Assessment of ambient background concentrations of elements in soil using combined survey and open-source data

Abstract

Understanding ambient background concentrations in soil, at a local scale, is an essential part of environmental risk assessment. Where high resolution geochemical soil surveys have not been undertaken, soil data from alternative sources, such as environmental site assessment reports, can be used to support an understanding of ambient background conditions. Concentrations of metals/metalloids (As, Mn, Ni, Pb and Zn) were extracted from open-source environmental site assessment reports, for soils derived from the Newer Volcanics basalt, of Melbourne, Victoria, Australia. A manual screening method was applied to remove samples that were indicated to be contaminated by point sources and hence not representative of ambient background conditions. The manual screening approach was validated by comparison to data from a targeted background soil survey. Statistical methods for exclusion of contaminated samples from background soil datasets were compared to the manual screening method. The statistical methods tested included the Median plus Two Median Absolute Deviations, the upper whisker of a normal and log transformed Tukey boxplot, the point of inflection on a cumulative frequency plot and the 95th percentile. We have demonstrated that where anomalous sample results cannot be screened using site information, the Median plus Two Median Absolute Deviations is a conservative method for derivation of ambient background upper concentration limits (i.e. expected maximums). The upper whisker of a boxplot and the point of inflection on a cumulative frequency plot, were also considered adequate methods for deriving ambient background upper concentration limits, where the percentage of contaminated samples is <25%. Median ambient background concentrations of metals/metalloids in the Newer Volcanic soils of Melbourne were comparable to ambient background concentrations in Europe and the United States, except for Ni, which was naturally enriched in the basalt-derived soils of Melbourne.

Ambient geochemical baselines for trace elements in Chernozems—approximation of geochemical soil transformation in an agricultural area

“…The ability to evaluate the “natural” background for soils is disputed, as soils have been impacted upon by human activity elsewhere since at least the industrial revolution. The term “ambient background” is sometimes suggested to describe the local modified “background” in areas impacted upon by human activity, where the metal concentrations in soils and sediments are slightly elevated, but do not generate any identifiable health or environmental risks (Reimann and Garrett 2005; Mikkonen et al 2017, 2018). Finally, the term “geochemical baseline” has been suggested to approximate the present ambient background values (Wang et al 2011).…”

“…To allow comparisons, the baseline has to be given as a single number (“line”) instead of a range of values. Various “baselines” are commonly reported: (1) more restrictive statistical measures such as mean or median metal concentration, and (2) relative (expected) maximum levels, such as mean + 2 standard deviations or median + 2 median absolute deviations (Gałuszka 2007; Matschullat et al 2000; Mikkonen et al 2017).…”

“…The “statistical” approaches approximate the ambient geochemical baseline based on soil samples collected in each area, where the baseline is required, but where the natural factors may be distinguished from the anthropogenic ones, using, e.g., regression analysis, fractal methods, probability plots, or outlier elimination (Dung et al 2013; Filzmoser et al 2005; Matschullat et al 2000; Reimann et al 2014; Zhou and Xia 2010). Among the latter methods, the iterative 2σ technique, more radical but less robust compared to the other tests, has become relatively popular (Gałuszka 2007; Mikkonen et al 2017). The technique aims at defining the background by approaching a normal range, where the values beyond the mean ± 2σ interval are subsequently omitted (Matschullat et al 2000).…”

Geochemical Baseline Values Determination and Evaluation of Heavy Metal Contamination in Soils of Lanping Mining Valley (Yunnan Province, China)

“…In the case of soils of the upstream area, concentrations of heavy metals from all the horizons were used to calculate the baseline, which was conducted by adding the geometric mean to twice the standard deviation [3,38,39] (Table 3). The proposed baseline values of Pb, Cu, Zn and Cd were 169.93, 31.81, 569.06 and 4.13 mg·kg −1 , respectively.…”

“…The application of the regional geochemical baseline values proposed in this study will allow the rapid identification of sites that could be affected by pollution processes due to current mining exploitation in the area [3]. In addition, this information will also be very useful for a restoration plan to try to reproduce the environmental conditions that existed prior to mining [39,42].…”