RESUMEN
Fifty years ago, the Penobscot Estuary was contaminated by mercury discharged from the chlor-alkali plant located in Orrington, Maine, USA. Almost all of the mercury was discharged from the plant during the late 1960s and early 1970s. Despite the much lower mercury discharges in recent decades, present-day concentrations in surface sediment remain high (averaging 350-1100â¯ng/gâ¯dw) and are still high in blood of marsh birds (up to 10.5⯵g/g), black duck muscle (0.8⯵g/g), and lobster muscle (0.4⯵g/g). Methyl mercury (MeHg) concentrations in marsh birds exceed levels that impair reproduction. There are health advisories for duck hunters and closures of shellfish fisheries. These continuing high mercury concentrations are caused by the trapping of legacy mercury in a mobile pool of sediment that is retained in the upper estuary above a tidally forced salinity front, which travels up and down the estuary each tidal cycle - slowing the transport of particulate mercury to Penobscot Bay. The trapped legacy mercury continues to be available for methylation 50â¯years after it first entered the estuary. This is demonstrated by the fact that rates of MeHg production are positively related to the inorganic mercury concentration in parts of the estuary with elevated concentrations of legacy mercury. Thus, remediation measures that would lower the THg concentration in surface sediment would lower the MeHg in birds, fish and shellfish. All of this new information leads us to recommend two remediation options. Addition of mercury binding agents may lower mercury concentrations in birds in some wetland areas. System-wide, we also recommend Enhanced Natural Recovery (ENR), a novel approach that involves the partial removal of the contaminated mobile sediment pool followed by replacement with clean-clay particulates to dilute inorganic mercury concentrations, which would lower methylation rates and mercury concentrations in biota.
RESUMEN
Computational models support environmental regulatory activities by providing the regulator an ability to evaluate available knowledge, assess alternative regulations, and provide a framework to assess compliance. But all models face inherent uncertainties because human and natural systems are always more complex and heterogeneous than can be captured in a model. Here, we provide a summary discussion of the activities, findings, and recommendations of the National Research Council's Committee on Regulatory Environmental Models, a committee funded by the U.S. Environmental Protection Agency to provide guidance on the use of computational models in the regulatory process. Modeling is a difficult enterprise even outside the potentially adversarial regulatory environment. The demands grow when the regulatory requirements for accountability, transparency, public accessibility, and technical rigor are added to the challenges. Moreover, models cannot be validated (declared true) but instead should be evaluated with regard to their suitability as tools to address a specific question. The committee concluded that these characteristics make evaluation of a regulatory model more complex than simply comparing measurement data with model results. The evaluation also must balance the need for a model to be accurate with the need for a model to be reproducible, transparent, and useful for the regulatory decision at hand. Meeting these needs requires model evaluation to be applied over the "life cycle" of a regulatory model with an approach that includes different forms of peer review, uncertainty analysis, and extrapolation methods than those for nonregulatory models.