Naturally occurring levels of heavy metals in soils coupled with anthropogenic influence has contributed to the accumulation of heavy metals in some regions, which has led in their concentrations exceeding natural levels, resulting in soil metal toxicity. As such, there are thousands of sites United States (and globe) that contain unacceptable levels of toxic metal(loid)s – arsenic (As), cadmium (Cd) and lead (Pb) – many of which are awaiting remediation and closure. Previous research has shown that the ubiquitous metal-sequestering properties of soil can significantly lower the bioavailability and risk of heavy metals to human and ecological receptors.
The objectives of the study were to (i) provide validation that the soil properties and soil extraction methods can serve as a screening tool for estimating toxic metal phytoavailability in contaminated soils, and (ii) provide validation that the relationships between soil properties and in vitro bioaccessibility methods can serve as a screening tool for estimating toxic metal bioavailability from incidental soil ingestion of contaminated soils.
Plant bioassays were used performed on both lettuce (Lactuca sativa), and ryegrass (Lolium perrene) to determine As, Cd and Pb contamination in 11 study soils. For ecological risk estimates, multiple linear regression and ridge regression models developed from a Environmental Security Technology Certification Program (ESTCP) study using bioaccumulation data from previous studies were used to predict metal phytoavailability of the contaminated soils. The measured metal phytoavailability values and the predicted values were then quantified using root mean square error (RMSE). Additionally, soil extraction methods were used to estimate metal phytoavailability, and then used to predict plant phytoaccumulation. This was accomplished by developing predictive equations using bioaccumulation data from a previous NCEA study were used. RMSE was used to compare the measured contamination phytoaccumulation data from the bioassays with the predicted values obtained from the extraction data. The ability of soil properties to predict in vitro gastrointestinal methods was also examined. Predictive equations used to predict %IVBA Pb and As from soil properties were assessed. Their accuracy in predicting %IVBA Pb and As was quantified again using RMSE.
The results showed that RMSE values obtained from the RR and MLR models for As, Cd and Pb plant tissue data all fell below the acceptable <25% limit. This indicated that the predicted and measured values were similar. Therefore, soil properties can be used as screening tools for estimating toxic metal phytoaccumulation in contaminated soils. Based on the extraction data, calcium nitrate extractions can be used as a predictive extraction method for Pb uptake in lettuce; The RMSE value 23.6%. Porewater and calcium nitrate extractions may be used to predict Cd uptake for ryegrass. The RMSE values were 12.4 and 10.1%, respectively. As uptake in lettuce and ryegrass may be predicted using both Mehlich 3 and porewater extractions all the RMSE values were <10%. Hence, soil extraction methods may serve as screening tools for metal phytoaccumulation in plants. However, plant species and extraction method can impact the accuracy of predicted data. Soil properties may be used to predict IVBA Pb and IVBA As in soils, depending on the source of Pb and As contamination. When the predicted and measured %IVBA Pb values were compared, the RMSE obtained was 9.22%. For As, the calculated RMSE was 27% when all soils were included, however, this decreased to 18.03% when the Deseret soil was excluded.
Risk assessors can use the soil properties of contaminated soils and predictive equations assessed for screening purposes. This will provide then with adequate data that can be used to determine if further investigations are necessary. An important outcome of this research is predictive equations can provide a basis for designing soil amendments based on soil properties. Based on prediction data, researchers can create amendments that will supplement soil properties and are designed to reduce Cd, As, and Pb phytoaccumulation and bioaccessibility.