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.