Fourmile Branch Research Articles

Mercury Accumulation in a Stream Ecosystem: Linking Labile Mercury in Sediment Porewaters to Bioaccumulative Mercury in Trophic Webs

Mercury (Hg) deposition and accumulation in the abiotic and biotic environments of a stream ecosystem were studied. This study aimed to link labile Hg in porewater to bioaccumulative Hg in biota. Sediment cores, porewaters, and biota were sampled from four sites along the Fourmile Branch (SC, USA) and measured for total Hg (THg) and methyl-Hg (MHg) concentrations. Water quality parameters were also measured at the sediment–water interface (SWI) to model the Hg speciation. In general, Hg concentrations in porewaters and bulk sediment were relatively high, and most of the sediment Hg was in the solid phase as non-labile species. Surface sediment presented higher Hg concentrations than the medium and bottom layers. Mercury methylation and MHg production in the sediment was primarily influenced by sulfate levels, since positive correlations were observed between sulfate and Hg in the porewaters. The majority of Hg species at the SWI were in non-labile form, and the dominant labile Hg species was complexed with dissolved organic carbon. MHg concentrations in the aquatic food web biomagnified with trophic levels (biofilm, invertebrates, and fish), increasing by 3.31 times per trophic level. Based on the derived data, a modified MHg magnification model was established to estimate the Hg bioaccumulation at any trophic level using Hg concentrations in the abiotic environment (i.e., porewater).

Estimates of Vadose Zone Drainage from a Capped Seepage Basin, F‐Area, Savannah River Site

Waste disposal into seepage basins has generated groundwater contaminant plumes at many locations. At the F Area within the Savannah River Site, Pu was extracted from depleted U from 1955 to 1988, with wastewater discharged into seepage basins. Basin 3 was the largest F-Area seepage basin, receiving acidic wastewater containing radionuclides (including 3H, 129I, and multiple isotopes of U, Pu, Sr, and Cs), elevated NO3, and some metals. Contaminants transported into the groundwater migrate toward Fourmile Branch, a tributary to the Savannah River. We developed a two-compartment model and used 20 yr of groundwater quality data to estimate the post-closure drainage of waste solutions through its vadose zone into the aquifer. Tritium, NO3−, and specific conductance were used as tracers in the model to estimate drainage rates. Our calculations indicate that early stages of post-closure waste drainage occurred with high water fluxes (~0.5 m yr−1) and quickly declined. Even 20 yr after basin closure, however, drainage continues at several centimeters per year. While the magnitude of this late-stage drainage rate is low, its impact is large because of the high concentrations of contaminants it continues to supply to the groundwater. These estimated drainage fluxes constrain predictions on the waste plume behavior, especially with respect to its trailing gradient and time scales suitable for monitored natural attenuation. Our methodology requires only groundwater monitoring data and a small number of well-constrained input quantities. This approach can be useful for understanding contaminant dissipation at other locations as well, especially where the hydrogeological setting is relatively simple.

A containment and disposition strategy for tritium-contaminated groundwater at the Savannah River Site, South Carolina, United States

A containment and disposition water management strategy has been implemented at the Savannah River Site to minimize the discharge of tritiated groundwater from the Old Radioactive Waste Burial Ground to Fourmile Branch, a tributary of the Savannah River. This paper presents a general overview of the water management strategy, which includes a two-component (pond and irrigation) system, and a summary of operations and effectiveness for the first 3 yr of operations. Tritiated groundwater seep discharge was impounded by a dam and distributed via irrigation to a 22-ac (8.9-ha) upland forested area comprised of mixed pines (loblolly and slash) and hardwoods (primarily sweetgum and laurel oak). As of March 2004, the system has irrigated approximately 133.2 million L (35.2 million gal) and prevented approximately 1880 Ci of tritium from entering Fourmile Branch via forest evapotranspiration, as well as via pond storage and evaporation. Prior to installation of the containment and disposition strategy, tritium activity in Fourmile Branch downstream of the seep averaged approximately 500 pCi mL1. Six months after installation, tritium activity averaged approximately 200 pCi mL1 in Fourmile Branch. After 1 yr of operations, tritium activity averaged below 100 pCi mL1 in Fourmile Branch, and a range of 100–200 pCi mL1 tritium activity has been maintained as of March 2004. Complex hydrological factors and operational strategies influence remediation system success. Analyses may assist in developing groundwater management and remediation strategies for future projects at the Savannah River Site and other facilities located on similar landscapes.

Rodents as receptor species at a tritium disposal site

New methods are being employed on the Department of Energy's Savannah River Site to deal with the disposal of tritium, including the irrigation of a hardwood/pine forest with tritiated water from an intercepted contaminant plume to reduce concentrations of tritium outcropping into Fourmile Branch, a tributary of the Savannah River. The use of this system has proven to be an effective means of tritium disposal. To evaluate the impact of this activity on terrestrial biota, rodent species were captured on the tritium disposal site and a control site during two trapping seasons in order to assess tritium exposure resulting from the forest irrigation. Control site mice had background levels of tritium, 0.02 Bq/mL, with disposal site mice having significantly higher tritium concentrations, mean = 34.86 Bq/mL. Whole body tritium concentrations of the mice captured at the disposal site were positively correlated with tritium application and negatively correlated with precipitation at the site.

Comparison of Single‐Domain and Dual‐Domain Subsurface Transport Models

Subgrid modeling of some type is typically used to account for heterogeneity at scales below the grid scale. The single-domain model (SDM), employing field-scale dispersion, and the dual-domain model (DDM), employing local hydrodynamic dispersion and exchange between domains having large hydraulic conductivity contrasts, are well-known examples. In this paper, the two modeling approaches are applied to tritium migration from the H-area seepage basins to a nearby stream--Fourmile Branch--at the Savannah River Site. This location has been monitored since 1955, so an extensive dataset exists for formulating realistic simulations and comparing the results to data. It is concluded that the main parameters of both models are scale-dependent, and methods are discussed for making initial estimates of the DDM parameters, which include mobile/immobile porosities and the mass exchange coefficient. Both models were calibrated to produce the best fit to recorded tritium data. When various attributes of the dataset were considered, including cumulative tritium activity discharged to Fourmile Branch, plume arrival time, and plume attenuation due to closure of the seepage basins in 1988, the DDM produced results superior to the SDM, while causing no unrealistic upgradient dispersion. A sensitivity analysis showed that only the DDM was able to accurately produce both the instantaneous activity discharge and cumulative activity with a single parameter set. This is thought to be due to the advection-dominated nature of transport in natural porous media and the more realistic treatment of this type of transport in the DDM relative to the SDM.

Influence of riparian alteration on canopy coverage and macrophyte abundance in Southeastern USA blackwater streams

Two tributary streams (Fourmile branch and Pen branch) located on the US Department of Energy's Savannah river site in west-central South Carolina, USA received thermal discharges from nuclear production reactors for over 30 years. Effluent releases produced stream water temperatures of over 50°C and stream flows of ten times above their base level. Consequently, existing plant and animal communities within the stream channels were killed and riparian zones largely destroyed. We compared canopy coverage and macrophyte abundance in these disturbed streams after 7–13 years of ambient flows and compared them to two similar, undisturbed streams. We also examined the effects of a more recent woody canopy removal associated with a restoration effort in ‘treated’ sections of Pen branch. We collected data in Spring and Fall from May 1995 through May 1997. A gradient in canopy cover existed, ranging from a fully open herbaceous canopy in the treated sections of Pen branch, through a moderately closed canopy of herbaceous plants, shrubs, and willows in the post-thermal ‘control’ streams, to a nearly closed hardwood tree canopy in the undisturbed streams. Even though Fourmile branch had 3 more years of growth, Pen branch had a more closed upper canopy. Total aquatic macrophyte abundance was negatively related to canopy cover producing nearly the reverse gradient among streams as did the canopy cover. However, control sections of Pen branch had a more closed canopy than Fourmile branch, but total macrophyte abundance was higher in Pen branch. This can be attributed to the presence of submergent macrophytes in Pen branch that were absent in Fourmile branch. Different structural types of macrophytes varied in their degree of limitation by canopy coverage and in their seasonal patterns of growth. Stream habitats remain severely altered due to destruction of the riparian vegetation by past thermal effluents. The full range of effects of the alteration of canopy coverage and the resultant macrophyte abundances on these streams should be the focus of future analyses.

Spatial and temporal patterns of carbon storage and species richness in three South Carolina coastal plain riparian forests

The distribution of organic matter within a floodplain is a controlling factor affecting water quality, habitat, and food webs. Accordingly, development of vegetation in the riparian zone can be expected to influence ecosystem functions, and organic matter storage patterns are believed to be indicators of functional recovery in disturbed riparian zones. Our objective was to compare the distribution and allocation of organic matter among microsites within the floodplain and with temporal changes (successional status) associated with community development. Three third order streams in the upper coastal plain of South Carolina were selected. Measurement transects were established across three floodplains of varying successional status, Meyer's branch; a mature riparian hardwood forest; Fourmile branch; a mid-successional riparian forest; and Pen Branch, an early successional riparian forest. Overall, measurements of aboveground biomass, soil carbon, and stand structure indicate that the early and mid successional stands are becoming more similar to the mature stand and that microsite differences within the braided, riparian stream systems are small.

Species selection trials and silvicultural techniques for the restoration of bottomland hardwood forests

Since 1990, a series of experiments has examined the appropriateness of 24 tree species for restoring a bottomland and swamp forest in the delta of Fourmile Branch in the coastal plain of South Carolina, USA. In addition, various silvicultural techniques used to maximize the survival of tree plantings, have been appraised for effectiveness. While the topographic relief on the delta is small (dm differences between most sites), it is sufficient to utilize woody species with different flood tolerances. Hence, a diverse forest community can be established, using these elevation differences. In the wettest sites where water persists virtually continuously and may be one to two meters deep during large flood events, only the most flood-tolerant species, such as Taxodium distichum and Nyssa aquatica, can survive. These species will also survive very well at higher elevations, because the permanent water table never fell below one meter deep. In sites that are not flooded, unless the entire swamp is flooded, several additional species can be used. These include Fraxinus pennsylvanica, Carya aquatica, and Quercus lyrata. Finally, in slightly higher areas, Q. michauxii, Q. nuttallii and Q. phellos would have adequate survival. To minimize herbivory and maximize survival, tree shelters should be used if herbivore pressure is high. The prime herbivore of concern is beaver. Thus, any plantings that are frequently flooded may require protection. Other silvicultural techniques, including fertilization and control of herbaceous and willow competition, were not essential to ensure growth and survival. Finally, survival of the least expensive planting stock, bare root saplings, was nearly as good as balled and burlapped stock. Thus, unless absolute maximal survival is required, bare root stock will produce good survival at a fraction of the cost. One critical characteristic of the bare root stock is height, which must exceed the flood depth during the growing season. In the case of the Fourmile Branch delta, this was at least 45–60 cm tall.