 milligrams per liter as nitrogen near the boundaries of the plume. Concentrations near the boundaries of the plume were diluted by mixing with ambient ground water, which contained relatively little nitrogen. Nitrate was the primary form of nitrogen in the septage-effluent plume. Lower concentrations of dissolved ammonium were present. The center of the septage-effluent plume in ground water was anoxic. Much of the nitrate in ground water was produced by nitrification of effluent ammonium in the unsaturated zone. The nitrification process in the unsaturated zone decreased the pH (7.2 in effluent and 5.0 in the septage-effluent plume) and produced high concentrations of dissolved nitrous oxide (as much as 0.4 milligram per liter as nitrogen). Ammonium sorption was the primary process affecting nitrogen transport in the saturated zone. Sorbed ammonium was about 17 percent of the total nitrogen in the plume in December 1992. Ammonium sorption was best described by a Langmuir isotherm and was strongly dependent on the ionic strength of the aqueous phase. Denitrification, the microbiological reduction of nitrate to dinitrogen gas, was limited by the low dissolved and solid-phase concentrations of organic carbon in the plume. The mean denitrification rates measured in aquifer-core incubations and from dinitrogen production were 9.6 and 3.0 nanograms of nitrogen per cubic centimeter of sediment per day, respectively. The dinitrogen-production rate, which best reflects in situ rates, corresponds to a nitrate reduction of 1.5 milligrams per liter as nitrogen per 100 feet of travel distance under anoxic conditions. Significant attenuation of nitrate concentrations from denitrification was not observed in the field due to the short distance traveled by anoxic ground water and the masking effect of variations in the effluent-nitrogen concentrations during the study.