Abstract
Backfills of dredged sediments onto a former sand and gravel mine site in Charles City County, VA may have the potential to contaminate local groundwater. To evaluate the mobility of trace elements and to identify the potential contaminants from the dredged sediments, a sequential extraction scheme was used to partition trace elements associated with the sediments from the local aquifer and the dredged sediments into five fractions: exchangeable, acidic, reducible, oxidizable, and residual phases. Sequential extractions indicate that, for most of the trace elements examined, the residual phases account for the largest proportion of the total concentrations, and their total extractable fractions are mainly from reducible and oxidizable phases. Only Cd, Pb, and Zn have an appreciable extractable proportion from the acidic phase in the filled dredged sediments. Our groundwater monitoring data suggest that the dredged sediments are mainly subject to a decrease in pH and a series of oxidation reactions, when exposed to the atmosphere. Because the trace elements released by carbonate dissolution and the oxidation (e.g., organic matter degradation, iron sulfide and, ammonia oxidation) are subsequently immobilized by sorption to iron, manganese, and aluminum oxides, no potential contaminants to local groundwater are expected by addition of the dredged sediments to this site.
Highlights
INTRODUCTIONThe mobility of trace elements in sediments, as well as their potential toxicity, depends on their aqueous and solidphase speciation and the chemical and physical processes to which these elements are subjected
A former sand and gravel mine site in Charles City County, VA was mined in the 1980s, backfilled with 10 m of sandy mine spoils, and reclaimed to farmland use
To ascertain the potential mobility of any contaminants added to a dredged spoil placement site by back-filling with the Potomac River dredged sediments, batch extractions were performed on the local aquifer sediments and on back-filling sediments dredged from the upper Potomac River estuary
Summary
The mobility of trace elements in sediments, as well as their potential toxicity, depends on their aqueous and solidphase speciation and the chemical and physical processes to which these elements are subjected. Trace elements bound to iron and manganese oxides, which will be reduced and solubilized under anoxic conditionsi.e., low Eh. Fraction 4 ͑Oxidizable phase. Eight samples of local aquifer sediments and the Potomac River dredged sediments were sequentially extracted using the techniques outlined by Tessier et al.[5] The various batch extraction fluids were subsequently analyzed by inductively coupled plasma mass spectrometryICP-MS; Finnigan MAT Element IIfor eleven trace elementsAg, Cd, Sb, Tl, Pb, Cr, Fe, Ni, Cu, Zn, and As. From the analytical results, the percentage of the total concentration of each trace element in each fraction is calculated. Any potential mobility of any contaminants from the dredge sediments as in situ conditions change following disposal at the replacement site is evaluated
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