A comparison of methods used to calculate normal background concentrations of potentially toxic elements for urban soil
Graphical abstract
Introduction
Soil geochemical background surveys are a fundamentally important source of data in the geological sciences and historically they have been undertaken for a range of uses. The geochemistry of soils provides a link between the atmosphere, lithosphere, hydrosphere, biosphere and anthroposphere and soils act as the repository of potentially toxic elements (PTEs) within the environment. Therefore, soil geochemical background surveys are vital to allow the impact of human activities and natural variation on soil chemistry to be understood, particularly in geochemically complex urban areas. Initially, geochemical background studies were undertaken with the aim of prospecting mineral resources whereas more recently they have become relevant for environmental studies, notably within the fields of medical geology and land quality assessment (Garrett et al., 2008b).
However, the definition of a geochemical background is not straight forward (ISO, 2011). Reimann et al. (2005) highlight the difficulties of defining ‘background’ concentration values within natural systems. They suggest that it is more appropriate to use the term ‘ambient’ rather than ‘natural’ background to account for the influence of diffuse anthropogenic emissions as well as natural, geological variation. As concentrations of chemical elements within soils are heterogeneous and vary as a function of time, in reality background concentrations comprise a range of values (Matschullat et al., 2000) and the calculated range of background variation is dependent on the scale and scope of the study (Salminen and Tarvainen, 1997, Salminen, 2000). As such, many terms are used to describe background contaminant concentrations. Within English legislation, the term ‘normal background concentration’ (NBC) is used and will be adopted in this study.
The Statutory Guidance concerning the contaminated land regime for England was updated in March 2012 (DEFRA, 2012). A key feature of this new guidance concerns the use of NBCs of contaminants. Land should not be considered contaminated unless contaminant levels exceed the upper NBC threshold for the area, which is required to be defined by Local Authority bodies, and remedial targets should be set according to NBCs.
The British Geological Survey (BGS) were commissioned to define and calculate NBCs in English soils (Johnson et al., 2012d) using seven geochemistry databases from a number of soil and sediment sampling campaigns (Salminen et al., 1998, Black et al., 2002, Oliver et al., 2002, Johnson et al., 2005, Barraclough, 2007, Reimann et al., 2008, Tóth et al., 2013). These datasets were collected and analysed using a systematic method with a wide spatial coverage for eight priority contaminants (Ander et al., 2013). However, gaps exist in the database and coverage is notably limited for some major English urban centres, leaving the unique geochemistry of these urban areas currently unmapped (Johnson and Ander, 2008).
This study compares three methods of calculating NBC levels for the Metropolitan Borough of Gateshead, England (Fig. 1), working in conjunction with Gateshead Council, using inorganic priority contaminants lead, cadmium and arsenic as test species (Martin et al., 2008). Sample data in the national database in the Gateshead urban area exist only at the ‘G-BASE rural’ scale and the National Soils Inventory (NSI) sampling scale: one sample per 2 km2 and one sample per 25 km2 respectively (Oliver et al., 2002, Johnson et al., 2005). The resolution of these data sets is too sparse to reflect the level of heterogeneity present in geochemically complex urban environments and is not considered suitable for calculating accurate NBC levels (Johnson and Ander, 2008). Given the lack of high resolution systematically collected data for Gateshead it was proposed that site investigation (SI) soil chemistry records collected during site development and held by the Local Authority as part of the planning process could be used, similarly to the TAPIR online database currently used in Finland (Jarva et al., 2010), as these records include data for all priority contaminants across a wide geographic area. However, a number of limitations may exist concerning the use of SI data, which although were not collected systematically, were collected according to set protocols defined by British Standard Legislation (British Standard Institution, 1999) and analysed using accredited laboratories to strict industrial standards (British Standard Institution, 1995, British Standard Institution, 1999, British Standard Institution, 2001). Briefly, the protocols require representative samples of the soil horizon (i.e. topsoil horizon in this instance) to be taken and the < 150 μm fraction is subjected to elemental analysis by ICP-AES following an aqua-regia digestion. The data may contain bias as more brownfield than greenfield sites may be represented within the data, more data points are likely to be available for more densely populated urban areas relative to more rural, undeveloped areas and temporal variation may exist. As such, this study secondly aims to determine whether SI records are a useful and robust source of data within the context of geochemical mapping, which would provide a cost effective method of calculating Local Authority Area specific NBC values and therefore more appropriately represent natural and anthropogenic geochemical heterogeneity within urban areas than national surveys.
Section snippets
Study area
The Borough of Gateshead is located within the North East of England (Fig. 1) and has an area of approximately 140 km2 and a population of 200,000 (Gateshead Council, 2014). It is a predominantly urban and historically industrial region although relatively rural areas are located within the south and west of the borough. The area is underlain by the Carboniferous Pennine Lower and Middle Coal Measures Strata comprising interbedded mudstones, siltstones, sandstones and coals (Mills and Holliday,
Use of site investigation data
Fig. 3 illustrates the distribution of sampling locations incorporated into the SI database from urban and rural wards.
In total 4029 sampling points from 209 sites were included in the database, averaging one site per 0.7 km2 although as shown in Fig. 4, more sites are located within urban areas, specifically along the northern, historically industrial, quayside area relative to rural areas in the south and west. All MSOA wards are represented although some 2 km2 grid squares (G-BASE regional
Conclusions and research implications
NBCs for lead, arsenic and cadmium were successfully calculated for the Borough of Gateshead using SI soil chemistry records. The ‘MAD’ method (Reimann et al., 2005) was chosen to calculate the NBC values for Gateshead, according to the precautionary principle as this removed the highest number of outliers from the dataset.
The SI data gave high resolution coverage of the area and Mann–Whitney tests confirmed statistical similarity for the lead and arsenic undisturbed comparison samples and the
Acknowledgements
We would like to thank Mike Poremba and the contaminated land team at Gateshead Council for their advice and allowing access to their archives and Phil Hartley at Newcastle City Council and Lindsay Bramwell for their comments and help with soil analysis. We would also like to thank Christine Jeans and Professor David Manning for their assistance in producing figures and proof reading respectively. We would like to thank two anonymous reviewers for their time and help in improving this paper.
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