Transport, air-surface exchange and landscape accumulation of airborne mercury deposited onto South Florida Everglades: A simulation study

Dynamics of airborne mercury deposited to terrestrial soils and wetland areas are investigated within the framework of a process-oriented but screening-level simulation modeling effort. Two recently developed models are applied to the South Florida Everglades, an area that has received much attention in the last few years due to the implications of its mercury contamination problem. Model dynamics are driven by simple hydrological, agrometeorological, biogeochemical and eco-physiological principles. Simulations were designed to calculate the long-term buildup and the recent seasonal dynamics of mercury for both the agricultural area and the wetland areas. Atmospheric deposition and hydrologic data were specified from available regional databases. Model process rates and coefficients were initially specified from previous studies, and modified in some cases with site-specific experimental results. Computations of transport fluxes, surface-atmosphere exchange rates and landscape accumulation are compared to observational data provided by a region-wide statistical sampling program. The simulation models were found successful to calculate many of the observed mercury concentration levels and fluxes. The sensitivity of model output to external and internal loadings and to process formulations was also examined and average mass balance fluxes were calculated.

Conclusions were drawn but must be considered as tentative due to the screening-level character of this study. For the agricultural area, mercury mass balance is characterized by the intensive exchange of mercury between atmosphere and ground surface. Atmospheric deposition of divalent mercury is the main source. Deposited mercury displays a strong tendency to be remobilized into the atmosphere as elemental mercury formed by the reduction of divalent mercury in the surface soil. Transport flux via runoff reflects the collective influence and interaction of the various hydrological, agroclimatological, land cover and scale characteristics of the field. In the wetland areas, the major source of mercury is from direct atmospheric deposition while the major sink is to the underlying soils; mercury evasion and downstream drainage are minor mass balance components. Periphyton and macrophytes are important agents in sequestering open water mercury in the wetland and depositing it to the soil. There is no discernible seasonal response to weather conditions and atmospheric loadings in the predicted soil mercury. The predicted wetland open water response is, however, strongly seasonal, with a minimum in spring (dry season loads, high vegetation) and a maximum in summer (wet season loads, low vegetation). Finally, in terms of lanscape accumulation of airborne deposited mercury, results showed that mercury increases in concentration from background, asymptotically approaching close to steady-state levels in about 50 years, for both the agricultural and the wetland soils.

It is expected that the integration of the ongoing model development, monitoring, and process studies will lead to more-predictive models, and therefore to advances in scientific understanding of dynamics of airborne deposited mercury in the South Florida Everglades.