Natural gradient, lakebed tracer tests using nitrite in a nitrate-contaminated groundwater discharge zone in Ashumet Pond, Massachusetts

The U.S. Geological Survey Toxic Substances Hydrology Program at Cape Cod has been investigating the fate and transport of a treated-wastewater, groundwater contaminant plume. A portion of the contaminated groundwater discharges into Ashumet Pond, a kettle hole, freshwater lake. A study was conducted from June 2013 to June 2015 to document transport, transformation, and discharge of dissolved inorganic nitrogen species (DIN; nitrate, nitrite, and nitrous oxide) from the contaminant plume to the lake, across the groundwater-surface water interface. As part of that study, in October 2014, two natural gradient tracer tests were conducted within the lake bed sediments using nitrite as a reactive tracer and bromide as a conservative tracer. Tracer solutions were injected into the lakebed sediment at a depth of 75 centimeters below the sediment surface and sediment porewater was collected with time from pushpoint samplers installed in a 3-dimensional grid as the tracer cloud moved upward through the sediment and discharged into the lake. Presented here are the sample grid coordinates, the injectate composition, and results of the tracer test sample analyses.