INVESTIGATING RATES OF COASTAL LAGOON SEDIMENT ACCUMULATION AFTER 1963 DREDGING ON MARTHA'S VINEYARD, MA

Along the coast of Martha’s Vineyard, MA, lies an artificially-created lagoon from 1963 that was dredged as part of a residential development. Although the originally intended yacht harbor was never constructed, a tidal inlet has been maintained since that time to preserve recreational, ecological, and scenic values. A local residents’ environmental committee (EC) has long-term concerns for the area as it appears the lagoon has been infilling with sediment and shallowing in recent years. We measured fallout radionuclides (FRNs) in sediment cores to quantify sediment accumulation in the lagoon since 1963. We also theorized sediment sources based on their physical properties in the sediment profile. Our findings will support the EC in management decisions related to future inlet and/or lagoon maintenance, dredging, and excavation.

Gamma-detected Cs-137 and excess Pb-210 provide temporal and depth constraints on the depositional conditions of modern detrital sediments. Atmospheric nuclear weapons testing from the 1950s-1960s allows Cs-137 to be used as a tracer of surface material exposed during that era; its peak fallout (1963) lines up with the conclusion of lagoon dredging. Excess Pb-210 can be used as a tracer for recent surface material deposition due to its continual atmospheric decay from U-238. We calculated FRN activity in subsamples and utilized upcore trends to determine sediment accumulation rates over time.

On average, 44 cm of sediment has accumulated in the lagoon since 1963. Bulk accumulation rates suggest the average rate of infilling is 0.117 g per square cm per year, suggesting sediment accumulation is not occurring rapidly enough to completely fill in the lagoon in the next few decades. Assuming infilling has been steady over time, this will take at least 360 years. However, upcore trends of Cs-137 activity do not match its atmospheric fallout, creating an expected case of sediment deposition that unknown biological or anthropogenic causes may explain. Ongoing measurements of excess Pb-210 will refine our accumulation rates and determine if/when sediment accumulation has fluctuated over the past six decades. We presume the EC is witnessing invasive Phragmites australis encroach rapidly upon the lagoon’s boundary, making it appear smaller and shallower in recent years.