Toxicity of sediments and porewaters
Survival of H. azteca exposed for 28 d to sediments collected from canals associated with the 2 landfills was similar, although survival from EL1 was significantly (p < 0.05) less than that shown for CDRef, the reference site located closest to the C/D Landfill (Table 1). Length (an indicator of growth) of H. azteca, however, was significantly reduced at sampling locations CD3, CD5, and CDRef when compared with the laboratory reference (OGRef). Contrasted with the reference site closest to the East Lake Landfill (ELRef), length was also significantly (p < 0.05) less in animals exposed to sediments from locations CD1, CD3, CD5, CDRef, and EL1. The length of H. azteca exposed to sediments from CD3 was significantly reduced when compared with CDRef.
Considerable variation often exists among reference sites, and the sites are not necessarily free from contaminants or other stressors (Ingersoll et al. 1994; Hughes 1995). To at least account for some of this variability, multiple references sites (3 in this case, 1 laboratory reference and 2 field references) were used for statistical comparisons between metrics. Each comparison was considered a different line of evidence. Concordance among test endpoints was good, with 3 of the 5 sites showing impaired growth from the solid-phase exposures and reduced survival in porewater (Table 1). Acute toxicity in the 96-h porewater tests occurred at sampling locations CD1, CD2, EL1, and CDRef. Based on tests with solid-phase sediment and porewater, sediment quality was reduced relative to at least 1 of the 3 reference sediments at 5 locations associated with the C/D Landfill (CD1, CD2, CD3, CD5, and CDRef) and the sampling location closest to the East Lake Landfill (EL1). Survival and growth of H. azteca exposed to sediment and pore water from these 6 sites ranked the lowest, demonstrating impaired conditions when compared with the other sites.
Toxicity (survival) was not influenced by exposure of the test animals to ultraviolet light subsequent to the sediment and porewater tests (therefore, data are not shown). Animals exposed to some PAHs have been shown to elicit a marked increase in mortality following a short exposure to ultraviolet light (Ankley et al. 1994). The absence of increased toxicity to animals from the solid-phase and porewater tests with exposure to ultraviolet light suggests that PAH (at least the compounds that are photoactivated by ultraviolet light) contamination may not be contributing to the toxicity shown by these sediments.
Table Table 1.. Survival and length (with standard deviations) of Hyalella azteca in 28 d, solid-phase sediment and 96 h, porewater tests using sediments collected in canals associated with 2 landfills at Alligator River National Wildlife Refuge, North Carolina, USAa
| ||Solid-phase sediment test results||Porewater test results|
|Sampling location||% Survival||SD||Length (mm)||SD||% Survival||SD|
Table Table 2.. Physical characteristics of sediments and chemistry of overlying water in solid-phase sediment tests with Hyalella azteca exposed to sediments collected from canals associated with 2 landfills at Alligator River National Wildlife Refuge, North Carolina, USA
| ||Sampling location|
|Total organic (%)||4.1||6.1||12.5||5.5||9.5||3.8||22.7||4.7||3.5||9.4||12.1||29.0||25.6||21.3||7.6|
|Dissolved oxygen (mg/L)||8.1||8.0||7.7||7.9||7.6||7.5||8.2||7.8||8.1||8.3||8.3||8.4||8.3||8.4||8.0|
|Alkalinity (mg/L CaCO3)||92||78||70||76||70||72||72||72||82||76||76||70||70||76||78|
|Hardness (mg/L CaCO3)||128||112||108||116||112||104||100||108||112||116||112||112||104||104||92|
|Total ammonia (mg/L)||0.7||1.4||1.1||0.6||0.7||1.9||0.5||0.5||1.8||1.0||1.1||0.6||0.2||0.5||0.6|
Factors influencing toxicity
The significance of specific contaminants that may be responsible for the impaired growth and toxicity shown by H. azteca exposed to sediments associated with the landfills depends not only upon the concentration but also on the matrix and route of exposure (sediment or porewater) and the physical and chemical factors that influence the availability of the toxicant to the organism. Primary factors that could influence the availability include the basic chemistry (e.g., pH, alkalinity, hardness, and redox potential) and physical composition of the sediment (e.g., percentage of organic matter and sediment particle-size distribution). Characteristics (total organic material, particle size, and percent moisture) of the sediments evaluated during this study showed some variability among stations but were within ranges that would be expected from these types of systems (Table 2). The percentage of fines (silt and clay) at the sampling locations that elicited toxicity tended to be higher than that at other locations with lower toxicity. Acid-volatile sulfides varied from low (1.4 μmol/g) to high (221.8 μmol/g) in the sediments, but concentrations of the SEM were uniformly low, and the SEM to AVS ratios were less than 1. SEM to AVS ratios greater than 1 are generally predictive of metal toxicity, and ratios less than 1 indicate that metal-sulfide forming metals (Cd, Cu, Hg, Pb, Ni, and Zn) would not be expected to elicit toxicity (Di Toro et al. 1992; Ankley et al. 1993). Chemistry of the overlying water from the solid-phase exposures showed no anomalies corresponding to locations considered to have impaired sediment quality. Concentrations of ammonia in the overlying water from the solid-phase exposures were low (< 2 mg/L). Basic characteristics (e.g., pH, hardness, and conductivity) of porewater were within normal ranges, although alkalinity and ammonia were elevated at several sampling locations (Table 3).
Exceedences of screening values
Factors responsible for the toxicity shown by H. azteca exposed to sediments and porewater are not clearly identified, although concentrations and correlative associations suggest that metal concentrations in the sediments and porewaters may be implicated (Table 4). Metal concentrations in sediments from several sampling locations were elevated above those from reference locations, and some exceeded specific guidelines or criteria and may have contributed to the reduced growth and survival of H. azteca shown in the chronic tests and acute tests (Table 5).
Concentrations of As, Cu, and Zn in sediments at several locations, for example, exceeded published TECs (MacDonald 2000). Arsenic concentrations at CD1 (38.7 μg/g) and CD2 (37.1 μg/g) exceeded both the TEC (9.79 μg/g) and PEC (33.0 μg/g) screening values. No other sediment samples exceeded PECs for any contaminant. Sediments with metal concentrations that exceeded consensus-based TECs would be expected to have adverse effects on sensitive sediment-dwelling organisms. Concentrations of Cr, Cu, and As were elevated (Table 5) at the 3 locations down-gradient from the C/D Landfill (CD1, CD2, and CD3). Average concentrations of Cr (30.0 μg/g), Cu (35.4 μg/g), and As (31.5 μg/g) at these 3 locations exceeded levels measured during baseline monitoring in 1988 before development of this area as a landfill: average (n = 3) Cr = 14.8 μg/g, Cu = 8.7 μg/g, and As = 4.2 μg/g (North Carolina Division of Water Quality, unpublished data). A possible source for these contaminants may be from lumber treated with chrome-copper-arsenic (Weis and Weis 1996) disposed of at the landfill.
Porewater is generally considered a major route of contaminant exposure to sediment-dwelling organisms (Adams et al. 1985), and the elevated concentrations of metals in this matrix, particularly at several of the sampling locations showing toxicity, may have contributed to the impaired conditions (Table 6). In this study, SEM to AVS ratios were <1 at all locations, suggesting that metal concentrations in the porewater would be low and not expected to elicit toxicity (Di Toro et al. 1992; Ankley et al. 1993). The lack of correlation between survival of H. azteca in porewater and concentrations of metals in the porewater is consistent with this observation (Table 4). However, porewater concentrations at several locations exceeded water quality criteria for As, Cr, Cu, Pb, Se, and Zn (USEPA 2002) for the protection of aquatic life. The lack of correlation does not preclude the possibility that elevated concentrations of these metals could adversely affect survival. Survival in porewater was, however, significantly (p < 0.05), negatively correlated with concentrations of As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn in the sediments, as well as with total metal concentrations. Concentrations of several of these metals in porewater exceeded those that have been shown to be harmful to aquatic organisms. For example, concentrations of As ranging from 19 to 48 μg/L have been found to adversely impact aquatic species (Eisler 1994), and Miller and Hendricks (1996) reported that aqueous Zn concentrations of 62 μg/L reduced the growth of Chironomus riparius. Porewater from 10 of the 14 sampling locations had Zn concentrations that exceeded 62 μg/L (Table 6). Similarly, Cr concentrations exceeded levels that have been shown to cause toxicity (Eisler 1986), and concentrations >20 μg/g in the sediments have been shown to cause toxicity to fish in the overlying water (Gendusa et al. 1993). Copper at concentrations of 5 to 10 μg/L are toxic to sensitive species (Eisler 1997).
Table Table 3.. Chemistry of porewater used in 96-h exposures with Hyalella azteca. Porewater was extracted from sediments collected from canals associated with 2 landfills at Alligator River National Wildlife Refuge, North Carolina, USA
| ||Sampling location|
|Dissolved oxygen (mg/L)||8.9||8.9||8.8||8.9||8.9||8.9||8.9||8.8||8.9||9.0||9.0||9.0||9.0||8.9||8.7|
|Alkalinity (mg/L CaCO3)||458||368||200||208||176||162||128||244||310||252||200||178||98||162||240|
|Hardness (mg/L CaCO3)||578||280||145||232||130||72||141||176||272||164||170||297||161||213||312|
|Total ammonia (mg/L)||5.3||10.4||6.0||3.8||3.1||14.2||3.2||1.9||14.4||8.5||9.0||4.8||1.7||3.2||2.0|
|Total organic carbon (mg/L)||108.7||122.4||38.9||26.6||37.7||47.8||60.9||25.5||55.3||40.54||55.1||26.3||44.3||61.6||49.9|
Table Table 4.. Spearman rank correlation between sediment and porewater toxicity with Hyalella azteca, sediment, and porewater characteristics; metal concentrations in porewater and sediments (normalized to organic content); and total PAH concentrationa
|Arsenic in sediment||−-0.352||0.516C||−-0.756B|
|Cadmium in sediment||−-0.453||0.616C||−-0.557C|
|Chromium in sediment||−-0.273||0.614C||−-0.798A|
|Copper in sediment||−-0.266||0.495||−-0.717B|
|Mercury in sediment||−-0.025||0.459||−-0.513|
|Nickel in sediment||−-0.351||0.662B||−-0.619C|
|Lead in sediment||−-0.233||0.649B||−-0.565C|
|Zinc in sediment||−-0.373||0.608C||−-0.625C|
|SQQ in sediment||−-0.522C||0.406||−-0.439|
|Total metals in sediment||−-0.273||0.608C||−-0.708B|
|Total PAHs in sediment||0.114||0.179||−-0.374|
|Arsenic in porewater||−-0.443||0.176||−-0.238|
|Cadmium in porewater||0.022||−-0.571C||0.385|
|Chromium in porewater||−-0.592C||0.137||−0.415|
|Copper in porewater||−-0.272||−-0.311||−-0.061|
|Mercury in porewater||−-0.022||−-0.169||−-0.311|
|Nickel in porewater||−-0.296||0.308||−-0.261|
|Lead in porewater||−-0.097||−-0.305||0.466|
|Zinc in porewater||−-0.260||0.044||0.174|
|Total metal in porewater||−-0.316||−-0.179||0.045|
|SQQ in porewater||−-0.367||−-0.105||−-0.158|
Figure Figure 2.. Relationship between porewater toxicity to Hyalella azteca and the sum of the sediment quality quotients for metals in sediments collected from areas draining 2 landfills at the Alligator River National Wildlife Refuge, North Carolina, USA.
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Other factors or contaminants not tested in this study could have contributed to or influenced the toxicities observed in the acute and chronic tests. In addition, chemical conditions associated with the basic water chemistry of the test waters could also have influenced the toxicity shown in the porewater (Table 3). Alkalinity, which has been shown to be a confounding factor associated with porewater testing (Lasier et al. 1997), was elevated at several of the sampling locations that showed the highest toxicities. Ammonia, which was elevated at some locations where toxicity was high (>10 mg/L at CD2 and EL1), has also been identified as a confounding factor in porewater testing. Ammonia concentrations, however, were below levels that are generally considered toxic to H. azteca (Ankley et al. 1990).
Table Table 5.. Metal concentrations (μg/g dry wt) in sediments collected from canals associated with 2 landfills at Alligator River National Wildlife Refuge, North Carolina, USA and used for 28-d chronic-toxicity tests with Hyalella azteca and 28-d bioaccumulation studies with Lumbriculus variegatus (locations in bold were used in the bioaccumulation study)a
| ||Sampling location|
|Mercury||<0.17||0.20||0.23||0.22||< 0.17||< 0.17||0.27||< 0.17||< 0.17||0.26||0.34||0.38||0.18||0.28||< 0.17|
Table Table 6.. Concentrations of metals in porewater from sediments collected from canals associated with 2 landfills at Alligator River National Wildlife Refuge, North Carolina, USAa
| ||Sampling location|
Table Table 7.. Concentrations of organic (phthalates, polynuclear aromatic hydrocarbons, and aliphatic hydrocarbons) contaminants (μg/g dry wt) in sediments collected from canals associated with 2 landfills at Alligator River National Wildlife Refuge, North Carolina, USA. Sediments from locations that are in bold were included in the bioaccumulation studya
| ||Sampling location|
|Phthalates|| || || || || || || || || || || || || || || |
|Aromatics|| || || || || || || || || || || || || || || |
|Aliphatics|| || || || || || || || || || || || || || || |
Table Table 8.. Concentrations of metals (μg/g dry wt) and bioaccumulation factors (in parentheses) for Lumbriculus variegatus exposed for 28 d to selected sediments from canals associated with 2 landfills at Alligator River National Wildlife Refuge, North Carolina, USA
| ||Sampling location|| |
|Aluminum||467.0 (0.04)||245.0 (0.01)||184.0 (0.01)||729.0 (0.03)||97.8 (0.01)||22.6|
|Arsenic||107.0 (2.76)||25.4 (1.35)||16.1 (1.85)||10.2 (0.73)||2.43 (0.71)||1.57|
|Boron||1.95 (0.29)||0.65 (0.16)||<0.51 (0.27)||0.59 (0.11)||0.65 (0.04)||2.52|
|Barium||106.0 (2.24)||101.0 (1.56)||100.0 (1.98)||87.5 (0.92)||77.0 (2.51)||37.8|
|Cadmium||0.29 (1.03)||0.31 (0.81)||0.23 (0.88)||0.25 (0.58)||0.28 (1.55)||<0.22|
|Chromium||2.29 (0.07)||1.14 (0.04)||1.01 (0.06)||1.29 (0.03)||0.73 (0.04)||0.90|
|Copper||48.4 (1.37)||51.2 (1.70)||46.5 (3.14)||33.7 (1.80)||35.4 (3.40)||12.3|
|Iron||2,408 (0.01)||2,224 (0.07)||1,890 (0.09)||1,869 (0.05)||965 (0.07)||713|
|Magnesium||993 (0.20)||889 (0.40)||896 (0.60)||957 (0.20)||2,541 (0.50)||763|
|Manganese||16.6 (0.07)||8.26 (0.09)||8.81 (0.01)||9.08 (0.06)||9.25 (0.15)||5.81|
|Molybdenum||0.53 (0.92)||0.36 (0.78)||0.39 (0.95)||0.42 (0.34)||0.49 (0.24)||0.58|
|Nickel||1.99 (0.13)||1.01 (0.10)||1.03 (0.12)||1.20 (0.05)||1.07 (0.12)||0.68|
|Lead||3.25 (0.17)||2.37 (0.11)||1.56 (0.13)||2.12 (0.10)||4.50 (0.02)||0.89|
|Selenium||5.63 (8.15)||4.38 (4.42)||4.45 (6.26)||4.71 (5.11)||3.38 (2.33)||2.02|
|Strontium||86.4 (2.74)||80.6 (2.76)||81.6 (3.90)||88.7 (1.63)||88.2 (1.35)||46.6|
|Vanadium||3.93 (0.15)||2.81 (0.12)||2.91 (0.15)||3.28 (0.05)||2.68 (0.13)||4.00|
|Zinc||448.0 (3.58)||476.0 (4.32)||429.0 (5.64)||355.0 (3.28)||245.0 (7.65)||278.0|
Table Table 9.. Uptake of organic contaminants (μg/g dry wt) by Lumbriculus variegatus exposed during a 28-d bioaccumulation study to sediments collected from canals associated with 2 landfills at Alligator River National Wildlife Refuge, North Carolina, USAa
| ||Sampling location|| |
|Butyl benzylphtha late||ND||ND||ND||ND||ND||ND|
The toxicities shown in the solid-phase sediment and porewater tests may be in response to the combination of metals (As, Cr, Cu, Pb, Se, and Zn) present. Metal mixtures have been shown to be important in eliciting toxicity (de March 1988; Kraak et al. 1994). Eisler (1997) also reported that Cu has been shown to have more than additive toxicity in the presence of Al, Cr, Fe, Mn, Mo, and Zn. A significant relationship was shown between survival of H. azteca in porewater and the total SQQ calculated for metal concentrations in the sediments (Figure 2). However, there was not a statistically significant relationship between the total SQQ for metals in porewater with survival in porewater or between survival/length from solid-phase tests and the corresponding SQQ for either matrix. Filtering of the porewater samples in preparation for submission for analytical testing may have contributed to the lack of correlation between survival in porewater and concentrations of metals. The test animals were exposed to porewater that had not been filtered, and filtering has been shown to remove metals from porewater samples (Winger et al. 1998). Ankley and Schubauer-Berigan (1994) found that unfiltered porewater was more toxic to test species than filtered porewater.
Table Table 10.. Quality criteria for protection of fish and wildlife resources from contaminants of concern addressed in this study
|Contaminant||Sediment threshold effects criteriaa||Water quality criteriab (100 mg/L hardness)||North Carolina water quality criteriac||Protection levelsd||Dietary limitse|
|Arsenic||9.79 mg/kg dry wt1f||150 μg/L2||50 μg/L3||190 μg/L4||50 μg/g4|
|Cadmium||0.99 mg/kg dry wt1||2.2 μg/L2||2 μg/L3||1 μg/L5||200 μg/g5|
|Chromium||43.4 mg/kg dry wt1||11 μg/L2||50 μg/L3||10 μg/L6||10 μg/g6|
|Copper||31.6 mg/kg dry wt1||9 μg/L2||7 μg/L3||12 μg/L7||200 μg/g7|
|Lead||35.8 mg/kg dry wt1||2.5 μg/L2||25 μg/L3||1.3 μg/L8||2.8 μg/g8|
|Mercury||0.18 mg/kg dry wt1||0.77 μg/L2||0.012 μg/L3||0.001 μg/L9||0.5 μg/g10|
|Nickel||22.7 mg/kg dry wt1||52 μg/L2||88 μg/L3||25 μg/L11||200 μg/g11|
|Selenium||1–4 mg/kg dry wt12||5 μg/L2||5 μg/L3||2 μg/L13||3 μg/g13|
|Zinc||121 mg/kg dry wt1||120 μg/L2||50 μg/L3||5 μg/L14||178 μg/g14|
|Anthracene||57.2 μg/kg dry wt1||0.01 μg/L15||NAg||4 μg/L15||NA|
|Fluoranthene||423 μg/kg dry wt1||0.01 μg/L15||NA||4 μg/L15||NA|
|Phenanthene||204 μg/kg dry wt1||0.01 μg/L15||NA||0.03 μg/L15||NA|
|Pyrene||195 μg/kg dry wt1||0.01 μg/L15||NA||0.02 μg/L15||NA|
|Total PAHsg||1,610 μg/kg dry wt1||0.01 μg/L15||NA||0.1 μg/L16||1.6 μg/g16|
Organic contaminants could also have contributed to the toxicities shown at several of the sampling locations (Table 7). The presence of phthalates (plasticizers in plastic material) at all locations may be characteristic of landfill leachates, but these chemicals are generally considered relatively nontoxic and would be expected to have minimal impact to resident biota, especially at the concentrations measured in this study. The more toxic PAHs (aromatics) were measured at several locations, particularly CD1 and CD3. Interestingly, the highest total concentration of PAHs occurred at ELRef, the reference for the East Lake Landfill, but toxicity was not shown at this location. The reference site is adjacent to US Highway 64 where road runoff, a significant source of PAHs (Eisler 1987a), may occur. This location also had high concentrations of Pb in the sediments and porewater, which is consistent with proximity to a highway (Harrison et al. 2003). Total PAHs in the sediments did not exceed the SQG (1.61 μg/g dry weight) (MacDonald et al. 2000) at any location, although total concentrations surpassed 1 μg/g dry weight at CD1, CD3, and ELRef. Individual PAHs (anthracene, fluoranthene, phenanthrene, and pyrene) exceeded SQGs at several locations. Alkanes (aliphatic hydrocarbons) were ubiquitous in the sediment samples collected around the landfills. Paraffins (straight-chain hydrocarbons common in the environment and also present in petroleum products) have relatively low toxicities and would not be expected to adversely impact aquatic systems at the concentrations measured in this study. Individual or total PAHs were not significantly correlated with survival or length of H. azteca exposed to the sediments or porewater (Table 4). Other chemicals, such as volatile compounds (benzene, toluene, ethylbenzene, and xylene), priority pollutants, and industrial chemicals may have been present in the landfill leachate but were not included in the chemical analyses.
Four of the sediments that demonstrated acute and/or chronic toxicity, plus a reference sediment (CD1, CD3, CD5, EL1, and ELRef), were further assessed through an evaluation of biological uptake of contaminants by L. variegatus during a 28-d bioaccumulation study (Table 8). Residue concentrations of contaminants in oligochaetes from the laboratory cultures were used as the baseline reference. For field comparisons, the sediment from the reference site located at ELRef was used. With the exception of B, Mo, and V, metal residues in the oligochaetes increased during the 28-d exposure to the sediments when compared with metal levels measured in the culture animals. Increases in concentration varied with each metal, but most ranged between 2 and 4 times higher, and some increased over 20 times (Al and As). Concentrations of metals were also generally lower in oligochaetes exposed to the ELRef sediment than in animals exposed in sediments from other locations, with the exceptions of Mg, Pb, and Sr. Metal residues were generally highest in the oligochaetes exposed to sediments from sampling location CD1. Sediment from CD1 also had the highest concentrations of metals (Table 5). Bioaccumulation factors, calculated as the ratios of concentration of metals (w/w) in the oligochaetes and concentrations in the sediments, were highest for As (0.7:2.7), Ba (0.9:2.5), Cu (1.4:3.4), Se (2.3:8.2), Sr (1.3:2.7), and Zn (3.3:7.6). Mercury, a heavy metal that bioaccumulates and biomagnifies (Eisler 1987b) was present in the sediments and at levels exceeding SQGs at several of the locations (Table 5), but uptake by the oligochaetes was insignificant (i.e., residues were at or below analytical detection levels).
Uptake of organic contaminants from the sediments by L. variegatus was limited to a few PAHs; however, aliphatic hydrocarbons were not included in analytical testing of oligochaete tissues (Table 9). Concentrations were low and the bioaccumulation factors for the various PAHs ranged from 0 to 6. Perylene bioaccumulated in oligochaetes at all locations except ELRef; bioaccumulation factors ranged from 2.28 to 5.25. Total PAHs in oligochaetes were highest at CD1 (1.2 μg/g), with residues at the other locations reaching only about half of this total level. The significance of PAH residues and bioaccumulation factors for aquatic organisms or wildlife are not known (Eisler 1987a). The limited bioaccumulation of organic contaminants by the oligochaetes probably reflects the relatively low concentrations present in the sediments, but their presence in the sediments and bioaccumulation does raise concern that the potential exists for transport of contaminants from both landfills that could result in further impairment of sediment and habitat quality.
The accumulation of some metals may be influenced by the organism's ability to regulate them (Rainbow 1996). Essential elements (e.g., As, Cu, Cr, Ni, Se, and Zn) are often regulated in tissues and organs (Miller and Hendricks 1996; Borgmann and Norwood 1997). Low doses are beneficial and are often needed physiologically, but at higher concentrations, essential elements can be detrimental or toxic to the organisms. Just as importantly, some organisms may bioaccumulate these contaminants to relatively high levels without significant adverse effects, but the levels that accumulate in tissue may exceed biological-effects levels for animals that feed upon them. Interpretation of the significance of body residues and the level of bioaccumulation shown in these tests is not clear cut, but the fact that there was accumulation above baseline levels suggests the availability of these contaminants and the potential for adverse biological impacts.
Environmental significance of tests
The routes of exposure may also be important in understanding the potential impact of contaminants to the organisms. Potential sources of contaminants to the aquatic organisms are ingestion of the sediments directly, the overlying water, and the sediment porewater (Power and Chapman 1992). Ingestion of the sediments and sediment organic matter is the most likely route of contaminant exposure to organisms such as L. variegatus that burrow and feed in the sediment. However, some organisms can also be adversely affected through direct exposure to the sediment and sediment porewater whereas others, especially those that reside in the water column, may take up the contaminants from the overlying water. During this study, the sediments did not appear to elicit an adverse response by L. variegatus, but a number of metals bioaccumulated through exposure to the sediments (Table 8). Growth of H. azteca (an epibenthic organism that occasionally burrows into the sediments but also spends time in the water column) was reduced in sediments from several locations. One or more of the metals, as well as PAHs, in the solid-phase sediment or porewater tests could have contributed to the toxicity elicited (Tables 1, 5, and 6). Direct exposure of H. azteca to porewater isolated from sediment from several locations also elicited a toxic response. Elevated concentrations of metals could have contributed to the observed toxicity, although alkalinity and ammonia may also have been confounding factors.
Concentrations of metals and PAHs that exceeded specific guidelines or criteria have a high probability of being responsible for the toxic responses shown by H. azteca exposed to sediment and porewater. The environmental impact or significance of these contaminants in the sediments from the canals located down-gradient from the landfills can only be inferred from the multiple lines of evidence shown by the laboratory evaluations. The demonstration that the sediments elicited toxic reactions in test organisms and that contaminants were bioaccumulated from these sediments provides strong support for these inferences. The comparison of contaminant concentrations in the sediments and pore-waters, in addition to the residues taken up by the oligochaetes during the bioaccumulation study, with SQGs and protection levels is further evidence that habitat quality at several locations receiving drainage from the landfills has been impaired (Table 10). Of particular significance is the number of metals (As, Cu, Hg, Ni, and Zn) and PAHs (anthracene, fluoranthene, phenanthene, and pyrene) that exceed SQGs (Tables 5 and 7). Water quality criteria and aquatic-life protection levels were also exceeded by As, Cr, Cu, Pb, Se, and Zn in the porewater (Table 6). An assessment conducted in October 2000 of the benthic macroinvertebrate communities in canals associated with the landfills also revealed evidence of biological impairment immediately down-gradient from the C/D Landfill near location CD-2 (North Carolina Division of Water Quality 2000). This information provides additional evidence of contaminant effects on aquatic resources situated down-gradient from the landfills.