66
R. Ferrara
4.3
Atmospheric Mercury Concentration
Several data of atmospheric mercury concentration are available in the literature,
performed after the cessation of mining and refining activity (Ferrara et al. 1982;
Breder and Flucht 1984; Ferrara et al. 1986).
The range of mercury levels in the air is very wide (10-1000 ng/m 3 ) and varies
with the characteristics of the sampling sites and the presence and type of
mercury sources. High mercury concentration values were found near the
exhaust pipe of the ventilation system of the mines ("'500 ng/m 3 ) and above the
deposits of roasted cinnabar (30-200 ng/m3). The highest mercury values were
measured around and inside the area of abandoned mine structures (100015000 ng/m3; Ferrara et al. 1998).
Of course, mercury levels are strictly dependent on the ambient temperature
and consequently show daily and seasonal behaviour. In Fig. 10 a typical daily
trend of atmospheric mercury concentration, measured over a roasted cinnabar
deposit, is reported.
The observations made near the geothermal power plants confirm the presence
of a distinct mercury emission from hot steam wells (from 900 to 10 ng/m3). The
atmospheric mercury levels decrease sharply with increasing distance from the
geothermal power plant (500-600 m) in an asymmetric distribution depending
on the prevailing wind direction (Ferrara et al. 1994).
The mean daily values of atmospheric mercury determined in the winter
season for the Amiata district proved to be less than 5 ng/m\ with values of the
order of 10 ng/m3 in the surroundings of the main sources. This distribution is
basically the same in the summer; the concentrations are significantly higher due
160
140
120
100
. .
i 80
~ 60
40
20
0
-- -- to
4
--+
8
-- ---- t
.--- ._----j -
12
16
Hour
----- 25
20
15
roc
10
5
o
24
Fig. 10. Daily variation of atmospheric mercury concentration ( ) and soil temperature (- - -) over a
roasted cinnabar bank (May 1987)