Monitoring and Mass Budget for Mercury in the Everglades Nutrient Removal Project

Abstract

The Everglades Nutrient Removal Project (ENRP) is a 1,545-ha prototype wetland constructed to test a larger system of stormwater-treatment areas that is being used to remove agricultural nutrients and help restore the remnant Everglades. Concerns that the ENRP may lead to mercury bioaccumulation as observed in other newly created reservoirs resulted in an effort to construct a mercury mass budget and measure bioaccumulation in aquatic organisms. For the first 2 years of operation, total mercury (THg) and methylmercury (MeHg) concentrations in water decreased significantly a short distance into the ENRP, probably through particle settling. During both years, THg concentrations in largemouth bass were about 0.1 &mgr;g/g throughout the ENRP while the adjacent Water Conservation Area bass were at or above the state health advisory level of 0.5 &mgr;g/g. THg levels in mosquitofish were lower than levels found in bass, and mosquitofish mercury concentrations in the interior of the ENRP were significantly lower than the inflow and outflow sites. The major THg pool was the sediment while less than 1% the total mass was stored in vegetation, water, and fish. The major MeHg pool also was the sediment but vegetation, water, and fish were significant fractions. Mass budget estimates showed the inflow pump and rainfall to be the major sources of THg, while the outflow pump was the major output. The inflow pump was the major source of MeHg while the outflow pump was the major output. Evasion of elemental mercury estimated by the stagnant-film model indicated a minor loss, but field measurements over cattails suggested that evasion may be the major pathway. When all inputs and outputs were considered, the ENRP removed about 70% of the THg and MeHg masses in the first 2 years of operation.

Extreme Spatial Variability and Unprecedented Methylmercury Concentrations Within a Constructed Wetland

“…The U.S. Environmental Protection Agency (USEPA) had encountered only two slightly higher MeHg values in 100s of samples of surface water collected in South Florida from 1995-1999 (D. Scheidt, USEPA, personal communication). Further, 4.8 ng MeHg/L represented 83% of the 5.8 ng THg/L present in the paired sample and this %MeHg was elevated relative to other areas (Southworth et al, 2002) and even relative to high values typically found in the Everglades Miles and Fink, 1998). MeHg concentration in the adjacent Cell 2 was only 10% of that detected in Cell 1 (Figure 3).…”

“…Although studies elsewhere continued to report both "first flush" of inorganic mercury and MeHg biomagnification leading to a so-called "reservoir effect" (Kelly et al, 1997;Paterson et al, 1998;Bodaly and Fudge, 1999), results from monitoring the prototype STA, the Everglades Nutrient Removal (ENR) Project, found no evidence of these problems (Watras, 1994;Miles and Fink, 1998). Quite the opposite, the ENR Project was found to be a net sink for both total mercury (THg) as well as MeHg.…”

“…Quite the opposite, the ENR Project was found to be a net sink for both total mercury (THg) as well as MeHg. What is more, fish from the ENR Project had some of the lowest tissue Hg levels in south Florida (Miles and Fink, 1998;Fink, 2000). Nevertheless, monitoring programs at STAs constructed subsequently have revealed transitory spikes in MeHg production (Rumbold, 2004a).…”

A Screening Level Probabilistic Risk Assessment of Mercury in Florida Everglades Food Webs

“…Reported Hg concentrations in the large fish trophic group ranged from 0.030 to 0.056 ppm Hg in bluegill in WCA 1 (Miles and Fink, 1998) to 1.7-1.8 ppm Hg in largemouth bass in Shark Slough (Loftus and Bass, 1992). Concentrations as high as 7 ppm have been reported in large predatory fish in the Everglades (Stober et al, 1995).…”

“…Because of the high levels of Hg found in largemouth bass and other freshwater gamefish, fish consumption advisories have been issued for the state of Florida (Ware et al, 1990). Also, alligator (Alligtor mississippiensis) hunting in the Everglades was restricted because of elevated levels of Hg (Loftus and Bass, 1992), and the Florida Department of Environmental Protection criterion for total Hg in freshwater (12 ng/L; class III water) has been exceeded in the Everglades (Miles and Fink, 1998). The bioaccumulation of Hg in the aquatic food web has also been associated with health problems in top predators of the Everglades (Frederick and Spalding, 1994;Roelke et al, 1991).…”