Agricultural practices and the management of surface water flow, occuring in and near the Everglades Agricultural Area (EAA) south of Lake Okeechobee, are important contributing factors in observed recent changes in the sawgrass prairie. The flow of waters enriched in nutrients is thought to play a role in mercury accumulation and cycling and in changes in vegetation communities (especially changes in algal populations and an increase in cattail and the decrease in sawgrass). It is believed that a variability gradient in this influence occurs primarily from north to south; however, the relative importance of surface vs. ground water flow patterns, and the associated hydro-geochemistry, has not been defined.
Objectives of Current USGS Trace Metal Studies. Using the north-south nutrient gradient hypothesis: 1. Define the importance of the solid phase (organic-rich sediments) on mercury cycling--that means, understand the role and inter-relationship of depth, time of deposit, organic matter decomposition, pore water trace element chemistry, and other geochemical parameters on the movement of mercury through the system. 2. Determine what effect assemblages of trace elements (including the environmentally important elements Pb, Cd, As, Cr, Cu, and Zn) have on mercury transport mechanisms. Can this information help explain mercury's bioavailability? 3. Using the sediment record, determine whether sources of trace elements (including mercury) have changed over time.
Results of initial studies. Enrichment factors: Using ratios of trace element levels in sawgrass (normalized to some non-metabolic element like aluminum) to the chemistry of the peat substrate shows that metabolic elements are being bioconcentrated in living tissue (their concentration in sediments is proportionally less than in living tissue). This means that P, K, Cu, Fe, and Zn are in forms readily absorbed by sawgrass. In contrast, concentrations of non-essential metals (such as Cr, Co, Pb, and Hg) are generally not being concentrated in living tissue over what is in the sediments. This means that non-essential metals are not being cycled but are accumulating in the peat.
Vertical Metal Concentration Variability: Accumulation rates (calculated in grams per square centimeter per year) for metals show a dramatic decrease with depth in the sediment core. This decrease is independent of whether the core was taken north (close to EAA and its drainage into Water Conservation Area 2A) or south (nearly 15 miles from the drainage from EAA). A baseline metal concentration seems to be reached in sediments that are about 100 years old. The accumulation rates of metals in the individual sediment column varies north to south and may reflect the influence of EAA. For example, mercury shows slightly higher accumulation rates in sediment cores collected in the north part of Water Conservation Area 2A when compared to cores in the south part of 2A. Accumulation rates for Ni, V, and Zn show very little difference between the areas whereas Pb is highest in the near surface layers of cores from the south. Definitive conclusions and interpretations must wait until data is received for additional cores currently under investigation.
Research plan--1996: Field studies will emphasize the geochemical processes occurring in organic-rich sediments within the Taylor Slough, Everglades National Park. This has two purposes, the slough represents the eastern-most major non-canal surface drainage feature and is the "end of the hydro-geological pipe". It is also a major contributor of fresh water to the eastern side of Florida Bay and ties in with ongoing water and sediment studies within the Bay. Results from the Taylor Slough study will help direct a similar bio- geochemical characterization of processes in Shark River Slough, the major surface water drainage feature in the Everglades system.