The impact of urbanisation on metals in soils: A case study from Leicester

By Nere Ruiz: September 2009

Supervisors: Dr Nick Voulvoulis and Dr Martin Head
Collaborator: British Geological Survey

Introduction

Increased concentrations of metals in soils are normally expected in urban areas as a consequence of human activities.

However, the scale and extent of pollution can't be assumed from urban geochemical data alone; it is necessary to compare it with the natural geochemical background. This is because trace elements occur naturally in soils’ parent materials, and so an increased concentration may not be due to human pollution.

Comparisons of urban geochemical data with local rural geochemical background can help to better assess soil contamination associated with the urban environment, and give insights into the different processes that may cause elevated concentrations of metals in urban soils.

This study uses soil data from the British Geological Survey’s national mapping programme, the Geochemical Baseline Survey of the Environment (G-BASE).

Objectives

  1. Describe the geochemistry of Leicester
  2. Compare urban and rural geochemical data for arsenic and lead, quantify their differences and find out their origins
  3. Investigate what is affecting heavy metal concentrations in soils, underlying geology, anthropogenic pollution or both?
  4. Identify the implications

Method

  • Study the spatial distribution of lead and arsenic concentrations in the area
     
  • Analyse the data to compare the level of contamination in the urban environment relative to the rural background
     
  • Select subsets of geochemical data based on different geological units to allow comparisons between urban and rural areas over the same parent material (avoiding the effect that the parent material may have on the element in the soil)
     
  • Explore the implications - where are the points which exceed the Soil Guideline Values (SGV)?

Results and conclusion

As the geochemical data from Leicester did not follow a normal or lognormal distribution in general, in urban or rural areas, non-parametric tests and exploratory data analysis techniques were selected for the statistical treatment of the data.

Without taking into account the underlying geology, median concentrations of arsenic and lead were higher in urban environments. Data for lead in urban soils showed the highest spread according to the Median Absolute Deviation (MAD) values.

Data outliers were identified as they could reflect sites affected by point pollution. More data outliers were found in urban areas than in rural ones, and more outliers in the case of lead than arsenic. The majority of outliers were high concentration anomalies, and an important number of them occurred next to the river Soar.

The rural-urban comparisons over the same underlying geology showed that lead concentration was always higher in the urban environment, while in the case of arsenic, no significant differences were found between urban and rural data in most of the parent materials.

Finally, regardless of the rural or urban environment, arsenic and lead concentrations in alluvial and river terrace deposits were very high compared to other geological units. In addition, an important number of outliers for both elements tended to occur in these sediments.

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