Spatial variation and behaviour of heavy metals (Pb and Zn) in the urban soils of north-central Newcastle, Tyneside

Dilks, Caroline Frances (2000) Spatial variation and behaviour of heavy metals (Pb and Zn) in the urban soils of north-central Newcastle, Tyneside. Doctoral thesis, University of Northumbria at Newcastle.

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Abstract

This thesis focuses on the spatial variation, partitioning and behaviour of the heavy metals, Pb and Zn, within the soil of open public access areas in north-central Newcastle. The study examines metal concentration variation and the complexity of spatial variability, using geostatistical techniques to identify the level of spatial dependence and continuity between samples, collected at a variety of scales. It investigates the mobility and availability of Pb and Zn using sequential extraction to determine partitioning, examining top : sub soil enrichment ratios, in addition to using various statistical techniques to explore metal behaviour in relation to selected key soil properties. The study goes on to examine the severity of any contamination by comparison with published guidelines and trigger concentrations, and explores some of the wider legislative implications. Top soil total Pb and Zn concentrations were determined at four sites and three plots, in addition to metal fractionation, pH, organic content and CEC within the top and sub soil of plot 1. Standard laboratory procedures were used where possible, with quality control checks employed to determine the reproducibility and efficiency of any non-standard methodologies.
The spatial distribution of contamination was characterised at all sampling scales by hot spot patterns of elevated concentrations, indicating that coarse scale sampling may be
inadequate to reveal the degree of soil Pb and Zn variation. Spatial dependence / continuity of Pb and Zn concentrations was low, a dense sampling network being required (0.5 X 0.5m) before clearly structured variograms could be produced. The two metals exhibited different partitioning characteristics; the reducible and oxidisable
fractions being dominant for Pb, and the residual fraction for Zn. Pb was classed as being of higher potential hazard than Zn, displaying substantially higher concentrations. The soil properties did not consistently influence metal fractionation, although an increase in acidity increased both Pb and Zn mobility, whilst organic matter acted as an efficient scavenger for Pb, in addition to providing more exchange sites for the nonspecific adsorption of Zn cations. CEC, associated with soil organic matter and clay content, was positively correlated with the oxidisable Pb fraction. The relative severity of Pb and Zn contamination varied according to the trigger thresholds used, although under the UK ICRCL system only three samples collected from the four sites, and approximately 25% of samples from the three plots, were classed as contaminated with Pb. No equivalent threshold exists for Zn within open public access areas. The implications of these findings for the assessment and management of contaminated sites are discussed.

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