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) 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 and, when combined with a best estimate of the area of groundwater discharge (11,000 m) and average seepage/porewater mercury concentration (242 ng L, UCL95), yielded a loading of 0.11 g day 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 individually, nor was the aggregate loading of all such sources >3 g day (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, respectively, to the Penobscot River. Based on sampling (total Hg ~ 2 to 4 ng L) and historical mean discharge data (340-460 m s), the Penobscot River upstream of the plant site contributes as much as 160 g day 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.