Quantifying Diffuse Contamination: Method and Application to Pb in

Apr 28, 2017 - Because the contamination source is local, only a small proportion of all samples (here 10%) will be contaminated with unusually high e...
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Quantifying diffuse contamination: Method and application to Pb in soil Karl Fabian, Clemens Reimann, and Patrice De Caritat Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 28 Apr 2017 Downloaded from http://pubs.acs.org on May 1, 2017

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Environmental Science & Technology

Quantifying diffuse contamination: Method and application to Pb in soil Karl Fabian ,∗,† Clemens Reimann,† and Patrice de Caritat‡ 1

†Geological Survey of Norway (NGU), P.O. Box 6315 Sluppen, N-7491 Trondheim, Norway ‡Research School of Earth Sciences, Australian National University, Canberra ACT 2601, Australia E-mail: [email protected] Phone: +47 7390 4203

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Abstract

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A new method for detecting and quantifying diffuse contamination at the conti-

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nental to regional scale is based on the analysis of cumulative distribution functions

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(CDFs). It uses cumulative probability (CP) plots for spatially representative data

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sets, preferably containing >1000 determinations. Simulations demonstrate how dif-

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ferent types of contamination influence elemental CDFs of different sample media. It

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is found that diffuse contamination is characterized by a distinctive shift of the low-

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concentration end of the distribution of the studied element in its CP plot. Diffuse con-

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tamination can be detected and quantified via either

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of the contaminating element to that of an element with a geochemically comparable

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behavior but no contamination source (e.g., Pb vs. Rb), or (2) comparing the top soil

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distribution of an element to the distribution of the same element in sub-soil samples

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from the same area, taking soil forming processes into consideration. Both procedures

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are demonstrated for geochemical soil data sets from Europe, Australia, and the USA.

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Several different data sets from Europe deliver comparable results at different scales.

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(1) comparing the distribution

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Diffuse Pb contamination in surface soil is estimated to be 1000 determinations. These are

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provided by geochemical mapping projects from various geological surveys.

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Data sets and method

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Data sets

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Five published data sets from different soil geochemical mapping projects at the continental

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(10,000,000 km2 ) to regional (25,000 km2 ) scale are used:

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1. The continental scale GEochemical Mapping of Agricultural Soil project (GEMAS) 18 ,

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2. The sub-continental scale Baltic Soil Survey project (BSS) 19 ,

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3. The continental scale North American Soil Geochemical Landscapes project (NASGL) 20 ,

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4. The continental scale National Geochemical Survey of Australia project (NGSA) 21,22 , 3

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5. The regional scale Nord Trøndelag project (NTR) 23 .

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Sampling and analytical procedures are described in the cited sources. All data sets contain

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between 750 and almost 5000 samples. Further requirements for the investigated elements

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are low detection limits and high analytical precision at the lower end of the data distribution

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.

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CDF signatures of contamination and sample media

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Three different types of contamination will be distinguished: (1) diffuse, (2) strong local to

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regional, and (3) from scattered local sources. All three processes lead to over-abundant con-

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centrations of the studied element which characteristically distort its pristine CDF (Fig. 1a-

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c).

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Diffuse contamination results from large scale atmospheric transport and mixing of ma-

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terial from many different sources of local or widespread anthropogenic activities at present

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and in the past, e.g., industrial and traffic emissions, or the use of fertilizers , including

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manures, sewage sludges, and other wastes. In contrast, local contamination is deposited in

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the vicinity of the source and causes a clear, most often exponential (e−d ) or power-law ( e.g.,

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1/d2 ) concentration gradient in the vicinity of a point source

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at the continental scale will not necessarily create such a concentration gradient and should

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not be expected to result in exceedingly high element concentrations.

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, diffuse contamination

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How diffuse contamination affects the pristine CDF of an element can be modeled in

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a simple way. To do this, Fig. 1a first shows the distribution of Ce, an element that is

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to date not significantly influenced by anthropogenic activities. Its CP plot is based on

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the GEMAS data 18 for European agricultural soil (Ap horizon) with a median of about 30

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mg/kg. The other curves in Fig. 1a show the effects of adding various amounts of constant

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‘diffuse contamination’ to all data points (2, 5 and 10 mg/kg). They demonstrate that even

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minor amounts of diffuse contamination ( 50 ppm) and at low (1% of determinations with Pb concentration

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below the detection limit, whilst the CDF for P btop shows few samples with concentration

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of