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Assessing the Ecological Health of the D-Area Ash Plume Wetland
David Scott, Brian Metts, Tracey Tuberville, and Bill Hopkins
The ash plume wetland (APW).
The APW received coal combustion wastes from a breach in a receiving basin in the 1970s.
Several trace metals are elevated in sediments of the APW area, including arsenic, selenium, strontium and copper.
Coal-fired facilities have been in operation on the SRS since the early 1950s. After combustion, coal combustion waste (CCW, sometimes also called fly ash) was mixed in a slurry and sent to receiving basins where most CCW settled out and was periodically moved from the receiving basins to terrestrial storage pits. Remaining CCW in the water was removed in a series of primary and secondary settling basins. At the D-Area coal facility adjacent to the Savannah River, CCW was sluiced into settling basins (hereafter referred to as “Ash Basin,” or AB) <500 m from a floodplain forest and associated wetlands. Through the early 1970s, CCW from the Ash Basin (see Figure) was released onto the Savannah River floodplain, resulting in a CCW plume that extends over 40 ha of floodplain at depths up to 2.7 m, including a natural wetland (hereafter referred to as “Ash Plume Wetland,” or APW). The D-Area coal plant operated until early 2012, when it was replaced by a biofuels cogeneration facility.
Until 2012 the Ash Basin received fresh sluiced CCW, whereas the APW has not received CCW discharge for >35 years, and the Savannah River floodplain area (including the APW wetland) has become revegetated with a mixed floodplain plant community that includes a thin organic soil layer. The University of Georgia’s Savannah River Ecology Lab (SREL) has conducted research in the D-Area system for more than 20 years, resulting in more than 70 published papers.
Most SREL research has focused on the active D-Area settling basins, where contaminant levels are highest, rather than on the floodplain where natural attenuation has been allowed to occur. Research on the settling basins demonstrated that amphibians and other wildlife inhabiting the basins and discharge streams can accumulate elevated concentrations of trace elements that cause adverse effects on survival, growth and development, energy acquisition and allocation, behavior or performance, and recruitment of young into the population. Active CCW settling basins may be ecological traps, attractive to wildlife species but unsuitable for their long-term survival, but the ecological status of floodplain spill sites is unknown. Thus, more recent investigations have targeted the APW.
Although trace element uptake and accumulation has been well documented in different species and life stages of amphibians at the APW, potential biological effects have been little studied. Earlier sampling of the amphibian and reptile community revealed that the number of species found at the APW is comparable that of the nearby reference site (Ellenton Bay). However, with the exception of the leopard frog (Rana sphenocephala), newly metamorphosed individuals of amphibians were not found in earlier studies, possibly indicating that the APW is not suitable for successful egg and larval development of some pond-breeding amphibians. If the APW is an ecological trap for herpetofauna, the lack of recruitment of juveniles into the population may be connected to the elevated trace metal concentrations.
Pilot Studies -- We combined aquatic and drift fence sampling with artificial mesocosm studies to 1) determine the species utilizing the APW and 2) assess biological effects of the APW environment on focal species of pond-breeding amphibians. These studies began in mid-March 2008 and continued (primarily mesocosm studies) through 2011. We observed newly metamorphosed juveniles of five species at the APW: R. sphenocephala, Bufo terrestris (southern toad), Pseudacris crucifer (spring peeper), P. brimleyi (Brimley’s chorus frog), and Ambystoma opacum (marbled salamander) In contrast, at the reference site during the same time frame four additional species (P. ornata, ornate chorus frog; A. tigrinum, tiger salamander; A. talpoideum, mole salamander; Scaphiopus holbrookii, spadefoot toad) reproduced successfully, at least in part due to its longer hydroperiod and absence of fish.
The pilot mesocosm studies investigated egg hatching and larval survival in five species of amphibians (S. holbrookii; P. nigrita, southern chorus frog; B. terrestris; R. sphenocephala; and Gastrophryne carolinensis, eastern narrowmouth toad). The reference site for both of these mesocosm studies was Ellenton Bay. Eggs of each species were reared water containing water and either clean or contaminated sediments, with each treatment replicated five times. Different numbers of clutches were used depending upon species average clutch size and the availability of eggs. We observed 100% hatching success in the APW trial for all five species; results for Ellenton Bay trials were nearly the same (4 of 5 species had 100% hatching success), with only B. terrestris showing reduced success (64%). Methods for larval survival studies were similar. Larval survival was generally 15-25% lower for larvae reared in the APW buckets; given the limited sample size in these pilot studies, however, survival differences between reference and APW habitats were non-significant (Fig. 1).
Larger Scale Studies -- Beginning in 2009, Brian Metts (a graduate student at UGA—degree completed in 2012) expanded our efforts and focused more intensely on a single species. We conducted three independent studies regarding parental and larval exposure effects on southern toads (Bufo [Anaxyrus] terrestris), one of the most common amphibian species in the system. The first addressed maternal transfer and subsequent effects on reproductive success. The second addressed individual and interactive effects of maternal and larval exposure. The third addressed effects of paternal exposure. These extended studies included not only the APW, but also the ash settling basins (AB). Ellenton Bay, Rainbow Bay, and Craigs Pond were used as reference sites.
Our maternal transfer studies indicated that that females collected from the ash basin and APW maternally transferred increased concentrations of Cd, Cu, Ni, Pb, Se, and Sr to their eggs. In addition, we found reproductive success was reduced 33% and 35% in females collected from the APW and ash basin, respectively, compared to females collected from uncontaminated reference sites (Metts et al., In review). Moreover, reproductive success negatively correlated with elevated trace element concentrations, most notably Se, in females and their eggs (Figure 2 below; Metts et al., In review).
The mesocosm study to assess the individual and interactive effects of previous maternal exposure and larval exposure to trace element-laden sediments on southern toads indicated that previous maternal exposure to CCW reduced larval survival to metamorphosis up to 57% compared to larvae of unexposed females (Metts et al., 2012). Moreover, previous maternal exposure to contaminants coupled with larval exposure to fresh CCW from the AB interacted to reduce survival to metamorphosis by 85% compared to reference conditions (Figure 3 below; Metts et al., 2012). We observed that larval survival of offspring from reference females (the comparison most like the pilot study methods) reared in the APW mesocosms was >40% lower than in the reference mesocosms (32% compared to 78%), a greater effect than the general trend we observed in the pilot studies. Interestingly, however, we also saw that APW offspring survived better than reference tadpoles in the APW mesocosms.
Next we investigated the contribution of paternal exposure. Compared to reference pairs, reproductive success was reduced by 40% when either the male or female was exposed to CCW and by 58% when both adults had been exposed (Metts, in review). Reductions in reproductive success were primarily attributed to reduced hatching success, not effects on clutch size or abnormalities. Our study provides evidence that paternal exposure to contaminants can influence reproductive success in amphibians, demonstrating that parental effects are not limited to mothers.
Taken together, these studies yield further evidence that disposal of CCW in aquatic basins such as the AB potentially creates ecological traps for some amphibian populations. Sediment and water chemistry comparisons, coupled with results from the larval studies, generally show that the APW elemental levels and biological effects are intermediate between the clean reference site and the fresh CCW of the AB, perhaps suggesting that impacts are attenuated in the aged sediments of the APW. Future studies should investigate post-metamorphic stages to determine effects of parental and larval exposure to CCWs on the terrestrial juvenile stage. In addition, other amphibian species with varying life history and ecological traits should be examined to determine their sensitivity to CCWs.
ACKNOWLEDGEMENTS: We would like to thank the Area Completions Project of Savannah River Nuclear Solutions for funding this study; work was also partially supported by the Department of Energy under Award Number DE-FC09-07SR22506 to the University of Georgia Research Foundation.