Monitored Natural Recovery for Onondaga Lake: A Progress Report

摘要

Background/Objectives. The Onondaga Lake (in Central New York State) Record of Decision specifies monitored natural recovery (MNR) as the remedy to achieve sediment performance criteria established for the profundal zone (deep-water areas that comprise 70 percent of the lake surface area). To evaluate the effectiveness of MNR, site‐specific numerical modeling supported by several types of ongoing monitoring data was used to predict mercury concentrations into the future and estimate the time to recovery and to determine the likely future effectiveness of MNR. The remedy is predicted to successfully meet the sediment performance criteria prior to the end of the prescribed MNR period, which is the 10 years following the remediation of upland sources and the littoral zone (2027). Approach/Activities. A site-specific model, developed in Microsoft? Excel using Visual Basic for Applications (VBA), is based on sediment processes that are well established in literature and incorporates site-specific monitoring data on sedimentation rates, sediment mixing depth, and surface sediment mercury concentrations. Mixing and rate of sedimentation are primary processes resulting in natural recovery in the profundal zone. Both typical and innovative monitoring data collected on these processes included sediment traps, high-resolution mercury cores, radioisotope cores, frozen cores, and most recently, deployment and monitoring of fluorescent microbead markers that marked the mudline elevation to quantify the thickness of newly deposited material. Supported by this monitoring data, the model was calibrated to mercury concentrations from surface sediment collected between 1992 and 2011 at almost 100 locations. Results/Lessons Learned. Although previously available lines of evidence, such as macroinvertebrate sampling and current shear-stress evaluations, indicate that very little mixing is occurring in the profundal zone sediments, data from frozen cores and radioisotope cores were used to specify the site-specific mixing depth in the model. Visual inspection of frozen cores and the majority of radioisotope cores indicated that mixing of sediment in most areas of the profundal zone is taking place in the top few centimeters only. Sedimentation rates were also examined using the radioisotope cores, highresolution mercury cores, and sediment trap data. Average sedimentation rates from these sampling programs ranged from 0.26 to 0.34 grams per square centimeter per year (g/cm~2/yr). Cores collected from microbead marker plots in summer 2012 show the fluo