Distributions of mercury and phosphorous in Everglades soils from Water Conservation Area 3A, Florida, USA

Soils in the southern half of Water Conservation Area 3A are mostly peats with some organic-rich marls. Mercury contents of 64 surface samples over a 500 km(2) area average 28.7 ng cc(-1) (209 ppb dry sediment), which is typical of organic-rich soils. High Hg contents in Everglades fish are therefore not caused by anomalously high soil Hg. Hg contents show no systematic lateral variation, consistent with deposition from well-mixed atmospheric sources rather than nearby point sources or runoff from canals. Cores from 9 sites contain more Hg and P at or near the surface than at 20-30 cm depth. Hg and P contents of individual cores correlate well and define separate background and anomalous populations. The subsurface distribution of P is determined largely by uptake by sawgrass and other plants. The correlation between P and Hg suggests that, although atmospheric Hg deposition has undoubtedly increased in recent decades, postdepositional mobilization of Hg may be important in Everglades soils. This finding, together with recent direct measurements of atmospheric Hg deposition, indicates that previous estimates of Hg deposition rates based on Everglades peat cores, which assumed that Hg is immobile in peat after deposition, have yielded large overestimates.