Mercury inputs and redistribution in the Penobscot River and estuary, Maine

We examined total mercury (Hg) distributions in sediments from the Penobscot River and estuary, Maine, a site of extensive Hg releases from HoltraChem (1967–2000). Our objectives were to quantify: (1) bottom sediment Hg inventories (upper ~ 1 m; 50–100 y); (2) sediment accumulation rates; and (3) contemporary Hg fluxes to bottom sediments; by sampling the Penobscot River (PBR), Mendall Marsh (MM), the Orland River (OR) and the Penobscot estuary (ES). Hg was rapidly distributed here, and the cumulative total (9.28 metric tons) associated with sediments system-wide was within the range released (6–12 metric tons). Evidence of sediment/Hg remobilization was observed in cores primarily from the PBR, and to a lesser extent the ES, whereas cores from MM, most of the OR, the ES, and half from the PBR exhibited sharp peaks in Hg concentrations at depth, followed by gradual decreases towards the surface. Based on background PBR sediment Hg concentrations (100 ng g^{− 1}), “elevated” (300 ng g^{− 1}), or “highly elevated” (600 ng g^{− 1}) Hg concentrations in sediments, and resulting inventories, we assessed impact levels (“elevated” ≥ 270, or “highly elevated” ≥ 540 mg m^{− 2}). 71% of PBR stations had “elevated” and 29% had “highly elevated” Hg inventories; 45% of MM stations had “elevated” and 27% had “highly elevated” inventories; 80% of OR stations had “elevated” inventories only; and 17% of ES stations had “elevated” inventories only. Most “highly elevated” stations were located within 8 km of HoltraChem, in MM, in the PBR, and in the OR. Near-surface sediments in the OR, PBR and MM were all “highly elevated” while those in the ES were “elevated” on average. Mean Hg fluxes to bottom sediments were greatest in the OR (554), followed by the PBR (469), then MM (452), and finally the ES (204 ng cm^{− 2} y^{− 1}).