769 Environmental Toxicology and Chemistry, Vol. 25, No. 3, pp. 769–775, 2006  2006 SETAC Printed in the USA 0730-7268/06 $12.00 + .00 EFFECT OF SOIL PROPERTIES ON LEAD BIOAVAILABILITY AND TOXICITY TO EARTHWORMS KAREN D. BRADHAM,*² E LIZABETH A. DAYTON,‡ NICHOLAS T. BASTA,‡ JACKIE SCHRODER,§ MARK PAYTON, and ROMAN P. LANNO# ²U.S. Environmental Protection Agency, 109 T.W. Alexander Drive, Research Triangle Park, North Carolina 27711 ‡School of Natural Resources, The Ohio State University, Columbus, Ohio 43210, USA §Department of Plant and Soil Sciences, Oklahoma State University, Stillwater, Oklahoma 74078, USA Department of Statistics, Oklahoma State University, Stillwater, Oklahoma 74078, USA #Department of Entomology, The Ohio State University, Columbus, Ohio 43210, USA ( Received 1 November 2004; Accepted 28 February 2005) Abstract—Soil properties are important factors modifying metal bioavailability to ecological receptors. Twenty-one soils with a wide range of soil properties (USA; http://soils.usda.gov/technical/classification/taxonomy/) were amended with a single concen- tration of Pb (2,000 mg/kg) to determine the effects of soil properties on Pb bioavailability and toxicity to earthworms. Earthworm mortality ranged from 0 to 100% acute mortality following exposure to the same total concentration of Pb (2,000 mg/kg) in amended field soils. Internal Pb concentrations in earthworms ranged from 28.7 to 782 mg/kg, with a mean of 271 mg/kg. Path analysis was used to partition correlations in an attempt to discern the relative contribution of each soil property. Results of path analysis indicated that pH was the most important soil property affecting earthworm mortality ( p  0.01) and internal Pb ( p  0.05). Soil pH was related inversely to mortality and internal Pb, soil solution Pb, and Pb bioavailability. The most important soil property modifying reproduction was amorphous iron and aluminum oxides (FEAL). Because FEAL is rich in pH-dependent cation-exchange sites, several soil properties, including pH, FEAL, and cation-exchange capacity, have a causal effect on Pb adsorption and soluble Pb. Path analysis is useful for assessing contaminated soils with a wide range of soil properties and can assist in ecological risk assessment and remediation decisions for contaminated sites. Soil properties are important factors modifying metal bioavailability and toxicity and should be considered during the ecological risk assessment of metals in contaminated soils. Keywords—Lead Ecological risk assessment Bioavailability Earthworm Soil properties INTRODUCTION Current routine practices for investigating the nature and extent of contamination at metal-contaminated sites involve determining total metal content in soils as an estimate of ex- posure. The exposure point concentration (i.e., dose) in a risk calculation may be an assumed percentage of the total metal content, reflecting the metal bioavailability, and this often ap- proaches 100%. Whereas this assumption is conservative in terms of being protective of human health and the environment, it may not be a reasonable estimate of site conditions, because the actual bioavailability of metals may be much lower. The resulting risk may overestimate the true risk of exposure to site media and result in lengthy and costly site remediation that could be unwarranted. Lead toxicity is seldom related directly to the total soil Pb concentration because of the modifying effects of soil phys- icochemical properties (e.g., pH, organic carbon [OC], amor- phous iron and aluminum oxides [FEAL], and cation exchange capacity [CEC]) on Pb bioavailability. Risk to soil inverte- brates based on total Pb concentration may not predict adverse effects accurately [1], because exposure expressed as total Pb does not consider the effect of these soil-modifying factors on Pb bioavailability, which often is much less than 100% [2–4]. It is difficult to determine the effects of individual soil * To whom correspondence may be addressed (bradham.karen@epa.gov). Presented at the Symposium on Risk Assessment of Metals in Soils, 14th Annual Meeting, SETAC Europe Meeting, Prague, Czech Republic, April 18–22, 2004. properties on metal bioavailability, because many soil prop- erties are intercorrelated to some degree. Various regression models have been suggested to segregate the effects of various physicochemical soil properties [1,4–6]. The reactions be- tween clay minerals and metals are attributed primarily to cation-exchange or ligand-exchange (i.e., specific adsorption) reactions that occur on FEAL clay. For this reason, soil clay content typically is highly correlated with FEAL content and CEC. Hydrous oxides of iron are contained in the clay fraction of soils and have a high sorption capacity for Pb [7]. Heavy metals are bound strongly to soils that are rich in organic matter or clay [8]. However, because many soil parameters have more than one binding mechanism and are strongly intercorrelated with other soil parameters, it is difficult to distinguish causal effects from intercorrelation. The bioavailability and toxicity of metals to soil inverte- brates are affected by many factors, including organic matter content, clay, and pH [4,9–13]. Janssen et al. [5] studied the impact of soil properties on metal uptake by earthworms in 20 contaminated (total Pb concentrations ranged from 0.34 to 4.09 mmol/kg) field soils. Using regression techniques, they found that soil pH was the most important soil property af- fecting Pb partitioning between the soil solid phase and the pore water and metal accumulation in earthworms. Further- more, their regression equations for Pb bioaccumulation fac- tors and soil characteristics showed that clay and amorphous iron oxides were the most important soil properties influencing bioaccumulation. Peijnenburg et al. [1] studied the uptake ki- netics of Pb in earthworms in the same 20 soils used during