Use of diffusive gradients in thin films (DGT) to measure potentially bioavailable metals in southeastern USA blackwater streams.

We used diffusive gradients in thin films (DGT) to measure potentially bioavailable metals in coastal plain streams in the southeastern USA that exhibited strong to moderate blackwater characteristics. Metals were partitioned into particulate metals, DGT-inert metals (i.e., colloidal and refractory organic complexes not accumulated by DGT), and DGT-labile metals (i.e., free metal ions, small inorganic complexes, and labile organic complexes). We also examined the influence of different DGT deployment times using data collected from the field and a follow-up laboratory study. The DGT-measured fraction of dissolved metals in the streams was 15% for Cd, 21% for Zn, 33% for Cu, 37% for Pb, and 98% for Mn. Metals bound to particulates predominated only for Pb. Most of the Cd, Pb, Zn, and Cu were associated with colloids, refractory organic complexes, or particles. Relatively small amounts were in free ion or labile complexes likely to be bioavailable through respiratory surfaces. Modeled concentrations of free and inorganically bound Cu and Pb were lower than the DGT fraction indicating that DGT accumulated some organically bound Cu and Pb that might not have been bioavailable. DGT-exposure times in excess of 5 days may have contributed to the accumulation of partly labile organic-metal complexes and were associated with substantial biofouling that caused metal uptake by DGT to depart from linearity.

Removal, distribution and retention of metals in a constructed wetland over 20 years.

The A-01 wetland treatment system (WTS) was designed to remove metals (primarily copper) from the effluent at the A-01 National Pollution Discharge Elimination System (NPDES) outfall at the Savannah River Site, Aiken, SC. This research investigated metal removal, distribution and retention in the A-01 WTS over a period of 20 years. The findings are important for ensuring continued metal sequestration in the A-01 WTSs over time, providing management guidance for constructed wetlands, and investigating changes in metal remediation effectiveness as a wetland ages. During 20 years of operation, systematic water and sediment sampling validated the wetlands' performance. After passage through the treatment cells, Cu concentrations were well below permit limits during all years of operation, often falling below 10 µg L-1. Cu removal has been consistent over time, averaging about 80% despite large changes in influent Cu concentrations. Most divalent metals were rapidly removed from the water and held in the sediments shortly after the water entered the treatment wetland. Average removal of Pb from water by the wetland system was 67 and 74% in 2004 and 2020, respectively. Comparable values for Zn were 52 and 65%, respectively. Generally, the highest concentrations of Cu, Pb, and Zn were found in the sediment from the first cell in each pair of cells suggesting that most of the Cu, Pb, and Zn in the A-01 effluent was bound to the sediment quickly. Diffusive gradients in thin films (DGT) measurements of Cu and Zn in the sediments were much lower than bulk sediment concentrations. These results suggest that most of the Cu and Zn in the A-01 WTS sediments was not bioavailable, hence not toxic to aquatic organisms, as a likely consequence of adsorption to sediment particles and complexation with organic and inorganic substances.

Effect of bioturbation on contaminated sediment deposited over remediated sediment.

A challenge to all sediment remediation technologies is the continued influx of contaminants from uncontrolled sources following remediation. However, contaminants deposited on sediments remediated with chemically active sequestering agents may be affected by the sequestering agents resulting in reduced impacts. We deposited sediment contaminated with As, Cd, Cu, Ni, Pb, and Zn over clean sediment capped with the sequestering agent, apatite, and clean uncapped sediment in laboratory mesocosms to simulate the recontamination of remediated sediment by influxes of particle-bound contaminants. Cap effectiveness was assessed in the presence and absence of the bioturbating organism Corbicula fluminea based on metal fluxes to sediment pore water and surface water, the distribution of mobile contaminants in sediment and surface water measured by Diffusive Gradients in Thin Films, and contaminant bioaccumulation by Lumbriculus variegatus. The metal sequestration capacity of apatite caps was unaffected or improved by bioturbation for all elements except As. Effects with uncapped sediment were metal specific including reductions in the bioavailable pool for Ni, Cd, and to a lesser extent, Pb, increases in the bioavailable pool for As and Cu, and little effect for Zn. It is likely that the reductions observed for some metals in uncapped, clean sediment were the result of burial and dilution of contaminated sediment combined with chemical processes such as sequestration by minerals and other compounds. These results indicate that apatite caps can control recontamination by metals regardless of bioturbation but point to the complexity of sediment recontamination and the need for further study of this problem.

Assessing effects of dissolved organic carbon and water hardness on metal toxicity to Ceriodaphnia dubia using diffusive gradients in thin films (DGT).

We evaluated the ability of diffusive gradients in thin films (DGT) to assess the effects of water hardness and dissolved organic carbon (DOC consisting of humic acids) on Cu and Zn toxicity (i.e., 48 h LC50s) to Ceriodaphnia dubia. When DOC was high, Cu concentrations measured by DGT (DGT Cu) were significantly lower than Cu concentrations in water (dissolved Cu), and DGT LC50s were significantly lower than LC50s based on dissolved concentrations. When DOC was low, differences between dissolved Cu and DGT Cu were small, as were differences between dissolved LC50s and DGT LC50s. Differences between DGT and dissolved measurements of Zn were small compared with the differences observed for Cu, and DGT Zn LC50s were relatively similar to dissolved Zn LC50s. Humic acids formed strong organic-Cu complexes that were both inaccessible for biological uptake and excluded by DGT, which selected for free or weakly bound Cu. In contrast, Zn did not form strong complexes with DOC that greatly affected either toxicity or the measurement of Zn by DGT. The effects of hardness on DGT measurements of Cu and Zn were smaller and more complex than the effects of DOC. Large, statistically significant differences between DGT measurements of Cu in low and high DOC water accurately reflected the strong effects of DOC on Cu toxicity. However, the effects of DOC were inconsistent for Zn, and DGT provided less information about the toxic fraction of this metal.

Removal of low levels of Cu from ongoing sources in the presence of other elements - Implications for remediated contaminated sediments.

Mesocosms were used to investigate the effects of Cu influx, alone and in the presence of other elements, on sediments remediated by active caps, passive caps, and in situ treatment. Competitive interactions between Cu and other elements were investigated because contaminants often co-occur. Elements in surface water remained at significantly lower concentrations in mesocosms with apatite and mixed amendment caps than in mesocosms with passive sand caps or uncapped sediment. Element concentrations in Lumbriculus variegatus were significantly higher in untreated sediment than in active caps and significantly related to element concentrations in sediment measured by DGT probes. The cumulative toxicity of Cu mixed with other elements was greater than the toxicity of Cu alone in treatments without active caps, but the ability of active caps to control Cu was not affected by the presence of other elements. Active caps can protect remediated sediments by reducing bioavailable elements in ongoing contamination.

A Stream Multimetric Macroinvertebrate Index (MMI) for the Sand Hills Ecoregion of the Southeastern Plains, USA.

A macroinvertebrate multimetric index is an effective tool for assessing the biological integrity of streams. However, data collected under a single protocol may not be available for an entire region. We sampled macroinvertebrates from the full extent of the Sand Hills ecoregion Level IV of the Southeastern Plains with a standard protocol during the summers of 2010-2012. We evaluated the performance of 94 metrics through a series of screening criteria and built 48 macroinvertebrate multimetric indexs with combinations of the best performing metrics, representing richness, habit, functional feeding guild, sensitivity, and community composition. A series of narrative-response tests for each macroinvertebrate multimetric index was used to find the best performing macroinvertebrate multimetric index which we called the Sand Hills macroinvertebrate multimetric index. The Sand Hills macroinvertebrate multimetric index consisted of the measures Biotic Index, % Shredder taxa, Clinger taxa(2)/total taxa, Plecoptera and Trichoptera richness, and Tanytarsini taxa(2)/Chironomidae taxa. Comparison of the Sand Hills macroinvertebrate multimetric index with existing assessment tools calculated with our data indicated that the Sand Hills macroinvertebrate multimetric index performs at a high level with regard to identifying degraded sites and in its response to stress gradients.

Environmental impact of ongoing sources of metal contamination on remediated sediments.

A challenge to all remedial approaches for contaminated sediments is the continued influx of contaminants from uncontrolled sources following remediation. We investigated the effects of ongoing contamination in mesocosms employing sediments remediated by different types of active and passive caps and in-situ treatment. Our hypothesis was that the sequestering agents used in active caps and in situ treatment will bind elements (arsenic, chromium, cadmium, cobalt, copper, nickel, lead, selenium, and zinc) from ongoing sources thereby reducing their bioavailability and protecting underlying remediated sediments from recontamination. Most element concentrations in surface water remained significantly lower in mesocosms with apatite and mixed amendment caps than in mesocosms with passive caps (sand), uncapped sediment, and spike solution throughout the 2520h experiment. Element concentrations were significantly higher in Lumbriculus variegatus from untreated sediment than in Lumbriculus from most active caps. Pearson correlations between element concentrations in Lumbriculus and metal concentrations in the top 2.5cm of sediment or cap measured by diffusive gradient in thin films (DGT) sediment probes were generally strong (as high as 0.98) and significant (p<0.05) for almost all tested elements. Metal concentrations in both Lumbriculus and sediment/cap were lowest in apatite, mixed amendment, and activated carbon treatments. These findings show that some active caps can protect remediated sediments by reducing the bioavailable pool of metals/metalloids in ongoing sources of contamination.

Defining the reference condition for wadeable streams in the Sand Hills subdivision of the Southeastern Plains ecoregion, USA.

The Sand Hills subdivision of the Southeastern Plains ecoregion has been impacted by historical land uses over the past two centuries and, with the additive effects of contemporary land use, determining reference condition for streams in this region is a challenge. We identified reference condition based on the combined use of 3 independent selection methods. Method 1 involved use of a multivariate disturbance gradient derived from several stressors, method 2 was based on variation in channel morphology, and method 3 was based on passing 6 of 7 environmental criteria. Sites selected as reference from all 3 methods were considered primary reference, whereas those selected by 2 or 1 methods were considered secondary or tertiary reference, respectively. Sites not selected by any of the methods were considered non-reference. In addition, best professional judgment (BPJ) was used to exclude some sites from any reference class, and comparisons were made to examine the utility of BPJ. Non-metric multidimensional scaling indicated that use of BPJ may help designate non-reference sites when unidentified stressors are present. The macroinvertebrate community measures Ephemeroptera, Plecoptera, Trichoptera richness and North Carolina Biotic Index showed no differences between primary and secondary reference sites when BPJ was ignored. However, there was no significant difference among primary, secondary, and tertiary reference sites when BPJ was used. We underscore the importance of classifying reference conditions, especially in regions that have endured significant anthropogenic activity. We suggest that the use of secondary reference sites may enable construction of models that target a broader set of management interests.

Impact of forest seral stage on use of ant communities for rapid assessment of terrestrial ecosystem health.

Bioassessment evaluates ecosystem health by using the responses of a community of organisms that integrate all aspects of the ecosystem. A variety of bioassessment methods have been applied to aquatic ecosystems; however, terrestrial methods are less advanced. The objective of this study was to examine baseline differences in ant communities at different seral stages from clear cut to mature pine plantation as a precursor to developing a broader terrestrial bioassessment protocol. Comparative sampling was conducted at nine sites having four seral stages: clearcut, 5 year recovery, 15 year recovery, and mature stands. Soil and vegetation data were also collected at each site. Ants were identified to genus. Analysis of the ant data indicated that ants respond strongly to habitat changes that accompany ecological succession in managed pine forests, and both individual genera and ant community structure can be used as indicators of successional change. Ants exhibited relatively high diversity in both early and mature seral stages. High ant diversity in mature seral stages was likely related to conditions on the forest floor favoring litter dwelling and cold climate specialists. While ants may be very useful in identifying environmental stress in managed pine forests, adjustments must be made for seral stage when comparing impacted and unimpacted forests.

Amendments for the in situ remediation of contaminated sediments: evaluation of potential environmental impacts.

Active sediment caps represent a comparatively new technology for remediating contaminated sediments. They are made by applying chemically active amendments that reduce contaminant mobility and bioavailability to the sediment surface. The objective of this study was to determine if active cap amendments including organoclay, apatite, and biopolymers have the potential to harm benthic organisms. Methods included laboratory bioassays of amendment toxicity and field evaluations of amendment impacts on organisms held in cages placed within pilot-scale active caps located in Steel Creek, a South Carolina (USA) stream. Test organisms included Hyalella azteca, Leptocheirus plumulosus, Lumbriculus variegatus, and Corbicula fluminea to represent a range of feeding modes, burrowing behaviors, and both fresh and saltwater organisms. In addition to the laboratory and field assays, chemical extractions were performed to determine if the amendments contained harmful impurities that could leach into the ambient environment. Laboratory bioassays indicated that 100% apatite had minimal effects on Hyalella in freshwater and up to 25% organoclay was nontoxic to Leptocheirus in brackish water. Field evaluations indicated that pilot-scale caps composed of up to 50% apatite and 25% organoclay did not harm Hyalella, Lumbriculus, or Corbicula. In contrast, organisms in caps containing biopolymers died because of physical entrapment and/or suffocation by the viscous biopolymers. The extractions showed that the amendments did not release harmful concentrations of metals. These studies indicated that apatite and organoclay are nontoxic at concentrations (up to 50% and 25% by weight, respectively) needed for the construction of active caps that are useful for the remediation of metals and organic contaminants in sediments.

An Overview of 137Cs Contamination in a Southeastern Swamp Environment.

In the early 1960's, an area of privately owned swamp adjacent to the Savannah River Site was contaminated by site operations. Studies conducted in 1974 estimated that approximately 925 GBq of Cs and 37 GBq of Co were deposited in the swamp. Subsequently, a series of surveys was initiated to characterize the contaminated environment. These surveys-composed of 52 monitoring locations-allow for continued monitoring at a consistent set of locations. Initial survey results indicated maximum Cs concentrations of 19.5 Bq g in soil and 8.7 Bq g in vegetation. By the 2004-2005 surveys, maximum concentrations had declined to 1-2 Bq g in soil and 0.4 Bq g in vegetation.

Concentration ratios for small mammals collected from the exposed sediments of a 137Cs contaminated reservoir.

(137)Cs concentration ratios were computed for small mammals collected from the dried sediments of a partially drained, contaminated reservoir. Soil (137)Cs activity concentrations were heterogeneous on small and large spatial scales and had a geometric mean of 253 (range 23-2110) Bq/kg dry weight. Mean (137)Cs activity concentrations in composite cotton rat Sigmodon hispidus and cotton mouse Peromyscus gossypinus samples averaged 2480 (range 556-6670) and 471 (range 96-1000) Bq/kg whole body dry weight, respectively. About 50% of the variance in cotton rat tissue (137)Cs activity was explained by variation in soil (137)Cs activity. Soil-to-animal dry weight concentration ratios averaged 6.0 for cotton rats and 1.2 for cotton mice and were generally similar to (137)Cs concentration ratios for herbivorous, homeothermic animals from other contaminated ecosystems. In the RESRAD-BIOTA dose model, the default wet-weight concentration ratio for (137)Cs in terrestrial animals is 110 resulting in an estimate of internal and external radiation doses to terrestrial biota that is 44 times more than the dose calculated with the actual measured wet-weight concentration ratio for cotton rats (1.6). These results show that site-specific concentration ratios can significantly affect the estimation of dose.

Metal distribution and stability in constructed wetland sediment.

The A-01 wetland treatment system (WTS) is a surface flow wetland planted with giant bulrush [Schoenoplectus californicus (C.A. Mey.) Palla] that is designed to remove Cu and other metals from the A-01 National Pollution Discharge Elimination System (NPDES) effluent at the Savannah River Site near Aiken, SC. Copper, Zn, and Pb concentrations in water were usually reduced 60 to 80% by passage through the treatment system. The Cu concentrations in the wetland sediments increased from about 4 to 205 and 796 mg kg(-1), respectively, in the organic and floc sediment layers in cell 4A over a 5-yr period. Metal concentrations were higher in the two top layers of sediment (i.e., the floc and organic layers) than in the deeper inorganic layers. Sequential extraction was used to evaluate remobilization and retention of Cu, Pb, Zn, Mn, and Fe in the wetland sediment. Metal remobilization was determined by the potentially mobile fraction (PMF) and metal retention by the recalcitrant factor (RF). The PMF values were high in the floc layer but comparatively low in the organic and inorganic layers. High RF values for Cu, Zn, and Pb in the organic and inorganic layers indicated that these metals were strongly bound in the sediment. The RF values for Mn were lower than for the other elements especially in the floc layer, indicating low retention or binding capacity. Retention of contaminants was also evaluated by distribution coefficient (Kd) values. Distribution coefficient (Kd) values were lower for Cu and Zn than for Pb, indicating a smaller exchangeable fraction for Pb.