M E R C U R Y P O L L U T I O N IN THE SOIL A N D M O S S E S A R O U N D

A G E O T H E R M A L P L A N T

F R A N C O B A L D I

Dipartimento di Biologia Ambientale, UniversiM degli Studi di Siena, Via Mattioli, 4; 1-53100 Siena, Italy

(Received June 10, 1987; revised November 17, 1987)

Abstract. Samples of soils and mosses were collected near a geothermal power plant, which is located in one of the most active geothermal fields of the world (Southern Tuscany). High concentrations of Hg (up to 1.8 Ixg g - 1 d.w.) occur in mosses as far as 0.6 km from the geothermal plant, and the Hg uptake of mosses is unrelated to the species except for Bryum torquescens. The Hg in the soil was lower than in mosses, and the greatest concentrations in soil were near the Travale-22 well, which feeds the most powerful geothermal plant (30 MW) in the area. In addition, the concentrations of Hg detected in soil cores and in old bottom leaves of several specimens ofHypnum cupressiforme show that this element has been emitted into the air also in the past probably since the discovery of T-22 well in 1972.

1. Introduction

Atmospheric Hg can be introduced into the air by degassing of the earth crust, volcanos, fumaroles, and hot springs (Eshlemann etal. 1971; Siegel and Siegel, 1975, 1978; Breder and Flucht, 1984), by Hg mining (Lindberg et al., 1979) and industries (Lodenius and Tulisalo, 1984).

The active volcanism in the Etna, Vulcano, and V esuvio areas, the almost 400 hydrothermal springs, wells and vents (Buat-Menard and Arnold, 1978; Duchi, 1978; Fanelli etal., 1982; Sabrouw, 1982; Breder and Flucht, 1984; Legittimo etal., 1986) from Tuscany to Sicily, the many geothermal power plants (Larderello, Travale, Monte Amiata, Torre Alfina e Vico) in western central Italy (Baldi et al., 1982), the Hg mines degassing (Monte Amiata and Idria) and the chlor-alkali plants (Breder and Flucht, 1984; Bargagli et al., 1985; Ferrara et aL, 1986), are responsible for the high concentra- tions of this element in the air of Italy.

Recently, here and in other countries, because of the development of alternative energy resources, intensifing the surveying of new geothermal fields has led to an increase in the production of geothermal power from 1100 MW in 1974 to the 3400 MW in 1983 (Guglielmetti, 1985). Today, 40 countries are interested in geothermal energy exploration and 15 countries are exploiting their geothermal fields to produce electricity. In 1982 Italy was the third largest producer of geothermal power, generating 440 MW yr- 1, after the United States with 936 MW and the Philippines with 580 MW (Barbier, 1985). World-wide, geothermal power provides 0.15 ~ of the total produced electricity (UN Economic Commision for Europe, 1982). In Italy this percentage is 0.9~o and, in a privileged region like Tuscany, the percentage reached 43~o in 1976 (Cataldi et aL, 1978). Recently the Italian government issued Law No. 896 (Gazzetta Ufficiale Italiana, 1986) to regulate the geothermal field exploration and exploitation.

Water, Air, and Soil Pollution 38 (1988) 111-119. © 1988 by Kluwer Academic Publishers.

112 FRANCO BALDI

The control of environmental risks from the emission of toxic elements is not mentioned, probably because there has been very little done in the way of environmental impact of geothermal survey.

Elsewhere it has been reported (Weissberg and Zobel, 1973; Axtmann, 1975; Siegel and Siegel, 1975; Robertson et al., 1977) that geothermal activity causes a long-term environmental hazard by emitting Hg and other toxic elements, and to date in Italy such studies are lacking. The aim of this research is to demonstrate that Hg is emitted only by an important geothermal plant (30 MW) in an area (Radicondoli), where there is not mixing in the air between the Hg from cinnabar deposits (degassing), and that one from the geothermal emission, as at Monte Amiata region (Ferrara et aL, 1986).

2. Study Area

The sampling area is located in the geothermal area of Radicondoli, 22 km south of Siena and 15 km east of Larderello (where the first geothermal hydroelectric plant in the world was installed in 1904). It is far enough (70 km) from the cinnabar deposits of Monte Amiata soil not to be affected by their degassing (Figure 1).

The Travale-22 well (T-22) blew out in January 1972 (Atkinson et aL, 1978) and in 1979-1980 a power plant with a 15 MW turboalternator was connected with this well by a vapor-pipeline. Then in 1976 other new wells were discovered and connected to the same geothermal plant raising the power to 30 MW (Wohlenberg, 1985) by adding another turboalternator. The soil of the study area was turned over during the last decade due to the borehole survey and the construction of the geothermal installation with its wells, vapor-pipelines, roads and tracks. The surrounding land has been laid bare; only small oak and elm lie to the North of the power plant. The sparse vegetation of this area consists mostly of mosses, which grow mainly on rocks or on the trunks of the surviving trees. Upon the discovery of the T-22 Well, the geothermal steam, probably with the synergism of the emission of toxic elements, started to injure the local vegetation. In contrast, the land is fully farmed about 1 km away from the power plant.

3. Materials and Methods

Thirty-one surficial soil samples (Figure 2) were collected in this barren area by scraping the soil surface with a plastic knife, previously washed in acid (1N HNO3). The samples were stored in plastic bags at - 20 ° C, until their analysis. Three cores of soil C-l, C-2, and C-3 (Figure 2) were taken by drilling for several centimeters into the soil with a plastic tube. The storage was the same as that for surficial soil samples.

Thirty-three mosses (Figure 3) were removed from the rocks and tree trunks with an acid-washed plastic knife and then stored in plastic bags. The day after, in the laboratory the mosses were washed with a 0.1H HC1 solution and then dried out at 30 °C for 5 days, to avoid mercury losses. The percentage of water residue, heating up the sample to 105 °C, was 1~ and 3~ , respectively, for the brown and green part of the moss. The greenest parts were removed with scissors. In five samples ofHypnum cupressiforme,

MERCURY POLLUTION IN THE SOIL AND MOSSES 113

Fig. 1.

¢--, I " - A - ' ° 4 "3-N

®E 0

SIENA

o Radicondoli

o Sovicil le

I Chiusdino O,, '2 .4 Km

Location of the sampling area with the stations outside the geothermal power plant detail.

we analyzed the top (green) and bottom light brown parts of the moss for Hg. A subsample of mosses was used to determine the species: Hypnum cupressiforme, Campylium hispidulum, Homalothecium sericeum, Bryum torquescens, Bryum gem- miparum, Camptothecium aureum, Leocodon sciuroides, and Ctenidium molluscum on the basis of a recent review (Corley et al., 1981).

The total Hg in the soil was determined on the wet sieved pelitic fraction (< 63 ~tm),

by digesting about 300 mg of dry soil with 3 mL of an acid mixture of HNO3/H2SO 4 (1 : 1, V : V) in teflon vessels, under-pressure at 120 ° C (Stoeppler and Banckaus, 1978). The digested sample was made up to 10 mL with DDW (Deionized distilled water). The ionic forms of Hg were reduced to atomic Hg with a 25~ of SnC12 in 1N H2SO 4 and then the metal was determined by flameless atomic absorption spectrophotometry (Perkin Elmer 300S). The coefficient of variation (C.V.) of the five analyses of the same samples was 7.2~o. The accuracy of the method was evaluated by means of an internal

114

Fig. 2.

FRANCO BALDI

1 . c - 3 /

# ,edC'"' _..$4 ~ k iu~ c-,

~ " ; , ~ 7 - e ~ ~., _ 2°e ........... ~ 21-

Detail of the sampling area (Figure 1) with stations for surficial soil samples and soil cores in the vicinity of the geothermal power plant.

Form

0 01 03 I i i i

, o 5 o ~ Km

Plant

O T~2 2 well

®u

Fig. 3. Detail of the sampling area (Figure 1) with stations for mosses near the geothermal power plant.

s tandard used for the intercalibration exercise conducted on Hg determination in

sediment S D - M E D P O L - I / T M of the Internat ional Atomic Energy Agency (IAEA,

Monaco ; our laboratory number was 9).

MERCURY POLLUTION IN THE SOIL AND MOSSES 115

About 150 mg of dry moss were digested with 2 mL of conc. HNO3, following the same method as for soil analysis. The C.V. of five analyses of the same sample was 6.3%.

4. Results and Discussion

Mercury has been emitted from the T-22 well, since its discovery in 1972 and by the geothermal plant since its installation in 1980. The analysis of the distribution of this element in the surficial and deep soil and in mosses, clearly shows that Hg is accumulated in the environs of the geothermal plant as well as around the T-22 borehole.

Fig. 4.

04-

'~ 0 3 - m 131 :=.,

111 • •1~

O ° o ° 0 • • O 0 O • •

t i t

1"-22 wel l 025 (lSo (~75

• •

/ ! , i ,

1.0 5 10 15 Km D i s t a n c e

Concentrations of total Hg in surticial samples of soil vs the distance from the Travale-22 well installation.

The highest concentrations of Hg (0.3 gg g-1) in the surficial samples of soil (Figure 4) occur close to the T-22 well, which continuously emits steam. The concen- tration of Hg tends to decrease as one moves away from the T-22 well to 0.020 ~tg g - 1 at a distance of slightly over 0.6 km. The latter level of the metal in the soil is four times lower than what is considered the background for the cinnabar-rich Monte Amiata and in general for southern Tuscany (Bargagli and Baldi, 1984), but it corresponds to levels in unpolluted areas (Andren and Nriagu, 1979). This low concentration of Hg is due to the geochemical features of the soil at Travale (Cataldi et al., 1978; Batini et aL, 1985), which is low in organic matter and rich in carbonate content.

The analysis of Hg in the three cores confirms the data on the surficial soil samples. The Hg profile of the C-1 closest to the T-22 well shows the highest concentrations, despite high and low peaks in the data with respect to those of cores C-2 and C-3, which were further from the T-22 well (Figure 5). The Hg content of cores C-2 and C-3 is low

U

¢-

Fig. 5.

116

10

F R A N C O B A L D I

Hg. j.lg. g -1 01 02 03 04 Tot 01 02

I I = , I I !

4

6

C-1

e'

C-g

01.

1 0 ,

-i 12- C"3

Profiles of total Hg concentrations in soil cores C-l, C-2, and C-3 sampled at different distances from the Travale-22 well.

and close to the background level, especially in core C-3, where the values are almost 10 times lower than in core C-1.

Mosses were chosen because they were the predominant vegetation in this barren area, and they are well-known as good bioindicators for monitoring air-borne trace elements in industrial and mining areas (Rasmussen, 1977; Beckett et aL, 1982; Nieboer et al., 1982). The analysis of Hg in the mosses reveals concentrations that are at least six times higher than in the soil (compare Figures 4 and 6). All the species of mosses examined take up Hg to the same extent, except for Bryum torquescens (Figure 6). Even though the specimens of this bryophyte are from the same part of the sampling area as the others, they display higher Hg concentrations, perhaps due to the more compacted leaves and hence to a higher specific surface area of absorption.

The highest values of Hg in mosses occur around the geothermal plant rather than around the T-22 well. Soil and moss distributions are different and that is probably due to the both different locations of the sampling sites and to their differences in metal uptake. The latter has tittle affinity for Hg, because of the high carbonate concentration and low organic C content, so the metal is leached out from the soil easily by the heavy rains (860 mm yr- ~). In contrast, the mosses have a high 'sequestring power' due to a large specific area of the leaves and due to the high affinity as riving organic matter, so the Hg is retained by the bryophytes for a longer period of time than the soil. In addition, the analysis of Hg (gg g- 1 d.w.) in the top (green leaves) and the bottom fight brown leaves) of some moss specimens shows that Hg was more concentrated in the bottom

MERCURY POLLUTION IN THE SOIL AND MOSSES 117

1"610

1.41

~to- ol :z

®

ol.2- o O o =e

0 0 0 f i i I

P o w e r 025 0.50 0.75 1.0 P lan t

D i s t a n c e

| I I I

* * . I o o

, I , ) 5 10 15 Km

Fig. 6. Concentrations of total Hg in different species of briophytes: (Q)Hypnum cupressiforme, (0) Campylium hispidulum, (',)~) Homalothecium sericeum, ([]) Bryum torqueseens, and (A) others vs the

distance from the geothermal power plant•

61 J

1.2

-1-

0.4

o L . \ .t

/ ' w " r ~ i I ~ I i I i I I I ~v i -~-

0.2 (14 0.6 0.8 1,0 1.2 Km P o w e r

P l a n t

Fig. 7. Differences of Hg concentrations between the bottom leaves (black histograms) and top leaves (white histograms) of some speciments of Itypnum eupressiforme collected at various distances from the

geothermal power plant.

leaves (Figure 7). This result shows, only qualitatively, that in the past (since 1972; year of the T-22 well discovery) the emission of Hg near the geothermal plant was higher than today. Unfortunally no data are reported in litterature about the growth rate of Hypnum

118 FRANCO BALDI

cupressiforme in this area to date more accurately the Hg pollution in the moss. The finding that Hg is more concentrated in soil near the T=22 weU, confirms that the geothermal well is today responsible for the highest immission of Hg in the air.

5. Conclusions

Mercury is emitted by the geothermal plant and T-22 well. The highest concentrations of rig in soil and mosses are distributed up to 0.6 km from the pollution sources. In this case mosses are more useful indicators, because they take up Hg more easily than soil.

The unremitting borehole survey, in old and new geothermal fields in Italy and in other countries (Barbier, 1985), leads to a continuous and increasing input of elemental Hg in the air. It will be worthwhile to recalculate the entire balance not only for this metal, but also for other toxic elements such as As, which come out from the new intensive geothermal survey. In addition, it should be important to answer the question 'How clean is geothermal power?'. Perhaps, it is: unfortunately 'tittle data are available today to give such an answer'.

Acknowledgment

The author thanks Prof. Erminio Ferrarini for the consulting work, the classification of mosses and for his suggestions and comments.

References

Andren, A. W. and Nriagu, J. O.: 1979, 'The Global Cycle of Mercury', in J.O. Nriagu (ed.), The Biogeochemistry of Mercury in the Environment, Elsevier/North Holland Biomedical, Amsterdam, p. 1.

Atkinson, P., Barelli, A., Brigham, W., Celati, G., Manetti, G., Miller, F. G., Neri, G., and Ramey, H. J.: 1978, Geothermics 7, 145.

Axtmann, R. C.: 1975, Science 187, 795. Baldi, P., Buonasorte, G., Ceccarelli, A., Ridolfi, A., D'Offlzi, S., D'Amore, F., Grassi, S., Squarci, P., Taffi,

L., Boni, C., Bono, P., Di Filippo, M., Martelfi, M. G., Lombardi, S., and Tora, B.: 1982, Contributo alia Conoscenza delle Potenzialit3 Geotermiche della Toscana e del Lazio, R-15, Ed. Consiglio Nazionale Ricerche, Progetto Finalizzato Energetica.

Batini, F., Castellucci, P., and Neri, G.: 1985, Geothermics 5/6, 623. Barbier, E.: 1985, Geothermics 14, 131. Bargagli, R. and Baldi, F.: 1984, Chemosphere 13, 1059. Bargagli, R., Iosco, F. P., and Leonzio, C.: 1985, Inquinamento 2, 33. Beckett, P. J., Boileau, L. J. R., Padovan, D., Richardson, D. H. S., and Nieboer, E.: 1982, Environ. Poll.

Set. B 4, 91. Breder, R. and Flucht, R.: 1984, Sci. Tot. Environ. 40, 231. Buat-M6nard, P. and Arnold, M.: 1978, Geophys. Res. Lett. 5, 245. Cataldi, R., Lazzarotto, A., Muffler, P., Squarci, P., and Stefani, G.: 1978, Geotehrmics 7, 91. Corley, M. F. V., Crundwell, A. C., Dull, R., Hill, M. O., and Smith, A. J. E.: 1981, J. Bryol. 11, 609. Duchi, V.: 1978, Atti Soc. Tosc. Sc. Nat. Mem. 85, 127. Eshlemann, A., Siegel, S. M., and Siegel, B. Z.: 1971, Nature 233, 471. Fanelli, M., Bellucci, L., and Nachira, F.: 1982, Contributo alia Conoscenza delle Risorse Geotermiche del

Territorio ltaliano, Ed. Consiglio Nazionale Ricerche, Progetto Finalizzato Energetica. Ferrara, R., Maserti, B., Petrosino, A., and Bargagli, R.: 1986, Atmospheric Environ. 20, 125. Gazetta Ufficiale Italiana: 1986, Istituto Poligrafico di Stato 120, 1.

MERCURY POLLUTION IN THE SOIL AND MOSSES 119

Guglielmetti, M.: 1985, Geothermics 14, 157. Kudo, A. and Hart, S. J.: 1974, J. Environ. Quality 3, 273. Legittimo, P. C., Piccardi, G., and Martini, M.: 1986, Chemistry in Ecology 2, 219. Lindberg, S. E., Jackson, D. R., Huckabee, J. W., Janzen, S. A., Levin, M. J., and Ltmd, J. R.: 1979, J.

Environ. Qual. 8, 572. Lodenius, M. and Tulisalo, E.: 1984, Bull. Environ. Cont. ToxicoL 32, 439. Nieboer, E., Richardson, D. H. S., Boileau, L. J. R., Beckett, P. J., Lavoie, P., and Padovan, D.: 1982,

Environ. Poll. Ser. B 4, 181. Rasmussen, L.: 1977, Environ. Poll. 14, 37. Robertson, D. E., Crecelins, E. A., Fruehter, J. S., and Lndwick, J. D.: 1977, Science 196, 1094. Sabroux, J. C.: 1982, Bolletin Vulcanologique 45, 277. Siegel, S. M. and Siegel, B. Z.: 1975, Environ. Sci. Technol. 9, 473. Siegel, B. Z. and Siegel, S. M.: 1978, Environ. ScL Techndl. 12, 1036. Siegel, B. Z., Siegel, S. M., and Thorarinsson, F.: 1973, Nature 241, 526. Stoeppler, M. and Backhaus, F.: 1978, Fresenius Z. Anal Chem. 9, 116. U.N. Economic Commission for Europe: 1982, Seminar on the Medium-Term and Long-Term Prospects for

the Electric Power Industry, Erhevic, U.V. General Report. Weissberg, B. G. and Zobel, M. G. R.: 1973, Bull. Environ. Toxicol. 9, 148. Wohlenberg, J.: 1985, Geothermics 5/6, 615.