An enriched stable isotope approach was developed to evaluate Zn bioavailability to Lumbricus terrestris. The decrease in 68Zn/66Zn in organ tissues was used to assess the relative magnitude of the bioavailable soil Zn pool. This tool was then used to specifically evaluate bioavailability as a function of soil cation distribution. Storm-water pond soils were modified using two treatment regimens whereby H2O-extractable Zn was varied either by different ZnCl2 amendments or by constant ZnCl2 amendment followed by varying the soil cation distribution through salt amendments (NaCl or CaCl2). Earthworms previously equilibrated in 68Zn-spiked soil were introduced to experimental soils, and after 2 d, removed for analysis of isotopic ratios in specific tissues. Despite a wide range of H2O-extractable Zn values produced by the salt treatments (0.007–24.3 mg/kg), a significant relationship between Zn turnover rate in earthworm tissues and H2O-extractable Zn in the salt-treated soils was not observed. Rather, considering both treatment regimens, turnover rate better correlated with Zn present in broader pools, such as that extracted by 6M HNO3. The bioavailability of trace metals to earthworms may be poorly characterized by loosely bound fractions such as the pore water. Additionally, the turnover rate of 68Zn in anterior organ tissues may be an effective tool to evaluate the relative magnitude of the bioavailable soil Zn pool. Environ. Toxicol. Chem. 2011; 30:607–615. © 2010 SETAC
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