The Tisza River is the longest tributary of the Danube River, comprising 20% of the Danube's catchment, with about 30% of that area in Hungary. The Tisza has great ecological and economic importance in the region. Its floodplain is among the most biologically diverse areas in Europe, and the river provides drinking water, irrigation, and supports an economy based on agriculture, fishing and tourism. In January 2000, a dam burst at a mining facility near Baia Mare, Romania, releasing about 100,000 m3 of mining wastewater with up to 120 tonnes of cyanide and metals (Cu and Zn primarily) into the Lapos, Szamos, Tisza and Danube rivers. In March 2000, another mine tailings dam burst in the same region at Baia Borsa, releasing about 20,000 tonnes of tailing sludge with heavy metals (Cu, Pb and Zn primarily) into a Tisza River tributary. In April 2000, major floods mobilized and dispersed the mine effluents downstream and onto the floodplain. In March 2001, The Tisza River flooded again, leading to a declared state of emergency in Hungary. Flood dispersal after the second spill of heavy metal-laden mud led to severe environmental impacts in plankton and macroinvertebrate communities. Data from 3-m long soil cores, taken in July 2000 at four floodplain locations, indicated substantial Pb, Zn and Cu deposits to apparently 170-cm depth, with Cd showing a more mobile and deeper trend. Plant uptake data showed accumulation by shallow-rooted plants coincided with metal load in the topsoil. Longer term impacts of the spills are still uncertain. Two workshops, funded by the NSF's International Program and the Hungarian's OTKA/MTA, were convened in 2002 where scientists and engineers from the U. S. and Hungary discussed the ecological consequences of metal deposition from flooding on Hungarian floodplain ecosystems. Subsequent funding of a proof-of-concept “Biocomplexity in the Environment” proposal enabled the U. S.-Hungarian team to collect field data, establish research plan and firm up hypotheses on metal dynamics and bioavailability in a floodplain setting. Data from samples collected in the fall 2003 in floodplains along the Tisza River indicate substantial enrichment of metals in the examined soil pedons. Of the trace metals analyzed (Cd, Co, Cr, Pb, Cu, Mn, Ni and Zn), Cd and Zn are particularly concerning as their bioavailable (NH4OAc-EDTA extractable) fractions, in most sites, exceeded the allowable threshold values (0.50 mg/kg for Cd and 40 mg/kg for Zn) established by Hungary and WHO/FAO. These threshold values signal potential phytotoxicity for agronomic species, and the need for a remediation and management scheme according to land use. Significant correlations between bioavailable Cd and bioavailable Zn, and between their bioavailable fractions and total concentrations infer commonality in their source term. Another concerning observation from these data are the unusually high levels of sulfate in floodplain sites. As with the metals, high correlations between total S and 0.01 M CaCl2 soluble sulfate indicate a common source, most likely from pyrite-enriched mine effluents dispersed in the floodplain by major floods in 2000. These data, along with earlier samplings taken by the Hungarians, show that metal accumulations occurred primarily in surface soils, the bioavailable fraction constituted a substantial portion of the total, and that S was dominated by its soluble sulfate form. For ecologically important metals, such as Cd and Zn, the abundant bioavailable fractions and sulfate indicate that, although the soils presently have relatively high free CaCO3 content, the co-existence of S may eventually override the soil buffering capacity, thus producing a potentially serious “time bomb” phenomenon. Such a scenario is not unrealistic given the history of accidental releases of acidic mining/industrial effluents into the Tisza.