Estimation of dynamic load of mercury in a river with BASINS-HSPF model

Purpose Mercury (Hg) is a naturally occurring element and mately 80% of the anthropogenic sources of Hg are emissions of elemental Hg to air, primarily from fossil fuel combustion, mining, smelting, chlor-alkali plants, and from incineration of solid wastes (Hamasaki et al. 1995; Stein et al. 1996). Another 15% of anthropogenic Hg loading to the land is the result of direct application of fertilizers, fungicides, and municipal solid waste containing Hg residues. The remaining 5% of anthropogenic Hg load enters by direct discharge of commercial effluent to receiving water bodies (Stein et al. 1996). Transport of Hg from watersheds to a basin outlet reflects the collective influence and interaction of the various geological, climatic, hydrological, soil, and landuse characteristics of the watersheds (Bishop and Lee 1997). In watersheds disturbed by human activities, direct point source discharges of Hg to surface waters contribute to Hg contamination more than the atmospheric deposition in many cases (Balogh et al. 1998). Human activities resulting in landscape disturbance (agriculture, logging, urbanization, etc.) also enhance the delivery of Hg to surface waters. Watershed type can exert a strong influence on Hg transport. Hurley et al. (1995) investigated effects of land use and cover characteristics on total Hg and methylmercury concentrations in 39 rivers in Wisconsin. These authors concluded that atmospherically deposited Hg undergoes siteand seasonal-specific processing as it is transported from a watershed. For instance, in agriculturally dominated watersheds, Hg is mainly associated with particulate phases, while in the wetlanddominated watersheds, filtered phases (i.e., dissolved and soluble with passing through 45 mm filter) dominated. In anthropogenically influenced areas, point source discharges must be evaluated before the Hg cycle can be fully understood (Hurley et al. 1998; Sim and Francis 2008). Transport by rivers is an important pathway for mobilization of Hg (Gill and Bruland 1990; Babiarz and Andren 1995; Hurley et al. 1995, 1998; Fleck et al. 2011). Mercury may enter flowing waters from geological sources through groundwater, surface or near-surface weathering processes or fine particulates (Rasmussen 1994; Plouffe 1995), direct atmospheric deposition (Fitzgeral and Gill 1979), and through leaching of soils and plant material to groundwater and surface waters (Hultberg et al. 1995; Krabbenhoft et al. 1995). In addition to receiving geologic and atmospheric inputs of Hg, rivers serve as receptors for industrial and municipal discharges (Glass et al. 1990; Hurley et al. 1995). In many cases, rivers represent the major hydrologic sources for lakes and reservoirs. This is the case for Lake Michigan, which receives a substantial fraction of its water budget from tributary inputs (Glass et al. 1990; Hurley et al. 1998). Several Lake Michigan tributaries and associated harbors are currently listed by the US Environmental Protection Agency (US EPA) as areas of concern for priority pollutants. The discovery of elevated levels of Hg in freshwater fish in the Florida Everglades, the deaths of several of the endangered Florida Panther (suspected to have been the result of Hg toxicity), and the linking of Hg to the decline of wading bird populations in this sensitive ecosystem has prompted State and Federal agencies to investigate the sources of Hg to this region (Dvonch et al. 1998). Atmospheric sources of Hg to south Florida may include both natural and anthropogenic contributions. Potential natural Hg sources in south Florida include the dispersion and transport of Hg from the surrounding ocean by sea spray and from diffusion from soil into the Everglades waters themselves (Dvonch et al. 1998). To examine the potential impacts of local anthropogenic sources of Hg, a study was conducted to monitor south Florida atmospheric Hg deposition in southeast Florida from August 6 to September 6, 1995 (Dvonch et al. 1998). Daily event precipitation samples were collected concurrently at 17 sites across the study domain during the 1-month period. The area normalized volume-weighted mean concentrations of Hg measured at the 17 sites during the study ranged from 13 to 31 ng year. While these monthly means indicated a significant site-to-site variation in Hg concentration, even greater differences between sites were observed on an event basis. Concentrations of Hg in individual daily event precipitation samples ranged from 5 to 113 ng year. These observed spatial and temporal patterns suggest that local sources strongly influence atmospheric wet deposition across this region. Measurements of atmospheric Hg deposition in north Florida are very limited. As part of the Florida Atmospheric Mercury Study, Guentzel et al. (2001) measured wet deposition fluxes of total Hg at a series of 10 sites located throughout Florida, including a site at Lake Barco in north central Florida. Both wet-only and bulk deposition measurements of Hg were collected monthly from 1993 through 1996. Wet deposition of Hg ranged from 12.5 to 18.9 μg m y and averaged 14.9±2.9 μg m y. Dry deposition of Hg in Florida is believed to be largely due to scavenging of gaseous Hg(II), which is a particularly labile or reactive form of Hg and therefore often labeled as reactive gaseous Hg. While there are numerous studies correlating anthropogenic inputs to the environment with elevated levels in freshwater ecosystems (Lathrop et al. 1991; Lange et al. 1993; Driscoll et al. 1994; Eisler 2004; Lee et al. 2008), there is little information on the concentration and cycling of Hg in many estuarine systems such as the Lower St. Johns River (LSJR) Basin in Florida. In recent years, the St. Johns River Water Management District (SJRWMD) of Florida has sampled sediments in the LSJR and its 208 J Soils Sediments (2012) 12:207–216