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Assessment of Mercury in Needles of Forest Trees
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Assessment of Mercury in Needles of Forest Trees
In the Austrian forest law “Zweite Verordnung gegen Forstschädliche Luftverunreinigungen, BGBl. Nr. 199/1984”, there are threshold values specified for the concentrations of several pollutants in leaves and needles. However, there are no such threshold values specified for mercury. From the current results of the Austrian Bio-Indicator Grid a threshold value of 0.017 mg Hg/kg can be derived, above which mercury pollution must be assumed.

Mercury is not required by plants, but accumulates in the leaves and needles after direct uptake from the air – analogous to sulphur, fluorine and chlorine (Ericson et al 2003, Fleck et al 1999). The occurring concentrations of mercury don’t have a direct phytotoxic effect, but cause damage to the forest ecosystem. Up to now, the samples collected from the Austrian Bio-Indicator Grid in the years 1986, 1996 and 2006 were analysed (Fürst 2007, 2008).

The mercury contents detected in Austria were between 0.006 and 0.174 mg/kg in 1986, between 0.005 and 0.245 mg/kg in 1996 and lower in 2006, namely between 0.005 and 0.066 mg/kg in the needle material (Fürst 2008). From 1986 to 2006, the results showed a significant downward trend. These concentration ranges correspond very well to the contents detected in the needles of forest trees in other European countries. Around 90% of the results gathered in different European studies (n = 63) were in the concentration range between 0.005-0.100 mg Hg/kg (Grigal 2002).

How are these results to be assessed and what is the threshold for evidence of mercury pollution? In the VDI-working group for the definition of values for the assessment of standardised grass cultures, the following statistical approach was taken (Sachs & Hedderich 2006) to determine a robust upper limit of the data distribution and therewith fix a limit for the normal range: The interquartile range (IQR) of the distribution is calculated. The IQR is the distance between the 75th percentile and the 25th percentile of a data set. It is a robust measure of statistical dispersion.

First of all, potential outliers are identified. For this purpose, all values greater than three times the IQR added to the 75th percentile are eliminated as “outliers”. For the remaining values, percentiles and the IQR are determined again. The upper limit of the normal range (0,017 mg/kg) is now obtained by adding one and a half times the IQR to the 75th percentile. Above this range, an accumulation caused by mercury pollution is to be assumed.

Table 1: Rating System for Mercury in the Needles of Forest Trees

Rating
Mercury content in mg/kg
Low natural content
up to 0.012
Slightly elevated content
above 0.012 to 0.017
Incipient mercury pollution
above 0.017 to 0.022
Significant mercury pollution          
above 0.022


The following rating system for the needle set 1 results (table 1). The level “low natural content” encompasses the 75th percentile of the distribution (up to 0,012 mg/kg). Between the 75th percentile and the upper limit of the normal range (75th percentile + 1.5 x IQR) is the range of “slightly elevated content” (above 0,012 to 0,017 mg/kg). Above the upper limit of the normal range, mercury pollution must be assumed. Significant pollution can be assumed above 0,022 mg/kg (75th percentile + 3 x IQR). The results of the Austrian Bio-Indicator Grid from 2006 are presented in a map in figure 1.

Figure 1: Rated mercury contents (needle set 1) of the Bio-Indicator Grid from 2006



In figure 1, a significant zoning of the polluted areas is visible (central region of Upper Austria, Mur-Mürz-Valley, Graz Hinterland/southern Styria, Brückl region/Carinthia and the Inn Valley). Main source of mercury pollution in Austria is the sintering of mercury-containing iron ore (Fürst 2007). Other sources include cement- and brick works which process mercury-containing materials or use them as fuel, contaminated chloralkali process plant sites (amalgam electrodes) and soil gas emissions associated with former mining activities (e.g. silver mining in Schwaz/Tyrol, mercury mining in Stockenboi/Carinthia).

Generally, in the analysed needle samples from the Austrian Bio-Indicator Grid the current mercury contents are too low to be directly phytotoxic. However, they point out the environmental pollution with a toxic heavy metal.

For further validation of the rating system based on the current measured data, the analysis of the samples collected from the Bio-Indicator Grid in 2008 is to be continued.

Literature

Erickson, J.A., Gustin, M.S., Schorran, D.E., Johnson, D.W., Lindberg, S.E., Coleman, J.S. (2003): Accumulation of atmospheric mercury in forest foliar. Atmospheric Environment 37: 1613-1622.

Fleck, J.A., Grigal, D.F., Nater, E.A. (1999): Mercury uptake by trees: An observational experiment. Water, Air, and Soil Pollution 115: 513-523.

Fürst, A. (2007): Quecksilber in Fichtennadeln als Immissionsmarker. BFW-Wien ISSN 1815-5103 Forstschutz aktuell 41:18-20.

Fürst, A. (2008): Quecksilber in Fichtennadeln. Poster präsentiert bei der 17. Tagung der Austrian Society of Plant Biology (ATSPB).

Grigal, D. F. (2002): Mercury Sequestration in Forests and Peatlands. Journal of Environm. Quality 32:393-405.

Sachs, L., Hedderich, J. (2006): Angewandte Statistik, Springer Verlag ISBN: 978-3-540-32160-6.

Zweite Verordnung gegen Forstschädliche Luftverunreinigungen BGBl.:199/1984

Fürst, A.
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