**5. Conclusions**

This study investigated the bioavailability of metals aged in situ in roadside soil. None of the soil assays tested (total metals, DGT, soil solution and CaCl2 extractable metals) were good approaches for measuring the bioavailability of metals aged in roadside soils. Optimised linear models showed good potential for use in predicting bioavailable Mn, Ni and Zn in field contaminated aged soils. Soil concentration (CSOL) relationships in this current study were weak, while CaCl2 showed only one significant correlation with Cu (r = −0.582, *p* = 0.077). This study provided insights into the understanding of the kinetics of long-term metal aging in field contaminated soil, using roadside soils as the model. The dissociation time, Tc, was used for comparing soil metal aging effects. The results showed that Tc was pH dependent for most metals tested (Cd, Ni and Zn). Soil pH was likely to be the key parameter controlling aging of metals in roadside soil, thus metal bioavailability.

**Author Contributions:** Conceptualization, S.D.S., T.H., S.M.R., and A.S.B.; methodology, S.D.S., T.H., S.M.R., and A.S.B.; validation, S.D.S., T.H., S.M.R.; A.S.B., and D.V.I.; formal analysis, S.D.S, T.H.S. and D.V.I.; investigation, S.D.S.; resources, S.M.R. and T.H.; data curation, S.D.S. and S.M.R.; writing, original draft preparation, S.D.S.; writing, review and editing, S.D.S., S.M.R., T.H., A.S.B., D.V.I.; visualization, S.D.S.; supervision, S.M.R., T.H., and A.B.; project administration, S.M.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** S.D.S. would like to thank the School of Civil, Chemical and Environmental Engineering, RMIT University for providing the PhD scholarship and Higher degree by research (HDR) publication fellowship.

**Acknowledgments:** We would like to thank RMIT University, School of Civil, Chemical and Environmental engineering for SDS's PhD scholarship and Higher Degree by Research Publication Grant (HDRPG), during this work. The authors gratefully acknowledge the assistance provided by Paul Morrison for ICP-MS operation and Sandro Logano, Cameron Crombie and Babu Iyer for technical assistance in the laboratory

**Conflicts of Interest:** The authors declare no conflict of interest.
