Distribution of metals in soil particle size fractions and its implication to risk assessment of playgrounds in Murcia City (Spain)

Metal contents in bulk soil samples are commonly used to assess contamination but metal accumulations in soils are known to increase with decreasing particle size. We investigated the distribution of metals in various particle size fractions, particle morphology and metal enrichment factors to characterize the properties of soils in four Urban parks in Murcia City (SE, Spain). We extended our results to explain possible sources of metals and their likely significance of metals in fine fractions to the risk assessment of urban soils. Soil samples were fractionated and concentrations of Al, Si, Ti, Na. Mg, Ca, P, K, Fe, Cu, Cr, Zn, Ph, Co, Mn and Ni were determined in nine particle size fractions including particulate matter with aerodynamic diameter <10 (PM10) and 2.5 (PM2.5) pm. The contents of metals in playgrounds were below some European cities and low enrichment factors (EF) indicate limited anthropogenic metal depositions, However, the consistently high EF (>3.0) for Ni in particulate matter (PM10 and PM2.5) in four parks together with EF>2.0 for Pb and Co in the 0-75 mu m in Malecon and Rio parks were attributed to vehicular depositions. In the study area, the predicted amount of Ni in relation to particle size is given by Ni (mg kg(-1))=-13 (In particle size in mu m)+92, r(2)=0.76. The presence of high amounts (60-80%) of calcite and dolomite in fine fractions might have contributed to metal accumulation through the formation of metal-carbonate complexes. Goethite, especially in PM10 and PM2.5 can also serve as sink for metals. Using EF for bulk soils indicates that Ni is not enriched (i.e., EF<1.0) in four parks but EF>3.0 for PM10 and PM2.5 indicate otherwise. Similar cases are true for the 0-75 mu m fractions for Zn in all parks. We therefore suggest that risk assessment of urban soils in Murcia City (and elsewhere) should be based on (fine) particle size because of the tendency of metal to accumulate in fine particles. In addition, the mineral composition of fine particles should also be considered in risk assessment. In the study area, the inhalation and ingestion of calcite can potentially irritate eyes and respiratory tract. Our results provide important information that might be useful in assessing risks associated with soils on playgrounds where children can unknowingly ingest and inhale dust while actively playing in urban parks. (c) 2008 Elsevier B.V. All rights reserved.