Long-term Stability of Trace Element Concentrations in a Spontaneously Vegetated Urban Brownfield With Anthropogenic Soils

Trace element (TE) contamination of soil is a persistent problem in urban environments, particularly hindering the reuse of abandoned land. While phytostabilization is a cost-effective approach to managing TE-contaminated soil, little is known about the effects of these practices on soil TE concentrations decades after plant establishment. This study analyzes soil data collected from 1995, 2005, and 2015 (28, 38, and 48 years since site abandonment) in a spontaneously vegetated urban brownfield contaminated with As, Cr, Cu, Pb, and Zn. No change in concentration of any of these TE in the upper 30 cm of soil at this site was observed from 1995 to 2005. However, from 2005 to 2015, As and Cr concentrations increased in the soil C1 horizon at this site (approximately 5- to 25-cm depth), whereas Cu, Pb, and Zn remained stable. We propose that the observed increases in As and Cr resulted from downward migration from the upper 5 cmof soil and subsequent immobilization in the C1 horizon. Increasing soil pH from 2005 to 2015 could have increased As and Cr solubility, while reducing Cu, Pb, and Zn solubility. In addition, significant correlations were found between the five TE and Fe or Mn, which are known to play a role in TE sorption. This study shows the ability of a phytostabilization site to retain some TE in its upper soil horizons for several decades following plant community establishment, although continued monitoring is needed to ensure soil conditions continue to favor stability.