Geochemical Characterization And Assessment Of Stabilization Mechanisms For Mercury Contaminated Riverbank Sediments From The South River Virginia Usa

Elevated concentrations of mercury (Hg) in aquatic systems can be a threat to ecosystems and human health. Mercury-bearing sediment particles from eroding riverbanks can be an ongoing source of bioavailable Hg to aquatic ecosystems. Hyporheic zones in particular can be important sources of both inorganic and organic-complexed Hg, which can be rapidly transported to adjacent surface waters. The objective of this study was to investigate the release and treatment of dissolved and particle-bound Hg in water derived from the riverbank sediments of the South River, Virginia.

This thesis presents two methods of calculating bank erosion rates along the South River, Virginia. In section I, digitized stream channel boundaries based on visual interpretations of georeferenced aerial imagery from 1937 to 2005 are compared to calculate a minimum estimate of the total area of bank sediment eroded between Waynesboro and Port Republic, Virginia. Estimates of riverbank height were extracted from aerial LIDAR data, allowing aerial estimates of bank retreat to be converted to volumes. Because of historical industrial contamination, bank sediments store mercury (vertically averaged concentrations range from 5 to 140 ppm, with higher values in the upper 50 cm), which is delivered to the stream channel through bank erosion and is of environmental concern. Nominal annual rates of bank retreat (averaged over the 68-year period) for several example locales along the study reach are very low, ranging from 3 to 15 cm per year. Bank erosion occurs at the outside banks of bends, through the development of islands, where deposition on confluence bars pushes the main flow into the opposite bank, and in small areas along the channel that are difficult to classify or explain. A minimum estimate of the total volume eroded for the study reach is approximately 161,000 m 3; the corresponding annual mass of mercury supplied to the channel by bank erosion is 110 kg yr-1. This work demonstrates that a careful analysis of aerial imagery and airborne LIDAR data can provide detailed, spatially explicit estimates of mercury loading from bank erosion, even when rates of riverbank erosion are unusually low.

Mercury (Hg) is a contaminant of concern due to the very high toxicity and bioaccumulating nature of organic Hg and the persistent leaching of Hg to water bodies from contaminated soils and sediments. The deleterious properties of Hg pose challenges for remediation as point source contamination can expand over time to affect much wider areas. Saturated, flow-through column experiments were conducted with riverbank sediment and floodplain soil collected from a contaminated reach of the South River near Waynesboro, VA. In one experiment, the composition of input solutions was varied to observe relationships among mobilized Hg, aqueous parameters and effluent constituents and identify dominant mechanisms and controls on Hg transport. Effluent Hg concentrations increased and remained elevated when a higher pH and alkalinity solution was input to the column. Effluent Hg and DOC concentrations were generally positively correlated. Increased effluent Hg concentrations broadly coincided with increased effluent iron (Fe) and manganese (Mn) concentrations and redox (Eh) minima. The lowest effluent Hg concentrations were observed upon decreasing the input solution pH from ~8.7 to ~6, whereas an increase in input pH from ~6 to ~12 coincided with the highest effluent Hg concentrations along with spikes in effluent Fe, Mn and DOC concentrations. Saturated flow-through column experiments with floodplain soil were conducted under both aerobic and anaerobic environments. Greater concentrations of effluent Hg were observed from the column operated in an aerobic environment as opposed to in an anaerobic environment. Two distinct effluent Hg concentration maxima were observed from the aerobic column with increased Hg concentrations observed together with a relatively high Eh (490 mV compared to average Eh of 360 mV) and low Fe and Mn concentrations, whereas the latter and greater Hg maximum broadly coincided with a sharp decrease in Eh (85 mV) and increased effluent Fe and Mn concentrations. The maximum effluent Hg concentration from the anaerobic column also broadly coincided with an increase in effluent Fe and Mn and a minimum Eh but the Hg release was of a much lower magnitude than from the aerobic column. Despite higher total effluent Hg concentrations from the aerobic column, methylmercury (MeHg) concentrations were consistently higher from the anaerobic column. A potassium polysulfide (KPS) solution (1 mM S) was applied to a fully-saturated, flow-through column of floodplain soil for approximately 10 pore volumes (PVs) under anaerobic conditions to assess the potential for polysulfide to stabilize Hg. Effluent Hg concentrations were very high during the application of the KPS solution and remained elevated above the control for the remainder of the experiment after the KPS application ceased; most other parameters were similar in the KPS and control column effluents for the duration of the experiment. An increase in effluent Hg from the KPS column was observed post-KPS application that broadly coincided with a decrease in Eh and increased effluent Fe and Mn. The relationship between increased Hg, Mn and Fe and decreased Eh was also observed in the control column, but the magnitude of Hg release was lower than from the KPS column. XANES sulfur spectra collected from the KPS-treated soil and the control were similar indicating that there was not an apparent change in solid-phase sulfur in the KPS-treated soil compared to the control soil. Dissolution of HgS and formation of highly mobile HgSx2- was likely the dominant mechanism for the Hg release. In situ immobilization of Hg in the floodplain soil was not achieved with the flow-through application of a polysulfide solution; contrary to past studies where immobilization was achieved by in situ formation of HgS via polysulfide application to elemental Hg0 in a glass bead medium.

In this text, drawn from presentations and discussion at a May 2005 NATO Advanced Research Workshop, current approaches to the assessment and remediation of contaminated sediments are discussed with emphasis on in-situ management. The text addresses physical, chemical and biological approaches for the assessment and remediation of sediments. The development of regulatory and strategic approaches is discussed with emphasis on the potential for biological remediation in the management of contaminated sediments.

Three rivers in the Shenandoah Valley of Virginia are contaminated with mercury and have been designated as “impaired” on Virginia's 303d list of contaminated waters due to fish consumption advisories issued by the Virginia Department of Health. These rivers, the South River, South Fork Shenandoah River, and the Shenandoah River between Front Royal and the confluence with Craig Run (fig. 1), are regulated by the Virginia Department of Environmental Quality (VDEQ) under the Total Maximum Daily Load (TMDL) Program, which develops plans to restore and maintain water quality for impaired waters.

Mercury-contaminated sediments are found in many locations throughout North America and the world. Release of Hg from such sediments and subsequent biological uptake can result in biomagnification in associated ecosystems. This study focused specifically on a stabilization technique involving the addition of reactive media to the sediment matrix to immobilize Hg and reduce its bioavailability. A series of batch and column experiments was conducted over a range of physical and geochemical conditions to evaluate the propensity of a diverse set of reactive media to stabilize Hg in sediment with high organic carbon and clay content. The additives, selected to promote adsorption and precipitation of Hg, included natural attapulgite (palygorskite) clay, organically-modified clay, elemental sulfur, a strong reductant, and mixtures thereof. The results of the batch experiments indicated that addition of reactive media to the sediment led to substantially lower aqueous concentrations of Hg relative to untreated sediment. The stabilization of Hg was observed to be dependent on mass of added reagent, with generally greater treatment observed for the higher masses of reagent evaluated. Aqueous concentrations of Hg were reduced from > 800 ng L-1 in control samples to 50 ng L-1 in treated samples for all of the reactive media at the highest mass proportions evaluated. The effectiveness of Hg stabilization using the sulfur-based blends was strongly affected by contact with atmospheric oxygen, with better treatment observed in oxygen-limited conditions. The results of the column tests showed that relatively low concentrations of Hg (