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

Abstract

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.