Current and historical rates of input of mercury to the Penobscot River, Maine, from a chlor-alkali plant.

Mercury inputs by surface and ground water sources to Penobscot River from a defunct Hg-cell chlor-alkali plant were measured in 2009-10 and estimated for the entire period of operation of this facility. Over the measured interval (422 days) approximately 2.3 kg (5.4 g day-1) of mercury was discharged to the Penobscot River by the two surface streams that drain the site, with most of the combined loading (1.8 kg Hg, 78%) associated with a single storm with rainfall in excess of 100 mm. Groundwater seepage rates from the site, as estimated from both a radon tracer and seepage meter methods were in the range of 3 to 4 cm day-1 and, when combined with a best estimate of the area of groundwater discharge (11,000 m2) and average seepage/porewater mercury concentration (242 ng L-1, UCL95), yielded a loading of 0.11 g day-1 for site groundwater. None of the municipal or other industrial point sources of mercury to the river between Veazie and Bucksport, Maine exceeded 1 g day-1 individually, nor was the aggregate loading of all such sources >3 g day-1 (based on State of Maine data). Mercury loadings for the three largest tributaries downstream of Veazie Dam were estimated to contribute 4.2, 3.7 and 2.5 g day-1, respectively, to the Penobscot River. Based on sampling (total Hg ~ 2 to 4 ng L-1) and historical mean discharge data (340-460 m3 s-1), the Penobscot River upstream of the plant site contributes as much as 160 g day-1 to the downstream reach depending on river discharge. Estimates of historical (1967-2012) mercury loading using both generic emission factors and measured releases ranged from 2.6 to 27 MT while the mass of mercury found in downstream sediments amounted to 9 MT.

Tidal fluxes of mercury and methylmercury for Mendall Marsh, Penobscot River estuary, Maine.

Tidal marshes are both important sites of in situ methylmercury production and can be landscape sources of methylmercury to adjacent estuarine systems. As part of a regional investigation of the Hg-contaminated Penobscot River and Bay system, the tidal fluxes of total suspended solids, total mercury and methylmercury into and out of a regionally important mesohaline fluvial marsh complex, Mendall Marsh, were intensively measured over several tidal cycles and at two spatial scales to assess the source-sink function of the marsh with respect to the Penobscot River. Over four tidal cycles on the South Marsh River, the main channel through which water enters and exits Mendall Marsh, the marsh was a consistent sink over typical 12-h tidal cycles for total suspended solids (8.2 to 41 g m-2), total Hg (9.2 to 47 µg m-2), total filter-passing Hg (0.4 to 1.1 µg m-2), and total methylmercury (0.2 to 1.4 µg m-2). The marsh's source-sink function was variable for filter-passing methylmercury, acting as a net source during a large spring tide that inundated much of the marsh area and that is likely to occur during approximately 17% of tidal cycles. Additional measurements on a small tidal channel draining approximately 1% of the larger marsh area supported findings at the larger scale, but differences in the flux magnitude of filter-passing fractions suggest a highly non-conservative transport of these fractions through the tidal channels. Overall the results of this investigation demonstrate that Mendall Marsh is not a significant source of mercury or methylmercury to the receiving aquatic systems (Penobscot River and Bay). While there is evidence of a small net export of filter-passing (<0.4 µm pore size) methylmercury under some tidal conditions, the mass involved represents <3% of the mass of filter-passing methylmercury carried by the Penobscot River.