U.S. Department of the Interior
U.S. Geological Survey
OFR-00-327

SUMMARY

We report progress on the task "Processes in Organic-Rich Sediments of South Florida–Mercury and Metals", part of the USGS-cooperative "Aquatic Cycling of Mercury in the Everglades" (ACME) project. Objectives of the task include: (1) definition of mercury and other element spatial concentration patterns and baselines, (2) definition of historical Hg and other element concentration trends in organic-rich sediments, and (3) development of an understanding of element uptake in sawgrass as it relates to element bioavailability in organic-rich sediments. The report focuses on work in Taylor Slough, Everglades National Park, because it is both the terminus of surface water flow for the eastern side of the Everglades and the main source of fresh water to the hyposaline eastern Florida Bay.

Surface water, sawgrass leaf and root material, and cores of organic-rich sediments were collected in 1996 at eight sites within Taylor Slough along a predominantly north/south trend. The upper-most site was near the canal water discharge gate to the Slough and the lower two sites were within several km of Florida Bay. All samples were collected, preserved, transported and analyzed for mercury and other elements using accepted USGS QA/QC procedures and protocols.

We present element concentration data for over thirty elements in the water, plant, and sediment samples collected. In addition to the cores used for element analysis, additional cores were collected at each site and used for nutrient and sulfur species studies, peat diagenesis studies, and geochronology (dating) determinations using ²¹⁰Pb. The core dates and dry bulk density data were used to calculate sediment accumulation rates which were in turn used, in conjunction with the element concentration data, to calculate element accumulation rates (EAR).

Very little variation in element concentration among cores was noted for the major elements. Several cores had higher Ca concentrations near the sediment surface which is attributed to localized biogenic marl accumulations. In addition, concentrations of Al and Fe in some cores increased with depth. In contrast, the concentration of minor (trace ) elements did present much greater regional and down-core variability. Our data, however, do not show strong near-surface trace element enrichment. This is in contrast to trends reported in the literature further north in the Water Conservation Areas (WCA). Only Pb and Zn show some enrichment in the top 10-15 cm for cores TS2, TS7E, TS9, and TS15. However, these elements, as well as Cu and Ni, often show an increase in concentration below about 30 cm. Our data, for the concentration of environmentally important trace elements in organic-rich sediments, when compared with more mineral-rich soils from throughout Florida, show levels that are about two- to eight-times greater.

Except for core TS15 (and possibly TS2) no strong pattern of higher total Hg concentration near the sediment surface was noted (~0.1-0.2 ppm). This tendency for Taylor Slough is contrary to trends reported for sediments in WCA further north where values as high as 0.41 ppm have been reported in the top 10 cm of similar sediments. The total Hg concentrations for all core sediments that we report (0.03-0.26 ppm) are , in general, two-to-three times less than those reported in WCA. For the five cores examined, our calculated Hg EAR’s showed an increase between about 8 and 12 cm in depth. Depending on the core, the depths corresponded to sediment ages from about 20 years to about 60-180 years. In contrast, the sediments in WCA have shown a general decrease in EAR with depth. Our highest EAR values (>200 µg m^-2 yr^-1) were found in the core from site 1 (TS1), the site furthest north in the Slough, and were fairly uniform with depth. These values directly reflect high sediment accumulation rates and not high total Hg concentrations. These EAR values (~200 to ~320 µg m^-2 yr^-1) are several-times greater than those found in post-1985 core material from WCA (23-141 µg m^-2 yr^-1). Progressing south in the Slough the EAR values for the cores decrease and the range of values for TS15 (furthest south) are ~2.2-10.

Sulfur concentrations and speciation were determined in sediments and pore water and are only peripherally discussed in this report. Sulfur is found to possibly influence the metal distributions in sediments. The environment of Taylor Slough ranges from a freshwater, low sulfate environment in the north, to a brackish water, high sulfate environment in the south. Metals showed generally poor correlations with total sulfur content in the freshwater areas whereas several metals (Ni, Cr, Fe, and Cu) showed reasonably strong correlations (R² = 0.8 or higher) in the brackish water zone. Correlation is not a proof of a mechanism, but does provide an indication that sulfur may play a role on metal distributions in sediments, especially in the southern part of the slough.

The uptake of elements from sediment by sawgrass (element bioavailability) was evaluated using enrichment factors (EF) normalized to Al. Except for Fe, the EF values in root tissue were lower than in leaf tissue. This difference reflects the greater concentration of most elements in root tissue relative to leaf tissue. The high EF values in general indicate that sawgrass tissue has a tendency to accumulate elements relative to the substrate within which it is growing. Because the sediment material is primarily decomposed sawgrass, the metals are being remobilized from dead tissue to living tissue. Further, the high EF values may indicate a physiological compensation mechanism that allows for element uptake and accumulation by sawgrass in the low nutrient sediments that are characteristic of the Everglades peat lands. Many elements (Hg, Mg, Na, and Zn) show strong decreasing north/south EF trends.

All total Hg analyses in surface water were below the 0.2 µg L^-1 detection limit for the analytical method used. This detection limit is two orders of magnitude above the values commonly reported for surface water in the canals that feed WCA. The data for other major and trace elements in water are regionally uniform and do not show pronounced areal trends.

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