GEOCHEMICAL CONTROLS ON TRACE METAL TRANSPORT IN AQUEOUS ENVIRONMENTAL SYSTEMS

Geochemical controls on the mobility of selected trace metals in two different environmental systems are examined. The two study areas are the Oak Ridge National Laboratory radioactive waste burial grounds in Oak Ridge, Tennessee, and the Pine Barrens swamps of southern New Jersey. The research at Oak Ridge evaluates the geochemical processes that are promoting or retarding the leakage of radionuclides from waste disposal pits and trenches. The Pine Barrens studies evaluate natural controls on trace metal geochemistry in ground, surface, and estuary water.; The Pine Barrens research included many of the same types of analyses that were carried out during the Oak Ridge study. Initially, organic controls on trace element transport and the removal mechanisms of trace elements were the principal subjects of interest. Later, additional topics were also investigated, including inorganic controls on iron and aluminum solubility, bacterial mechanisms of bog iron deposition, and the trace element and organic geochemistry of the estuaries.; The results of the bog iron deposition study suggest that iron-oxidizing bacteria are catalyzing the precipitation of bog iron in the Pine Barrens. Several species of iron-oxidizing bacteria were identified in surface water from the Batsto area, and scanning electron microscopy of the iron deposits revealed the presence of microfossils resembling several of these bacteria. Chemical arguments for the catalysis of Fe oxidation by bacteria are strong: (a) seasonal patterns of observed Fe oxide precipitation in surface waters correlate poorly with times of lowest theoretical Fe solubility; (b) the solubility of Fe in groundwaters examined increases with decreasing depth, whereas the traditional theory of bog iron deposit would predict exactly the opposite; and (c) theoretical calculations based on the rate law of Fe('2+) oxidation suggest that inorganic oxidation at pH 4.5, which is typical of Pine Barrens waters, is too slow to account for quantitatively significant deposits of bog iron.; Results of both of the above study areas lead to the generalization that some of the important geochemical controls on trace element transport in aqueous environmental systems are very selective reactions between only trace amounts of reactants. At Oak Ridge the EDTA concentration in groundwaters examined was only 3 x 10('-7) M, yet even this low a concentration was capable of promoting a significant ('60)Co dispersion problem. The interactions beween EDTA and heavy metals are so selective that, despite 10('-2) to 10('-3) M concentrations of other cations such as Na('+) and Ca('2+), EDTA was still binding significant amounts of radionuclides. Selective controls on trace metal geochemistry were evident in the adsorption mechanism aspect of the Oak Ridge research as well. Fe sesquioxides and clays, both well known for their cation sorption properties, were present in soils at concentrations averaging 1% and 25%, respectively. Yet Mn oxides, present in conentrations of only about 0.2% and as low as 0.02%, were dominating ('60)Co and actinide sorption. In the Pine Barrens waters, trace amounts of humic and fulvic acids were interacting strongly with Fe and Al. If an average molecular weight of 1,000 is assumed, the average dissolved humic/fulvic acid content of Cedar Creek and the Mullica River is about 6 x 10('-6) M. At this concentration, approximately 10-50% of the dissolved Fe present appears to be associated with the organics.