Science Blog: Chemistry of urban soils reflects both geology and anthropogenic sources

Timo Tarvainen, Senior Scientist

Urbanisation is one of the modern world’s megatrends. Currently, urban areas cover just 2.8% of the global land area, but they are home to more than half of the world’s population. In Europe, as much as 70% of the population is urban. At the same time, increasing environmental pollution is another global megatrend. Releases of pollutants to aquatic ecosystems, air and soils can lead to negative implications for biodiversity and ecosystems, and they can also lead to health risks.

Potentially harmful elements such as lead, cadmium, arsenic and mercury are common contaminants in the urban environment. They can be released, for instance, from traffic, industry, heating or incinerators. Urban soil is a depository of contamination over a long period of time. However, the same elements in soil can also be derived from geological sources. Naturally occurring concentrations of arsenic and other potentially harmful elements can be relatively high in the soil of so-called geochemical provinces.

In Finland, a Government Decree on the Assessment of Soil Contamination and Remediation Needs has generated a need for reliable and readily accessible data on geochemical baseline concentrations in Finnish soils. According to the Decree, baseline concentrations, referring to both the natural geological background concentrations and the diffuse anthropogenic input of substances, shall be taken into account in the soil contamination assessment process.

The Geological Survey of Finland (GTK) has carried out soil geochemical baseline mapping within cities and in natural soils around urban areas. These data, together with earlier regional geochemical datasets, have been stored in the national geochemical baseline database, TAPIR. The city of Hämeenlinna was one the first geochemically studied urban areas. GTK gathered almost 400 topsoil samples from Hämeenlinna in 2010. At the same time, the Geological Survey of Sweden (SGU) carried out a similar study in Karlstad.

GTK and SGU have recently compared the urban soil geochemistry of these two Nordic towns. Major rock-forming elements such as aluminium, magnesium and potassium have a geogenic origin in both towns. Cobalt, chromium and nickel are examples of trace metals that are probably derived from mafic or ultramafic rocks. The northern parts of Hämeenlinna belong to a geochemical metal province, and naturally elevated cobalt concentrations exceed the threshold values given in the Decree.

Cluster and factor analyses revealed a group of elements that probably have a mainly anthropogenic origin in both towns: silver, gold, bismuth, cadmium, copper, lead, antimony and tin. The source of arsenic differs between these towns: Hämeenlinna is located in the so-called Southern Pirkanmaa arsenic province, and arsenic concentrations are relatively high in both natural and urban soil. In Karlstad, arsenic concentrations are probably anthropogenic.

In Hämeenlinna, the naturally high baseline concentrations of arsenic and cobalt should be taken into account in the assessment of soil contamination and remediation needs.

Figure 1. Urban soil sampling site in Hämeenlinna. Samples were collected from a 10-cm-deep soil pit.

European Environment Agency. 2015. The European Environment State and Outlook 2015. Assessment of Global Megatrends. https://www.eea.europa.eu/soer-2015/global/action-download-pdf

Tarvainen, T. 2011. Hämeenlinnan taajamageokemia. Hämeenlinnan ympäristöjulkaisuja 17. 30 s.

Tarvainen, T., Ladenberger, A., Snöälv, J., Jarva, J., Andersson, M. & Eklund, M. 2017. Urban soil geochemistry of two Nordic towns: Hämeenlinna and Karlstad. Journal of Geochemical Exploration, Special Issue. https://doi.org/10.1016/j.gexplo.2017.07.018

Timo Tarvainen

Text: Timo Tarvainen

Dr Timo Tarvainen is a Senior Scientist in Environmental Geology, specializing in geochemistry and the urban environment. He has been at the Geological Survey of Finland (GTK) since 1986, carrying out local, national and international geochemical mapping projects, as well as statistical analysis of geochemical data. He is currently focusing on the development of the national soil geochemical baseline database.