E. A. Nelson
Westinghouse Savannah River Company
Aiken, SC 29808

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The F- and H-Area Seepage Basins received liquid waste from the F and H chemical separation facilities from 1955 through 1988. Tree mortality in seepline-fed wetlands downslope from the basins was observed in the late 1970’s, and investigations were conducted to determine the cause and source of the impacts (Loehle and Gladden, 1988; Mackey, 1988; Haselow et al., 1990; LeBlanc and Loehle, 1990; Greenwood et al., 1990). Analysis of the soil and water in the tree-kill zones demonstrated a strong chemical linkage with the F- and H-Area Seepage Basins (Killian et al., 1987a, 1987b). Although no single cause of the mortality was determined, it was believed to be the result of interactions of alterations in the hydrology and erosional deposition, along with lowering of pH and increased conductivity, sodium, aluminum, and nitrogen compounds (Looney et al, 1988). A mild drought during the growing season may also have increased the concentration of the chemical contaminants in the soil matrix.

In 1988, the F- and H-Area Seepage Basins were closed and covered with a clay cap to reduce the rate of dispersion of the contaminants in the soil beneath the basins. Subsequent studies of the chemical composition of the tree-kill zone groundwater (Dixon and Rogers, 1993) and toxicological characteristics of the seepline soil (Nelson and Westbury, 1994) have shown a reduced contaminant flux. In 1993, an initial vegetation study was undertaken to determine the level of recovery by the plant communities in the tree-kill zones (Nelson and Irwin, 1994). A subsequent study repeated the initial vegetation investigation in 1994 (Westbury and Nelson, 1994) in order to further analyze and characterize the recovery of plant communities in the zones after an additional year of growth.

As part of the groundwater cleanup program, the pump and reinject system was put in place above the F- and H-Area seepline locations. Preliminary modeling of the effect on the lowered water table in the seepline-fed wetlands indicated that the water table would be reduced significantly. To follow this effect, a series of piezometers were installed through the areas that may be impacted (Halverson, 2000). Results of 1999 monitoring indicate that the water table has been lowered, and that lowering is correlated to volume of water being extracted by the wells. 1999 was a drier than normal rainfall year and this may have added to the extraction effect. This monitoring effort will continue to follow the long-term effect. This report discusses only two piezometer locations and may not be indicative of the entire sampling results. The following vegetation characterization was to examine the change from 1994 to the present at previously reported plots in the tree-kill zones of the F- and H-Areas. The data were examined to determine if they reflected the changes in hydrology that have occurred in the immediate vicinity of the seepline near the two areas.

The two study sites that were examined in 1993 and 1994 were the most severely impacted wetlands in F- and H-Areas (Figure 1). These were defined as the complete mortality of the canopy trees. Prior to the occurrence of tree mortality, the study sites were swamp tupelo (Nyssa sylvatica var. biflora) wetlands located between naturally occurring seeplines and Fourmile Branch. All of the mature trees at these sites had been killed, completely eliminating the overstory.

A series of vegetation characterization plots were established in 1993 at uniform intervals along transects (Nelson and Irwin, 1994). The F-Area site was sampled along four transects (Figure 2) that crossed the zone laterally, and consisted of 40 plots, while the H-Area site was sampled along a single transect (Figure 3) that bisected the zone length-wise, and consisted of 56 plots. The transects were located in order to best characterize the variation present in the areas based on visual assessment and chemical data. Beginning and ending points of each transect are marked with PVC pipe and identified with metal tags.

Forty circular milacre plots (43.56 sq. ft./plot, 4.05 sq. m/plot) were examined at the F-Area site and 56 plots were examined at the H-Area site. These are the same plots as those established and measured in 1993. Within each plot, all individual were identified to the lowest possible taxonomic level, usually species. For each plot the height of tree and shrub seedlings, the number of vine stems, and the percent cover of herbaceous plants was recorded. Sampling was conducted during mid- to late-August 2000, whereas the 1994 sampling had occurred in June.

The F-Area site continues to exhibit minimal plant community development. The site was considerably drier during sampling than it has been historically. A recording piezometer, FPZ005 A/B, was placed in the middle of the tree-kill zone in 1998 (Halverson, 2000). Prior baselining had occurred in 1996 and 1997 for the areas. This indicated that prior to extraction of the groundwater in F-Area the tree-kill zone typically had two or more inches of standing water over the surface, and observations indicate that there was usually water flowing out of the area. The year 1996, when baselining was occurring, had a greater yearly rain deficit than did 1999. During that period the level of standing water in the tree-kill zone remained fairly constant. Current measurements from the piezometer indicate that the water table is often below the soil surface. This has allowed the previously unconsolidated soil to solidify and begin to have structure. The soil appears to contain a very high proportion of organic material; this was previously noted as a component of the unconsolidated soil.

Species composition of the area has not changed markedly in the six years since the study was last reported. The number of individuals in the sapling/shrub category was greatly reduced from the 1994 assessment. This is possibly due to performing the assessment at a later date (August 2000 vs. June 1994). The vast majority of individuals scored in this category were newly germinated seeds from the spring seed crop. Nearly all of them die during the summer months due to the stress that is still present in the area. The most prevalent woody species continued to be red maple (Acer rubrum), loblolly pine (Pinus taeda), and swamp tupelo. General vigor of most woody individuals was poor and survival through the first growing season is questionable. There were more individuals in the 2000 sampling that seem to be establishing themselves, with 22 saplings measuring at least one meter in height. These were predominately pine and red maple.

Greenbriers (Smilax spp.) continued to be the most common vine species encountered. Yellow jessamine (Gelsemium sempervirens) was also common, especially in the south end of the study area.

The herbaceous plant community in the F-Area tree-kill area has exhibited the greatest change from the prior condition. The grass species broomsedge (Andropogon virginicus) accounted for nearly all of the herbaceous cover in the plots. Average herbaceous cover went from approximately eight percent in 1994 to over 24 percent in the current assessment. Again, this was entirely due to invasion of broomsedge onto the drier soil surface that has resulted from the lower water table. Broomsedge is listed as a facultative negative (FAC-) species in the list of indicator species for the Southeastern region. This means that the species can occur in a wetland or upland situation, but that it prefers a drier habitat than would typically be found in a jurisdictional wetland. Because of its dominance of the herbaceous composition, the vegetative component of the wetland is beginning to shift away from the hydrophytic vegetation classification. The presence of hydrophytic vegetation is one of the three criteria required to classify an area as a wetland.

Continued observation of the hydrology of the area, through the piezometer location, and the vegetation development of this area is clearly needed to assess the impacts of the F-Area pumping program on these seep-fed Fourmile Branch wetlands.

The H-Area tree-kill site continued to be much more densely vegetated with more consolidated sediments of a higher organic content than the F-Area site. Extensive development of a herbaceous wetland plant community and general plant vigor indicates that the conditions within the tree-kill area have improved considerably. The general character of the plants in the area did not show visible signs of stress and the high percentage coverage indicates minimal impairment of plant establishment and growth.

The transect along which the sample plots are located consists of two distinct wetland types. The first part of the transect (Plots 1-15) is classified as a scrub-shrub, broad-leaved deciduous wetland (Cowardin et al., 1979), while the remainder of the zone (Plots 16-56) is open-canopy, persistent emergent wetland dominated by cattails (Typha spp.) and cutgrass (Leersia oryzoides). A piezometer, HPZ001A, is located along the transect near Plot 26. Data from this location indicates that the water table has been reduced from several inches of standing water to just below the soil surface. During sampling, the soil was found to be saturated just below the surface and little indication of ponded areas was seen.

Species composition was somewhat similar between the sampling periods, although the upper plots (1-15) generally had fewer total individual species identified than previously. This may have been due to the later sampling date (August vs. June), or due to the increased crown closure and the reduced amount of light reaching the forest floor. The lower plots (16-56) continued to be dominated by a limited number of herbaceous species.

The seedling and shrub layer of the upper series of plots has grown considerably in the six years since the last investigation. The seedling and shrub species composition was the same in both years. Dominant species include red maple, wax myrtle, and red bay (Persea borbonia). Many of the individuals are currently in excess of two meters tall. Swamp tupelo was only rarely present in the plots. It may be that there is not a seed source near enough to provide for the area of impact, while the lighter seeded species are represented on the slopes above the wetland and are easily blown to the soil surface. Rushes (Juncus sp.) and sedges (Carex sp.) tended to dominate the herbaceous layer of these plots, although it was generally sparse due to the shading by the woody vegetation.

The vegetation of plots 16 through 56 is markedly different than the first 15. There is a zone of several plots demarking the transition. These plots are dominated by herbaceous and early successional species; very few woody species were identified in the plot boundaries. This area has historically had several inches of standing water, and the nitrogen influx from the seepage basins makes this an ideal habitat for cattail (Typha latifolia) invasion. Also present in most of the plots was cutgrass. The percent cover by the herbaceous species has declined since the 1994 assessment, falling from over 90 percent to less than 63 percent. Cattail stands will typically begin to decline after a number of years in a location due to a reduction in soil nutrient availability. This reduction in cover may be a natural decline and may not be directly attributable to the water table decline. If this is a natural decline, the additional light getting to the soil surface should produce germination of woody species in the following years. Kinds of species to appear will be determined by seed availability and future hydrology of the area. This natural vegetational progression is also seen in other riparian and Carolina Bay restorations.

The vine layer was dominated by greenbrier and honeysuckle (Lonicera japonica) in the upper plots, while the lower plots were essentially devoid of vines, probably due to historic water levels and competition from established vegetation.

The H-Area tree-kill site continues to develop an extensive post-disturbance plant community that is dominated by herbaceous wetland plants. After the swamp tupelo canopy was removed by the initial impact, wetland herbaceous species would be expected to colonize the saturated sediments as soon as the toxicity of the sediments became sub-lethal. With the passage of time, shrub and then tree species would be expected to become dominant, eventually returning to the pre-disturbance composition. This will be a long-term transition towards a bottomland forest. The northern portion of the zone (Plots 1-15) has developed a sub-canopy of shrubs and saplings indicating a more advanced successional stage than the remainder of the zone. The main tree species present is red maple, which is an early successional invader. If the lower zone continues to become more open, movement of additional species into the area is anticipated. Continued monitoring of the water table and vegetation changes over time will help answer these questions.

General conditions at both tree-kill zones have begun to show some differences since the 1994 assessment. Differences due to sampling date (August 2000 vs. June 1994) were apparent. The F-Area site continues to exhibit toxicity or stress to the vegetation in the sediments, as is seen in the visible stress on the newly germinated individuals. Sampling at the seepline continues to indicate low pH and therefore higher levels of toxic metals. Herbaceous species typical of drier habitats have begun to colonize the slightly elevated portions of the tree-kill area now that the water level is below historic elevations. This has exposed soil surfaces that were previously under water and allowed seed germination. The consolidation of the sediments, now that they are not constantly under water, may be a positive development. A new round of sediment and water sampling is presently occurring, and these may give a clearer picture of continuing toxicity candidates in the soil. The shifting of the vegetation away from the normally hydrophytic species is of some concern, and should be followed up in future years. It this pattern continues, there may be a future need to assess loss of wetland functionality in the most severely impacted regions.

The H-Area site exhibits far greater plant community development and successional recovery than the F-Area site. The presence of a shrub sub-canopy in the northern portions of the zone, and the generally high percentage cover of herbaceous wetland plants, suggest lowered toxicity of the substrate to germination and establishment of vegetation. The opening up of the southern portion (cattail beds) may allow establishment of additional woody and herbaceous species and begin developing towards a more complex wetland community.

1. Cowardin, L.M., V. Carter, F.C. Golet and E.T. LaRoe. 1979. Classification of Wetland and Deepwater Habitats of the United States. FWS/OBS-79/31. United States, Fish and Wildlife Service, Biological Services Program.
2. Dixon, K.L. and V.R. Rogers. 1993. Results of the second quarter tritium surveys of the F- and H-Area seeplines: September 1992. WSRC-TR-93-129, Westinghouse Savannah River Co., Aiken, SC 29808.
3. Greenwood, K.P., M.E. Hane, A.L. Lander, C. Loehle, and C.J. Richardson. 1990. Assessment of tree toxicity near the F- and H-Area Seepage Basins of the Savannah River Site (U). WSRC-TR-90-253, Westinghouse Savannah River Co., Aiken, SC 29808.
4. Halverson, N.V. 2000. Monitoring of the water levels in the wetlands of Fourmile Branch near the F and H Areas of SRS: January 1999 to December 1999 (U). WSRC-TR-2000-00122, Westinghouse Savannah River Co., SRTC, Aiken, SC 29808.
5. Haselow, J.S., M. Harris, B.B. Looney, N.V. Halverson, and J.B. Gladden. 1990. Analysis of soil and water at the Four Mile Creek seepline near F & H Area of SRS (U). WSRC-RP-90-0591, Westinghouse Savannah River Co., SRL, Aiken, SC 29808.
6. Killian, T.H., N.L. Kolb, P. Corbo, and I.W. Marine. 1987a. Environmental Information Document: F-Area seepage basins. DSPT-85-704, E.I. duPont de Nemours & Co., SRL, Aiken, SC 29808.
7. Killian, T.H., N.L. Kolb, P. Corbo, and I.W. Marine. 1987b. Environmental Information Document: H-Area seepage basins. DSPT-85-706, E.I. duPont de Nemours & Co., SRL, Aiken, SC 29808.
8. LeBlanc, D.C. and C. Loehle. 1990. The effect of contaminated groundwater on tree growth: a tree-ring analysis (U). WSRC-RP-90-552, Westinghouse Savannah River Co., SRL, Aiken, SC 29808.
9. Loehle, C. and J. Gladden. 1988. Preliminary assessment of tree mortality near F- and H-Area seepage basins. DPST-88-260, E.I. duPont de Nemours & Co., SRL, Aiken, SC 29808.
10. Looney, B.B., J.E. Cantrell and J.R. Cook. 1988. Sampling and analysis of surface water in the vicinity of the F- and H-Area seepage basins. DSPT-88-229, E.I. duPont de Nemours & Co., SRL, Aiken, SC 29808.
11. Mackey, H.E. Jr. 1988. Initial evaluation of photographic data of F- and H-Area seepage basin outcrops. DSPT-88-314, E.I. duPont de Nemours & Co., SRL, Aiken, SC 29808.
12. Nelson, E.A., and J.E. Irwin. 1994. Current vegetation characteristics within tree-kill zones of the F- and H-Areas (U). WSRC-TR-94-0203, Westinghouse Savannah River Co., SRTC, Aiken, SC 29808.
13. Nelson, E.A., and H.M. Westbury Jr. 1994. Lettuce seed germination and root elongation toxicity evaluation of the F-Area seepline soils (U). WSRC-TR-94-0510, Westinghouse Savannah River Co., SRTC, Aiken, SC 29808.
14. Westbury, H.M. Jr. and E.A. Nelson. 1994. Plant community development within the F- and H-Area tree-kill zones (U). WSRC-TR-94-0544, Westinghouse Savannah River Co., SRTC, Aiken, SC 29808.