ALTERNATIVES EVALUATION REPORT Penobscot River Phase III Engineering Study Penobscot River Estuary, Maine Prepared for: United States District Court District of Maine Prepared by: Amec Foster Wheeler Environment & Infrastructure, Inc. 511 Congress Street, Suite 200 Portland, Maine 04101 Project No. 3616166052 September 2018 ALTERNATIVES EVALUATION REPORT Penobscot River Phase III Engineering Study Penobscot River Estuary, Maine Prepared for: United States District Court District of Maine Prepared by: Amec Foster Wheeler Environment & Infrastructure, Inc. 511 Congress Street, Suite 200 Portland, Maine 04101 Project No. 3616166052 September 2018 _____________________ __________________ Nelson Walter, P.E. Eugene Shephard, P.E. Principal Project Manager Associate Engineer US District Court – District of Maine Alternatives Evaluation Report Penobscot River Phase III Engineering Study EXECUTIVE SUMMARY In January 2016, the United States District Court for the District of Maine (the Court) selected Amec Foster Wheeler Environment & Infrastructure, Inc. (Amec Foster Wheeler) to conduct the Penobscot River Phase III Engineering Study (Phase III Engineering Study), to identify and evaluate feasible, effective and cost-effective measures to remediate mercury in the Penobscot River Estuary (the Estuary). The geographic area to be addressed within the Phase III Engineering Study is described by the Court as “the region from the site of the former Veazie Dam south to Upper Penobscot Bay, including Mendall Marsh and the Orland River." Beginning in 1967, a chlor-alkali facility located in Orrington, Maine released mercury into the Estuary. Releases of mercury at overall declining concentrations continued throughout facility operation and ceased with facility closure in 2000. In 2002, the Court ordered an independent scientific study, the Penobscot River Mercury Study, to assess the spatial distribution and impact of mercury discharge in the Penobscot River. As of 2017, two phases of the study have been completed: Phase I in 2008 (PRMSP 2008) and Phase II in 2013 (PRMSP 2013). The Phase I Report (PRMSP 2008) concluded that there was enough scientific evidence to conclude that the Penobscot River is contaminated with mercury to an extent that poses risks to some wildlife species, and possibly some limited risk for human consumers of fish and shellfish. The Penobscot River Mercury Study Panel recommended that the study proceed to a second phase (Phase II). The Phase II Study estimated that although the Estuary has recovered significantly since the period of peak mercury discharge, it will take over 100 years for mercury concentrations in Estuary sediment to decrease to a level consistent with regional background concentrations in sediment at the current rate of system recovery (PRMSP 2013). The slow rate of decline of mercury concentrations in the Estuary is attributable, in part, to the presence of a large pool of mercury- affected mobile sediment in the Estuary. This mobile sediment is retained in the Estuary by natural processes that result in the landward flow of both bottom water and associated sediment under the influence of tides. This large volume of contaminated sediment is referred to in the Phase II Study as “the mobile pool” (PRMSP 2013). With these studies as background, and following additional sampling and analysis conducted by Amec Foster Wheeler in 2016–2017, this Alternatives Evaluation Report presents the results of the development, evaluation, and comparison of remedial alternatives that could be implemented to reduce ecological and human health risks resulting from the discharge and subsequent accumulation of mercury in the sediments and biota of the Estuary. Alternatives were developed, evaluated, and compared based on six evaluation criteria as established by the Court Order and the Phase III Engineering Study process. These criteria are: (1) viability of remedy; (2) whether the proposed solution has been successfully attempted previously or is innovative; (3) the likely Project No.: 3616166052 September 2018 ES-i US District Court – District of Maine Alternatives Evaluation Report Penobscot River Phase III Engineering Study cost of the solution; (4) the length of time to complete the recommendations; (5) the likely effectiveness of the solution; and (6) any potential environmental harm that may be caused by the proposed solution. The remedial strategy recommended as the result of the alternatives assessment presented in this report is presented in the Phase III Engineering Study Report (Amec Foster Wheeler 2018a). As a component of the evaluation process, bench-scale treatability studies were conducted to provide data for the development and evaluation of alternatives. Data generated from the bench- scale treatability studies were used to support selection of process options and technologies; refine engineering assumptions used as the basis for the detailed evaluation of alternatives; support cost estimation; and assess feasibility, limiting conditions and/or aspects of uncertainty associated with the implementing various remedial technologies. These studies included evaluation of: (1) the physical properties of sediments to determine whether physical separation techniques can be used to reduce the volume of sediment requiring treatment or removal; (2) the chemical properties of sediments, to assess the need for sediment treatment, removal, and containment, and subsequent material handling, dewatering, and water treatment or disposal requirements following removal (if applied); and (3) the toxicity of carbon-based amendments under consideration for application on marsh platforms. Regarding the current site understanding and material transport in the Estuary, the processes that control the internal cycling of sediment within estuaries will significantly influence the recovery time of the system. For estuaries like the Penobscot River Estuary that have been historically impacted by chlor-alkali discharge, recovery times have been documented to vary from years to decades, depending on how recovery is defined. Modelling of 2017 geochronology data and calculation of apparent recovery half times for the Estuary suggest that the apparent natural recovery rate is slowing relative to what was calculated in 2009 during the Phase II Study. The term ‘apparent’ is used herein consistent with its use in the Phase II Study in which the calculation of recovery rates is dependent on data extrapolation and assumptions regarding temporal mixing and redistribution of mercury in the Estuary. Increasing apparent recovery half times calculated in 2017 relative to the apparent recovery half time calculated in 2009 indicate that the rate of change in sediment mercury profiles over the 21-year interval from 1996–2017 is decreasing relative to the rate of change in sediment mercury profiles over the 21-year interval from 1988– 2009 used in the Phase II modeling. For cores collected in 2009 from locations defined as reflecting representative physical mixing and chemical attenuation within the Estuary (i.e., cores from locations in communication with the larger system), surface sediment concentrations in 2009 appeared to be converging toward 600–700 nanograms per gram (ng/g). For cores collected in 2017 from similarly defined locations, surface sediment total mercury concentrations do not Project No.: 3616166052 September 2018 ES-ii US District Court – District of Maine Alternatives Evaluation Report Penobscot River Phase III Engineering Study appear to have changed significantly from this average, and in some reaches of the Estuary remain higher than 700 ng/g. For data used in this report in the assessment of remedial alternatives, the general consistency in calculated average total mercury concentrations over much of the Estuary supports a hypothesis that the Estuary is achieving some level of homogenization or equilibrium redistribution of mobile mercury-affected sediment and wood waste. In attempting to evaluate or predict system- wide ecological recovery, the extent to which mobile sediments are a mixture of mineral sediment and wood waste—two distinct phases with differing particle sizes and densities, mercury concentrations, and transport properties—impacts the ability to accurately project recovery rates for the Estuary. Likewise, if sediment mercury concentrations in those portions of the system that are not in communication with the larger system are elevated relative to a homogeneously mixed concentration for other parts of the system, then changes to the hydrodynamic processes controlling sediment mixing or erosion (e.g., increases in wind/wave action, changes to flow regime) will also impact projections for system-wide recovery. The remedial evaluation presented in this report includes the delineation of the Estuary into reaches and hydrodynamic zones, and calculation of area weighted average total mercury concentrations within each reach/zone unit. Calculation of area weighted average total mercury concentrations included all total mercury data in the project database from 2000–2017, with the exception of data for which either the analytical laboratory, the analytical method, or sampling details were unclear. Data were grouped into discrete depth increments using an interval participation weighted concentration approach. This approach allows for the integration of data from a project database that includes a range of sampling types (e.g., grab samples and sediment cores) that may have been collected for differing objectives and