A COMPARISON OF SEDIMENT GEOCHEMISTRY FROM ABOVE AND BELOW A TIDAL RESTRICTION IN THE BACK RIVER CREEK MARSH, WOOLWICH, ME

This study examines the geochemistry of sediment cores collected from above and below the approximately 150-year-old Dike Road and Route 1 tidal restriction across the Back River Creek Marsh (BRCM) in Woolwich, ME. Two 1 m long sediment cores were collected north of the restriction, one 1 m core was collected south of the restriction, and a fourth core was taken downstream at a reference site. Sediments were described visually and analyzed for Pb concentration, carbon content, and stable carbon isotope composition. The Pb concentrations were highest at 24 cm below the surface; assuming this reflects the onset of the phasing out of leaded gasoline in the US starting in the 1970s, average sediment deposition rates are estimated to be 0.5 cm/year north of the restriction. The carbon concentrations were higher north of the restriction than south of the restriction, reflecting the current ecosystems in place; organic rich Typha sp. wetlands dominate the system north of the restriction, and sparsely vegetated tidal flats are present south of the restriction. The δ¹³C values from all the cores range between -24‰ and -30‰, falling well within the typical C3 plant range, thus indicating a freshwater or grassland setting across the entire marsh since the late 1700s. Significant carbon and nitrogen isotope shifts occur in the early 2000s reflecting the establishment of current wetlands and shifts in nitrogen cycling immediately after partial tidal flow restoration with the removal of a tide gate and the addition of a new culvert by Bath Iron Works. This study provides important baseline information on the nature of the marsh prior to the building of Route 1 tidal restriction and is useful for evaluating recent and future restoration efforts.