Long-Term Distribution and Transport of Nitrate and Ammonium Within a Groundwater Sewage Plume, Cape Cod, MA, After Removal of the Contaminant Source.

Disposal of treated sewage for 60 yrs. onto infiltration beds at a site on Cape Cod, MA produced a groundwater contaminant plume >6 km long. The plume was characterized by an anoxic ammonium-containing core, surrounded by an oxic-suboxic outer zone within the sand and gravel aquifer. In Dec. 1995 the sewage treatment facility ceased operation. A long-term study to characterize the distribution of sewage plume constituents was conducted along a 500 m-long transect (source to 3 yrs. groundwater travel distance). Prior to sewage-disposal cessation, total inorganic N within 30 m vertical profiles decreased from 6.6 moles N/m² (92% NO₃^-, 8% NH₄⁺) at the point of discharge to 3.3 moles N/m² (77% NO₃^-, 23% NH₄⁺) at the furthest point along the transect. Post-cessation nitrate concentrations increased within the first 6 mo. and then gradually decreased. The nitrate decrease was accompanied by an initial nitrite increase, an indication that denitrification was reducing nitrate after the oxygenated sewage discharge was discontinued. There was also an apparent increase in ammonium concentration in the first 6 mo. after cessation. Previous laboratory experiments on pre-cessation cores showed that nitrification was important in converting sorbed ammonium to nitrate under the sewage beds. However, with the removal of the oxygenated sewage source, nitrification ceased, allowing ammonium to initially increase. This increase was correlated with dissolved organic carbon concentrations within the groundwater. Ammonium concentrations decreased dramatically after a year, but subsequently increased in the core of the plume to pre-cessation levels through mineralization of organic N. Recent laboratory core experiments and extractions show that there is a large pool of sorbed organic carbon, although dissolved organic carbon concentrations have been consistently less than 3 mg/L for 6 yrs. Seven yrs. after cessation of the sewage disposal, there is still a significant amount (0.6 moles N/m²) of fixed inorganic N present in groundwater at the disposal site. It is apparent that sorbed constituents now contribute significantly to the dissolved N pool and natural restoration of the contaminant plume, particularly with respect to N, will likely take several more years.