Recent trends in Hg emissions, deposition, and biota in the Florida everglades: A monitoring and modelling analysis

Abstract

With the discovery in 1989 of widespread, severe contamination of biota by mercury in the Florida Everglades, the Florida Department of Environmental Protection and its many collaborators mounted a sustained program of monitoring, modelling and research to plumb the causes of this problem and propose solutions. This analysis describes the 15-year trend records and reconstructions developed to put the research findings in historical context, whereas the process-level research findings are reported elsewhere. At the time these studies began neither the causes of this problem were apparent nor was there any expectation among the scientists that any would live to see the fruits of their labors-viz., a significant decline in mercury in Everglades biota. However, and most gratifyingly, beginning mid-decade of the 1990's and continuing into the new millennium, progressive, statistically significant declines in mercury concentrations have been observed in both largemouth bass and great egret nestlings at a number of sites located throughout the Everglades (Pollman et al., 2002; Frederick et al., 2001). It was not until later that retrospective studies determined that marked declines in local emissions and deposition of mercury (RMB, 2002; Husar and Husar, 2002) from the major point emissions sources in southern Florida antedated the declines in emissions and deposition. Atmospheric sources of wet and dry deposition are now accepted as the major sources of mercury to the Everglades (Stober et al., 2001), and because local emissions have been inferred to be the predominant source of mercury deposited in south Florida rainfall, including the Everglades (Dvonch et al., 1999), the question arises whether the observed declines in biota mercury concentrations can be related to declines in local emissions. This chapter reviews the existing data on mercury emissions, deposition, and biota trends in south Florida in order to address this question. Much of this discussion is based on work previously published by Pollman et al. (2002) and Pollman and Porcella (2003), but extends that work by including more recently available, longer time series for biota concentrations, as well as incorporating new analyses on wet deposition trends of mercury and some exploratory model hindcasting to examine the relationship between emissions and deposition, and aquatic biota response. This analysis integrates information from numerous studies to evaluate the causal relationship between mercury emissions, deposition and biotic response in the Florida Everglades. We began first with an examination of the recent trends in both mercury emissions and, as a surrogate to test the robustness of the emissions trends, mercury use. The mercury emissions and usage trends data were compiled by RMB Consulting & Research (RMB; 2002) and Husar and Husar (2002), respectively, for the major sources of mercury emissions in south Florida including municipal waste combustion (MWC) and medical waste incineration (MWI) facilities. These results indicate that large reductions in emissions (approximately 90% relative to peak emissions), occurring ca. 1991. Second, we statistically analyzed wet deposition fluxes for mercury from November 1993 through December 2002 for samples collected in Everglades National Park to determine whether these trends can be related to changes in the atmospheric signal, or are related to changes in rainfall patterns. Although the monitoring period for wet deposition began well after the largest fraction of the reductions in local emissions had occurred, the wet deposition signal still showed a significant decline (ca. 25%; p = 0.0413) that agrees reasonably well with the emissions declines during the same period. Third, we compare the biota trends to examine whether the timing and magnitude of changes in largemouth bass mercury observed in the Everglades are consistent with predicted changes produced by the changing deposition trajectory. These results suggest that the biota changes are indeed consistent with the estimated declines in local emissions and deposition, although additional analyses to test other hypotheses should be conducted before more definitive conclusions are reached.