Sediment accumulation and mixing in the Penobscot River and estuary, Maine K.M. Yeager a, ⁎, K.A. Schwehr b , K.J. Schindler a , P.H. Santschi b a Sedimentary and Environmental Radiochemistry Research Laboratory (SER 2 L), Department of Earth and Environmental Sciences, University of Kentucky, Lexington, KY 40506, United States b Laboratory for Oceanographic and Environmental Research (LOER), Department of Marine Science, Texas A&M University at Galveston, Galveston, TX 77554, United States HIGHLIGHTS • Sediment variability increased from Mendall Marsh/Orland River b Penob- scot estuary b Penobscot River. • Generally, sediment mixing was limited (maximum Db ranged from 3-6 cm 2 y -1 ; mixed layers of ~1-3 cm). • A wide range in sediment accumulation rates was observed (0.10-1.85 cm y -1 ), and was expected. • Extensive lateral transport of sediment (and total Hg) was limited to parts of the Penobscot River. • Sediment (and Hg) accumulation in the Penobscot River was most random, fo- cused inside meanders, and coves. GRAPHICAL ABSTRACT Composite sediment accumulation rates determined by radionuclides and total Hg for each part of the Penobscot River an/d estuary system (uncertainties reported at 1 σ). abstract article info Article history: Received 29 June 2017 Received in revised form 26 March 2018 Accepted 3 April 2018 Available online xxxx Mercury (Hg) was discharged in the late 1960s into the Penobscot River by the Holtra-Chem chlor-alkali produc- tion facility, which was in operation from 1967 to 2000. To assess the transport and distribution of total Hg, and recovery of the river and estuary system from Hg pollution, physical and radiochemical data were assembled from sediment cores collected from 58 of 72 coring stations sampled in 2009. These stations were located throughout the lower Penobscot River, and included four principal study regions, the Penobscot River (PBR), Mendall Marsh (MM), the Orland River (OR), and the Penobscot estuary (ES). To provide the geochronology re- quired to evaluate sedimentary total Hg profiles, 58 of 72 sediment cores were dated using the atmospheric ra- dionuclide tracers 137 Cs, 210 Pb, and 239,240 Pu. Sediment cores were assessed for depths of mixing, and for the determination of sediment accumulation rates using both geochemical (total Hg) and radiochemical data. At most stations, evidence for significant vertical mixing, derived from profiles of 7 Be (where possible) and porosity, was restricted to the upper ~1–3 cm. Thus, historic profiles of both total Hg and radionuclides were only mini- mally distorted, allowing a reconstruction of their depositional history. The pulse input tracers 137 Cs and 239,240 Pu used to assess sediment accumulation rates agreed well, while the steady state tracer 210 Pb exhibited weaker agreement, likely due to irregular lateral sediment inputs. © 2018 Published by Elsevier B.V. Keywords: Geochronology Sedimentation rates River-estuarine system Sediment transport Penobscot River 1. Introduction The Penobscot River is approximately 425 km in length (including the West and South branches), drains a basin of ~22,300 km 2 , and is the second largest river system in New England, after the Connecticut Science of the Total Environment 635 (2018) 228–239 ⁎ Corresponding author. E-mail addresses: kevin.yeager@uky.edu, (K.M. Yeager), schwehrk@tamug.edu, (K.A. Schwehr), santschi@tamug.edu. (P.H. Santschi). https://doi.org/10.1016/j.scitotenv.2018.04.026 0048-9697/© 2018 Published by Elsevier B.V. Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv