A 290 YEAR RECORD OF LEAD DEPOSITION AT TAYLOR POND, AUBURN, MAINE

Little is known about the timing, magnitude and sources of atmospheric lead (Pb) deposition in northern New England, a region often referred to as “the tailpipe of the nation.”  In this study a detailed 290 year record of Pb deposition was constructed by measuring Pb concentration in a lacustine sediment core collected from Taylor Pond (TP) in Auburn, Maine.  TP is a dimictic mesotrophic lake with a surface area of 2.6 km² in a 37.8km² watershed.  A 38 cm gravity surface core was collected from TP in May, 2007, and subsampled at 1 cm intervals.  Freeze dried sediments were digested using a microwave digester (EPA Method 3051) prior to analysis for Pb and scandium (Sc) by inductivity coupled plasma-optical emission spectrophotometer.  Additionally, ^239-240Pu radionuclide dating was used to determine average sedimentation rates for the core and timing of Pb deposition.  Total organic carbon values were determined to normalize for changes in organic deposition through time. 

Elevated ^239+240Pu activities are derived from atmospheric nuclear weapons testing which began in 1953/1954 and reached a maximum in 1963/1964.  At TP, the ^239+240Pu activity began to rise between 7.5 cm and 6.5 cm, and peaked at 4.5 cm, which corresponds to an average sedimentation rate of ~1.3 mm/yr.  Assuming constant sedimentation rates, the TP record represents ~290 years of deposition.  

Between 1720-1800, Pb and Sc concentrations remain constant at 1.46 +/- 0.22 ppm and 3.46 +/- 0.23 ppm, respectively.  Because anthropogenic inputs of Sc to the natural environment are minimal, we surmise that these values represent background levels of Pb and Sc input to the watershed.  Sc remains relatively constant throughout the record.  Between 1800 and 1900, Pb concentrations increase steadily to ~13 ppm and between 1900 and 1980, Pb concentrations increase rapidly to ~74 ppm.  Pb values decrease only slightly between 1980 and the present.  The most rapid increase in Pb deposition occurred through the 20^th century, and likely represents widespread use of leaded gasoline.  A significant decrease in Pb deposition after passage of the 1973 Clean Air Act was expected and not measured at TP, possibly due to the masking effects of bioturbation.