Remediation Effectiveness
Blackwell smelter soil was contaminated predominantly with Zn, although Cd and Pb levels were elevated compared with Taloka soil (Table 1). The addition of remediation amendments did not change total Cd, Pb, or Zn levels in Blackwell soil; the remediation goal was not to remove metals from the soil, but immobilize available metal species via changes in soil physicochemical properties. The metals present in the remediation amendments (Gradwohl, 1998) elevated total Cd, Pb, and/or Zn in Taloka soil. The remediation amendments significantly increased organic matter (SS and LSB) and pH (LSB) (Table 1).
Municipal sewage sludge biosolids amended with lime (LSB) significantly reduced the toxicity of the Blackwell smelter soil to the earthworms, with all Blackwell smelter soil amendment–combinations lethal to the earthworm E. fetida except for LSB (Fig. 1). Applying Fisher's protected LSD tests to transformed mean cumulative mortality at 16, 24, 48, and 72 h, we concluded that the SS amendment was the most toxic Blackwell soil–amendment combination, followed by None and RP. The relative toxicity of the soil–amendment combinations to earthworms was very similar to that of lettuce (Gradwohl, 1998). However, earthworms appeared to be more sensitive as lettuce was able to survive in the Blackwell RP soil–amendment combination. We cannot explain why the Blackwell SS soil–amendment combination was more toxic than the unamended soil, although Edwards and Bohlen (1996) suggest that fresh anaerobic sewage sludge may have additional components aside from heavy metals that are toxic to earthworms. Soil pH appeared to be more important than organic matter in immobilizing bioavailable metals, since LSB (higher pH, lower organic matter) reduced toxicity while SS (lower pH, higher organic matter) seemed to increase toxicity. Sequential extraction results suggest increased pH in soil treated with LSB increased adsorption and/or precipitation of Cd, Pb, and Zn and decreased their availability (Basta et al., 2001). Since Fang and Wong (1999) found that the addition of lime to sewage sludge biosolids reduced DTPA- and water-extractable metals, it follows that LSB-amended soils would also have reduced metal availability compared with SS-amended soils (Basta and Gradwohl, 2000; Sloan and Basta, 1995). The RP amendment did reduce toxicity slightly, but not as effectively as LSB. The main component of RP is carbonated fluorapatite, a mineral that is very effective in immobilizing available Pb (Ma et al., 1993), but not Cd or Zn (Basta et al., 2001). Although RP did not reduce the lethality of the Blackwell soil to earthworms, it was least toxic among the lethal soil–amendment combinations (Fig. 1) and may be useful in reducing long-term bioavailability of metals in some contaminated field soils, especially where Pb is of concern. There was no lethality associated with the three remediation amendments themselves since no mortality rates >15% were observed in the Taloka soil–amendment combinations.
Calcium Nitrate Extractions as Surrogate Measures of Metal Bioavailability
Metal availability as determined by the Ca(NO3)2 extraction was significantly different for Cd, Pb, and Zn among the Blackwell soil–amendment combinations and depicted large differences between the lethal Blackwell soil–amendment combinations and the nonlethal Blackwell LSB and Taloka combinations (Table 1). Zinc had the highest availability of all three metals, on a molar basis, with Ca(NO3)2–extractable concentrations more than 10-fold greater than Cd and Pb in the lethal Blackwell soil–amendment combinations. The 0.1 M Ca(NO3)2 fraction is the most available metal fraction in contaminated soil and has been shown to be well related to earthworm toxicity in metal-spiked artificial soil (Conder and Lanno, 2000) and phytotoxicity to lettuce (Basta and Gradwohl, 2000). Calcium nitrate–extractable Zn levels in the Blackwell soil–amendment combinations are compared with the incipient lethal levels (ILLs: time-independent LC50s) based on Ca(NO3)2–extractable Zn in the Zn artificial soil toxicity test (Fig. 2). Calcium nitrate–extractable Zn levels in the lethal Blackwell soil–amendment combinations were two to three times higher than the ILL (95% CI) of 6.33 (6.17–6.49) mmol Ca(NO3)2–extractable Zn/kg soil measured in artificial soil (Fig. 2). In contrast, Ca(NO3)2–extractable Cd and Pb were present at only 1% of Ca(NO3)2–extractable ILLs: 9.8 (9.4–10.3) and 1.16 (1.11–1.22), respectively (Conder and Lanno, 2000). Thus, we believe that Zn was the most likely source of toxicity. Furthermore, the two lowest lethal exposure levels in the artificial soil Zn test (15.6 and 35.2 mmol Zn/kg soil; Conder and Lanno, 2000) were very similar to the Ca(NO3)2–extractable Zn levels found in the lethal Blackwell soil–amendment combinations (Fig. 2). Time at which 100% cumulative mortality was reached was also very similar (36 to 120 h for lethal field soils, 72 to 144 h for Zn-spiked artificial soils [Conder and Lanno, 2000]). However, Ca(NO3)2–extractable Zn levels in the nonlethal Blackwell LSB combination were far below the ILL for Ca(NO3)2–extractable Zn in artificial soil and Ca(NO3)2–extractable Zn levels in lethal Blackwell soil–amendment combinations. All soil treatments reduced Ca(NO3)2–extractable Cd, Pb, and, with one exception, Zn (Table 1), with the greatest reductions in the Blackwell smelter soil–LSB amendment combination. Compared with Blackwell soil receiving no amendment (None), Ca(NO3)2–extractable Zn in the LSB amendment decreased by a factor of 25, Cd by 10, and Pb by 7. In the other Blackwell soil–amendment combinations, SS and RP, Ca(NO3)2–extractable Cd and Pb decreased by approximately half, while Ca(NO3)2–extractable Zn remained the same in SS, and decreased slightly in RP. As with toxicity, the RP amendment did reduce Ca(NO3)2–extractable metal slightly, but not as effectively as LSB (Table 1, Fig. 1). Treatment of contaminated soil with SS showed slight increases in Ca(NO3)2–extractable Zn but decreases in Ca(NO3)2–extractable Cd and Pb (Table 1). Increased Ca(NO3)2–extractable Zn may be responsible for increased toxicity of the SS treatment compared with the unamended Blackwell soil (Fig. 1). Although nonalkaline organic amendments (i.e., manures) can reduce Zn extracted by neutral salt solutions (Pierzynski and Schwab, 1993), the inability of our SS treatment to reduce Ca(NO3)2–extractable Zn can be attributed to the high Zn content (26 mmol/kg) in the biosolids. The LSB amendment contained Zn levels commonly found in municipal biosolids (USEPA, 1988). Apparently, organic amendments with low Zn are necessary to reduce potentially toxic Zn.
The large differences in the Ca(NO3)2–extractable metal concentrations of the lethal and nonlethal soils, as well as the similarity in lethal Ca(NO3)2–extractable Zn levels of the smelter soil and Zn-spiked artificial soil suggests the possibility of developing universal, soil-independent ILLs for Zn based on Ca(NO3)2–extractable metal levels. Further testing with a wide range of Ca(NO3)2–extractable metal levels in metal-contaminated field soils is necessary to validate this concept.
Plant Root Simulators as Surrogate Measures of Metal Bioavailability
Zinc uptake by PRSs exposed in the Blackwell and Taloka soil–amendment combinations is shown in Fig. 3. Cadmium uptake by PRSs in the Blackwell and Taloka soils was very low (2 and 5% of PRS Zn uptake, respectively), while Pb uptake was even lower (0.8 and 0.6% of PRS Zn uptake, respectively). The mean CV (95% CI) for PRS Zn uptake was 39% (22–56) compared with 1.9% (0.5–3.2) for Ca(NO3)2–extractable Zn, suggesting that Zn availability determined by PRSs was not as precise as Ca(NO3)2–extraction determinations. In addition to reduced precision, PRS measurements were not able to discriminate soil–amendment combinations lethal to earthworms, and thus were poorly related to toxicity, as found by Conder and Lanno (2000) for earthworms exposed to Cd, Pb, and Zn-spiked artificial soils. However, PRSs did indicate high levels of available Zn in all of the Blackwell soil–remediation amendments, despite the very distinct differences in toxicity of these combinations to earthworms. The PRS-chelate, DTPA, may not be suitable for estimating high levels of available metals in soils (Conder and Lanno, 2000). The use of the conventional DTPA soil extraction to measure high levels of available heavy metals may not even be appropriate since it was designed for soil fertility measurements in soils deficient in Zn, iron (Fe), manganese (Mn), or copper (Cu) (O'Connor, 1988). Differences in soil pH may alter the chelating tendencies for individual metals (O'Connor, 1988). When used for metals other than Zn, Fe, Mn, and Cu, the DTPA extraction tends to overestimate plant-available metals (O'Connor, 1988). Its application is also discouraged for estimating plant-available heavy metals in sludge or sludge-amended soils (Hooda and Alloway, 1993; O'Connor, 1988). Regardless, PRSs were able to indicate large differences (two orders of magnitude) in metal bioavailability between the Taloka reference and Blackwell smelter soils.
Answers to all your Biology Questions
