Estimates of recovery of the Penobscot River and estuarine system from mercury contamination in the 1960's.
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abstract
Mercury (Hg) was discharged in the late 1960s into the Penobscot River by a chlor-alkali production facility, HoltraChem. Using total Hg concentration profiles from 56 stations (58 sediment cores) in the Penobscot River (PBR), Mendall Marsh (MM), Orland River (OR) and Penobscot Estuary (ES), and sediment accumulation rates derived using detailed profiles of total Hg concentrations and radionuclide activities (137Cs, 239,240Pu, 210Pb), recovery from system-wide Hg pollution was assessed. Total Hg concentration profiles showed sharp maxima at depths attributed in time to a 1967 release date, and were divided into two sections: the first 21years (1967-1988; rapid recovery), and the recent 21years (1988-2009; slower recovery). The recent 21years of Hg input were used to estimate 'apparent' recovery rates, yielding exponentially decreasing total Hg concentrations. Apparent recovery half-times (T1/2=ln2/) were calculated from an exponential fit of Hg(t)=Hg(t=21)exp(-t)+Hg() to total Hg concentration profiles over the past 21years (assuming Hg() of 0, 100, or 400ngg-1). Mean T1/2 values were, at PBR 31years (16 of 24 cores), at MM 22years (9 of 11 cores), at ES 20 to 120years (mean of 78years; 12 of 18 cores), and at OR 69years (3 of 5 cores). In 18 out of 57 cores, concentrations either increased towards the surface or remained the same, indicating slower or incomplete 'communication' with the larger system. The Penobscot River and Estuary system has recovered substantially since 1967, and top 1cm sediment Hg concentrations (Hg(0)) from areas in rapid communication with the larger system are converging to 600-700ngg-1 (1967 maxima of 70,000+ngg-1). However, to recover from Hg(0) of 700ngg-1 to a Hg() of <100ngg-1 would require 3 or more half-times.
