Estimates of recovery of the Penobscot River and estuarine system from mercury contamination in the 1960's

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 21 years (1967–1988; rapid recovery), and the recent 21 years (1988–2009; slower recovery). The recent 21 years of Hg input were used to estimate ‘apparent’ recovery rates, yielding exponentially decreasing total Hg concentrations. Apparent recovery half-times (T_1/2 = ln2/α) were calculated from an exponential fit of Hg(t) = Hg(t = 21) ∗ exp(− α ∗ t) + Hg(∞) to total Hg concentration profiles over the past 21 years (assuming Hg(∞) of 0, 100, or 400 ng g^{− 1}). Mean T_1/2 values were, at PBR 31 years (16 of 24 cores), at MM 22 years (9 of 11 cores), at ES 20 to 120 years (mean of 78 years; 12 of 18 cores), and at OR 69 years (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 1 cm sediment Hg concentrations (Hg(0)) from areas in rapid communication with the larger system are converging to 600–700 ng g^{− 1} (1967 maxima of 70,000⁺ ng g^{− 1}). However, to recover from Hg(0) of 700 ng g^{− 1} to a Hg(∞) of < 100 ng g^{− 1} would require 3 or more half-times.