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Chemical and Biological Measures of Heavy Metal Bioavailability and Toxicity in Soil

Conder, J.M. 1997. M.Sc. Thesis. Dept. Zoology, Oklahoma State University, Stillwater, OK

Abstract

This study was conducted to better understand the bioavailability and toxicity of cadmium (Cd), lead (Pb), and zinc (Zn) to soil organisms, both as individual contaminants and as mixtures. To adequately protect or restore soil ecosystems there is a great need to characterize soils presumed to be contaminated with heavy metals. Typical chemical analyses of soils, which determine total heavy metal concentrations of soils, are not well correlated with soil organism toxicity due to a host of modifying factors such as pH, organic matter content, and clay content. Due to these modifying factors, no soil will have 100% of its metal content bioavailable to organisms. Only bioavailable metals in soil are able to exert toxic action. Bioavailable metals cannot be directly measured using chemical analyses- only living organisms determine bioavailability. Laboratory toxicity tests, in which soil organisms are exposed to contaminated soils, are routinely used to evaluate toxicity and/or contaminant bioavailability, but are time consuming, expensive, and often difficult to interpret. The goal of this research was to investigate surrogate measures of metal bioavailability that would not only be inexpensive and precise, but also relate directly to toxicity. Three methods of measuring metal bioavailability in soils were investigated: i) earthworm metal residues; ii) weak electrolyte soil extractions; and iii) ion-exchange membrane uptake. Single- and multiple-metal toxicity tests using the earthworm Eisenia fetida and ion-exchange membrane exposures were conducted in artificial soil spiked with metal salts and remediated/unremediated Zn-smelter contaminated and reference field soils. Toxic units were calculated from the single-metal tests in artificial soil in order to evaluate mixture toxicity of the multiple-metal test. During all artificial soil toxicity tests, dead earthworms were analyzed to determine critical body residues (CBRs) for lethality for each metal. CBRs are concentrations of toxicants in an organism associated with a toxic endpoint, providing a link between the measure of bioavailability (the residue) and toxicity. CBRs were also used to further investigate mixture toxicity. All soils were extracted with a weak electrolyte (0.1M Ca(NO3)2) hypothesized to extract exchangeable or weakly bound "available" metals in soil. Plant Root Simulators™ (PRS™, Western Ag Innovations, Saskatoon, SK, Canada), ion-exchange membranes coated with disodium-diethylenetriaminepentaacetic acid (DTPA), were exposed to artificial soils at the same concentrations as the earthworm toxicity tests and remediated field soils. Ca(NO3)2-extractable metals and PRS™ uptake were compared to toxicity (mortality) and/or earthworm metal uptake to assess their use as surrogate measures of bioavailability. The acute mixture toxicity of Cd, Pb, and Zn was additive (LC50of approximately 1 toxic unit) regardless of the manner in which toxicity was quantitatively expressed. Earthworm metal concentrations were well related to lethality, and enabled the estimation of LC50s using critical body residues (CBRs, LCBR50s), which were 5.72 (3.54-7.91), 3.33 (2.97-3.69), and 8.19 (4.78-11.6) mmol/kg (±95% confidence interval) for Cd, Pb, and Zn, respectively. Zn concentrations of dead earthworms exposed to a lethal remediated Zn-smelter soil were 3-fold above the LCBR50s for Zn and comparable to earthworm concentrations in lethal Zn-spiked artificial soils, despite a 14-fold difference in total soil Zn concentration between the lethal field and artificial soils. Metal concentrations in different methodological fractions of the earthworm tissue showed promise as measures of bioavailability at sublethal exposure levels. Weak-electrolyte (0.1M Ca(NO3)2) extractable metals in soil were precisely related to toxicity in all artificial toxicity tests. Ca(NO3)2-extractable Zn was comparable between the lethal Zn-spiked artificial and lethal remediated Zn-smelter soils, despite the aforementioned differences in soil parameters. Zn uptake in both live and dead earthworms was also well related to Ca(NO3)2-extractable Zn regardless of soil type (R2 = 0.96), but poorly related to total soil Zn. Ion-exchange membrane metal uptake in soil was not well related to toxicity or earthworm metal uptake in metal-spiked artificial or remediated Zn-smelter soil.