Relating soil geochemical properties to arsenic bioaccessibility through hierarchical modeling.

Interest in improved understanding of relationships among soil properties and arsenic (As) bioaccessibility has motivated the use of regression models for As bioaccessibility prediction. However, limits in the numbers and types of soils included in previous studies restrict the usefulness of these models beyond the range of soil conditions evaluated, as evidenced by reduced predictive performance when applied to new data. In response, hierarchical models that consider variability in relationships among soil properties and As bioaccessibility across geographic locations and contaminant sources were developed to predict As bioaccessibility in 139 soils on both a mass fraction (mg/kg) and % basis. The hierarchical approach improved the estimation of As bioaccessibility in studied soils. In addition, the number of soil elements identified as statistically significant explanatory variables increased when compared to previous investigations. Specifically, total soil Fe, P, Ca, Co, and V were significant explanatory variables in both models, while total As, Cd, Cu, Ni, and Zn were also significant in the mass fraction model and Mg was significant in the % model. This developed hierarchical approach provides a novel tool to (1) explore relationships between soil properties and As bioaccessibility across a broad range of soil types and As contaminant sources encountered in the environment and (2) identify areas of future mechanistic research to better understand the complexity of interactions between soil properties and As bioaccessibility.

Relationship Between Total and Bioaccessible Lead on Children's Blood Lead Levels in Urban Residential Philadelphia Soils.

Relationships between total soil or bioaccessible lead (Pb), measured using an in vitro bioaccessibility assay, and children's blood lead levels (BLL) were investigated in an urban neighborhood in Philadelphia, PA, with a history of soil Pb contamination. Soil samples from 38 homes were analyzed to determine whether accounting for the bioaccessible Pb fraction improves statistical relationships with children's BLLs. Total soil Pb concentration ranged from 58 to 2821 mg/kg; the bioaccessible Pb concentration ranged from 47 to 2567 mg/kg. Children's BLLs ranged from 0.3 to 9.8 µg/dL. Hierarchical models were used to compare relationships between total or bioaccessible Pb in soil and children's BLLs. Total soil Pb concentration as the predictor accounted for 23% of the variability in child BLL; bioaccessible soil Pb concentration as the predictor accounted for 26% of BLL variability. A bootstrapping analysis confirmed a significant increase in R2 for the model using bioaccessible soil Pb concentration as the predictor with 99.0% of bootstraps showing a positive increase. Estimated increases of 1.3 µg/dL and 1.5 µg/dL in BLL per 1000 mg/kg Pb in soil were observed for this study area using total and bioaccessible Pb concentrations, respectively. Children's age did not contribute significantly to the prediction of BLLs.

Influences of Coal Ash Leachates and Emergent Macrophytes on Water Quality in Wetland Microcosms.

The storage of coal combustion residue (CCR) in surface water impoundments may have an impact on nearby water quality and aquatic ecosystems. CCR contains leachable trace elements that can enter nearby waters through spills and monitored discharge. It is important, therefore, to understand their environmental fate in affected systems. This experiment examined trace element leachability into freshwater from fly ash (FA), the most common form of CCR. The effects on water quality of FA derived from both high and low sulfur coal sources as well as the influences of two different emergent macrophytes, Juncus effusus and Eleocharis quadrangulata, were evaluated in wetland microcosms. FA leachate dosings increased water electric conductivity (EC), altered pH, and, most notably, elevated the concentrations of boron (B), molybdenum (Mo), and manganese (Mn). The presence of either macrophyte species helped reduce elevated EC, and B, Mo, and Mn concentrations over time, relative to microcosms containing no plants. B and Mo appeared to bioaccumulate in the plant tissue from the water when elevated by FA dosing, while Mn was not higher in plants dosed with FA leachates. The results of this study indicate that emergent macrophytes could help ameliorate downstream water contamination from CCR storage facilities and could potentially be utilized in wetland filtration systems to treat CCR wastewater before discharge. Additionally, measuring elevated B and Mo in aquatic plants may have potential as a monitoring tool for downstream CCR contamination.