Sulfur Contamination and Geochemistry of the Everglades
William Orem, Harry Lerch, Anne Bates, Margo Corum, and Marisa Beck
U.S. Geological Survey, Reston, VA, USA
Sulfur is an important water quality issue in the Everglades because of its role in
microbial sulfate reduction and the methylation of mercury. Microbial sulfate
reduction produces toxic hydrogen sulfide (H2S) as an endproduct, and
accumulation of H2S in sediment porewater may change sediment redox
chemistry and the ability of aquatic macrophytes and other marsh plants to
maintain necessary oxygen levels in root systems. Microbial sulfate reduction also
produces methylmercury (MeHg), a neurotoxin that is bioaccumulated. MeHg has
been found in high concentrations in freshwater fish from the Everglades, and
poses a potential threat to fish-eating wildlife and to human health (especially
pregnant women) through fish consumption. Based on USGS results, sulfur
appears to play a key role in regulating both the magnitude and distribution of
MeHg within the ecosystem.
Freshwater wetlands typically have low sulfur concentrations, but we discovered
high concentrations of sulfate sulfur in surface water of the northern Everglades.
Independent studies conducted by the U.S. EPA and the South Florida Water
Management District have also documented high concentrations of sulfate in large
parts of the northern Everglades. Marshes in portions of the Water Conservation
Areas (WCA’s) have surface water sulfate concentrations that average nearly 60
mg/l, compared to concentrations of 1 mg/l in pristine areas of the Everglades.
Areas with high surface water sulfate concentrations are concentrated in the
northern WCA’s, and especially near sites of canal discharge and along canal
levees. Even higher concentrations of sulfate (average >70 mg/l and sometimes
approaching 200 mg/l) were found in canal water draining from the Everglades
Agricultural Area (EAA). This canal water appears to be the major source of
excess sulfate entering the Everglades. The high loads of sulfate entering the
ecosystem stimulate microbial sulfate reduction, and areas with high
concentrations of sulfate in surface water also have very high levels of toxic H2S
in sediment porewaters. Porewater sulfide concentrations range from 5,000 ppb at
marsh sites near canal discharge, to <0.05 ppb at pristine sites. Porewater sulfide
concentrations were highly correlated with surface water sulfate concentrations.
We used sulfate concentration data and the sulfur (34S) and oxygen (18O)
isotopic composition of sulfate in marsh surface water, canal water, rainwater, and
groundwater to trace the source(s) of the excess sulfate entering the Everglades.
Although the canals originate in Lake Okeechobee, the lake on average
contributes only about 20% of the sulfate observed in the canals. Rainwater has
too little sulfate to account for the high sulfate concentrations observed in the
canals and in large portions of the Everglades. Groundwater beneath the
Everglades has either too low a sulfate concentration or a 34S signature that is
inconsistent with that of surface water in the Everglades. Both sulfate
concentration data and the 34S values of sulfate in surface water from the
Everglades and canal water confirm that canals in the EAA are the major source
of excess sulfate entering the ecosystem. Furthermore, results showed that canal
water with the highest sulfate concentrations had 34S values of +16 per mil,
which is consistent with the 34S signature of agricultural sulfur (a soil
amendment used in the EAA). Sulfate extracted from the upper 10 cm of soil in a
sugarcane field in the EAA also had a 34S value of about +16 per mil. These data
suggest that agricultural sulfur is the principal source (but not the only source) of
excess sulfate entering the canals and Everglades wetlands.
The high levels of sulfate entering the Everglades from canal discharge have had
important effects on the ecosystem. The H2S buildup in sediment porewater at
sites in the northern Everglades has significantly lowered redox potentials in the
sediments. The lower redox conditions and high levels of H2S in the sediments
may have an impact on vascular plant growth in the Everglades by limiting
oxygen penetration to roots. It is noteworthy that cattails have replaced sawgrass
as the dominant macrophyte at sites heavily impacted by sulfur, although factors
other than sulfur (e.g. eutrophication and high water levels) may also be
influencing this change. Similarly, tree islands have disappeared mostly from the
northern Everglades, in areas heavily impacted by sulfur.
The effect of excess sulfur load on MeHg production in the Everglades is
complex. Sulfur appears to stimulate MeHg production through increased
microbial sulfate reduction, but buildup of sulfide in sediment porewater inhibits
MeHg production. The balance between these two effects of sulfur influences
both the magnitude and location of MeHg production in the Everglades, and
produces the interesting effect that the zone of maximum MeHg production
occurs in areas with sulfur loads only moderately higher than natural. This MeHg
“Goldilocks” zone (where sulfur levels are just right) occurs in the central portion
of WCA 3A, at the front end of the northern Everglades sulfur contamination
plume. The complex relationship between sulfur geochemistry and MeHg
production in the Everglades was first hypothesized from field studies at various
sites in the ecosystem. This hypothesis has been further solidified from
experimental studies in the Everglades using mesocosms. These mesocosm
studies show that increasing sulfate concentrations increases MeHg production up
to sulfate concentrations of about 10 mg/l. At sulfate concentrations >10 mg/l
buildup of sulfide from microbial sulfate reduction inhibits the methylation of Hg.
The principal sink for sulfur entering the Everglades ecosystem is long-term
storage in sediments. Sequestration of sulfur in sediments typically results from
microbial reduction of sulfate to sulfide, and subsequent reaction of sulfide with
either organic matter, to form organic sulfur compounds, or metals, to form
insoluble metal mono- and disulfides. The dominant sulfur species in Everglades
sediments is organic sulfur. Sulfur accumulation rates (g m-2 day-1) in Everglades
sediments ranges from 9.0 x 10 –3 to 0.38 x 10-3, a difference of more than 20 fold.
The highest sulfur accumulation rates occur in brackish water mangrove (average
of 6.0 x 10-3), and sulfur-contaminated freshwater marsh areas (average of 4.5 x
10-3). Sulfur accumulation rates average about 1.6 x 10-3 at pristine sites.
Sulfur in sediments represents a reservoir of reduced sulfur that may be
reoxidized and remobilized during drought/fire and subsequent rewetting.
Because drought and fire are frequent occurrences in the Everglades ecosystem,
the remobilization of sulfur from sediments is an important factor influencing
sulfur and mercury geochemistry. Studies of sulfur remobilization were conducted
in northern WCA 3A following a burn and subsequent rewetting in that area
during 1999. The average sulfate concentration in surface water at 14 sites was
58.6 mg/l immediately after the burn and rewetting of this area, compared to an
average sulfate concentration of 5.10 mg/l before and over a year after the burn.
The sulfate remobilized after the burn and rewet stimulated sulfate reduction and
extreme levels of MeHg production (see abstract by Krabbenhoft et al., this
volume). Cores from burned and control (e.g. unburned) sites in northern WCA
3A showed that burned sites had significantly lower total sulfur contents in
sediments. A simulated laboratory experiment was conducted in which mini-cores
were dried under simulated natural sunlight in controlled laboratory conditions for
a specified period of time and then rewetted to further examine the effects of
fire/drought on S geochemistry and MeHg production. Results verified the
remobilization of sulfate and stimulation of MeHg production following
fire/drought events. Ecosystem and water managers must consider the effect of
drying and rewetting portions of the Everglades in order to avoid exacerbating the
already extensive MeHg problem in the Everglades.
William Orem, U.S. Geological Survey, 956 National Center, Reston, VA 20192
703-648-6273 (office); 703-648-6419 (fax); borem@usgs.gov