A nitrogen-rich septage-effluent plume in a coastal aquifer, marsh, and creek system, Orleans, Massachusetts; project summary, 1988-95

A report summarizing the major findings of an ongoing study of a nitrogen-rich septage-effluent plume in Orleans, Massachusetts has been published. The study is being conducted by the U.S. Geological Survey and the Massachusetts Department of Environmental Protection, Office of Watershed Management, with additional support from the Cape Cod Commission. The report describes the history of the septage-effluent plume in the coastal aquifer and the results of baseline water-quality, sediment, and vegetation monitoring in Namskaket Marsh and Namskaket Creek, the expected discharge areas for the effluent plume. Namskaket Marsh has been recognized by the Commonwealth of Massachusetts as an Area of Critical Environmental Concern, and Namskaket Creek, which flows through the marsh, has been designated as an Outstanding Resource Water. Peter Weiskel, U.S. Geological Survey hydrologist and principal author of the report, said that discharge of effluent from the treatment facility since February 1990 has resulted in a plume of nitrogen-rich (10 to 40 milligrams of nitrogen per liter) ground water that is travelling at about 0.4 feet per day westward and northwestward toward Namskaket Marsh and Cape Cod Bay. Seepage zones near the marsh-upland boundary and the bottoms of Namskaket Creek and tributaries are the most likely discharge areas for the effluent plume. High rates of ground-water discharge (averaging 0.30 gallon per square foot per hour) have been measured in these zones during the ebb-tide. Measurements of nitrate uptake by sediments in these zones indicate considerable potential for nitrate removal during plume discharge through these zones. The marsh vegetation is dominated by Phragmites australis (common reed) in the seepage zone near the upland boundary, and by Spartina patens (salt-meadow grass) in the interior areas of the marsh. Further study is required to (1) continue mapping the progress of the plume toward the marsh and creek, (1) define the potential nitrogen-removal capacity of the marsh, and (3) monitor the possible future effects of the plume on the water quality and vegetation of the marsh.